Romuald Lepers
email: romuald.lepers@u-bourgogne.fr

Maffiuletti Na

Age and gender interactions in ultra-endurance performance: insight from triathlon

Keywords: Swimming, Cycling, Running, Ultra-endurance exercise, Gender difference, Aging

Purpose: i) to investigate the effect of age on gender difference in Hawaii Ironman triathlon performance time, and ii) to compare the gender difference between swimming (3.8 km), cycling (180 km), and running (42 km) performances as a function of age. Methods: Gender difference in performance times and estimated power output in the three modes of locomotion were analysed for top 10 men and women amateur triathletes between 18 and 64 yrs of age for three consecutive years (2006 to 2008). Results: The gender difference in total performance time was stable until 55 yrs of age and then significantly increased. Mean gender difference in performance time was significantly (P  0.01) smaller for swimming (mean  CI 95%: 12.1  1.9%) compared to cycling (15.4  0.7%) and running (18.2  1.3%). In contrast, mean gender difference in cycling estimated power output (38.6  1.1 %) was significantly (P  0.01) greater compared to swimming (27.5  3.8 %) and running (32.6  0.7 %).

Conclusion:

This cross-sectional study evidenced that gender difference in ultra-endurance performance such as an Ironman triathlon was stable until 55 yrs of age and then increased thereafter and differed between the locomotion modes. Further studies examining the changes in training volume and physiological characteristics with advanced age for men and women are required to better understand the age-associated changes in ultra-endurance performance.

INTRODUCTION

Paragraph 1. The gender difference in endurance (events lasting less than 2-3 hours) and ultra-endurance (events lasting more than 3 hours) performance has attracted considerable attention over the last 30 yrs. The majority of studies on gender difference have focused on running performance (e.g. [START_REF] Coast | Do gender difference in running performance disappear with distance[END_REF][START_REF] Cheuvront | Running performance differences between men and women. An update[END_REF][START_REF] Hoffman | Ultramarathon trail running comparison of performance-matched men and women[END_REF][START_REF] Hoffman | Performance trends in 161-km ultramarathons[END_REF][START_REF] Leyk | Age-related changes in marathon and half-marathon performances[END_REF][START_REF] Sparkling | The gender difference in distance running performance has plateaued: an analysis of world rankings from 1980 to 1996[END_REF], but less have analysed the gender difference in swimming [START_REF] Stefani | The relative power out put and relative lean body mass of World and Olympic male and female champions with implications for gender equity[END_REF][START_REF] Tanaka | Age and gender interactions in physiological functional capacity: insight from swimming performance[END_REF], cycling [START_REF] Schumacher | Development of peak performance in track cycling[END_REF] or triathlon performance [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF][START_REF] Sultana | Effects of age and gender on Olympic triathlon performances[END_REF]. The 10-15% sex gaps in endurance performance appear to be biological in origin; as men possess a greater aerobic capacity and muscular strength, the gap in endurance performances between men and women is unlikely to narrow naturally (4).

Paragraph 2. Knowing that the physiological (e.g. muscle strength, oxygen carrying capacity) and morphological (e.g. percentage of body fat, muscle mass) functional characteristics change with advancing age, gender difference in endurance performance may also change with advancing age. Concerning the gender-specific differences in loss of muscle mass with age (sarcopenia), some studies have shown that elderly women may lose it more rapidly than do their male counterparts [START_REF] Phillips | Muscle weakness in women occurs at an earlier age than in men, but strength is preserved by hormone replacement therapy[END_REF][START_REF] Samson | Relationships between physical performance measures, age, height and body weight in healthy adults[END_REF]; even it is not a general finding. Few studies have however, investigated the combined interaction of age and gender on endurance performance. For example, the increase in 10-km running time with advancing age has been found to be greater in women compared with men [START_REF] Tanaka | Invited Review: Dynamic exercise performance in masters athletes: insight into effects of primary human aging on physiological functional capacity[END_REF]. Similarly, Tanaka & Seals [START_REF] Tanaka | Age and gender interactions in physiological functional capacity: insight from swimming performance[END_REF] reported that the rate of decline in swimming performance with age was greater in women than in men. Taken as a whole, these findings suggest the possibility of an increased sex gap in endurance performance with advanced age. Paragraph 3. Age and gender interaction in endurance performance could also be influenced by the type and duration of the physical task performed. Changes in running performance do not necessarily reflect aging and gender interactions in other types of endurance activities. In fact, Tanaka & Seal [START_REF] Tanaka | Age and gender interactions in physiological functional capacity: insight from swimming performance[END_REF] showed that endurance 1500-m swimming performance decreased with age in men and women but the pattern of decline was somewhat different from that observed with long-distance running [START_REF] Joyner | Physiological limiting factors and distance running: influence of gender and age on record performances[END_REF][START_REF] Tanaka | Invited Review: Dynamic exercise performance in masters athletes: insight into effects of primary human aging on physiological functional capacity[END_REF]. Indeed, the magnitude of overall reduction in swimming performance with advancing age appeared to be smaller than that observed in running performance. However, these results obtained from different populations limit the interpretation of the above findings. The effect of age on gender difference in endurance and ultra-endurance performances remains to be clarified for the different modes of locomotion (e.g. swimming, cycling and running).

Paragraph 4. In this regard, triathlon involving swimming, cycling, and running, represents an intriguing alternative model to analyse the age and gender interaction in endurance performance because gender difference can be analyzed in the same subjects collectively and also for the three disciplines separately [START_REF] Bernard | Age related decline in Olympic triathlon performance: effect of locomotion mode[END_REF][START_REF] Lepers | Age-related changes in triathlon performances[END_REF][START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF]. However, it should be pointed out that due to the fatigue accumulated during the previous disciplines, running performance in a triathlon can not be compared with running alone. A previous study conducted on elite triathletes at the Hawaii Ironman triathlon showed that mean gender difference tended to be smaller in swimming compared to cycling and running [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF]. However, this study focused only on the top 10 men and women overall finishers and did not provide a complete representation of gender difference with age.

Paragraph 5. Accordingly, the aims of the present study were: 1) to examine the effect of age on gender difference in Ironman triathlon total time, and 2) to compare the gender difference as a function of age among swimming, cycling, and running. To achieve these aims, a retrospective analysis of top amateur performance times aged from 18 to 65 yrs was performed at the Hawaii Ironman triathlon, which is the top race in the field of longdistance triathlon [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF]. We anticipated that gender differences in total time performance would increase with advancing age and that gender difference in performance time would be lower in swimming compared to cycling and running. METHODS Paragraph 6. Approval for the project was obtained from the Burgundy University Committee on Human Research. This study involved the analysis of publicly available data so content was waived. Age and time performances data for all triathletes completing the races were obtained through the Ironman corporation web site: http://ironman.com/events/ironman. Paragraph 9. The magnitude of gender difference was examined by calculating the percent difference for the average totals of swimming, cycling, and running times between the top men versus women of each age group. Because of the non-linear relation between speed and power output from air or water resistance [START_REF] Stefani | The relative power out put and relative lean body mass of World and Olympic male and female champions with implications for gender equity[END_REF], percent differences in time do not equate to percent differences in power output. Therefore, we estimated the percent difference in power output between the men and women for each discipline according to Lepers [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF].

Briefly, for swimming the mechanical power (P) depends on the third power of velocity (V); according to the model proposed by Stefani (27), the swimming power ratio between men (m) and women (w) is: 3 Similarly for cycling, the mechanical power also depends on the third power of velocity; assuming level terrain, no wind, and a negligible contribution of rolling resistance to total power demand, the cycling power ratio between men and women is:

P w / P m = 0.91 (V w / V m )
P w / P m = 0.93 (V w / V m ) 3
For running, the mechanical power depends on the velocity and on the body mass [START_REF] Stefani | The relative power out put and relative lean body mass of World and Olympic male and female champions with implications for gender equity[END_REF]; accordingly, running power ratio between men and women is:

P w / P m = 0.79 (V w / V m )
Finally, the percentage difference in estimated power between women and men was calculated as 100 x (1-P w / P m ) for the three disciplines.

Paragraph 10. The percentage difference between the man and woman in swimming, cycling, running and total event performance times and in estimated power outputs for each of the top 10 places and for each year in each age group was determined. One-way ANOVA was used to compare gender difference in total time across ages. Two-way ANOVAs (age group x mode of locomotion) with repeated measures on locomotion mode were used to compare gender difference in time and power output between swimming, cycling, and running across ages. Tukey's post hoc analyses were used to test differences within the ANOVAs when appropriate. A significance level of P < 0.05 was used to identify statistical significance.

