Kyoungro Yoon
email: yoonk@konkuk.ac.kr

Youngseop Kim

Je-Ho Park

Jaime Delgado
email: jaime.delgado@ac.upc.edu

Akio Yamada
email: yamada@da.jp.nec.com

Frederic Dufaux
email: frederic.dufaux@telecom-paristech.fr

Ruben Tous
email: rtous@ac.upc.edu

JPSearch: New international standard providing interoperable framework for image search and sharing

Keywords: JPEG, Image Search, Image Sharing, Social Tagging, Metadata

Introduction

Since the introduction of Query-by-Image-Content [START_REF] Flickner | Query by image and video content: the qbic system[END_REF], there has been various research and great advances in the field of content-based search and retrieval of images [START_REF] Shandilya | A survey on: Content based image retrieval systems[END_REF]. The effort of research on image search and retrieval was greatly increased with the popularity of digital cameras and phone cameras. Some efforts, which may not cover the complete scope of JPSearch Frame-work, related to parts of JPSearch standard can be found in [START_REF] Tesic | Metadata practices for consumer photos[END_REF] [4] [START_REF]Exif Version 2.2 Digital Still Camera Image File Format Standard (Exif) Version 2[END_REF] [6] [START_REF] Morand | Scalable object-based video retrieval in HD video databases[END_REF] [START_REF] Mrak | Fast analysis of scalable video for adaptive browsing interfaces[END_REF].

In 2003, 50 million units of digital cameras were sold worldwide according to Photo Marketing Association (PMA) (www.pmai.org) marketing research, and the digital camera market was growing rapidly. Stimulated by the explosion of digital camera market and JPEG images, JPEG working group of ISO/IEC started to explore the JPSearch project with the objective of specifying metadata format and related functionalities to support flexible and efficient still image search, in April 2004.

The idea of developing standard for image search and retrieval became concretely structured over time. Providing an abstract framework of search architecture that decouples the components of image search and a standard interface between these components became Preprint submitted to Signal Processing: Image Communication May 1, 2012 Manuscript the objectives of the JPSearch standard. The idea of searching images using metadata and developing related functionalities had been exchanged with MPEG working group. It evolved over time resulting in the idea of producing one technical report, four parts of technical specifications and finally a reference software from 2007 to 2011. The development of the JPSearch standard aimed at solving three main problems as the following:

• Lack of the ability to reuse metadata When a user annotates a collection of images using one system, it is almost impossible to adopt second type of annotation system with additional functionality without redoing the annotation. Also, when a community of users are annotating a single shared image, which we call a social tagging, it is very hard to merge all the annotations into a single unified metadata.

• Lack of a common query format and search semantics Search is an essential functionality of large image repositories. However, the various systems providing image search functionality do not provide a common way of specifying a search. Therefore, a query given by a user may be interpreted in different ways. To provide homogeneous interface for the users and make searching over various image repositories consistent, a common query format and search semantics should be defined.

• Lack of a common format for handling context in searching The terms that people use for query may have different meaning from user to user and from case to case. The specific meaning of a term used may depend on the context of use. For the system to understand the context of a specific user there should be a common format of specifying the context of a certain term being used in the query.

About four years have been spent to provide a concrete specification of standard framework for image search systems. As a result, one technical report of JPSearch Part 1 and technical specifications of five parts are either published or on their way to publication as international standards, which are JPSearch Part 2: Registration, identification and management of schema and ontology [START_REF]Information technology -JPsearch -Part 2: Registration, identification and management of schema and ontology[END_REF], JPSearch Part 3: Query format [START_REF]Information technology -JPsearch -Part 3: Query format[END_REF], JPSearch Part 4: File format for metadata embedded in image data (JPEG and JPEG 2000) [START_REF]Information technology -JPsearch -Part 4: File format for metadata embedded in image data[END_REF], JPSearch Part 5: Data interchange format between image repositories [START_REF]Information technology -JPsearch -Part 5: Data interchange format between image repositories[END_REF] and JPSearch Part 6: Reference software and conformance [START_REF]Information technology -JPsearch -Part 6: Reference Software[END_REF].

Part 1 is the technical report providing an overview of the JPSearch standard with some usage scenarios.

Part 2 specifies three main tools to support interoperability between image descriptions based on different metadata schemas. The core metadata serves two purposes. One is to provide definitions of essential metadata describing images. The other is to provide a reference in describing third party metadata with help of translation rules. The JPSearch Translation Rules Declaration Language (JPTRDL) is defined to enable specifying guidelines for translating third party defined metadata schema to core metadata. In addition, management tools are defined to register, update and retrieve third party metadata with translation rule to the JPSearch metadata authority, so that users can understand metadata instances which are not defined using JPSearch Core Metadata.

Part 3 specifies message format in XML schema to be sent and received between information requestors (clients) and information providers (database servers). These messages are divided into three types of input query, query output, and query management tools. Input query specifies the syntax and semantics for composition of queries by specifying complex combinations of query conditions, such as boolean combinations of various query conditions, mix of query conditions of various media types as well as relevance feedback. Query output specifies the expected message format and information to be included in the returned data from the responder. Query management tools helps users or applications to select the service by defining message formats describing service capabilities of individual service provider.

Part 4 specifies file formats based on JPEG file format and JPEG 2000 file format to carry metadata with the images. These file formats allow multiple metadata to be embedded inside the JPEG file or JPEG 2000 file. Social tagging is supported by enabling multiple occurrences of metadata inside an image file.

Part 5 specifies a data interchange format for the exchange of image collections with metadata which can be provided at the individual image level as well as the collection level. By defining the interchange format, JPSearch allows synchronization between image repositories, independent of platform or device types.

Part 6 provides reference and utility software to show normative use of the JPSearch tools and various use cases.

Figure 1 shows the architecture of JPSearch Framework. When a query is composed, it can be represented 1) in the figure. The query can be based on the metadata or an image implementing query-by-example. When an example image is given as a query, either a feature extracted from the image or a metadata can be given as the query condition. If the metadata given in the query is defined by a metadata schema other than the JPSearch Core metadata, it can be translated into an instance of the JPSearch core metadata, shown in [START_REF] Mrak | Fast analysis of scalable video for adaptive browsing interfaces[END_REF], by following the interpretation of the translation rules defined in Part 2 and represented by [START_REF] Shandilya | A survey on: Content based image retrieval systems[END_REF]. The query can be sent directly or after going through the query preprocessor to the database. The database can be a collection of images and metadata as shown in [START_REF]Exif Version 2.2 Digital Still Camera Image File Format Standard (Exif) Version 2[END_REF] or a collection of images in which metadata is embedded using the file format defined in Part 4 as shown in (4). The database or the collection of images with metadata can be exchanged using the interchange format defined in Part 5 as shown in [START_REF] Tesic | Metadata practices for consumer photos[END_REF]. The image file format defined in Part 4 supports heterogeneous metadata instances as shown in [START_REF]Information technology -JPsearch -Part 5: Data interchange format between image repositories[END_REF] to be embedded in images in JPEG or JPEG2000 format. These heterogeneous metadata can have essential interoperability through the translation rules defined in Part 2 as shown in (7)(9)(10) [START_REF]Information technology -JPsearch -Part 4: File format for metadata embedded in image data[END_REF]. The processed query result ( 6) is formed into a query output as defined in Part 3 and returned to the user.

Introduction

JPSearch Part 2 concerns how users can search and retrieve images that include metadata within their physical image files. The images under consideration can be, moreover, spread out multiple repositories that might be managed by a number of independent authorities or systems. Here, the metadata schemas that are used for images do not need to be identical among the different systems as well as the systems maintaining the images do not need to understand the metadata schemas of other systems. In order to provide interoperable search functionality among the different repositories or systems, one common metadata schema, JPSearch Core Metadata, is utilized for delivering a user's query in an understandable manner among the systems that process search related operations using the different metadata schemas.

To retrieve images that satisfy a user's demand, a query referring information in images metadata would be composed by exploiting particular data elements or fields that are only known to the user. The composed query is submitted to a designated interface in order to be spread out the query to the systems that cooperate with the interface carrying the user's original intention. At this point, the submitted query is respectively transformed into a number of different queries that can be understood by the target system(s) regarding metadata syntax, particularly metadata schema terms. In order to support this scenario, JPSearch Part 2 provides the following:

• Description of JPSearch Core Metadata Schema in XML,

• Rules for the publication of machine-readable translations between proprietary metadata schema and JPSearch Core Metadata Schema, and

• Rules for the registration and request of metadata schema and its translation rules or links to them.

