DiPTiCC: Diversity and Productivity of Trees in the context of Climate Change (2017-2020)

1-Rationale and socio-economical context

Forests are the most represented ecosystems on Earth, covering about 30% of total land area. The number of people who use forest outputs to meet their needs for food, energy and shelter is in the billions (FAO, 2014). Contributions of forests to human well-being are extraordinary large and diverse. Forest ecosystems play a major role in combating rural poverty, ensuring food, livelihoods and energy supply. Wood energy is often the only energy source in rural areas of less developed countries but it is also increasingly used in developed countries with the aim of reducing dependence on fossil fuels. Forest products make a significant contribution to the shelter of at least 1.3 billion people, or 18 percent of the world’s population (FAO 2014). Forests deliver other vital environmental services such as clean air, water quality, biodiversity conservation and mitigation of climate change through carbon sequestration (FAO, 2014). The Paris Agreement of COP21 sets the global warming target to be “well below +2°C compared to pre-industrial levels”. The reach this objective the Agreement indicates in its Article 5 that there is a need to “conserve and enhance […] sinks and reservoirs of greenhouses gases […] including forests”. Carbon is not only stored in living trees, deadwood, litter and forest soil, but it is also held for decades in harvested wood products.

Forests are increasingly affected by climate-driven disturbances. A recent global assessment has revealed drought and heat-induced tree mortality in many of the world’s forested ecosystems, suggesting emerging climate change risks for forests (Allen et al. 2010). In recent decades, forest disturbance regimes have intensified markedly in Europe, resulting in strongly increasing damages from wind, insect outbreaks and wildfires (Schelhaas et al. 2003, Lindner et al. 2010), with climatic changes identified as key drivers of this increase (Seidl et al. 2011). This intensification of disturbances is therefore likely to increase the release of C into the atmosphere. Worldwide, insect herbivores are responsible of ca. 8% of annual loss of foliar biomass (Kozlov et al. 2015). Even minor but chronic damage to long-lived plants like trees can result in significant growth reductions (Zvereva et al. 2012). The predicted 2 – 5% increase in background herbivory due to climate warming (Wolf et al. 2008) may then produce previously underestimated negative impacts on forest productivity. In France, the percentage of trees with moderate to severe foliar loss has been growing steadily in the last decades (MAAF, IFN, 2016). These observations underline that future forest management must include practices to enhance forest resistance and resilience to climate change and associated biotic and abiotic disturbances.

To meet the rising demand for wood products and carbon sequestration in forests, the forestry sector increasingly relies on planted forests (Paquette and Messier 2010). Even if forest areas are increasing in some developed countries (e.g. France), a net loss of natural forest of 7 million ha per year was observed from 2010-2015 worldwide (FAO 2015) and was mainly due to development for agriculture, including oil palm plantation. In contrast, planted forest area increased by over 110 million ha since 1990 with an average annual rate of increase of
4 million ha (FAO 2015).  To counteract the increasing pressure on natural forests and the associated loss of ecosystem services (incl. habitat for biodiversity), more efficient production practices must be adopted in forest plantation, hence increasing their net productivity. However compared to crops or grasslands, species composition of planted forests remains fixed for decades or even centuries, which may strongly reduce their ability to respond to changing conditions (Loreau et al. 2013). It is therefore critically important to design and manage forests in order to maintain the stability of their productivity over time rather than to maximize annual yields under the environmental conditions at planting time, which are susceptible to change.

Mixed-species plantations are considered one of the main options for adapting to and reducing risks of climate change (Paquette and Messier 2013). While more diverse forests are on average less prone to pest damage (Jactel and Brockerhoff 2007) they are also considered more productive than monocultures of particular tree species in the long term (Firn et al. 2007, Piotto 2008, Zhang et al. 2012). In addition, higher tree species diversity may increase the stability of above ground biomass production on the long term (Morin et al. 2014, Jucker et al. 2014). Yet, diversity –productivity relationships (DPRs) seem to interact with local environmental conditions in a complex manner: positive DPRs were found over large gradients of climate conditions, including under wetter conditions (Finegan et al., 2015; Poorter et al. 2015) but also in sites with limited water availability (Jucker et al. 2015) or low fertility (Toïgo et al. 2015). However, current results based on observational studies also suffer from methodological limitations, including covariation between site fertility and tree species richness, variation in successional stages, unbalanced coverage of low- and high-diversity mixtures, or forest management practices (Vila et al. 2005). Furthermore, the processes underlying the diversity effects on stability of forest processes remain weakly understood (Loreau et al 2013, Morin et al. 2014). More critically, the influence of environmental drivers cannot be separated from true diversity effects. It is thus of crucial importance to better determine how climate interacts with tree diversity effects on forest productivity.

2-Objectives and hypotheses

The general objective of the project is to better quantify and predict the effect of tree species diversity on the productivity of forest under climate change (Fig.1).