RESULTS

Paragraph 11. The swimming, cycling, running and total performance times of the 10 top men and women are presented in Figure 1. Compared to the fastest men's age groups (30-34 or 35-39 yrs), time performance of the slowest men's age group (60-64 yrs) was longer for total event (27%), swimming (30%), cycling (22%), and running (32%). Compared to the fastest women's age groups (25-29 or 30-34 yrs), time performance of the slowest women age group (60-64 yrs) was longer for total event (38%), swimming (43%), cycling (31%), and running (47%). For both men and women, standard deviation (indicating within-group variability) for total event and for the three disciplines increased considerably with advancing age, particularly for women. Paragraph 12. Figure 2 shows that gender differences in total event time were affected by age (F = 78.8; P < 0.001). Gender difference for age groups 55-59 and 60-64 yrs were significantly (P < 0.01) greater compared to younger age groups, and gender difference for age group 55-59 yrs was significantly (P < 0.05) lower than for age group 60-64 yrs. Paragraph 13. There was no significant age group x locomotion mode interaction for gender difference in performance time (F = 1.02; P = 0.42). Independently of age, gender differences in time differed between the locomotion modes (F = 16.17; P < 0.001). Gender difference in swimming time was significantly (P < 0.01) lower than for cycling and running (Figure 3A). Gender difference in cycling time was significantly (P < 0.05) lower compared to running. There was no significant age group x locomotion mode interaction for gender difference in estimated power output (F = 0.94; P = 0.52). Independently of the age, gender differences in estimated power output differed between the locomotion modes (F = 20.88; P < 0.001). Gender difference in cycling power output was significantly (P < 0.01) greater than for swimming and running (Figure 3B). Gender difference in swimming power output was significantly (P < 0.01) lower compared to running. DISCUSSION Paragraph 14. The main findings of the present study were first that gender differences in Ironman triathlon total performance time were stable until 55 yrs of age after which they significantly increased. Second, gender difference in performance time (swim < cycle < run) and estimated power output (swim < run < cycle) differed between the disciplines (i.e. mode of locomotion) of triathlon.

Paragraph 15. Although it lacked a number of physiological parameters, such as aerobic capacity, body weight, lean body mass, and training volume, this study offers valuable data. Elderly triathletes are able to maintain a high degree of physiological plasticity late into life and represent a fascinating model of exceptionally successful ageing [START_REF] Tanaka | Endurance exercise performance in Masters athletes: age-associated changes and underlying physiological mechanisms[END_REF]. The approach that consists of examining the changes in endurance performance with age in highly trained and competitive athletes represents an effective experimental model because changes observed with advancing age are thought to reflect mainly the results of primary (physiological) aging [START_REF] Tanaka | Invited Review: Dynamic exercise performance in masters athletes: insight into effects of primary human aging on physiological functional capacity[END_REF]. The nature of the event (a world championship) meant that athletes likely performed to their maximal capacity, from both a mental and physical perspective.

Gender difference in total performance time increases with age

Paragraph 16. The results suggested that the best age for the fastest Ironman total performance time is comprised between 25 and 39 yrs for both men and women. This was also true for cycling and running performance, where men and women over the 25-39 yrs age range were faster compared to younger and older age groups. In contrast, the swimming performance times for both men and women remained quite similar between a larger range of years i.e. between 18 and 44 yrs. It has been suggested that the age-related decrease in maximal oxygen uptake seems to be the most clear and consistent contributor to the decline in endurance performance with advancing age [START_REF] Tanaka | Endurance exercise performance in Masters athletes: age-associated changes and underlying physiological mechanisms[END_REF]. Reductions in lactate threshold may also contribute, whereas submaximal exercise economy seems preserved with ageing in endurance athletes [START_REF] Tanaka | Endurance exercise performance in Masters athletes: age-associated changes and underlying physiological mechanisms[END_REF]. Our results showed that gender difference in total event performance time increased significantly with advancing age from 55 yrs. Men triathletes at the age of 60 yrs were on average 27% slower than the 30-40 yrs old triathletes, while the difference reached 38% for women. The exact reasons for these sex-related differences are not clear but could be due to a greater decline of one or more physiological determinants of endurance performance for women compared to men (e.g. maximal oxygen uptake, lactate threshold, and exercise economy). Even if one previous study [START_REF] Holloszy | Exercise. In: Bethesda, MD. Handbook of Physiology[END_REF] suggested that the relative rates of decline in maximal oxygen consumption with age are similar between men and women, age-related changes in the physiological determinants of endurance and ultra-endurance performances between men and women have not received considerable attention. Paragraph 17. However, interpretation of such cross-sectional comparisons of triathlon performance times across ages and sexes must be made carefully. It is possible that non-physiological factors may have contributed to these observations. For example, the widening of gender difference with advancing age could be due at least in part to the fewer number of women triathletes in the older age groups. The percent of women participating at the Hawaii Ironman triathlon during the studied period corresponded on average to 27%, but "finisher" women in the age group 60-64 yrs were usually less than twenty. This participation difference will no doubt diminish over the next couple of decades (as the % of women participating to Hawaii was only 10-15% in the 90s, http://ironman.com/events/ironman). As a result, triathlon performances of the oldest women will probably improve more rapidly than those of the oldest men, as the new generation of well-trained young women athletes moves into older age group competition.

Paragraph 18. It is well established that the reduction in training volume and intensity

may contribute to the larger declines in endurance performance of the elderly athletes [START_REF] Fitzgerald | Age related decline in maximal aerobic capacity in regularly exercising vs. sedentary females: a meta-analysis[END_REF][START_REF] Pollock | Twenty-year follow-up of aerobic power and body composition of older track athletes[END_REF]. Some studies of older endurance athletes have indicated that the training volume of master athletes often is up to 50% lower compared to their younger peers [START_REF] Fitzgerald | Age related decline in maximal aerobic capacity in regularly exercising vs. sedentary females: a meta-analysis[END_REF][START_REF] Pollock | Twenty-year follow-up of aerobic power and body composition of older track athletes[END_REF]. Differences in terms of years of training, training volume and intensity between elderly men and women triathletes performing Ironman triathlon may exist but further work is required to clarify this.

It would also require long-term follow up of training diaries for all participants.

Gender difference differs between locomotion modes

Paragraph 19. For 3.8-km swimming the magnitude in gender difference over the 18-64 yrs age range was 12% in performance time and 28% in estimated power output. These values are in agreement with previous studies that examined long-distance swimming performance [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF]. Tanaka & Seals (29) examined gender differences in swimming performance time for distances from 50 to 1500 m, and found that gender difference became smaller as swim distance increased ranging from  19% for 50-m to  11% for 1500-m freestyle event. Because swimming is the first discipline of triathlon and therefore triathletes perform it in a relatively non fatigued state, in the future, it would be interesting to compare gender difference in swimming performance during an Ironman distance triathlon with gender difference during a long distance swim race. Independently of age, gender difference in performance time was smaller for swimming compared to cycling and running. One explanation for these differences between locomotion modes is the higher economy and mechanical efficiency of swimming in women compared to men. In contrast to running where the oxygen cost appears to be similar between women and men (e.g. [START_REF] Pate | Cardiorespiratory and metabolic responses to submaximal and maximal exercise in elite women distance runners[END_REF], the energy cost of freestyle swimming has been shown to be significantly higher (i.e. lower economy) in men [START_REF] Pendergast | Quantitative analysis of the front crawl in men and women[END_REF]. The higher mechanical efficiency of women has been attributed to smaller body size (resulting in smaller body drag), smaller body density (greater fat percent), and shorter lower limbs, resulting in a more horizontal and streamlined position and therefore a smaller underwater torque [START_REF] Lavoie | Applied physiology of swimming[END_REF][START_REF] Pendergast | Quantitative analysis of the front crawl in men and women[END_REF].

Paragraph 20. For cycling, the gender difference over the 18-64 yrs age range was 15% for performance time and 39% for estimated power output. Compared to elite triathletes ( 14), these values are slightly greater for performance time (13% in Lepers ( 14)) but in a similar range for estimated power output (38% in Lepers ( 14)). However, power output gender difference in the present study might be underestimated because total elevation (1400 m positive elevation over 180 km) is not accounted for in the cycling power model.

There is a paucity of data in the literature concerning gender difference in cycling performance. In track cycling, where women are weaker than men in terms of power/weight ratios, the performance gap between men and women appeared constant (11%), and independent of the race distance from 200 to 1000 m [START_REF] Schumacher | Development of peak performance in track cycling[END_REF]. Official time-trial road-cycling championships generally take place on distances much shorter than 180-km and distances are greater for men than for women. Greater muscle mass and aerobic capacity in men, even expressed relative to the lean body mass (e.g. 15), may represent an advantage during long distance cycling, especially on a relative flat course such as Ironman cycling, where cycling approximates a non weight-bearing sport. Indeed, it has been shown that absolute power output (which is greater for men than for women) is associated with successful cycling endurance performance, because the primary force inhibiting forward motion on a flat course is air resistance [START_REF] Lucia | Physiological response to professional road cycling: climbers vs time trialists[END_REF].