JPSearch Core Metadata Schema

JPSearch Core Metadata Schema mainly consists of four types:

PersonNameType, PublisherType, SourceType and JPSearchCoreType.

The PersonNameType is used for the simple description of human beings within the image or image regions. The PublisherType describes publisher related information that can be any public or private organization or person. The SourceType is devised for the description of sources of an image especially when the image is created from an artificial work or product, such as painting, book, video and so on, that exists before the creation of the image.

The JPSearchCoreType is devised in order to describe the information about an image in metadata and contains the most important fields that are necessary for image maintenance work flow such as identifier, modifier, creator, publisher, time related values, description, source, keyword, title, rating and collection related values, GPS position and region related information. In order to support such a role of JPSearchCoreType, additional types are provided in the form of XML types. These additional types are used for description of ownership right, place, person, organization, event, object inside an image, region of interests, region location, external description, rating term definition, image identifier related information and GPS location.

If a system supporting a particular metadata schema wants to be included within an image retrieval operation, the information regarding the system's metadata schema as well as its correspondence relationship concerning JPSearch Core Metadata Schema is required to be registered at a JPSearch Registration Authority (JP-SRA).

Metadata and Translation Rule Maintenance

The list of the registration authorities would be provided at a site maintained by ISO/IEC JTC1. The registering system or organization needs to keep the registered schema and translation rules available online to allow validation of external independent applications. When JPSRA verifies the provided information and approves the necessary correctness for JPSearch compliance, JPSRA supports requests for registered information and for replacement of existing information. All these processes are managed by using XML encoded form that is described in SchemaManagment element. For example, registration of a metadata schema or an application needs to provide the following information:

• provider information such as name, description, contact information, and provider capability,

• schema information such as description, name, version, identifier, and location, and

• translation rules that specifies the rules which are necessary for reformulating a query from the reference metadata model to the registered target metadata model.

After review of the registered application, according the review results, an identifier would be assigned to the registered metadata and translation rules or necessary processing would be started in order to complete the registration process.

The schema registration and maintenance process utilize the SchemaManagment element that include information according to the undergoing process: registration, request of registered information or replacement of existing information. Moreover, the corresponding result for a specific operation also utilizes the similar XML structure.

Translation Rule Description

A translation rule describes correspondence relationship between JPSearch Core Metadata Schema and a registered metadata schema such as oneto-one, one-to-many, or many-to-one relationship in the context of XML element level.

To support the relationship description, JPSearch defines three types:

OneToOneFieldTranslationType, ManyToOneFieldTranslationType, and OneToManyFieldTranslationType. The position of a particular XML element is described by using XPath expression. Moreover, JPSearch metadata translation rule system provides mechanism for complex selection of elements in correspondence relationship definition by using a type FilteredSourceFieldType that supports regular expression, variable usage, position based binding and list based binding. Here, we are going to demonstrate two scenarios regarding utilization of JPSearch Core Metadata Schema and Translation rules for interoperability. For the first scenario, we assume that there exist two sites, Service A and Service B, that maintain two distinctive image databases, Database A and Database B, and provide image retrieval functionality from the respective databases. We also assume that each service site maintains image related information by using a specific information structure, schema. Hence one site does not understand the schema used in the other site. As a result, there exists no interoperability between two service sites. Figure 2 demonstrates the above-mentioned situation.

JPSearch Core Metadata Schema here, plays a role for interoperability among the sites. If each site can understand and maintain the mapping rules between Core Metadata Schema and its own information schema based upon semantic correspondence in the constituent element level, the two service sites are able to communicate in terms of image retrieval request and resulting image group. For example, a user who tries to retrieve images from both sites submits a request through a specific site, say Service A. Then, an agent process, say Agent A, that accepts the request written in Service A's specific schema structure generates two queries for the local service engine and the remote agent in Service B.

The Service Engine A processes the submitted query and responses to the agent with a result without further additional operation. Agent A here translates the submitted original query to a query by using Core Metadata Schema elements referring the information that is locally maintained. This translated message described using JPSearch Core Metadata Schema is being sent to Agent B that translates the received query into the query using Service B's schema. After this translation, Agent B submits the translated query to Service Engine B that generates a result. This result will be sent to Agent A through Agent B. After collecting two results from both sites, Agent A merges the results and generates a inte-Figure 3: Situation with interoperability grated response for the user. The reverse case can be processed by switching the workflow's direction. This situation is demonstrated in Figure 3.

The above example assumes two service sites. In the case of more than two sites, the second scenario can be a general case of utilization of JPSearch Core Metadata schema and translations rules. In second scenario, a representative site is known to users as a portal for retrieval of images from multiple image database service sites. The sites that want to be involved in this heterogeneous system need to register their mapping rules between JPSearch Core Metadata and respective schema by using the specified manner that is described in JPSearch Part 2.

When all the members in the system become JPSearch compliant after demanded registration process, the representative site can replicate the user's request by using registered correspondence relationship information that is maintained in the representative site. As in the first example, the correspondence relationship information is described in the context of correspondency between a specific metadata schema and JPSearch Core Metadata Schema in the level of elements taking into account the semantic similarity. The representative site also takes responsibility of merging the results from multiple member sites. This scenario is illustrated in Figure 4. MPQF allows combination of Data-Retrieval-like conditions (XPath, XQuery) with Information-Retrieval-like conditions (Query-by-Example, etc.). In other words, combined query of text-based keywords or metadata and image examples is allowed. It is an XML application with one XML schema describing messages both from requester to responder (Input Query Format) and from responder to requester (Output Query Format), and also management messages (service properties, etc.). MPQF and JPQF instances are XML documents that can be validated against MPQF and JPQF schema, respectively. Although it was initially developed in the MPEG-7 context (in fact, it is Part 12 of the MPEG-7 standard), it is metadata-neutral.

In turn, the goal of JPQF is to facilitate and unify access to search functionalities in distributed digital image repositories. To achieve this goal, the JPQF standard also specifies precise input and output parameters to express requests, and uniform client side processing of result sets. Moreover, the management component of JPQF covers searching and the choice of the desired services for retrieval. For this purpose, the standard provides a means to describe service capabilities and to undertake service discovery.

Hence, JPQF is essentially an XML-based query language that defines the format of the queries and replies exchanged between clients and servers in a distributed image search and retrieval system (see Figure 5). There are two main benefits in the standardization of such a language: • Platform independence (which also offers benefits for non-distributed scenarios).

As a result, developers can build applications handling image queries independently of their internal format and even of the offered service. In the example of Figure 5, the Client application only needs to know about JPQF in order to query external databases that may have, or not, JPQF as their native query interface.

JPQF instances are XML documents that can be validated against the JPQF XML Schema. A JPQF instance always starts with the JPEGQuery element as the root element and has the InputQuery element, the FetchResult element, the OutputResult element or the Management element as the top level elements (See Figure 6). JPQF instances with the InputQuery element or the OutputResult element are the usual requests/responses of an image multimedia search process. The InputQuery element is used if the document is a request, and the OutputResult element is used if the document is a response. may have OutputDescription element to specify the format of the message to be returned as the search results. A special query input constitutes the FetchResult element, which is also a top level element, to be used in asynchronous mode for the collection search results. Alternatively, below the root element, a JPQF document can include the Management element. Management messages (which can be both requests and responses) provide a means for requesting service-level functionalities such as discovery of multimedia services or other kinds of service provision, interrogating the capabilities of a service, or configuring service parameters.

Taken from the current Working Draft of the second edition of JPSearch Part 1, the example in Figure 7 shows a JPQF query which asks for images created on 7th October 2009 at 8:46:45. It is worth mentioning the use of the OutputDescription element (requesting some features for the expected answer) before the QueryCondition. In this example, maximum of 30 items are reqested by using the The Figure 9 shows an snippet of an example instance of JPQF showing a relevance feedback query. In this example, the user selected that result number 4, 8 and 10 from the query result with id IDofPreviousQuery are relevant to the intended query. In this example, other part of query is intentionally not shown to give focus on the condition part of the relevance feedback query.

JPSearch Part 4: File Format

Overview

JPSearch Part 4 focuses on the interoperability of image metadata and enables metadata portability between image repositories as well as various kinds of image handling applications.

