Résumé
The interactions between hydrological and ecological processes are key
issues to improve our predictions of ecosystem responses to increasing
droughts. However, predicting the dynamics and the impacts of vegetation
water stress remains challenging because of complex ecohydrological
feedbacks. The ecohydrological optimality approach proposes that
functional adjustments within plant communities may buffer the increase in
vegetation water stress despite local water shortage. This study aimed to
test whether vegetation water stress may be invariant across contrasting
plant communities, reflecting possible optimality processes. We addressed
the following question: does a lower soil water storage capacity under the
same climate generate greater vegetation water stress over time? We
hypothesized that vegetation water stress would be buffered around a low
and constant level through the adjustment of vegetation biomass
productivity net primary productivity (NPP), evapotranspiration (ET)
and/or water-use efficiency (WUE) in relation with local soil water
storage capacity. We monitored 12 native plant communities distributed
along a gradient of soil water storage capacity (ranging from 20 mm to 120
mm) during five successive years. Net primary productivity, ET, WUE as
well as soil water dynamics were assessed and modelled for each plant
community throughout the 5 years of study. Vegetation water stress was
determined for each plant community as the deviation of between actual ET
and their maximum ETm rate achieved under non-limiting conditions. We
found that NPP and ET were together proportionally related to local soil
water storage capacity across the 5 years of study while WUE did not
differ between plant communities. Vegetation water stress was found quite
similar for all plant communities whatever the soil water storage
capacity. These results suggested that vegetation water stress was
strongly buffered by the community-level plant growth rates and total
water use along the soil gradient, but not by WUE. Our results suggest
that stressful environments rarely exist for plant communities. A dynamic
scaling relationship between NPP and ET may underpin the control of
vegetation water stress over seasonal and pluriannual time-scales. Such
results could contribute to better understanding processes associated with
ecohydrological optimality and improve the predictions of vegetation
dynamics under increasing droughts.