Paragraph 21. In running, gender difference over the 18-64 yrs age range was equal to 18% in performance time and 33% in estimated power output. The gender difference in performance time is greater than the difference found between elite men and women triathletes at the same Ironman triathlon (13% in Lepers ( 14)) but is similar to the values observed by Tanaka & Seals (29) for a 10-km running in masters athletes. It may be problematic to compare gender difference in an Ironman triathlon marathon with running performance during a marathon alone. However, Lepers [START_REF] Lepers | Analysis of Hawaii ironman performances in elite triathletes from 1981 to 2007[END_REF] showed that for the top 10 finishers the gender difference in running at the Hawaii Ironman marathon was similar to running at the New-York marathon (13%) suggesting that the swim and cycle portions of the triathlon do not exacerbate the gender difference in running. However, it could also be interesting in future to compare for the same athletes the personal best time in a marathon alone and the best performance time in Ironman triathlon marathon, in order to examine whether fatigue differs between genders during a marathon of an Ironman triathlon. Previous studies suggested that gender difference in running could diminish as distance increases past the marathon (e.g. 1). Possible physiological advantages for women over men in long distance running include greater fat utilization (and thus better carbohydrate conservation) [START_REF] Venables | Determinants of fat oxidation during exercise in healthy men and women: a crosssectional study[END_REF], higher proportions of maximal oxygen uptake [START_REF] Speechly | Differences in ultraendurance exercise in performance-matched male and female runners[END_REF], or greater muscle fatigue resistance [START_REF] Hunter | Men are more fatigable than strengthmatched women when performing intermittent submaximal contractions[END_REF]. However, recent analysis of ultramarathon running performance suggested that gender difference remained stable for distances ranging from 50-km to 161-km [START_REF] Hoffman | Ultramarathon trail running comparison of performance-matched men and women[END_REF], with the fastest women running about 14-20% slower than the fastest men [START_REF] Coast | Do gender difference in running performance disappear with distance[END_REF][START_REF] Hoffman | Performance trends in 161-km ultramarathons[END_REF]. Morphological (body fatness) and physiological differences between sexes such as greater maximal oxygen uptake and oxygen carrying capacities (haemoglobin concentration) may partly explain the gender difference in distance running performance [START_REF] Levis | Physiological differences between genders. Implications for sports conditioning[END_REF]. In contrast, running economy and lactate threshold do not seem to differ between men and women (e.g. 26).

Paragraph 22. Gender difference in actual performance time was greater for running (18%) compared to swimming (12%) and cycling (15%), but in contrast gender difference in estimated power output was greater for cycling (39%) compared to swimming (28%) and running (33%). The calculation of power in swimming and cycling is accurate only if the triathletes do not draft (i.e., swim or cycle behind another athlete). However, drafting during swimming was not controlled and therefore it might have influenced the assumed drag coefficient, thus invalidating the estimation of power output in swimming. At Hawaii Ironman, triathletes are not allowed to draft in cycling (drafting is consistently checked by marshals) suggesting that the estimated power output in cycling is not affected by this confounding variable. Power output in swimming and cycling is related to an exponential expression of velocity while it is directly related to velocity for running. A difference of 15% between men and women in cycling performance time (or velocity) does not therefore correspond to a difference of 15% in physiological capacity since power output in cycling depends on the third power of velocity. Because power output is proportional to oxygen uptake, the magnitude of gender difference in power output provides a more correct representation of underlying gender difference in physiological capacity [START_REF] Seiler | The fall and rise of the gender difference in elite anaerobic performance 1952-2006[END_REF].

Conclusion

Paragraph 23. The present study shows that the relative difference in performance time between women and men for an ultra-endurance event such an Ironman triathlon is stable until 55 yrs of age and then increases. The reasons for this increase are not clear and further studies examining in particular the changes in the training characteristics (volume, intensity) in elderly triathletes are required to better understand the age-related changes in Ironman triathlon performances. This study, conducted on amateur triathletes, also confirms previous findings obtained on elite triathletes showing that gender difference differs between the locomotion modes (14), i.e., gender difference for swimming performance being smaller compared to running and cycling. However, the magnitude of gender difference for swimming, cycling and running varies when performance time or estimated power output are considered. Further investigations are required in order to analyse gender differences in performance time for ultra-endurance events of different modes of locomotion such as openwater ultra-swim, ultra-cycling and running events (e.g. 11), to see if gender differences change with length and nature of the endurance events. LEGENDS Figure 1 Swimming, cycling, running, and total performance times for the top 10 men and women in each age group at the Hawaii Ironman triathlon (2006, 2007, 2008 pooled data).

Values are mean  SD.

Figure 2 AveragedFigure 3

 23 Figure 2

amateur of nine age groups for women and men, respectively were analyzed for three consecutive Hawaii Ironman races in 2006, 2007 and 2008. We focused our attention on

	10:14:37  12:50 h:min:s; men: 9:04:01  8:20), 2007 (women: 10:22:10  10:18; men:
	9:10:34  6:11), and 2008 (women: 10:17:16  12:56; men: 9:11:57  10:06), data were
	pooled together.
	The Hawaii Ironman triathlon (3.8 km swim, 180-km cycle, and 42-km run) has been held
	each year since 1982 in Kailua-Kona (Big Island, Hawaii, USA). The Hawaii Ironman
	triathlon is the last in a series of nearly 25 Ironman triathlons held in the world that serve as
	qualifying races. Only best amateur triathletes in their age group and elite triathletes can
	participate in the final in Hawaii that is considered as the Ironman World Championship for
	both amateur and elite triathletes.
	Paragraph 7. Averaged swimming, cycling, running and overall time performances of
	the top 10 these three years because we felt that it would represent a better insight into current
	performance. The Ironman age groups (http://ironman.com/events/ironman) distinguish the
	categories as follows: 18-24 yrs, 25-29 yrs, 30-34 yrs, 35-39 yrs, 40-44 yrs, 45-49 yrs, 50-54
	yrs, 55-59 yrs, and 60-64 yrs.
	Paragraph 8. The performances of 30 triathletes per age group (i.e. the top 10 of each
	age group in 2006, 2007 and 2008) were considered. Because one-way ANOVA showed no
	significant difference for the top 10 of the best performing age group between 2006 (women:

Acknowledgments

The author would like to extend his gratitude to Paul Stapley (Mc Gill University, Montreal, Canada) for helpful suggestions concerning the manuscript.

None funding has been received for this work from NIH, Wellcome Trust, HHMI or others.


Benoît Gabrielle
email: benoit.gabrielle@agroparistech.fr

Nicolas Nguyen The
email: nicolas.nguyen-the@fcba.fr

Pauline Maupu
email: pmaupu@invivo-group.com

Estelle Vial
email: estelle.vial@fcba.fr

Life-cycle assessment of eucalyptus short-rotation coppices for bioenergy production in Southern France

Keywords: Short Rotation Coppice, Life Cycle Assessment, Carbon storage, Eucalyptus, Biomass, GHG emissions

Numerous international and national policy frameworks were recently put into place to promote renewable energy sources, including biomass. Among the wide range of possible feedstocks, dedicated energy crops such as short rotation coppices (SRCs) are considered prime candidates. They produce good-quality biomass that is easy to harvest, while reducing the competition for forest products between energy and other end-uses when grown on agricultural land. Besides technical, social and economical aspects, environmental issues are important to take into account when developing SRCs. For this purpose, a life cycle assessment (LCA) was implemented to provide an accurate and comprehensive estimate of the environmental impacts of delivering 1 GJ of heat from SRC wood chips. The LCA was applied to various scenarios of eucalyptus SRC in France, based on the established SRC pulp scheme and extended to more theoretical systems of very short rotation coppices (VSRCs) with 3-year rotations.

Compared to equivalent fossil chains, all eucalyptus scenarios achieved savings of fossil energy and greenhouse gas (GHG) emissions in the 80%-90% range. The transportation of wood chips contributed the highest share of fossil primary energy consumption and GHG emissions. The second most important item was fertilization, especially in the case of the VSRC schemes due to the evergreen character of eucalyptus.

The possibility of including ecosystem carbon dynamics was also investigated, by translating the temporary sequestration of atmospheric CO 2 in the above-and below-ground biomass of eucalyptus, relative to a reference land use (in this case a land parcel reverting to wilderness after removal of a vineyard) as CO 2 savings using various published equivalence factors. This offset the life-cycle GHG emissions of heat provision from eucalyptus SRCs by 70 to 400%.

Introduction

The recent European Directive on renewable energy set ambitious targets for all Member States, in order for the EU to reach a 20% share of energy from renewable sources by 2020 (European Commission, 2009). Amongst renewable energy sources, the biggest contribution (63%) may come from biomass, as suggested by a foresight analysis in Europe (European Commission, 2005). At present, biomass already contributes about 4% of the total EU energy supply, predominantly as heat, and combined heat and power applications to a lesser extent. The production of liquid biofuels for transport from biomass increased severalfold in the last decade, and is currently a major issue.

Among various sources of biomass (organic waste, forestry products, cereal straw, etc.), dedicated crops such as short rotation coppices are currently being investigated. These systems involve the cultivation of a fast growing ligneous species with short to very short harvesting cycles. Species with a capacity to sprout after cutting are particularly interesting as they make it possible to harvest the same plantation several times over the lifetime of the trees. Eucalyptus (Eucalyptus sp.) is one of the most widely known species used for biomass oriented short rotation coppice, particularly for pulp and paper industries (Iglesias-Trabado and Wisterman, 2008). Poplar (Populus sp.) and willow (Salix sp.) have been used more recently for energy purpose for example in northern Europe [START_REF] Lindroth | Assessment of regional willow coppice yield in Sweden on basis of water availability[END_REF]Båth, 1999, Wilkinson et al., 2007) or in Italy [START_REF] Manzone | Energetic and economic evaluation of a poplar cultivation for the biomass production in Italy[END_REF].