One of the biggest problems in existing image management system is the lack of the ability to reuse metadata. A user, who wants to establish well-organized image archive supporting image retrieval, is forced to make a heavy investment to annotate images. There are so many efforts for automatically annotating images, however, in the current market, they are not very popular due to the lack of accuracy and flexibility. After spending a long time for annotation, he/she may find <Input> <QueryCondition> <Condition xsi:type="QueryByRelevanceFeedback" answerID="IDofPreviousQuery"> <ResultItem>4</ResultItem> <ResultItem>8</ResultItem> <ResultItem>10</ResultItem> </Condition> </QueryCondition> </Input> another problem that he is unexpectedly locked-in a service provider since there are no easy way to carry out his annotation efforts into other services. JPSearch Part 4 provides metadata carriage capability by embedding the metadata in image data itself so that the user never worries about the lost of his achievements when he wants to move his content from a specific service provider to another. The benefits of the usage of Part 4 could be concluded that

• The standardized format allows metadata carriage along with content itself.

• The standardized format is completely compatible with corresponding existing file formats so that any systems/tools/applications used in a current market are fully applicable to JPSearch file format.

• The standardized format can carry any kinds of metadata instances. If their schema is registered into JPSearch registration authority, the instances are understandable by other users.

• The standardized format allows multiple instantiation of metadata to support social tagging for the use of images in a community. Multiple instantiation of metadata of different schema is also available to use an image for various applications.

There are two types of JPSearch file format; one is fully compatible to JPG (JPEG) file format and the other is to J2K (JPEG2000) file format. Figure 10 shows the overall structure of these two types. Both formats have a common format-independent blocks named as JPSearch metadata block (JPS-MB) to carry metadata instances. A JPS-MB is a binary bitstream and designed to carry multiple instances of Elementary Metadata (JPS-EM), which is a basic carriage unit of a metadata instance as described in 4.

JPS-MB carriage in JPEG compatible file format

JPS-MB is stored in JPEG file format using application marker segments, APP3, as shown in Figure 11. Multiple instantiation of APP3 block is allowed to store a large size metadata instance in a file since an APP3 segment has a limitation of size. A JPS-MB shall not be divided across several APP3 segments. When the size exceeds the limitation, several sets of JPS-MB, each of which has different JPS-EM, shall be employed. An APP3 segment can have only one JPS-MB inside.

JPS-MB carriage in JPEG-2000 compatible file format

JPS-MB is stored in JPEG-200 file format using UUID (Universally Unique Identifier) boxes with specific universally unique identifier as shown in Figure 12. Since JPEG-2000 file format allows multiple JPEG-2000 codestreams inside, two different types of UUID boxes can be instantiated. The UUID box at the toplevel in the file structure is used to describe globally applicable metadata and those in codestream-layer headers are used to describe the corresponding local codestream. If local information is different from the global one, the local description overwrites the global one. ;;<=$./0$>!"%-),6?$>2/6)2@@! ;;<=$./0$>!"%-),6?$>2/6)2@@! For example, if there are several codestreams basically capturing identical scene with different coding parameters or coding schemes in a single JPEG-2000 file, the set of metadata describing the scene captured in the provided JPEG-2000 file may be provided in the UUID box at the top-level and individual set of metadata specific to a single codestream with a certain coding parameters or coding scheme may be provided in the UUID box of corresponding codestream header box, respectively.

!"#! $%%&! $%%&! '%()*+,-./0123045! ! ! ! ("#!

Metadata carriage using JPS-EM

A JPS-EM is the most fundamental structure to describe a metadata instance according to a specific schema by a single metadata author. It consists of (1) identifier of the used schema,

(2) various descriptive information of metadata creation process such as creation/update date/time, author information, and confidence of the description, and

(3) metadata itself.

Any type of metadata schema can be used even if the schema is not registered into the JPSearch Registration Authority, so a user can embed his/her metadata using his/her original schema without losing interoperability of a file. Note that if the schema is not registered, the JPS-EM, unfortunately, may not be understandable by other users, although this unknown (unregistered) metadata block can still be securely delivered along with image itself (in compressed image bitstream).

If another instance using different schema is required, the user can easily add another JPS-EM into a JPS-MB. For an example, a metadata for medical purpose can coexist with a metadata for content sharing purpose in a single file entity. Also, if another user has different view from the existing metadata instance, he can instantiate additional JPS-EM to describe his own idea as a metadata. Therefore, in the case of social tagging joining 10 participants, there can be a compressed image data and ten JPS-EMs in a single file.

JPSearch Data Interchange Format Between Image Repositories

Scope

Part 5 of JPSearch [START_REF]Information technology -JPsearch -Part 5: Data interchange format between image repositories[END_REF] provides a data interchange format for the exchange of image collections and respective metadata between JPSearch compliant repositories. The metadata can be at the level of the image or an image collection. By providing a solution for the carriage of image collections and associated metadata between compliant devices and platforms, the JPSearch data interchange format enables the synchronization of repositories in order to facilitate simple and fully interoperable exchanges across different devices and platforms.

The JPSearch data interchange format should enable the easy and reliable transfer of data between different hardware and software systems. In particular, it should support functions such as

• exchange of data between JPSearch repositories on different devices and platforms,

• consolidation of metadata generated on different systems,

• transfer of data to a newer and better system,

• consolidation of selected data to a centralized repository, and

• archive of data in a format which will survive current products.

The JPSearch data interchange file format encompasses both internal or external resources, namely encoded image bitstreams, along with collection-level and item-level metadata, namely entity of descriptive data of image data.

File format and resource

The JPSearch data interchange file format encapsulates all the collection-level and item-level metadata into a valid XML descriptor in a single text-based format. The aim is to enable easy exchange of metadata between repositories.

The collection-level and item-level metadata should follow the interchange format schema as defined in the specifications (see Sec. 5.3.2). Besides a plain-text XML representation, the descriptive metadata can optionally be stored using BiM [START_REF]Information technology -MPEG systems technologies -Part 1: Binary MPEG format for XML[END_REF].

Only JPEG-conforming [START_REF]Information technology -Digital compression and coding of continuous-tone still images: Requirements and guidelines[END_REF], JPEG 2000conforming [16] and JPSearch-conforming codestreams may be used as internal or external resources in the JPSearch data interchange file format. The ImageDataType type allows linking between metadata and each corresponding internal or external resource when used within the binary file format, or just external resources when used within the XML metadata interchange format.

Internal resources shall be stored in InlineMedia of ImageDataType, an instance of an image codestream encoded in hex binary of base64 binary format. Alternatively, external resources can be identified by MediaUri of ImageDataType, which describes where the image file, to which these metadata refer, is located.

As a special case, a file can have all images defined as external resources. As a result, the file only contains metadata (collection-level and item-level).

Schema and metadata

In this section, we describe the metadata which is supported by the JPSearch data interchange format.

JPSearch collection metadata schema

We first discuss the schema that facilitates the composition of XML metadata descriptions about image collections, for the purpose of information exchange between image repositories. Specifically, the type hierarchy of the schema and the root element are described.

The Collections element serves as the root element of the collection metadata schema. The root element shall be used as the topmost element when collections metadata appears in an independent way. This applies when collections metadata appears within the top-level metabox of the binary file format.

The CollectionsType type serves as the root element of the collection metadata part of the metadata interchange format schema.

The CollectionType type allows expressing metadata related to one image collection. It includes descriptive elements from the JPSearch Core Schema in JPSearch Part 2.

JPSearch XML metadata interchange format schema

We now discuss the schema that facilitates the interchange of XML metadata descriptions about images and collections. The following description tools are specified: The type hierarchy of the schema and the root element.

The ImageRepository element serves as the root element of the XML metadata interchange format schema. It corresponds to the movie box as defined in the ISO base media file format [17].

The ImageRepositoryType type allows expressing metadata related to images and image collections. It is composed by zero-to-many CollectionsMetadata elements, and one-to-many Image elements. It corresponds to the movie box type as defined in the ISO base media file format [17].

The CollectionsMetadataType type allows expressing metadata related to image collections. It is composed by multiple Collection elements from the CollectionType defined in the JPSearch collection metadata schema.

The ImageType type allows expressing metadata related to one image.

The ImageDataType type allows linking between metadata and each corresponding internal or external resource when used within the binary file format, or just external resources when used within the XML metadata interchange format.

The ImageMetadataType type allows expressing metadata related to one image (item level). It includes

JPSearch Reference Software and Applications

JPSearch Reference Software

JPSearch provides reference software in order to help implementers interpreting the ISO/IEC 24800 specifications, and also to enable them determining whether their products or systems are conformant to the standard. The JPSearch reference software, which is specified in Part 6, provides a specific implementation that behaves in a conformant manner. However, other implementations that conform to ISO/IEC 24800 are possible that do not necessarily use the same algorithms or the same programming techniques. The JPSearch reference software operates on and generates conformant JPSearch metadata and image files. It is entirely written in the Java programming language and it is divided in four different modules, one for each normative part of the standard (Parts 2, 3, 4 and 5). These software modules are defined in a way that they can be used independently or combined to form a basic JPSearch compliant system. Figure 13 depicts the architecture of ISO/IEC 24800 reference software.

