Project: DroughtForC
Funding: PEPR FairCarbon
Lead(s)/Coordinator(s): Jean-Marc Limousin
Duration:
Web link : https://www.pepr-faircarbon.fr/projets/projets-laureats-de-l-appel-a-projets-faircarbon/drought-forc
Forests are both strongly impacted by the on-going climate change and at the heart of the mitigation strategies, thereby urging the scientists and forest managers to better anticipate how the forest carbon cycle will be affected under future climate. In Drought-ForC, we identify three main scientific issues that are currently limiting our understanding of climate change effects on forest’s carbon sequestration.
- The first one relates to the allocation of carbon among the different tree organs and the link between carbon photosynthetic assimilation and sequestration in perennial tree biomass.
- The second one relates to the degradation of organic matter and the sequestration of soil organic carbon under the antagonist effects of warming and soil drying.
- The third issue relates to the interactive effects of water and nutrient limitations for tree growth and forest functioning.
We aim at tackling these three scientific issues by uniting in a common research consortium all the French forest experimental sites that use eddy covariance carbon fluxes measurements or in situ rainfall manipulation experiments within the research infrastructures ICOS and AnaEE, together with a wide and representative panel of process-based models simulating forest functioning. Our objectives are to use concurrently the existing research infrastructures and datasets, new targeted experiments and measurements, and a wide variety of modeling approaches in order to improve our knowledge of drought response mechanisms in forests, our quantification of drought impacts on forest C cycling, and our projections of future climate change effects on the forest C sink.
The project is organized into three topical work packages (WP2 on carbon assimilation and allocation, WP3 on organic matter degradation and soil carbon sequestration, and WP4 on nutrient limitations) designed to address scientific knowledge gaps, and embedded in two transversal work packages (WP1 on the integration of experimental infrastructures, and WP5 on the comparability and robustness of model projections) designed to improve, compare and integrate the available tools to study these questions, namely the field experiments and the process-based models.
WP1 will aim at homogenizing experimental protocols and datasets among experimental sites and building the first soil warming experiments in French forest ecosystems. WP2 will investigate the linkage between carbon assimilation and growth, and how assimilated carbon is allocated to the different tree organs (above- or below ground, perennial or short-lived) because this affects the carbon residence time in the ecosystem. WP3 will investigate the organic matter decomposition and the C storage in the soil under the effects of experimental soil drying and warming, and by considering the climate change effects on soil biological activity. WP4 will investigate the nutrient limitations to growth and photosynthesis by considering the nutrient status, the recycling of nutrients in trees and litter, and the nutrient immobilization in relation to C stock. WP5 will aim at validating model’s predictions on experimental data, projecting forest responses under future climate and improving the modeling of nutrient limitation.
Project: TAW-TREE
Funding: ANR
Lead(s)/Coordinator(s): Nicolas Delpierre
Duration: 54 months
Web link : https://anr.fr/Projet-ANR-23-CE01-0008
Evaporating water is vital for trees. Since trees are sessile organisms, they rely on the soil stores to access the water resource. In the context of climate change, terrestrial ecosystems are facing an increase in atmospheric evaporative demand and projections of reduced summer precipitation point to a likely increase in edaphic water stress in Western Central Europe and the Mediterranean zone over the coming decades. How forests will react to increased water deficit depend, to a large extent, on their access to soil water. Besides, there is evidence that trees can take up water deeper than just in the pedological soil. Therefore, we need comprehensive approaches and multi-disciplinary research to consider hydrological and hydrogeological parameters of the critical zone in the response of forests to water deficit.
For a long time, the so-called “available water content” (AWC, in millimeter of water) has been identified as a key parameter in the modeling of forest water balance, carbon balance, tree growth6 and survival. Yet, the concept of AWC has two main drawbacks that question its relevance in a context of increasing water deficit, particularly in the case of forests.
Project organization and illustration of how work packages are nested. In WP1 box, VWC stands for Volumetric Water Content. In WP2 box, please note that the Cumulative Water Deficit (CWD) axis increases downwards. In WP3 box, the background map illustrates the current AWC estimates from the InfoSol database down to a 2-m depth, with higher values in dark blue amounting “>200 mm”.
First, the estimation of AWC is classically based on integrating so-called “pedotransfer functions” (yielding soil volumetric water content at field capacity and at the wilting point as a function of soil texture) over an estimated rooting depth. While some pedotransfer functions have been shown valid on a variety of soils9, estimating the tree rooting depth remains a major challenge. As a consequence, AWC is usually determined over a shallow depth, typically down to 2-m. This considerably underestimates the capacity of trees to grow roots much deeper (down to 5-10 m in the temperate and Mediterranean zones), for example through cracks in the rock. Challenging the AWC concept, recent works have defined the Total Available Water (TAW) concept, that adds “deep water” extraction by trees to AWC considered over 1-m. Deep water has a key role in buffering the tree water deficit during successive drought years. However, this deep water resource remains poorly quantified because the soil volume actually explored by roots is unknown.
First, the estimation of AWC is classically based on integrating so-called “pedotransfer functions” (yielding soil volumetric water content at field capacity and at the wilting point as a function of soil texture) over an estimated rooting depth. While some pedotransfer functions have been shown valid on a variety of soils9, estimating the tree rooting depth remains a major challenge. As a consequence, AWC is usually determined over a shallow depth, typically down to 2-m. This considerably underestimates the capacity of trees to grow roots much deeper (down to 5-10 m in the temperate and Mediterranean zones), for example through cracks in the rock. Challenging the AWC concept, recent works have defined the Total Available Water (TAW) concept, that adds “deep water” extraction by trees to AWC considered over 1-m. Deep water has a key role in buffering the tree water deficit during successive drought years. However, this deep water resource remains poorly quantified because the soil volume actually explored by roots is unknown.
Second, the application of pedotransfer functions to estimate AWC requires digging soil pits which, by definition, is destructive and cannot be deployed over large areas. If estimating AWC with a few soil pits may be acceptable for an agricultural plot with a low heterogeneity, it may however be highly incorrect for a forest plot, the soil of which is often very heterogeneous due to the absence of soil preparation and the soil rockiness. Consequently, the access to soil water can vary greatly among conspecific and interspecific tree individuals in a forest, mirroring the large inter-individual variability in terms of functioning and vulnerability to water stress that is often observed but more rarely explained, and contributes to the forest resilience to climate extremes. At a larger spatial scale, evidence is increasing that the access to deep water (hence the estimation of TAW) will be central in the forecast of the functioning and vulnerability of temperate and Mediterranean forests to ongoing climate change.
In this context, the TAW-tree project aims to:
- Quantify TAW reserves in forest plots by combining geophysical and ecophysiological approaches.
- Extend TAW estimation to a regional scale using remote sensing, in order to quantify the impact of spatial variations in TAW on the functioning, growth, and vulnerability of temperate and Mediterranean forests in the face of climate change.
The working hypotheses are as follows:
- AWC generally underestimates, and sometimes significantly, the TAW in forests.
- Variations in TAW within a forest largely explain differences between trees in their response to water stress.
- TAW, particularly its deep component, plays a critical role in the functioning and vulnerability of forests subjected to heat and drought episodes.