In France, short rotation coppices (SRCs) were developed with poplar and eucalyptus in the mid 1980's on the initiative of pulp companies. Nowadays, some 2000 ha of pulp SRC are still present although only eucalyptus is still being used in the south-western part of France with an average rate of 100-200 ha planted every year (Nguyen The et al., 2004). The typical plantation scheme is based on 10-year rotations with a stand density of 1250 stems ha -1 (on a 4 m x 2 m grid). Three harvests in 30 years are expected with an average productivity of 10 oven-dry metric tons (ODT) ha -1 yr -1 with the currently-used specie: E. gundal, an hybrid between E. gunnii and E. dalrympleana [START_REF] Cauvin | Guide de culture du TCR Eucalyptus[END_REF]. The recent drive for renewable energy sources and concerns with the sustainability of biomass production [START_REF] Robertson | Sustainable Biofuels Redux[END_REF][START_REF] Scharlemann | How Green Are Biofuels?[END_REF] have sparked interest for SRC given its presumed low environmental impacts since it requires less inputs than agricultural crops (WWI, 2006).

The traditional pulp and paper SRC scheme may be directly transposed to biomass production for biofuel, heat or power production purposes. Since SRC is expected to be mainly grown on former cropland, silvicultural schemes with shorter cycles than the traditional 10 year pulp rotation, are being investigated in order to be closer to usual farming systems. Growing cycles may be shortened to 7 years with the same productivity as long as stand density is kept within a 2000-2500 stems ha -1 range, as was already tested with poplar (Berthelot et al., 2004). Similarly, so-called very short rotation coppice (VSRC) are being tested and developed with an objective of 3-year harvesting cycles. This scheme was illustrated with willow [START_REF] Dimitriou | Des saules pour l'énergie et la phytoremédiation en Suède[END_REF], and requires far higher stand densities, between 10 000 stems ha -1 and 15 000 stems -ha -1 . Such systems are currently being trialled in France with eucalyptus and poplar.

Independently of economic and technical issues, it is important to consider the environmental performance of these new energy crops. Several issues were raised regarding their actual GHG benefits, impacts on water resources or biodiversity [START_REF] Robertson | Sustainable Biofuels Redux[END_REF][START_REF] Monti | Cradle-to-farm gate life cycle assessment in perennial energy crops[END_REF]. Here, we chose the LCA methodology to address these issues for eucalyptus SRC, since it is widely-used for bioenergy assessment and is a multi-criteria, holistic method (von Blottnitz and Curran, 2007;[START_REF] Cherubini | GHG balances of bioenergy systems -Overview of key steps in the production chain and methodological concerns[END_REF]. No such assessments have been reported for eucalyptus SRC, to the best of our knowledge, although they exist for traditional eucalyptus forests [START_REF] Jawjit | An analysis of the environmental pressure exerted by the eucalyptus-based kraft pulp industry in Thailand[END_REF][START_REF] Lopes | Application of life cycle assessment to the Portuguese pulp and paper industry[END_REF]. There is also a growing literature on the LCA of other lignocellulosic feedstocks, whether annual arable crops [START_REF] Kim | Life cycle assessment of various cropping systems utilized for producing biofuels: Bioethanol and biodiesel[END_REF], perennial grasses such as miscanthus and switchgrass [START_REF] Monti | Cradle-to-farm gate life cycle assessment in perennial energy crops[END_REF][START_REF] Shurpali | Atmospheric impact of bioenergy based on perennial crop (reed canary grass, Phalaris arundinaceae, L.) cultivation on a drained boreal organic soil[END_REF], or other types of SRC such as willow and poplar [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF][START_REF] Goglio | A screening LCA of short rotation coppice willow (Salix sp.) feedstock production system for small-scale electricity generation[END_REF], whose performance may be compared with eucalyptus. The objectives of this work were two-fold: i/ to apply LCA to eucalyptus SRCs in southern France, based on the currently existing pulp scheme, and extended to very-short rotation coppices (VSRCs), and ii/ to investigate the possibility of including the temporary storage of atmospheric CO 2 in ecosystem carbon pools in the GHG balance of heat provision from eucalyptus SRC, following the approach suggested by Moura-Costa and Wilson (2000) for forest products. Eucalyptus biomass was used to generate heat, and compared to equivalent fossile energy sources.

Materials and methods

The eucalyptus pulp SRC system was chosen as a basis for the study. This species and its silvicultural scheme have been studied in France for almost 30 years and many technical references already exist [START_REF] Cauvin | Guide de culture du TCR Eucalyptus[END_REF]. This SRC was designed for pulp production but may easily be extended to bioenergy production.

Scope, functional unit and system boundaries for the LCA

The function studied here is heat production from the combustion of SRC wood chips in a boiler. The functional unit selected was therefore 1 GJ of final heat, which means that lifecycle impact indicators were calculated relatively to the production of 1 GJ of heat.

The system studied is described on Figure 1, and comprises five main stages:

1. The production of cuttings from selected eucalyptus clones, which corresponds to current practices. It includes the production of mother trees in a biotechnology facility and transportation to a nursery. In the inventory, we used data pertaining to a research laboratory, therefore not designed nor optimized an industrial-scale production of cuttings.

2. Plantation establishment and removal, including site preparation, fertilization, plantation and weed control during the first 2 years, as well as stump removal at the end of the project.

3. Harvest, including felling, forwarding and chipping for SRCs and silage harvester for VSRCs. This stage also includes the transportation of harvesting machines to the tree parcel.

4: Transportation of wood chips from the collection site to the boiler. We used a distance of 80 km corresponding to the actual average distance between eucalyptus plantations and the pulp mill of Saint-Gaudens (South-Western France).

5: Handling and combustion of wood chips in a boiler.

Management scenarios

The reference scenario was the pulp SRC scheme based on three 10-year harvest cycles (ie a total duration of 30 years), with a stand density of 1250 stems ha -1 . From this baseline we designed a scenario dedicated to biomass production for energy by doubling the stem density (2500 stems ha -1 ) with three harvests every 7 years for a total duration of 21 years. Next, a very short rotation coppice (VSRC) scenario was designed with a density of 5000 stems ha -1 , which represents in the present context the maximum possible density considering the costs of eucalyptus cuttings. The scenario plans harvests every 3 years, that is 7 successive harvests over the same 21-year time interval.

A set of technological variants technical aspects likely to influence LCA results were considered to enlarge the number of management scenarios:

1. Harvest mechanization: approximately 50% of pulp SRCs are currently harvested with felling machines rather than manual felling with chainsaws. Felling machines have a better productivity and make mechanical debarking possible in the field, which results in higher rates of nutrient returns to soils. On the other hand, felling machines consume more fuel and emit more GHGs. VSRCs are usually harvested with adapted agricultural harvesters.

2. Productivity: for SRCs, a yield of 10 oven dry metric tons (ODT) ha -1 yr -1 considered as a robust average value taking into account the mortality of trees and their partial ground cover. It corresponds to a final cut at a diameter of 7 cm (commercial cut). The full stem harvest leads to an extra 20% of biomass, including leaves (Nguyen The and Deleuze, 2004). For the 2nd and 3rd harvest, a 25% gain in biomass production is usually observed due to a faster growth (D. Lambrecq, Fibre excellence, Saint-Gaudens, pers. comm.). For VSRCs, for lack of more accurate references, we assumed the same average figure of 10 ODT ha -1 yr -1 3. Fertilizer inputs: Pulp SRCs are currently not fertilized in France because it is not considered as a relevant operation for the sustainability of biomass production. Nevertheless, this is a very critical point, especially for VSRCs whose nutrient exports are expected to be significantly higher thant SRCs. Therefore, we assumed in all scenarios fertilizer input rates corresponding to the estimated exports of nutrients at harvest. The differences between scenarios were particularly acute across harvesting techniques, whether including debarking (with the mechanical harvest) or harvesting full stems or logs. Eucalyptus being an evergreen species, harvesting full stems rather than wood logs would lead to far larger nutrient exports because of the high nutrient contents of the leaves. The amount of N, P and K applied were calculated using state-of-the-art knowledge and data on nutrient exports of VSC and VSRC with eucalyptus in France (Nguyen The et al., 2004 and2010a) and atmospheric deposition rates [START_REF] Croisé | RENECOFOR -Deux approches indépendantes pour l'estimation et la cartographie des dépôts atmosphériques totaux hors couvert forestier sur le territoire français[END_REF].

As a result of the above variants, a total set of 5 scenarios was implemented, whose characteristics are summarized in Table 1.

LCA methodology

The cut-off threshold for neglecting system components was set at 3.6 10 -6 %. The production of laboratory equipment was excluded because cuttings production was only a marginal part in the use of this equipment overt its total life cycle. The transportation of pesticides and fertilizers (N, P, K and Mg fertilizers in the nursery, herbicides for site preparation and plantation maintenance, field fertilization) were not taken into account due to a lack of accurate information.

Chemical inputs in the nursery were exclusively attributed to the production of cuttings, except for fungicides and hormones which were neglected due to the very low dosages used.

Nursery propagators were also excluded due to the lack of information on this material (jiffy pellets made from peat). Neither waste nor co-products are produced during the life cycle of SRCs, which alleviated the need for allocations. As usually assumed in the LCA of bioenergy systems, the global warming potential of the CO 2 emitted during the combustion of biomass was considered nil [START_REF] Cherubini | GHG balances of bioenergy systems -Overview of key steps in the production chain and methodological concerns[END_REF].