The Part 2 module (Metadata Registry, Validation and Translation) allows registering external metadata schemas, validating metadata instances and defining cross-schema metadata translations. This module performs syntactic and semantic schema/translation validation, allowing implementers determining whether their schemas and translations are conformant to the

JPSearch Applications

JPSearch is a recent standard, but there are already some real applications which are taking profit from it. One example is the federated image search system developed by the DMAG (Distributed Multimedia Applications Group), a research group of the Computer Architecture Dept. of the Universitat Politcnica de Catalunya (UPC BarcelonaTech) [START_REF]Distributed multimedia applications group[END_REF]. The aim of this project, further described in [START_REF] Toran | Image search based on a broker approach[END_REF], is to provide the ability to search images, from a central point, on different servers such as Panoramio, Picasa or Flickr, simultaneously. The system is compliant with Parts 2 and 3 of the JPSearch standard. It receives ISO/IEC 24800-3 (JPQF) queries addressing metadata in ISO/IEC 24800-2 format (extended with some EXIF fields, like the camera make and model) and rewrites them once for every metadata format that is supported (Panoramio, Picasa and Flickr in the first public version). In order to ensure that the results are correct, they are reprocessed (or filtered) based on the search constraints, whenever it is possible. This filtering also expands the search capabilities of each server because it allows searches that would not be possible without doing it. Figure 14 shows the system architecture. A demo of the system can be found in [START_REF]Distributed image search application demo[END_REF].

Another example usage of ISO/IEC 24800 is the BIOPSEARCH system [START_REF] Ferrer | Solo-medicine in optical biopsies -a way to practice telemedicine[END_REF], a content based medical image retrieval application specialized in optical biopsies. An optical biopsy is an optic diagnostic method capable to analyze the tissue in surface and in deepness without the need to extract it from the body. The system assists physicians and other medical personnel in the interpretation of optical biopsies obtained through confocal laser endomicroscopy (CLE). CLE is a novel technique for intravital microscopy during ongoing gastrointestinal endoscopy. However, most gastroenterologists are not trained to interpret mucosal pathology, and histopathologists are usually not available in the endoscopy suite. BIOPSEARCH may greatly facilitate CLE data base management and diagnosis. The system, also developed by DMAG, allows users navigating and searching over an image database containing optical biopsies of the human colon. Users are able to retrieve information about precedent diagnostics by providing an example CLE image for content based image retrieval (CBIR), by using keywords, or by filtering different fields for structured retrieval. The system is conformant with ISO/IEC 24800-2 and ISO/IEC 24800-3. Figure 15 shows a screenshot of the system's web frontend.

Conclusion and Discussions

In this paper, we introduced a new international standard of JPSearch, which provides a unified framework for interoperable search and sharing of images. Some of the major advantages of JPSearch are as follows:

• JPSearch provides tools for interoperable interpretation of heterogeneous metadata supporting social tagging. By registering their own schema and translation rules to the authority, based on the JPSearch core metadata and translation rule description language, people can add their own metadata such as key words or text description to the existing images and share the images with metadata;

• JPSearch provides unified messages between client and servers for query and result, called Query Format, to allow independent development of applications and standardized interfaces enabling simple aggregation service of multiple database services;

• JPSearch provides interoperable file format, within which arbitrary number of metadata can be embedded, based on JPEG and JPEG2000 file format, and provides straightforward binding mechanism of image and corresponding metadata. Using the JPSearch file format, multiple instances of metadata can be embedded inside the image file and transferred between devices without losing information;

• JPSearch defines interchange format for exchange of data between JPSearch image repositories.

For semantic interoperability of metadata within the JPSearch framework, the JPSearch Ad Hoc Group of ISO/IEC JTC1 SC29 WG1 (a.k.a. JPEG) is currently considering extension of JPSearch standard by including ontology related technology.

Acknowledgement

This article is a collaborative effort of editors of JPSearch standard. The authors would like to thank members of the JPEG committee who contributed to the development of JPSearch standard.

Figure 1 :

 1 Figure 1: Structure of JPSearch Framework

  In this paper, details of Part 2, Part 3, Part 4, Part 5 and Part 6 are provided in the following sections. Conclusion and discussions are provided in the last section.

2 .

 2 JPSearch Part 2: Registration, identification and management of schema and ontology

Figure 2 :

 2 Figure 2: Situation without interoperability

3 .

 3 JPSearch Part 3: JPSearch Query Format One of the key elements in JPSearch is the query format, which is specified in Part 3 of ISO/IEC 24800. The tools of the JPEG Query Format (JPQF) are adaptations of ISO/IEC IS 15938-12 (MPEG Query Format, or MPQF) for the still images domain. The MPQF was developed by the MPEG Committee (ISO/IEC JTC1

Figure 4 :

 4 Figure 4: Interoperability among multiple service sites

Figure 5 :

 5 Figure 5: Possible scenario for the use of the JPEG Query Format

Figure 6 :

 6 Figure 6: Schema overview of the uppermost elements of the JPQF

Figure 7 :

 7 Figure 7: Example of Input Query in JPQF

Figure 8 :

 8 Figure 8: Example of Combined Query in JPQF

Figure 9 :

 9 Figure 9: Example of Relevance Feedback Query in JPQF

4 .

 4 JPSearch File (JPEG/JPEG-2000 compatible)! !"#$%&'()$*%+%*% block! ,-$.$/*%&0)$*%+%*% block! An instance of schema A by author B! 1%*% block! An instance of schema C by author D! An instance of schema C by author E! * One of 1%*% block shall be an instance of JPSearch Core Metadata schema!

Figure 10 :

 10 Figure 10: Concept of JPSearch file format

Figure 11 :

 11 Figure 11: Structure of JPEG compatible JPSearch file format

Figure 12 :

 12 Figure 12: Structure of JPEG-2000 compatible JPSearch file format

Figure 13 :

 13 Figure 13: JPSerach reference software architecture

Figure 14 :

 14 Figure 14: Federated image search system architecture

Figure 15 :

 15 Figure 15: Screenshot of the BIOPSEARCH demo GUI client

  The InputQuery may have QueryCondition element to specify the search criteria of the database through various QueryType type elements such as QueryByMedia, QueryByDescription, QueryByFreeText and QueryByRelevanceFeedback, to name a few, and their Boolean combinations. The InputQuery also <?xml version="1.0" encoding="UTF-8"?> <JPEGQuery xmlns="urn:jpeg:jpqf:schema:2008" xmlns:mpqf="urn:mpeg:mpqf:schema:2008" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

	xmlns:jpcs="urn:jpeg:jpcs:schema:2010"
	xsi:schemaLocation="urn:jpeg:jpqf:schema:2008 24800-3.xsd
	urn:jpeg:jpcs:schema:2010 jpcs2010.xsd
	urn:mpeg:mpqf:schema:2008 15938-12-Cor1-Cor2-Amd1.xsd" jpqfID="http://www.mpqf.org/id1">
	<InputQuery>
	<OutputDescription maxItemCount="30" maxPageEntries="10"
	outputNameSpace="urn:mpeg:mpeg7:schema:2004">
	<mpqf:ReqField typeName="jpcs:uri"> ImageInstanceLocator[1]</mpqf:ReqField>
	<mpqf:ReqField typeName="jpcs:identifier">Identifier</mpqf:ReqField>
	<mpqf:ReqField typeName="jpcs:Creators/GivenName">Creators/GivenName</mpqf:ReqField>
	<mpqf:ReqField typeName="jpcs:Creators/FamilyName">Creators/FamilyName</mpqf:ReqField>
	<mpqf:ReqField typeName="jpcs:CreationDate">CreationDate</mpqf:ReqField>
	<mpqf:ReqField typeName="latitude">GPSPositioning/@latitude</mpqf:ReqField>
	<mpqf:ReqField typeName="longitude">GPSPositioning/@longitude</mpqf:ReqField>
	</OutputDescription>
	<QueryCondition>
	<EvaluationPath>/MovieBox/TrackBox/ItemMetaBox/JPSearchCore</EvaluationPath>
	<Condition xsi:type="mpqf:Equal">
	<mpqf:DateTimeField>CreationDate</mpqf:DateTimeField>
	<mpqf:DateTimeValue>2009-10-07T08:46:45</mpqf:DateTimeValue>
	</Condition>
	</QueryCondition>
	</InputQuery>
	</JPEGQuery>