DiPTiCC-2017-2020_inra_imageFig.1 Diagram showing the concept of the project: testing the interactive effects of tree diversity (both species richness and composition) and climate change (increased temperature and reduced precipitation in both trends and extreme events) on key above- and below-ground ecosystem processes driving tree growth and thus explaining the stability of productivity in mixed forests.

Towards this end, four specific objectives have been defined:

1. The first specific objective is to quantify the interactive effects of tree species diversity and climate conditions (temperature and precipitation) on overyielding in mixed forests (as compared to pure forests) and its stability over time. The main hypothesis is that association of functionally dissimilar species, i.e. with contrasting growth patterns and distinct responses to disturbances, is likely to promote both complementarity in resource use and asynchronicity in response to climatic fluctuation, thus resulting in higher mean and lower variance of productivity.

2. The second objective is to decipher fundamental above- and below-ground processes driving diversity – productivity relationships in mixed forests. The corresponding hypothesis is that tree species with contrasting traits more efficiently exploit light, water and nutrient resources. Furthermore, damage from primary consumers should be reduced in mixed forests, resulting in overall higher allocation of resources to growth.

3. The third objective is to better predict the productivity of mixed forests in response to climate change, including long term trends and extreme events. The working hypothesis is that weighing the relative influnce of key processes will help improve the process-based model FORCEEPS that will be used to test the effect of major climate disturbances on both the resistance (departure from mean trajectory) and the resilience (recovery time), i.e. the two main dimensions of stability, of the productivity of mixed forests.

4. The fourth objective is to use science-based knowledge to substantiate recommendations for forest managers who plan to develop new designs of mixed tree plantations. The hypothesis here is that compromises need to be found between advantages of new forest plantations based on tree species mixtures that are likely more productive on the long term and constraints related to silvicultural practices and wood market opportunities.

3-General structure of the project

All field tasks of the project will be carried out in two already established experimental platforms:

  • The ORPHEE experiment

It belongs to the worldwide Tree Diversity Network (http://www.treedivnet.ugent.be/ExpORPHEE.html). It is located 40 km south of Bordeaux (France) and was established in January 2008 on a 12ha of maritime pine clear cut. In total, 25,600 trees belonging to five native species were planted (European birch: Betula pendula, Roth; Pedunculate oak: Quercus robur, Linné; Pyrenean oak: Q. pyrenaica, Willdenow; Holm oak: Q. ilex, Linné; and Maritime pine: Pinus pinaster, Aiton). Eight blocks were established with 32 plots in every block corresponding to the 31 possible combinations of 1-5 species, with an additional replicate of the combination of the five species (www.facebook.com/orpheeexperiment). In 2015 an irrigation system was established  at the centre of each of the 32 plots in four out of eight blocks. The quantity of water supply has been set to compensate for water deficit during the driest months (from April to October).

OrpheeExperiment_inraFig.2 Structure of the Orphée experiment

  • The BIOPROFOR platform

This platform of field sites was initiated during the RPDOC – ANR project BioProFor (2011-2015, X. Morin), to study the effect of diversity in tree species on forest productivity along macro- and meso-climatic gradients. This platform focuses on Mediterranean and alpine forests because these ecosystems have been identified as particularly sensitive to climate change. More precisely, the sites are located in the external Alps, from the Provence (Sainte Baume) to Northern Alps (Bauges), on calcareous bedrock and northern aspects. The target species were beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and pubescent oak (Quercus pubescens Willd.). The platform includes 6 sites (Fig. 3), with elevational gradients of plots with various diversity (i.e. plots dominated by 1 vs. 2 species) replicated at three elevation steps.

BioProForExperiment_cnrsFig.3 Structure of the BioProFor experiment

The project is organized in four main tasks (Fig.4). The task 1 quantifies the interactive effects of tree diversity and climate on the stability of mixed forest productivity using tree measurements and tree-ring data. The task 2 deciphers the key ecosystem processes driving tree growth and analyses how they are affected by tree diversity and climate. The task 3 assembles the outcomes of task 1 and task 2 to better explain and predict long term productivity of mixed forests under climate change. The task 4 manages the project and organizes interactions with forest managers and stakeholders.

workflow_tasks_DiPTiCCFig.4 Workflows between the four tasks of the project

4-Research units involved in the project

INRA BORDEAUX – UMR 1202 BioGeCo Biodiversité Gènes et Communauté

INRA BORDEAUX  – UMR 1391 ISPA Interactions Sol-Plante-Atmosphère

INRA BORDEAUX – UEFP – Unité Expérimentale Forêt Pierroton

INRA NANCY – UMR 1137 EEF Ecologie et Ecophysiologie forestières

CEFE UMR 5175 – CNRS DR13 Centre d’Ecologie Fonctionnelle et Evolutive