LCA calculations were done with the TEAM 4.0 software package (Ecobilan-PWC, Paris) with the EcoInvent 2000 database (V2.01, St-Gallen, Switzerland). Field emissions related to the input of fertiliser N and P were calculated using the methods proposed in the Ecoinvent report (Nemecek et al., 2003). However, the model proposed for nitrate leaching was found unsuitable for eucalyptus, and this flux was thus neglected. The leaching risk was low because fertilizers are usually applied in spring after the winter drainage, and taken up before the onset of drainage in autumn. In addition, nitrate leaching under forests is generally minimal [START_REF] Gabrielle | Life-cycle assessment of straw use in bio-ethanol production: a case-study based on deterministic modelling[END_REF]. Impacts were characterized with the CML (2001) method, as described in [START_REF] Guinée | Handbook on life cycle assessment. Operational Guide to the ISO standards[END_REF], and the following categories considered: non-renewable energy consumption, global warming (with a 100-year timeframe), acidification, eutrophication, and photochemical ozone creation potential (POCP).

Accounting for ecosystem C dynamics and land-use changes

In a first variant relative to our baseline LCA calculations, we investigated the possibility of accounting for the temporary storage of atmospheric CO 2 in the biomass of eucalyptus stands.

The principle is to derive an equivalence factor with permanently-stored CO 2 based on the cumulative radiative forcing of atmospheric CO 2 over time. Moura-Costa and Wilson (2000) derived such as factor from the number of years over which the reduction in radiative forcing would be identical between the temporary and permanent storages. They estimated the duration for break-even to approximately 55 years, yielding an equivalence factor of 1/55 or 0.0182. However, other factors are presently under discussion in relation to carbon trading. Two other factors were thus tested here: a coefficient of 1/26 proposed by the French Ministry for Agriculture [START_REF] Map | methodology to account for the sequestration of carbon in wood stands[END_REF], corresponding to an economical calculation involving an annual discount rate of 4 %, and the 1/100 factor proposed by PAS [START_REF] Bsi | PAS 2050:2008 -Specification for the assessment of the life cycle greenhouse gas emissions of goods and services[END_REF] for consistency with the IPCC time horizon in the cilmate change scenarios (2100). Following the above approach, the temporary effect of C storage may be calculated as :

Mitigating effect (in t CO 2 eq.) = Qc x T x EF
where Qc is the amount of C stored in tree biomass (t C ha -1 ), T is the duration of storage (years), and EF the equivalence factor (unitless). The Qc x T component of the equation actually corresponds to the cumulative sum of C stored through time, except for the last year when the stand is harvested (Figure 1).

The C sequestration of eucalyptus SRC should be compared to a baseline scenario in terms of land-use. Here, we chose abandoned agricultural land (referred to as wildland in the following), which typically occurs after vineyard removal in southern France. Eucalyptus SRCs would therefore be established on former vineyards in our scenario, which excludes indirect land-use change effects. The global C storage was therefore calculated by substracting the C storage of SRC by C storage of wildland.

The carbon stored in the above-ground biomass (AGB) of the eucalyptus stands was calculated from the C content of harvested wood, considering that the C content of biomass was 47% (dry weight basis; Paixao et al., 2006 ;[START_REF] Tanabe | Prepared by the National Greenhouse Gas Inventories Programme[END_REF]. Below-ground biomass (BGB) was estimated with an allometric relationship as a fixed proportion of AGB, set to 30% [START_REF] Tanabe | Prepared by the National Greenhouse Gas Inventories Programme[END_REF].

For the wildland, aboveground biomass was considered constant at 0.9 t C ha -1 yr -1 , which is the peak value for grasslands in warm temperate, dry climates given in the IPCC guidelines for GHG inventories [START_REF] Tanabe | Prepared by the National Greenhouse Gas Inventories Programme[END_REF]. It is in the lower end of the 0.8 -3.2 t C ha -1 yr -1 range reported in Europe for former arable fields up to 3 years after abandonment [START_REF] Hedlund | Plant species diversity, plant biomass and responses of the soil community on abandoned land across Europe: idiosyncracy or above-belowground time lags[END_REF], ie in the early years of fallow regeneration. The belowground biomass was set at 2.0 t C ha -1 yr -1 [START_REF] Tanabe | Prepared by the National Greenhouse Gas Inventories Programme[END_REF], which is slightly lower than the 2.5 -3.5 t C ha -1 yr -1 range in annual returns to soils estimated in the classical Rothamsted (UK) long-term wilderness experiments, where arable fields were allowed to undergo natural woodland regeneration in the 1880's [START_REF] Jenkinson | Calculating net primary production and annual input of organic matter to soil from the amount and radiocarbon content of soil organic matter[END_REF]. In the beginning of the transition from arable to wildland, only herbaceous species are involved and their net annual biomass production is entirely returned to soils as litter. Further on during the 30-year life cycle of the eucalyptus plantation, it is likely that some woody species may also appear in the wildland and start accumulating biomass from one year to the next, although the exact dynamics of that transition has not been documented to the best of our knowledge. Over a longer time-frame, observations in the 'Geescroft wilderness' experiment in Rothamsted (UK), an arable field allowed to undergo natural woodland regeneration in 1885, may give us some insight into this process and provide an upper limit for this component. In this plot, the accumulation of AGB was estimated at 0.6 t C ha -1 yr -1 over the first 100 years of the transition [START_REF] Grogan | Review of the potential for soil carbon sequestration under bioenergy crops in the U[END_REF], which we considered as the upper limit of what would happen in the first 30 years of wildland growth after abandonment (the lower limit being no accumulation at all). A below-to above-ground biomass ratio of 1:3 was assumed for the wildland [START_REF] Grogan | Review of the potential for soil carbon sequestration under bioenergy crops in the U[END_REF], which is similar to the value used for eucalyptus trees.

It is likely that the differences in soil organic carbon (SOC) will appear between the SRC eucalyptus and the baseline land-use over time, due to differences in litter and below-ground inputs [START_REF] Grogan | Review of the potential for soil carbon sequestration under bioenergy crops in the U[END_REF]. However, since eucalyptus SRC systems are relatively recent, there are no long-term experiments documenting the dynamics of SOC after conversion to eucalyptus, let alone comparing them with other land-uses such as arable farming or wildlands. We therefore elected to exclude differences in SOC between eucalyptus SRC and wildland in our analysis. The effect of this hypothesis is addressed in the Discussion section.

Results

LCA results

Life-cycle consumption of non-renewable energy ranged from 77.0 to 92.7 MJ GJ -1 heat output from eucalyptus biomass (Table 2). It was lowest for the S1 scenario with lower stem density and manual harvest, and highest for the very short rotation scenario (S5). In all scenarios, wood chips transport represented the main energy consumption hotspot with a share of 46% to 55 %. Harvesting operations came second with 30 to 36 % of total energy consumption, except for the very short rotation scenario, where their share was only 3.3 %. This is due to the use of an adapted silage harvesting machine instead of heavy, fuelconsuming forestry machines. In the VSRC scenario, the most important steps were fertilization and plant production. Fertilizer inputs were larger than with the SRC schemes because the harvest of whole stems including leaves lead to higher nutrient export rates and enhanced fertilizer requirements. Stem density is also twice higher in the VSRC scenarios compared to the SRC energy scenarios, and this had a significant impact on energy consumption since the production of cuttings takes place in an energy-intensive biotechnology laboratory. The shorter rotations and higher stem densities associated with VSRCs further enhanced this trend, making this scenario the most energy-intensive. Its energy ratio (ratio of heat output to fossil energy inputs) was also the lowest of all scenarios, at 10.8. This ratio increased with decreasing harvesting frequency, leading to the pulp scheme achieving the highest value (13).

Life-cycle GHG emissions (excluding ecosystem C pools) varied in a narrow range for the four SRC scenarios, from 8.2 (S1) to 8.5 (S4) t CO2-eq. GJ -1 . They were 50% higher for the VSRC scheme (Figure 3), due to its requiring 2 to 3 times more NPK fertilizer inputs than the SRC schemes, altogether with a 20-30% lower productivity (Table 1). The relative importance of the various steps of the life-cycle followed a similar pattern for all scenarios with an important contribution of fertilisation (38 to 44 % of total), transport (32 to 33 %) and harvest (18 to 22 %). The very short rotation scenario (S5) had lower emissions than the short rotation scenarios in the harvest step due to the use of a agricultural harvesters. Its GHG emissions were thus dominated by fertilization, which accounted for 68% of the total emissions.

Indicators for the eutrophication impact ranged from 48 (S1) to 152 (S5) g PO 4 2-eq. GJ -1 (Figure 3), and were dominated by the fertilization phase. The losses of P from the plantation by runoff and erosion made up 90% of the impact related to fertilization, while ammonia volatilization contributed the remainder, the impacts of NO emissions from soils being negligible. Because of its larger fertilizer requirements, the very short rotation system had nearly 3-fold higher eutrophication impacts than short rotation ones. Although the latter also received varying rates of fertilizer inputs (Table 1), differences in productivities compensated for these variations and all short rotation schemes had a similar eutrophication impact within a 5% relative range. Interestingly, the best scenario was the one with the highest biomass productivity (S4) and not those that with the least fertilizer inputs per ha (S2) which only achieved a mid-range performance.