Clémentine Ols
email: clementine.ols@ign.fr

Martin P Girardin
email: martin.girardin@canada.ca

Annika Hofgaard
email: annika.hofgaard@nina.no

Yves Bergeron
email: yves.bergeron@uqat.ca

Igor Drobyshev
email: igor.drobyshev@uqat.ca

Monitoring climate sensitivity shifts in tree-rings of eastern boreal North America using model-data comparison

Keywords: boreal forests, North America, forest growth models, climate change, climate-growth relationships, black spruce, Picea mariana

come

Introduction

Tree growth rates are well correlated with spatial and temporal climate variability [START_REF] Gifford | Photosynthesis, carbon partitioning, and yield[END_REF]Rennenberg and others 2006;Wu and others 2012;Vlam and others 2014;Gričar and others 2015). Since the beginning of the Industrial Revolution, increasing anthropogenic activities have altered global climate and local weather dynamics, particularly during the last century (Mann and others 1998;IPCC 2014), thereby affecting tree growth processes. Tree growth in many boreal regions lost its positive response to rising temperatures during the late-20 th century (D'Arrigo and others 2008), a phenomenon often paralleling increased sensitivity of tree growth to precipitation and drought severity (Buermann and others 2014;Galván and others 2015;Latte and others 2015). Yet causes for changing climate sensitivity in tree-rings vary and may also result from responses to other phenomena that are associated with changing cloud cover, delayed snowmelt and increasing local pollution (Vaganov and others 1999;D'Arrigo and others 2008). Furthermore, links between temporal variations in tree responses to climate and climate change likely involve cross-scale interactions between abiotic and biotic variables, e.g., tree age/size and site characteristic effects on tree growth [START_REF] Carrer | Age-dependent tree-ring growth responses to climate in Larix decidua and Pinus cembra[END_REF]Rossi and others 2008;Ibáñez and others 2014;Navarro-Cerrillo and others 2014) and insect herbivory (Krause and others 2012;Fierravanti and others 2015). The evaluation of climate change effects on tree growth dynamics remains challenging (Girardin and others 2016b).

In the boreal forest of eastern North America, seasonal temperatures have increased by as much as 3ºC since the beginning of the 20 th century (Hansen and others 2010;Jaume-Santero and others 2016), while seasonal precipitation has shown variable patterns (Wang and others 2014). Studies have reported a decrease in tree growth sensitivity to growing season temperature in historically 'temperature-limited' high latitude and high altitude forests [START_REF] Jacoby | Tree ring width and density evidence of climatic and potential forest change in Alaska[END_REF]Briffa and others 1998;Galván and others 2015). In parallel, growth declines have been reported over the late 20 th century (Girardin and others 2016a), while the occurrence of years with extremely low growth in the boreal forest of eastern North America has increased throughout the 20 th century (Ols and others 2016). Both phenomena have been attributed to increased drought impacts on tree growth. During the 21st century, soil water availability, atmospheric water demand and heat stress in the boreal forest of eastern North America are projected to limit tree growth increasingly as a consequence of continuing warming (Girardin and others 2016b;Novick and others 2016). The degree to which this forest will adapt to warmer and drier conditions, e.g., by increasing its water use efficiency, is uncertain (Charney and others 2016). Therefore, it is important that these ecosystems are continuously monitored to detect early warning signs of changes in climatic controls on tree growth (Gauthier and others 2015).

However, such observation-based monitoring is complicated by the large spatial extent of the boreal forest of eastern North America.

Forest growth models can facilitate the exploration of tree growth processes and their expected relationships with the evolving local climate. Such models can be built upon sets of mathematical equations accounting for non-linear relationships between specific environmental and physiological variables that have been derived from empirical observations [START_REF] Landsberg | A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning[END_REF][START_REF] Misson | MAIDEN: A model for analyzing ecosystem processes in dendroecology[END_REF]). Studying the coherency of climatic signals that are contained in empirical tree growth data and simulated tree-growth data may help us understand whether variations in tree-growth responses to climate emerge from changing climate alone or from changes in tree-growth sensitivity to climate.

Modeling may also help the study of tree growth and its sensitivity to climate in areas where ground sampling is more difficult due to the remoteness of locations and the costs that are associated with this type of sampling.

In this study, we explore the possibilities of detecting shifts in tree growth sensitivity to climate in boreal black spruce forests of eastern North America by comparing observed and model-based climate-growth relationships over the period 1908-2013. Through this case study, we have proposed an experimental design that could be paired with ongoing national forest inventory programs (e.g., Girardin and others 2016a) to implement large-scale, systematic and long-term monitoring of tree growth sensitivity to climatic variations. Here, observed data consisted of a newly acquired network of 50 annually resolved and absolutely dated black spruce tree-ring width chronologies covering latitudinal and longitudinal gradients of eastern boreal North America (49°N-52°N, 58°W-82°W, Fig. 1). Regarding the model-based data, we used a stand-level forest growth model that was based on the Physiological Principle Predicting Growth (3PG) model [START_REF] Landsberg | A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning[END_REF] to simulate yearly site-specific net primary production (NPP) for the period encompassing the observed data (i.e., 1908-2013). Two hypotheses were formulated on the basis of the widely accepted evidence that temperatures have been rising in the study region:

(H1) Yearly variability in tree growth is under the control of climate. The validity of this hypothesis implies a significant correlation between tree-ring width data and climatically driven simulations of NPP;

(H2) The control of water on tree growth has increased over time along with the rise of temperature, particularly in high-latitude and high-altitude forests. This implies an increased positive sensitivity to precipitation, in both tree-ring width data and climatically driven simulations of NPP.

Material and methods

Study area

The study area consists of three latitudinal transects (western, central and eastern; Fig. 1a) that were established in northern boreal Quebec (Ols and others 2016). The terrain in this area is characterized by low plains in the west (200-350 m above sea level [a.s.l.]) and by mountains, where topographic relief is particularly pronounced in the north, central and eastern regions (up to 1128 m a.s.l. in the Otish Mountains). The two main climatic gradients in the study area are a decreasing temperature gradient from south to north and an increasing summer (June to August) precipitation gradient from west to east (Fig. 1b). The eastern region is regularly prone to spruce budworm (Choristoneura fumiferana [Clemens]) outbreaks [START_REF] Boulanger | Spruce budworm outbreaks in eastern Quebec over the last 450 years[END_REF].

Tree-ring width measurements

Tree growth data (n = 890 trees) were collected at 50 sites that were located along the three latitudinal transects (Fig. 1a; Table S1) (Ols and others 2016). All sites were pure, unmanaged old-growth black spruce (Picea mariana (Miller) B.S.P.) forests growing on xeric to meso-xeric soils (Direction des inventaires forestiers 2015). Between 10 and 27 dominant trees (standing living or dead) were sampled per site (one core per tree). Sampled cores were processed using standard procedures and the rings were visually and statistically cross-dated. Tree-ring width measurements were detrended using a 60year spline to eliminate noise that was caused by site-and biologically related effects (e.g., competition, self-thinning and aging) [START_REF] Cook | Calculating unbiased tree-ring indices for the study of climatic and environmental change[END_REF]. Detrended ring-width measurements were then processed using autoregressive modeling to remove autocorrelation (pre-whitening) and averaged into site-specific residual tree-ring width (RWI) chronologies using a robust bi-weighted mean.

Climate data

Climatological data that were used as inputs to the forest growth model and in the calculations of climate-growth relationships were monthly means of maximum (T max ) and minimum (T min ) temperatures, and monthly total precipitation (Prec), which were all extracted from the 0.5° x 0.5° CRU TS 3.22 database (Harris and others 2014). The climatic characteristics of each study site were extracted over the 1901-2013 period, using a site-centered 0.5° x 0.5° grid cell. We retrieved data from twenty-one grid cells, with each grid cell containing between 1 and 7 study sites. Consequently, some study sites exhibited identical climatic characteristics (Table S1). To test the influence of climate data type on model simulations and climate-growth relationships, site-specific climate data (T min , T max and Prec) were also extracted over the 1901-2013 period (using the same procedures as above) from three alternative databases: (1) the Canadian software BioSIM (Régnière and others 2014); (2) the combined 0.5° x 0.5° CRU TS 3.22 temperature (Harris and others 2014) and GPCC precipitation (Full Data Reanalysis Version 7, Schneider and others 2015); and (3) Twentieth Century Reanalysis (20CR, Compo and others 2011) datasets. The 20CR data are derived from oceanic temperature and surface pressure data, and do not incorporate precipitation and station temperature records (Compo and others 2011); 20CR may thus be viewed as being independent of all other climate products.

