| Resum: |
Understanding how plants regulate transpiration is a central issue in ecophysiology since its origins. Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy and carbon budgets at the land-atmosphere interface. Although transpiration represents the main terrestrial water flux, its response to environmental drivers is currently poorly defined by observations globally. In this thesis, I aim to give a global perspective on the ecohydrological variables driving the regulation of transpiration using sap flow data at the whole-tree level. To achieve this goal I have contributed to the development of the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET). In order to provide a harmonized sap flow database, compatibility between the different sap flow methods has to be ensured. To this end, the second chapter of this thesis deals with the uncertainty of different sap flow techniques by carrying out a meta-analysis of 290 individual calibration experiments gathered from the literature. Results suggest that Dissipation methods may be more appropriate to assess relative sap flow and Pulse methods may be more suitable to quantify absolute flows. All sap flow methods showed high precision, allowing potential correction of the measurements when a study-specific calibration is performed. In the third chapter, I present the SAPFLUXNET database, which contains 202 globally distributed datasets with sap flow time series for 2714 trees of 174 species. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. In the fourth chapter, I carried out a quantification of the importance of hydroclimatic drivers controlling tree transpiration globally. I found that transpiration regulation dynamics are better explained by vapour pressure deficit (VPD) than by soil water content (SWC) or radiation in most areas. I also found that whole-tree canopy conductance (G) of trees in dryland biomes are less coupled to all three hydrometeorological drivers compared to those in other biomes. Climate, soil, and vegetation structure were common controls of all three hydrometereological couplings with G, with wetter climates, fine-textured soils, and tall vegetation being associated with tighter coupling. Finally, in the fifth chapter, I characterized tree water use strategies across species emerging from the covariation between water use regulation and hydraulic traits, controlling also for the climatic effects produced by differences in precipitation. I found that reference canopy conductance and its sensitivity to VPD is coordinated with hydraulic and allocation traits (i. e. ΨP50 , maximum sapwood hydraulic conductivity, Huber value, water potential at turgor loss point, root depth, leaf surface and tree height) rather than being directly controlled by mean annual precipitation. Species with efficient xylem transport (higher hydraulic conductivity) had higher canopy conductance but also higher sensitivity to VPD. Moreover, I found that angiosperms had higher reference canopy conductance and higher sensitivity to VPD than gymnosperms. In conclusion, this approach allowed for a simplified global mapping of hydrometeorological drivers importance and species trait-based water use strategies. In addition, the use of simple measurable traits altogether with functional grouping can lead to a better approximation of species reference whole-tree water conductance and its sensitivity to VPD. |
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