The acidification indicator ranged from 39 (S1) to 110 (S5) g SO 2 eq. GJ -1 , following a pattern similar to eutrophication (Figure 3). The very short rotation scenario had again a 3-fold larger impact than the other scenarios, and for the same reason: its higher fertilizer inputs, which translated in higher field emissions of ammonia and nitric oxide, and indirect emissions due to fertilizers' manufacturing. However, the harvest and transport steps played a more impotant role than for eutrophication, and the breakdown differed between the scenarios. The share of harvest ranged from 20 to 30% for the SRC, while it was nearly negligible (at 2%) for the VSRC. This stems from the major advantage of the VSRC schemes, namely the use of agricultural machines in lieu of forestry ones which are far more resource-intensive. However the associated savings did not compensate for the large requirements of synthetic fertilizer inputs for the VRSC compared to SRC.

The photochemical ozone creation potential (POCP) indicator ranged from 2.4 (S5) to 6.8 (S1) g C 2 H 2 eq. GJ -1 , with harvest operations and wood chips transport contributing the most (Figure 3). The much higher emissions of photo-oxidants occurring with the scenario S1 is explained by the chainsaws used for manual felling. The chainsaws used in France are seldom equipped with catalytic exhaust pipes and release volatile organic compounds which have a high potential for ozone formation. These emissions also occur to a lesser extent with the mechanized felling option (in scenarios 2 to 4) because chainsaw operators are necessary for the 2 nd and 3 rd harvest to thin the coppice before felling machines can be used.

For all impact indicators, the results were strongly influenced by the distance between the plantation and the boiler, which was set at 80 km in the baseline calculations. Table 3 illustrates the influence of various distances on the five LCA impacts for scenario S1. Energy consumption was the most sensitive indicator: it dropped by 28% when halving the transport distance, while GHG emissions and acidification impacts were only reduced by 16%, photo chemical ozone formation by 10 % and eutrophication by 3%. The energy ratio increased from 13.0 to 18.0 when the transportation distance decreased from 80 km to 40 km, and reached 25.2 with a 10 km distance (Figure 4). The other indicators were less sensitive to this parameter, A comparison with fossil energy sources was carried out to assess the environmental advantages and drawbacks of using SRC biomass as a substitute to coal, fuel oil and natural gas (Figure 5). In all scenarios, the provision of heat from SRC biomass consumed 90% less fossile energy than when using fossile energy sources. Similarly, GHG emissions were reduced by more than 80 % with the SRC biomass. However, the patterns with the local to regional-range impacts (acidification, eutrophication and photochemical ozone formation) were less clear-cut. Biomass-derived heat had generally much lower acidification and photochemical ozone formation impacts than fossile-based heat except with natural gas, which out-performed the VRSC scenario for eutrophication and scenario SRC S1 (pulp SRC with manual felling) for ozone formation. Natural gas had 2 to 30 times lower impacts than the other fossile sources, especially coal. Conversely, the eutrophication impacts were in the 50-135 g PO 4 3-eq. GJ -1 range for the eucalyptus scenarios, and in the 5-40 g PO 4 3-eq. GJ -1 range for the fossiles, pointing to a weakness of the biomass-based chain. The two-fold higher eutrophication impacts of the VSRC compared to the other scenarios were clearly due to the larger fertilizer inputs required by the former.

Inclusion of ecosystem C dynamics

Figure 6 depicts the dynamics of aboveground and belowground biomass in the eucalyptus plantation (Scenario 1) and the baseline wildland representing the baseline alternative land-use. Over the 30-year period of the eucalyptus life-cycle, biomass accumulation was several-fold larger in the SRC than in the wildland, even when considering the appearance of ligneous species in the latter. This hypothesis had a significant impact since it leads to a 8-fold higher estimate of total biomass after 30 years compared a wildland solely composed of annual species. The larger biomass accumulation in the eucalyptus SRC was due to a higher net primary production and an important storage in the belowground compartment, which kept increasing though the cuts. When averaged over the 30 years of the SRC rotation, the differences between SRC and wildland range from 16 to 24 t C ha -1 for the aboveground biomass, and from 26 to 37 t C ha -1 for the total biomass (Figure 6). These gaps represent the net ecosystem CO 2 gains incurred when substituting wildland with SRC, for instance after the abandonment of a vineyard. They are related to land-use effects may be included in the life-cycle GHG emissions of eucalyptus biomass production by using equivalence factors to account the temporal value of C sequestration in the biomass. This lead to savings of 0.57 to 5.16 t CO 2 -eq ha -1 yr -1 (Table 4), depending on the equivalence factors and the carbon pools taken into account. Including these CO 2 savings the LCA of eucalyptus-derived heat offset GHG emissions by 70 to 400 % (Figure 7), and therefore had a large impact on the global warming indicators. With the most favorable equivalence factors (1/26 and 1/55), the C stored in eucalyptus biomass resulted in heat provision being a net GHG sink.

Discussion

Benefits and drawbacks of eucalyptus SRC

Substituting fossile sources with biomass from eucalyptus SRC leads to a 80-90% abatement of life-cycle GHG emissions and fossil energy consumption per MJ of heat supply, for all SRC management scenarios. These figures confirm the strong benefits of bioenergy chains and are consistent with other LCAs of heat from biomass. For instance, Reinhardtt et al.

(2000) reported a 95% abatement in GHG emissions and energy consumption when displacing oil or natural gas with short-rotation willow for district heating in several European countries. In addition, inclusion of the temporary storage of CO 2 in the plant biomass, which was ignored in previous literature, more than doubled the GHG savings compared to fossil sources. The relevance of this hypothesis is discussed in subsection 4.3.

Conversely, the benefits of SRCs were far from obvious for the other impact categories, especially when displacing natural gas which had 3 to 4-fold lower impacts per functional unit than the other fossile sources. This trade-off between global impacts (global warming and fossil energy consumption) and local impacts has often been reported for bioenergy chains [START_REF] Reinhardt | Bio-energy for Europe. Which ones fit best? Final report[END_REF][START_REF] Gabrielle | Life-cycle assessment of straw use in bio-ethanol production: a case-study based on deterministic modelling[END_REF], and is almost inevitable because of the gaseous and leaching losses of nutrient occurring upon the feedstock production phase.

Despite the relatively low fertilizer N requirements of eucalyptus stands compared to arable crops, none of the management scenarios achieved lower eutrophication impacts than the fossil-based alternatives. Furthermore, the impact estimates were conservative because some losses of nutrients were neglected, as discussed in subsection 4.2.

In terms of management scenarios, the very short rotation scenario (VSRC) was outperformed by the conventional SRC scenarios for all impact categories except ozone formation, by a factor of 50% to 250%. Since the economics of this system are also unfavourable (Nguyen The et al. 2010b), VSRCs do not emerge as a good candidate compared to short rotation scenarios. Thus, the benefits from a quicker biomass growth and simplified harvesting made possible by the 3-year growing cycle of VSRC were outweighed by their larger fertilizer input and stem density requirements. The only advantage of VSRCs over SRCs appeared in the photochemical ozone creation potential (POCP), in which harvesting operations were predominant. However, VSRCs only out-performed SRC systems by a margin of 20%, which is within the uncertainty range of this indicator given the uncertainties on the characterization factors of ozone precursors [START_REF] Labouze | Photochemical ozone creation potentials[END_REF].

To our knowledge, no LCAs have been carried out so far on eucalyptus SRCs, whether for energy or pulp and paper. Our results may still be compared with those pertaining to traditional eucalyptus plantations published by [START_REF] Jawjit | An analysis of the environmental pressure exerted by the eucalyptus-based kraft pulp industry in Thailand[END_REF] in Thailand. Their study used system boundaries and characterization factors similar to ours, but found much lower impact values in general. Plant-gate life-cycle GHG emissions were estimated at only 3.1 kg CO 2 -eq. GJ -1 , compared to the 8-12 kg CO 2 -eq. GJ -1 range we obtained here. The acidification impact was 22 g SO2-eq. GJ -1 in the Thailand study compared to our 40-110 g SO 2 -eq. GJ -1 range, while the photo-chemical ozone formation potential amounted to 1.6 g C 2 H 2 -eq. GJ -1 in Thailand compared to our 2.5-7.0 g C 2 H 2 -eq. GJ -1 range. Eutrophication was an exception with similar impacts between Thailand and France, at 41 g PO 4 2--eq. GJ -1 and an average of 50 g PO 4 3--eq. GJ -1 for the SRC systems, respectively. Some of these discrepancies are explained by the higher yields of 17.4 ODT ha-1 yr -1 achieved by eucalyptus under the tropical conditions of Thailand, compared to the 9.5 -14 ODT ha-1 yr -1 range assumed here. The eutrophication impact was relatively higher because 35% to 20% of the fertiliser N and P applied was supposed to leach to water bodies in this Thailand study, whereas those losses were neglected here, as they were in other LCAs on herbaceous and tree species for lack of specific references [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF][START_REF] Monti | Cradle-to-farm gate life cycle assessment in perennial energy crops[END_REF].