The boreal region of eastern Canada is not covered by a dense network of weather stations (Fig. S1). In many instances, the existing stations have been running intermittently (Girardin and others 2016b). To capture precipitation and temperature input data accuracy through space and time, the number and location of meteorological stations that were used for climatic interpolations within our study area were extracted. We also extracted the only long-running hydrological record that was available for the study area, i.e., the 1960-1993 De Pontois river flow from HYDAT 28.0 (Water Survey of Canada, http://www.ec.gc.ca/rhc-wsc) (Table S3), and used this record as a surrogate for drought conditions (Haslinger and others 2014).

Forest attributes

Biometric information necessary for the model simulation was obtained as follows. First, the aboveground biomass in Mg per hectare (𝑊 𝑎𝑏𝑔 ) was estimated at each study site using country-wide species-specific allometric equations (Paré and others 2013) that were applied to site-specific basal areas (Table S2). Second, site-specific topography data (slope and aspect values; Table S2) were extracted from Canada 3D, a digital elevation model (DEM) that was produced by the Canadian Forestry Service (Natural Resources Canada 2002) using ArcGIS® [START_REF]Norme de stratification écoforestière -Quatrième inventaire écoforestier du Québec méridional[END_REF]. Last, historical patterns of defoliation severity that was incurred by the spruce budworm , and which were compiled from Quebec's annual provincial surveys (Ministère des Forêts, de la Faune et des Parcs du Québec [MFFPQ] 2014), were extracted for each of our sites.

Net primary productivity data

Net primary production (NPP) at our 50 sites was simulated using the StandLEAP model (version 0.1 SVN, Girardin and others 2016b). StandLEAP is a generalized plot-level model that is based upon the 3PG model [START_REF] Landsberg | A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning[END_REF], which is applicable to relatively homogeneous forests. It was developed for the estimation of forest productivity over large areas (e.g., Girardin and others 2016b) but with a spatial resolution that was sufficiently fine for use in forest management (e.g., Raulier and others 2000;Coulombe and others 2009;Anyomi and others 2014). StandLEAP can be parameterized for individual species and its application to any stand does not require fine-tuning of the model to fit the data. The model has been tested against numerous independent tree-ring datasets in western, central and eastern Canada (Girardin and others 2008(Girardin and others , 2011b(Girardin and others , 2011a(Girardin and others , 2012(Girardin and others , 2014(Girardin and others , 2016b)).

StandLEAP runs on a monthly time-step. In StandLEAP, parameters are set up to fully characterize the effects of many interacting and non-linear modifiers of carbon flux quantities (e.g., growth and respiration). Absorbed photosynthetically active radiation (APAR) is related to growth primary production (GPP) using a radiation use efficiency (RUE) coefficient that differs among locations and through time as a function of environmental constraints. Constraints take the form of species-specific parameters (f 1 ,…,f n ) that take a value of 1 under average conditions; they are closer to zero to represent increasing limitations, or above 1 as conditions improve towards optimal. Constraints represent the effects of soil drought (Bernier and others 2002), frost (Aber and others 1996) (both limited to a maximum of 1.0), mean maximum and minimum air temperature, vapor pressure deficit (VPD), radiation, and leaf area index (where values greater than 1.0 are possible) on GPP. The following equation summarizes these functions:

) ... ( 2 1 n f f f RUE APAR GPP    , (1)
where RUE represents a species-specific mean value of RUE that is applicable to the entire species' range. Monthly canopy light absorption and photosynthesis parameters were derived from metadata that were generated using a more detailed multi-layer, hourly time-step model of canopy photosynthesis and transpiration that is called FineLEAP (Raulier and others 2000;Hall and others 2006). Representation of photosynthesis in FineLEAP is based upon the equations of Farquhar and others (1980). Additional details of the procedure and origin of the basic field measurements and procedure for estimation of parameters for radiation interception, radiation-and water-use-efficiency can be found in Hall and others (2006). NPP is computed monthly, after partitioning respiration into maintenance (Rm) and growth (Rg: a fixed proportion of the difference between GPP and Rm) quantities and subtracting these from GPP. Maintenance respiration is computed as a function of temperature using a Q 10 relationship [START_REF] Ågren | Population respiration: A theoretical approach[END_REF][START_REF] Ryan | Effects of climate change on plant respiration[END_REF][START_REF] Lavigne | Growth and maintenance respiration rates of aspen, black spruce and jack pine stems at northern and southern BOREAS sites[END_REF].

Acclimation of respiration to temperature is modelled using the equation of Smith and others (2016).

As is the case in 3-PG, part of NPP is first allocated to fine roots (Eq. ( 13) in Landsberg and Waring 1997) on a yearly basis and then to replacement of carbon biomass that is lost to leaf and fine woody litter turnover. The remaining NPP is then allocated to increments in stand carbon compartments of foliage, branches, coarse roots and stems. The modifier for soil water availability is based on modeled water balance, which is coupled to transpiration and NPP, as described by Bernier and others (2002).

The impact of CO 2 fertilization is included through a modifier of the potential water use efficiency (WUE), as described by Girardin and others (2016b). The active soil depth was set to 600mm at all sites (Table S2). An active soil depth between 300 and 900mm has generally been accepted as a desirable range for black spruce [START_REF] Viereck | Picea mariana (Mill.)[END_REF]Girardin and others 2016b). Three sites (viz., 39, 45 and 47; Table S2

Correlation between tree-ring and NPP metrics

Carry-over effects from the previous growing season have been reported to affect tree growth significantly the following season, and particularly in harsh environments (Babst and others 2014;Ols and others 2016). For instance, lower carbohydrate reserves in the following growing season, can notably decrease the capacity of trees to respond to favorable growth conditions. Accordingly, monthly NPP values that were obtained from modelling were summed from July of the previous year to June of the current year of growth to represent carbon quantities that were mobilized and allocated to growth from one year to the next (as in Girardin and others 2016b). The correspondence between annual RWI and NPP metrics were then explored through moving window correlations (one-tailed test) at site level. Correlations were computed in R (R Core Team 2015) using 21-year-long windows that were incremented in five-year steps from 1908 to 2013. The null hypothesis of no positive correlation between RWI and NPP was rejected when P < 0.05. Temporal stability in correlations between RWI and NPP metrics were also investigated at the regional level using the same movingcorrelation procedure as above. Regional RWI and NPP metrics were computed as a robust biweighted mean of all site-specific metrics. The significance of each 21-year correlation averaged across sites was evaluated using a competitive test, which combines the probabilities of dependent Shifts in tree growth sensitivity to climate 10 tests using Fisher's method (Dai and others 2014). When applied to our specific case, it compares, for each 21-year period the distribution of P values of all site-specific NPP and RWI correlations to the distribution of randomly selected 100,000 vectors of P-values of similar length. Competitive tests were computed in R using the competitive.test function that is available in the CombinePValue package [START_REF] Dai | CombinePValue: Combine a vector of correlated p-values. R package version 1.0[END_REF]. To test for coherence in year-to-year variability between the two metrics, moving correlations between first-differenced RWI and NPP metrics (subtraction of the value at year t-1 from the value at year t ) were also computed using the same methodology that was described above.

Climate-growth relationships

Coherency in the climatic signals that were contained in RWI and NPP metrics were investigated by correlation analyses. First, correlations between tree-growth metrics and monthly climate data were computed using 21-year-long windows incremented in five-year steps from 1908 to 2013 using the treeclim package [START_REF] Zang | treeclim: an R package for the numerical calibration of proxy-climate relationships[END_REF]. Climate data included monthly maximum and minimum temperatures and monthly total precipitation. Months spanned from May of the previous year to August of the current year of growth. Site-specific moving correlations were then averaged across all sites to characterize monthly climate-growth relationships at the scale of our study area. The following hypotheses, which were based upon the earlier work by Girardin and others (2016b), were postulated and tested using one-tailed tests: (1) growth is positively correlated with previous September through current May (hereafter September-May) temperatures;

(2) growth is negatively correlated with June-August temperatures; and (3) growth is positively correlated with precipitation, regardless of the month. Alternatively, we also tested the inverse versions of hypotheses 1 to 3: (4) growth is negatively correlated with September-May temperatures; (5) positively correlated with June-August temperatures; and (6) negatively correlated with precipitation. Hypotheses 1-6 were considered true both for months of the previous year and current growing season. These procedures were run for both RWI and NPP series. Note that stronger correlations observed with NPP can logically emerge from computation alone, since NPP is itself computed from these climate data. The significance of each 21year correlation averaged across sites was evaluated using the competitive test that was described earlier. We opted for six one-tailed hypotheses rather than three two-tailed hypotheses, because under two-sided testing the analysis is particularly sensitive when both strong positive and negative effects occur across sites [START_REF] Whitlock | Combining probability from independent tests: the weighted Z-method is superior to Fisher's approach[END_REF]. Last, the distributions of site-specific correlations with monthly climate variables of the two metrics were compared using Wilcoxon-Mann-Whitney (U-test) and Kolmogorov-Smirnov tests. To test for coherence between local drought conditions and forest growth, we also computed the correlation between tree growth metrics and the mean July-to-September De Pontois river flow, using both raw and first-differenced metrics.

