Conceptual frames and research strategies for integrated studies of adaptation to drought

International audience

English. This two-part session focused on: 1) ecophysiological studies aiming at understanding the adaptive value of xylem and leaf function, including approaches linking xylem traits such as hydraulic capacitance and parameters of hydraulic vulnerability across climatic gradients; 2) genetic studies aimed at estimating the potential for genetic adaptation to climate change in experimental trials as well as in natural populations; 3) investigating how specific functional traits may be fixed or adaptive within a species and related to climatic distribution more broadly; and 4) dendroecological studies aiming at retrospectively investigating tree responses to climate change. Fifteen oral presentations based on reviews and recent studies of novel and applied research were included in the session related to drought responses and adaptation. These were studied within a number of disciplines: ecophysiology, dendroecology, remote sensing and genetics. Some studies were integrated at the genus level (multi-species studies of eucalypts), genetic level (clone and provenance studies of eucalypts, lodgepole pine and radiata pine) or bioclimatic/environmental level (multisite studies of lodgepole pine and eucalypts). Studies employed drought treatments (piñon pine, juniper, radiata pine, and eucalypts) and inter-annual variation of response to drought by direct and retrospective approaches (oak, eucalypts, Siberian pine and fir). In addition there was one multi-generation study on radiata pine, and age effects were indirectly represented by height gradients of Douglas fir. Physiological studies of response to drought were presented at different levels, including: hydraulic vulnerability of leaf and branch xylem with reference also to carbon depletion and starvation. Several studies discussed interspecific variation in functional drought adaptation by comparing seasonal and controlled (water-exclusion or water-deficit) conditions. There was evidence of common plasticity in seasonal adaptation to declining water potential between eucalyptus species from different climatic zones. Other studies suggested within-species variation for drought-resistance strategies (lodgepole pine). In addition, fast natural selection (micro evolution) was apparent in drought resistance in radiata pine. In ecosystems, genus, and species, evidence of variation in drought resistance strategies were related to different levels and components of hydraulic vulnerability from leaves to xylem, with variable relative weights. These levels also varied with environment: between growing season, between sites, and between fertilization treatments. Reliable proxies or methodological improvement for the measurement of some physiological traits including resistance to cavitation (hydraulic vulnerability or hydraulic capacitance) and stomatal regulation of water potential should be investigated to develop large-scale studies involving many species and replicates and for better comparison of results from independent studies.