Our results on eucalyptus SRC may be compared more broadly to other ligenocellulosic feedstocks: willow in France [START_REF] Reinhardt | Bio-energy for Europe. Which ones fit best? Final report[END_REF] and Italy [START_REF] Goglio | A screening LCA of short rotation coppice willow (Salix sp.) feedstock production system for small-scale electricity generation[END_REF], poplar SRC in Italy [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF], reed-canary grass in Finland [START_REF] Shurpali | Atmospheric impact of bioenergy based on perennial crop (reed canary grass, Phalaris arundinaceae, L.) cultivation on a drained boreal organic soil[END_REF], and four perennial grasses in Italy [START_REF] Monti | Cradle-to-farm gate life cycle assessment in perennial energy crops[END_REF]. All of these studies used similar system boundaries with the exception of the combustion step, and relied on the same set of characterization coefficients (from [START_REF] Guinée | Handbook on life cycle assessment. Operational Guide to the ISO standards[END_REF]. Most of them also used the EcoInvent data base for the life-cycle inventory phase.

Compared to the poplar SRC system assessed by [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF] in Italy, the production and harvest of eucalyptus biomass consumed 1.8 to 2.5 more primary energy, essentially because the harvest was 3-fold less energy-intensive per ton of biomass than eucalyptus (for the SRC system) or because poplars required 4-fold less fertilizers (for the VSRC systems). Also, the data on fuel consumption by farm machinery were adapted from the EcoInvent database based on local records but the exact corrections were not given by the authors.

When including the transportation of wood chips, albeit with a shorter distance than our nominal hypothesis (25 vs. 40 kms), the GHG emissions of poplar totalled 1.93 kg CO 2 -eq. GJ -1 , kg CO2 GJ -1 , which is 4 to 6 times less than our 8-12 kg CO 2 -eq. GJ -1 range for eucalyptus. The gap was even wider for the other impact categories: the eutrophication impact of poplar was estimated at 3.4 g PO 4 3--eq. GJ -1 vs. 40-135 g PO 4 3--eq. GJ -1 for eucalyptus ; acidification amounted to 15.7 g SO 2 -eq. GJ -1 vs 40-110 g SO 2 -eq. GJ -1 for eucalyptus ; and POCP totalled 0.3 g C 2 H 2 -eq. GJ -1 for poplar compared to 2.4-7 C 2 H 2 -eq. GJ -1 for eucalyptus.

Besides differences in management and inventory data for farm machinery, these large discrepancies arise because direct field emissions contributed only a minor share of the impacts in the Gasol et al. study, whereas they predominated in our LCA. There are reasons to believe some of these emissions were somehow under-estimated: for instance, N 2 O emissions from Gasol et al. were similar to our estimates on a ha basis, whereas NO emissions were 2fold lower. This contradicts current literature, which indicates that NO and N 2 O emissions fall within a similar range [START_REF] Stehfest | N2O and NO emissions from agricultural vegetation: summarizing available of global annual emissions[END_REF]. Our estimate of N 2 O emissions also included background emissions (ie non anthropogenic) and the contribution of eucalyptus residues.

Our LCA results for eucalyptus are overall closer to those reported by [START_REF] Reinhardt | Bio-energy for Europe. Which ones fit best? Final report[END_REF] and Goglio and Owende (2010) for short-rotation willow in Germany and Ireland, respectively. These authors reported energy consumptions of 33 MJ GJ -1 heat and 56.4 MJ GJ -1 , respectively, compared to our 55.6 MJ GJ -1 figure for scenario 1 (S1) with a similar transportation distance (40 kms). The lower figure from [START_REF] Reinhardt | Bio-energy for Europe. Which ones fit best? Final report[END_REF] was due to a less energy-intensive harvest for willow, whereas the Goglio and Owende ( 2009) study involved a drying phase prior to combustion. The GHG emissions were very similar, at 7.13 kg CO 2 -eq. GJ -1 for willow in Germany vs 6.80 kg CO 2 -eq. GJ -1 for the S1 eucalyptus system here, while the eutrophication impact for willow was 94 g PO 4 3--eq. GJ -1 , well within the 40-135 g PO 4 3- -eq. GJ -1 range reported here for our systems, although it should be noted that the estimation of nitrate and phosphate losses was not explicitly described in the willow study. Lastly, the acidification emissions of willow in Germany totalled 174 g SO 2 -eq. GJ -1 , compared to a 40-110 g SO 2 -eq. GJ -1 range for eucalyptus SRCs. This is probably due to higher combustion emissions of acidifying compounds in the [START_REF] Reinhardt | Bio-energy for Europe. Which ones fit best? Final report[END_REF] study than listed in the EcoInvent database, which pertains to more recent technologies. For the same reason, POCP impacts were also larger with willow, at 18 C 2 H 2 -eq. GJ -1 in comparison to 6.1 C 2 H 2 -eq. GJ -1 g for the S1 system. Lastly, eucalyptus SRCs may be compared to the range of perennial grasses assessed by Monti et al. (2010), involving miscanthus, switchgrass, cynara and giant reed, with a cradle to farm-gate system boundary. Energy consumption ranges from 33 to 142 MJ GJ -1 biomass energy content, compared to approximately 35 MJ GJ -1 for eucalyptus SRC (Table 2), putting the latter on a par with the best performers, giant reed and miscanthus. However, their GHG emissions were significantly lower, at 1.75 kg CO 2 -eq. GJ -1 compared to 5.5 -9.4 for kg CO 2 -eq. GJ -1 eucalyptus. The same applied to eutrophication impacts, ranging from 4 to 20 g PO 4 3--eq. GJ -1 for grasses and from 45 to 132 g PO 4 3--eq. GJ -1 for eucalyptus, and also to acidification impacts, which are 2 to 2.5 lower for the grasses than eucalyptus. As with the [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF] study, it may be that field emissions were undervalued, since fertilizer N input rates were rather higher than the eucalyptus SRC systems (at 80 kg N ha -1 yr -1 compared to a 6-40 kg N ha -1 yr -1 range for eucalyptus). The Monti et al. (2010) paper does not mention direct emissions of nitrate or P in the field. Because of differences in local contexts, in the sources of life-cycle inventory data and estimation methods for field emissions, it is not possible to directly compare the eucalyptus systems tested here with other coppices or herbaceous plants since these differences are likely to overrule the differences between feedstocks per se. With the exception of the [START_REF] Gasol | LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario[END_REF] study, the LCA indicators of eucalyptus were within the range of impacts reported for other lignocellulosic feedstocks, but no robust patterns emerged in terms of ranking with other species.

Uncertainties in the life-cycle inventories

Field emissions are particularly difficult to correctly address in the LCA of agricultural or forestry systems as they depend to a large extent on local conditions (soil properties, climate) and on their interactions with management practices, which govern the fate of chemical or organic inputs. Since very little data on field emissions has been published for eucalyptus SRC in temperate zones, we used estimation methods developed for other species, or assumed some emissions were negligible. Such was the case for nitrate leaching and P losses, which may have lead to an under-estimation of eutrophication impacts. [START_REF] Lopes | Application of life cycle assessment to the Portuguese pulp and paper industry[END_REF] found these emissions negligible in their LCA of eucalyptus-derived paper, and so did [START_REF] Jawjit | An analysis of the environmental pressure exerted by the eucalyptus-based kraft pulp industry in Thailand[END_REF] although their estimates of nitrate and phosphate emissions from eucalyptus plantations were rather large: they assumed that 35% and 20% of fertilizer N and P inputs were leached to water bodies, respectively, according to the 1997 IPCC guidelines for GHG inventories. The 35% emission factor for nitrate (which was revised to 30% in the 2006 IPCC guidelines - [START_REF] Tanabe | Prepared by the National Greenhouse Gas Inventories Programme[END_REF] should in principle apply to managed forests, but no reference specific to forest or energy plantation is given in the literature base that served to determine this value. Further research is therefore warranted to provide a more accurate estimate of nitrate leaching for eucalyptus SRC. The same applies to P losses, and also to gaseous emissions of N 2 O, NH 3 and NO. The latter were calculated according to the IPCC (2006) guidelines for managed ecosystems, using default emission factors which are characterized by a large uncertainty range [START_REF] Stehfest | N2O and NO emissions from agricultural vegetation: summarizing available of global annual emissions[END_REF]. Unfortunately, no literature data were found for eucalyptus SRC or forests in Europe to refine those estimates.