Results

Climate sensitivities in tree growth metrics

RWI was generally positively correlated with current year spring and summer temperatures (Fig. 2a).

However, these correlations decreased substantially and became non-significant during the mid-to late-20 th century (Fig. 2a). This decrease in correlation corresponded with the emergence of significant negative correlations with previous summer and previous October temperatures (maxima and minima) from the 1940s to 1990s, and with current spring precipitation during a brief period covering the 1960s to 1980s (Fig. 2a). In addition, significant positive correlations between RWI and early winter temperatures were observed during the late-20 th century (Fig. 2a). The correlation between RWI metrics and the mean July-to-September De Pontois river flow of the year prior to growth, and over the period 1960-1993, was significantly positive (median correlation: r = 0.31), especially at high latitudes (> 51.5 ºN) (Fig. 3). In summary, annual growth variability in this boreal region of eastern North America has shifted from being positively correlated with growing-season temperature early in the 20 th century, to being negatively correlated with summer temperature during mid-century, and then back to being positively correlated with temperature during the late-20 th century. There was no clear evidence of a strengthening of tree growth sensitivity to precipitation throughout the 1908-2013 period, using CRU precipitation data (Fig. 2a).

Relationships between NPP and monthly climate variables were similar to those observed for RWI metrics (Fig. 2b,c). NPP correlated positively with current year temperatures, but this relationship was much weaker from 1958 to 1988 compared to all other periods (Fig. 2b). Between temperature (Fig. 2b). Unlike RWI, there was a period of sustained significant positive correlations with July or August precipitation during the year contemporaneous to growth from 1933 to 1998 (Fig. 2b). Wilcoxon-Mann-Whitney tests indicated that the distribution of correlations between NPP and monthly temperature (minimum and maximum) was generally more homogeneous than with RWI (Fig. 2c). In contrast, the distributions of correlations with precipitation for both metrics were mostly similar. This was also observed using Kolmogorov-Smirnov tests (Fig. S2). NPP metrics and the mean July-to-September De Pontois river flow were not significantly correlated (P > 0.05) (data not shown).

However, these variables were significantly correlated after a first-difference transformation in 33 of the 50 study sites, mostly north of 51.5ºN and west of 74.0ºW (median correlation: r = 0.32; correlation pattern was similar to the RWI pattern exhibited in Fig. 3d, but with both variables taken on their non-lagged calendar years; results not shown). In summary, NPP variability shifted from being temperature-driven in the early-20 th century (an indication of temperature limitation of the rate of photosynthesis), to precipitation-driven during the mid-to late-20 th century (i.e., the influence of available moisture), and then again temperature-driven during the late-20 th century.

Synchronicity in tree growth metrics

Correlations between site-specific RWI and NPP metrics at the regional level were often positive and significant during the early-20 th century, and throughout the late-20 th to early-21 st centuries (Fig. 4).

However, a clear desynchronization was observed in the middle of the century at almost all sites, when correlations substantially decreased to become negative and occasionally significantly negative (Fig. 5, left-hand panels). Although its duration and timing differed across sites, this desynchronization was most prominent in mountainous north easternmost sites (Fig. 6d), i.e., in areas ongoing the most rapid warming (Fig. 1a). First-differencing of the RWI and NPP data enhanced correlations during the late-20 th century but decreased correlations during the early-20 th century across the whole area (Fig. 5, right-hand panels). It is noteworthy that the same mid-to late-20 th century desynchronization between observed and simulated tree-growth metrics was obtained using alternative climate datasets, albeit with variations in the onset and duration of this desynchronization depending upon the data products (Fig. S3). For instance, correlations between NPP and RWI were improved during the Shifts in tree growth sensitivity to climate 13 period when fed by simulations of NPP that were driven by the 20CR data, although they did not fully compensate for model-data desynchronization (Fig. S3d).

Discussion

The purpose of this study was to document shifts in tree growth sensitivity to climate in temperaturelimited boreal forest ecosystems of eastern boreal North America over 1908-2013 using a comparative study of field and modeled data.

Despite climate warming in the study area (Figs. 1a andS4), there was no clear evidence for a strengthening of radial tree growth (RWI) sensitivity to precipitation during recent decades (Fig. 2).

The post-1980 significant positive correlations between (1) growing-season temperature and radial growth (Fig. 2c), and between (2) radial growth and modeled productivity (NPP) across the entire study region (Fig. 4) indicate that both photosynthesis (the main driver of the model) and xylem formation (the main driver of radial growth) have until recently remained temperature-limited. The response of tree radial growth in our study area is therefore different from the response that has been frequently reported in the literature for the boreal forest. A likely reason for this lack of increased sensitivity in tree growth to precipitation may be that despite rising temperature (Fig. 1a), atmospheric water demand may have decreased over the course of the 20 th century in our study area (Fig. S5; also see Fig. S9 in Girardin and others 2016b). A decrease in water demand, coupled with a potential increase in water use efficiency under elevated atmospheric CO 2 concentrations, may have contributed to the stabilization of tree dependence upon incoming precipitation that is necessary for soil moisture recharge.

Our results revealed that while modeled productivity remained somewhat spring temperaturesensitive over the entire study period, the positive influence of growing season temperature on radial growth briefly disappeared during the mid-20 th century. This phenomenon occurred in parallel with an increased sensitivity to moisture, as indicated by the negative correlation between radial growth and summer temperature, by the positive correlation between radial growth and the mean July-to-September De Pontois river flow, and by the positive correlation between modeled productivity and July precipitation. During this same period, the overall region-wide significant synchrony between modeled productivity and radial growth also dropped to become insignificant, particularly at eastern high-latitude mountainous sites, i.e., those undergoing the strongest warming (Fig. 1a). This sudden model-data desynchronization is noteworthy and deserves attention, as it may impair our capacity to monitor shifts in tree growth sensitivity to climate in these forests. We subsequently discuss three factors that may be involved in this desynchronization: accuracy of input climate data; the advent of external factors in the ecosystem; and model uncertainties.

Climate data uncertainties have a large influence on model-based estimations of historical and ongoing ecosystem processes. The choice of climate dataset, with those of precipitation being of particular concern, affects the capacity to identify drivers of variability in empirical data products and model results (e.g., Daly and others 1994;Ito and others 2017;Wu and others 2017). For example, the probability of a false negative result (e.g., a significant RWI-precipitation correlation that was not detected when a true relationship exists) could theoretically be higher at sites where climate data quality is lowest (Wilson and others 2007). In the current study, the quality of climate data is likely to be a critical factor explaining the drop in model-data correspondence during the mid-to late-20 th century. First, the desynchronization in model-data correspondence was most prominent at sites located above 51.5 ºN (Fig. 5c), i.e., where station density is low (Figs. 5 and S1). Therefore; there is an apparent relationship between station density and climate signal degradation. Additionally, weak model-data correspondence was clearly linked to altitudinal differences between reference stations (mainly located along the coast) and mountainous sampled sites, with a higher capacity to detect a positive correlation between radial growth and modeled productivity at low altitude sampling locations (Fig. 6d). This bias finds explanation in the fact that CRU temperature interpolations over eastern North America do not depict altitudinal climate gradients (Fig. S6). Our capacity to model forest growth in mountainous regions is thus very likely hindered by inaccuracies of temperature and precipitation estimates at these high-altitude sites. The variations in the onset and duration of the desynchronization between radial growth and modeled productivity across climate data products illustrate the problem of climate data uncertainties, which are particularly present in precipitation estimates (Figs. S1c andS7). Cross-correlations between CRU and the 20th Century Reanalysis (20CR) precipitation gridded data show strong inconsistencies during the period 1940-1980 (Fig. S7).

This pinpoints a significant error in data estimation in one or the other climatic product, or both. It is noteworthy that the onset of desynchronization between radial growth and modeled productivity coincides with a period of high weather station density in the region (Fig. 6a). Yet, many of these stations have recorded observations intermittently, which can be observed from the fluctuations in the annual number of stations contributing to the climate interpolation algorithm (Fig. S1b&c).

Outbreaks of eastern spruce budworm, which are recurrent in the boreal forest of eastern North America but currently not accounted for during simulations, could have contaminated field data by disrupting radial growth responses to local climate, thereby causing the model-data desynchronization (Girardin and others 2016b). We did not observe any obvious abrupt growth decline or release, which is typical of severe outbreak defoliation on these site chronologies (Figs. 4 and S7). Also, most of the sites showing desynchronization were located in an area that did not show historical evidence of spruce budworm outbreaks during the periods of 1947-1958, 1975-1992[START_REF] Wilson | A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data[END_REF]-2016 (Fig. 6b (Fig. 6b). Furthermore, the sampled stands were free of the main budworm host-species

Abies balsamea (L.) Miller. Although the influence of such disturbance on growth cannot be ruled out entirely, its role in the observed model-data desynchronization can be only minor. It may, however, become an important concern if this experimental design is to be applied in regions where this disturbance is recurrent (Girardin and others 2016b).

Our experimental design assumes that radial growth and modeled productivity are directly comparable analogs. However, this disregards documented evidences that shifts in radial growth sensitivity, to temperature for instance, emerge from changes in allocation of assimilates within a tree (Lapenis and others 2013). Assimilate allocation is strongly sensitive to stand density (e.g., denser stands favoring allocation to terminal buds to increase access to light) and climate (e.g., drier climate favoring allocation to roots to increase access to water). The period of desynchronization was characterized by a decrease in spring precipitation and an increase in spring temperature (Fig. S4). Such climatic conditions may have temporarily favored the allocation of assimilates to the root system against radial growth, disrupting radial growth responses to climate. In a sensitivity analysis where the productivity fraction allocated to the stem was substituted for total annual productivity (as in Girardin and others 2008), we noted an improved model-data correspondence during the first half of the desynchronization period (Fig. S9). Yet the correspondence deteriorated substantially during the second half (Fig. S9). Moreover, many of the sites showing model-data desynchronization were located in open-canopy stands adjacent to the limit between spruce-moss and spruce-lichen domains, which were located north of 51.5°N [START_REF] Robitaille | la Direction de la gestion des stocks forestiers et la Direction des relations publiques du Ministère des ressources naturelles du Québec[END_REF]. Thus, there are no reasons to believe that height growth would have been favored over diameter growth during the mid-to late-20 th century. Model-data desynchronization during the mid-to late-20 th century, therefore, does not appear to be linked to shifts in field growth allocation patterns.

Finally, the formulation of the model used herein may be missing important dynamic processes that are associated with snow accumulation and thawing, initiation of leaf-out and growth processes. The sudden negative sensitivity to spring precipitation in radial growth over the mid-to late-20 th century, which was not observed with modeled productivity, may be indicative of a stronger negative impact of spring precipitation on tree growth (Huang and others 2010;Girard and others 2011;Ols and others 2016). Sites exhibiting a sudden negative sensitivity to spring precipitation were the ones undergoing the strongest warming (Fig. 1a) and were mainly located in high-altitude mountainous areas (Fig. 6d). Snow dynamics (snowfall and snowmelt) influence tree growth and climate-growth relationships at boreal latitudes (Frechette and others 2011;[START_REF] Verbyla | Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska[END_REF], particularly along altitudinal gradients (Trujillo and others 2012). Indeed, a thick spring snow cover may delay the start of the growing season through delayed snow melt (Vaganov and others 1999). The current formulation of the water balance within the model does not include a dynamic snow model, as is the case in Terrier and others (2013). This may mask the onset and duration of the drought season, and ultimately, affect the capacity to uncover drivers that are associated with the water balance at highaltitude sites.

Conclusion

Climate change and its impact on high-latitude boreal ecosystems are now recognized. There is no doubt that, in the near future, intensive efforts will be required to monitor these impacts to pave the way for adaptation and mitigation solutions (Gauthier and others 2015). This will require tools to link ecosystem dynamics adequately to atmospheric properties. In this study, we showed that forest growth models can reasonably track processes leading to forest growth variability in the northernmost boreal forest of eastern North America through space and time when growth remains temperature-limited (Girardin andothers 2008, 2016b). It is conceivable that the analysis proposed here be deployed over larger areas for monitoring, in particular through integration with national forest inventories that specifically aim at providing large-scale systematic, timely and coherent information on the extent, composition and characteristics of forests and their evolution over time (e.g., Girardin and others 2016a). Our work nevertheless illustrated some of the challenges that hinder the capacity to monitor high-latitude boreal ecosystems at fine-scale across a diversity of landscapes. Among various issues, uncertainties in climate data are of particular concern. Many of the temperature-limited regions of boreal Canada are covered by a scarce network of weather stations, which affects the accuracy of local climate variability estimates and that makes it difficult to relate climate to ecosystem dynamics. The significance of each averaged correlation across sites was evaluated under two sets of climategrowth hypotheses using one-sided competitive tests (see 2.7 Climate-growth relationships). Open circles and black dots on panels (a) and (b) identify significant (P < 0.05) correlations under hypotheses 1-3 and 4-6, respectively. Black dots on panel (c) stand for no significant (P > 0.05) differences in the distribution of correlations. -1933, (b) 1933-1953, (c) 1953-1973, (d) 1973-1993, and (e) 1993-2013. Correlations were computed using original (left-hand panels) and first-differenced (right-hand panels) metrics. Blue and red circles represent negative and positive correlations, respectively. The larger the circle, the higher the value of the correlation (|r|). Black contours delineating circles indicate significant correlations (P < 0.05). Black crosses indicate the position of meteorological stations available for that period.

Figure 1 .

 1 Figure 1. (a) Location of the black spruce forests under study, eastern Canada. The sampling sites (n =

Figure 2 .

 2 Figure 2. Average correlations between monthly climate data and (a) RWI and (b) NPP metrics across

Figure 3 .

 3 Figure 3. (a) Mean of site RWI chronologies above 51.5°N. (b) Mean July-to-September river flow

Figure 4 .

 4 Figure 4. (a) Average tree-ring width indices (black line) and net primary production (red line)

Figure 5 .

 5 Figure 5. Pearson correlations (r) between site-specific RWI and NPP metrics during five different

Figure 6 .

 6 Figure 6. Potential factors involved in the low RWI-NPP correlation from 1953 to 1993. (a) Changes

  ) had their above-ground biomass truncated to a maximum value of 110 Mg/ha because of estimated field values that reached higher than typical conditions for which the model was calibrated. All carbon flux quantities used in this study were made insensitive to changing forest age over time, by fixing constant forest attributes (e.g., biomass and stem densities) across all simulation years. Carbon flux quantities solely express direct climate influences on plant growth, avoiding the influence of post-fire stand dynamics on fluxes (e.g., Girardin and others 2011a; Pan and others 2011) and allowing a direct comparison with climate driven tree-ring width measurements that were collected from old-growth forest stands. The model does not simulate soil processes other than water balance, since it implicitly assumes constant soil nutrient properties and turnover. Furthermore, computations assume the absence of insect outbreaks.

Figure 3.

Figure 4.

Acknowledgments

We thank Emeline Chaste for GIS analyses, Xiao Jing Guo for assistance with StandLEAP, Williams F. J Parsons for language revision, and two anonymous reviewers and the Associate Editor for helpful comments on an earlier version of this manuscript. This study was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC Strategic and Discovery Grants), the Nordic Forest Research Cooperation Committee (SNS), the Canadian Forest Service (CFS) and the Research Council of Norway (grant 160022/E50). This work was also supported by a fellowship from the Forest Complexity Modelling program (NSERC Strategic and Discovery Grants). The authors have no conflicts of interest to disclose.

Shifts in tree growth sensitivity to climate

Availability of climate data, therefore, may critically limit our ability to monitor climate change impacts on high-latitude forest ecosystems while drought severity is projected to rise. Through remote sensing, recent estimates of climate data, notably those of precipitation and snow cover could help address some of these issues in the future.

 (Régnière and others 2014). If the four nearest stations did not all present climate records over the entire period, additional stations were added until the full period was covered. Site-specific 1953Site-specific -1993 RWI-NPP correlations are plotted using transect-specific symbols: circles for West, squares for Central and triangles for East. Blue and red symbols represent negative and positive correlations, respectively; the larger the symbol, the higher the correlation value. Black contours indicate significant correlations (P < 0.05). (d) Altitudinal gradient (map) versus the distribution of the 1953-1993 NPP-RWI correlations (Jarvis and others 2008). The larger the circle, the higher the value of the correlation. Note that the altitudinal scale was truncated to 800 m to enhance contrasts between lowand high-altitude sampling sites.