Relevance of including ecosystem C dynamics

Accounting for variations in ecosystem C stocks, compared to the alternative land-use (wildland in our case) had a drastic effect on the GHG balance of eucalyptus-derived heat, whose magnitude depended on the factor chosen for the equivalence between C stored in ecosystem pools and atmospheric CO 2 . Even when using the most conservative value of 1:100 (ie that least favorable to eucalyptus), ecosystem C pools offset GHG emissions by 50 to 70%, depending on the inclusion of below-ground biomass. This made net eucalyptus a nearly carbon-neutral source of heat, and stresses the influence of ecosystem C dynamics in relation to land-use changes (LUC) in LCAs, already noted by Ndong et al. (2009) for biodiesel from jatropha in West Africa, and [START_REF] Shurpali | Atmospheric impact of bioenergy based on perennial crop (reed canary grass, Phalaris arundinaceae, L.) cultivation on a drained boreal organic soil[END_REF] for reed-canary grass in Finland. Note that the latter authors effectively used an equivalence factor of 1:1, since they used measurements of net ecosystem exchanges of CO 2 over reed-canary grass, as cumulated over one year, as a measure of the C sink strength of the field where this crop was grown. Such hypothesis was also implicit in the GHG budgets of farmland and woodland management computed by [START_REF] Palm | Identifying potential synergies and trade-offs for meeting food security and climate change objectives in sub-Saharan Africa[END_REF] in 2 villages in Africa, or by [START_REF] Ceschia | Management effects on net ecosystem carbon and GHG budgets at European crop sites[END_REF] for cropping systems across Europe. In both references, ecosystem C fixation was put on a par with CO 2 emissions from fossil sources or N 2 O emissions from soils. This may be justified on a short-term basis, but is misleading in the long-run since most of the C taken up by ecosystems on a given year will be released back to the atmosphere after a few years since it enters fresh organic matter pools with rapid turnover [START_REF] Jenkinson | The turnover of organic carbon and nitrogen in soil[END_REF]. From a life-cycle perspective, whereby one attempts at estimating the cumulated past and future effects of substituting one product by another, using such an hypothesis would have over-emphasized the sink capacity of SRC stands compared to wildland, and given wrong results on the actual GHG benefits of eucalyptus biomass. The use of equivalence factors, which are up to 2 orders of magnitude lower, is thus fully justified.

Of course the magnitude and direction of this effect strongly depends on the LUC hypotheses made in the LCA. Adverse effects were conversely noted for biofuels when including indirect land-use change effects whereby the displacement of food crops for biofuels in the US entailed the conversion of natural ecosystems to arable farming in other parts of the world [START_REF] Fargione | Land Clearing and the Biofuel Carbon Debt[END_REF]. Our scenarios for eucalyptus growth did not involve such effects since they considered the farming of eucalyptus SRC as an opportunity to value former arable land or vineyards that had been abandoned because of a drop in the market prices of wine.

Soil organic matter (SOM) pools were not included in the ecosystem pools for lack of robust estimates of SOM variations under both eucalyptus SRC and wildland. This pool was actually responsible for most of the land-use offset of GHG emissions in the LCA of Jatropha by Ndong et al. (2009). Similarly, given the differences in net primary production between the SRC stands and the wildland, it is likely that the former have a higher SOM content than the latter, and therefore further accrue their GHG benefits. [START_REF] Grogan | Review of the potential for soil carbon sequestration under bioenergy crops in the U[END_REF] thus argued from a very preliminary modelling study that 'short-rotation coppice systems have the capacity to sequester substantial amounts of carbon, comparable to, or even greater than, an undisturbed naturally regenerating woodland'. This results from C inputs from SRCs being higher than from the regenerated woodland, which is comparable to our wildland system here. Photochemical ozone formation (g C2H2-eq.) 6.8 6.1 5.7 5.5

Figure captions

Global warming (kg CO2-eq.) 8.16 6.80 6.11 5.77

Table 4. Carbon storage in the eucalyptus SRC stands (management scenario 1), relative to the baseline wildland, as averaged over the 30-year duration of the project, in the aboveground and above-and below-ground biomass pools (t CO 2 ha -1 ). The lower-end of the range corresponds to the emergence of woody species in the wildlands, which is ignored for the upper-end value. C stored in biomass pools are transformed into CO 2 sequestration rates using the 3 possible equivalence factors detailed in the text.

Ecosystem pools

Field

  samplings were carried out in our study area to estimate SOM contents under vineyards, eucalyptus SRC of various ages, wildlands and arable land. Although the comparison was confounded by soil clay content, SOM was clearly lowest under the vineyards and comparable between wildlands and SRCs. Conversion shortly after vineyard abandonment would therefore maximize the benefits of eucalyptus SRCs in terms of SOM gains from landuse change. Further work (in particular SOM modelling) is nevertheless required to provide more robust estimates of the magnitude of these potential gains. von Blottnitz H, Curran MA (2007) A review of assessments conducted on bioethanol as a transportation fuel from a net energy, greenhouse gas, and environmental life cycle perspective. Journal of Cleaner Production, 15, 607-619. Wilkinson JM, Evans EJ, Bilsborrow PE, Wright C, Hewison WO, Pilbeam DJ (2007) Yield of willow cultivars at different planting densities in a commercial short rotation coppice in the north of England. Biomass and Bioenergy, 31, 469-474. WWI (2006) Biofuels for transportation -Global potential and implications for sustainable agriculture and energy in the 21st century. Worldwatch Institute, Washington D.C.

Figure 1 :

 1 Figure 1: Calculation of the cumulative amounts of carbon stored in eucalyptus biomass over time in the 10-year interval between two cuts.

Figure 2 .

 2 Figure 2. System boundaries and steps of the life-cycle.

Figure 3 .

 3 Figure 3. LCA results for global warming, eutrophication acidification, photo-chemical ozone creation potential, per GJ of heat delivered.

Figure 4 .

 4 Figure 4. Energy ratio as a function of the transportation distance from the eucalyptus plantation to the boiler for scenario S1.

Figure 5 .

 5 Figure 5. LCA indicators weigthed by the average impact of an European inhabitant and compared to fossil energy sources.

Figure 6 .

 6 Figure 6. Dynamics of above-ground (top) and above-and below-ground (bottom) C storage by pulp eucalyptus SRC (solid line) and wild land with (dotted line) or without (dashed line) consideration of C accumulation in woody species, in the years following conversion to SRC.

Figure 7 .

 7 Figure7. Greenhouse gas emissions (g CO 2 eq. GJ -1 heat) due to sowing and harvesting operations, fertilization and transport of chips, and CO 2 savings from CO 2 sequestration in ecosystem biomass using various equivalence factors and the lower and upper estimates.

Figure 1 :

 1 Figure 1: Calculation of the cumulative amount of carbon stored in eucalyptus biomass over time in the 10-year interval between two cuts.

Figure 2 .Figure 3 .

 23 Figure 2. System boundaries and steps of the life-cycle.

Figure 4 :Fig. 6 .

 46 Figure 4: Energy ratio as a function of transportation distance from field to boiler for scenario S1.

Figure 7 .

 7 Figure7. Greenhouse gas emissions (g CO2 eq. GJ -1 heat) due to sowing and harvesting operations, fertilization and transport of chips, and CO 2 savings from CO 2 sequestration in ecosystem biomass using various equivalence factors and the lower and upper estimates.

Table 2 .

 2 Non-renewable energy consumption per life cycle stage of the various SRC systems (MJ GJ -1 ), and ratio of energy delivered to primary energy consumption.

	Non emitted CO2 -high
	Non emitted CO2 -low
	Woodchip haulage
	Harvest
	Fertilization
	Other stages

Table 3 .

 3 Influence of woodchips transportation distance from plantation to boilers on LCA indicators for scenario S1, per GJ of heat.

	Transportation distance (km)
	80	40	20	10

Acknowledgements

These results were obtained in the framework of the CULIEXA project funded by the ENERBIO fund of the TUCK foundation (Rueil-Malmaison, France).

Adaptation Strategies for Global Change, 5, 51-60. Nemecek T, Heil A, Huguenin O, et al. (2003) Life cycle inventories of agricultural production systems. Final report Ecoinvent 2000 No. 15, FAL Reckenholz, FAT Tänikon, Swiss center for life cycle inventories, Dübendorf, CH. Ndong R, Montrejaud-Vignoles M, Saint-Girons O, Gabrielle B, Pirot R, Domergue M, Sablayrolles C (2009) Life cycle assessment of biofuels from Jatropha curcas in West Africa: a field study. Global Change Biology Bioenergy, 1, 197-210. Nguyen The N, Lambrecq D, Sionneau J (2004) Eucalyptus establishment in France: 20 years of collaboration between research and industry. In: Proceedings of the IUFRO conference "Eucalyptus in a changing world". Aveiro, Portugal, 11-15 October 2004 (ed Borralho N), p. 484, RAIZ, Eixo. Nguyen The N, Deleuze C (2004) Biomass and nutrient accumulation for three short rotation coppices of E. gundal in southern France. In: Proceedings of the IUFRO conference "Eucalyptus in a changing world". Aveiro, Portugal, 11-15 October 2004 (ed Borralho N), pp. 324-325, RAIZ, Eixo. Nguyen The N, da Silva Perez D, Melun F, Bouvet A (2010) Evaluating the potential of biomass production, nutrient export and woodchips quality by eucalyptus in a perspective of culture in VSRC. In: Proceedings of the 18th European biomass conference. Lyon 5-9 May 2010. Nguyen The N, Gaspard M, Ménard P, Kephaliacos K, Ridier-Martos A, Gabrielle B (2010) The Culiexa project: analysis of technical and socio-economic drivers for the development of woody biomass crops in agricultural farms. Final project report, FCBA, Paris (in French).

Table 1. Selected characteristics of the eucalyptus management scenarios for the short rotation (SRC) and very short rotation (VSRC).