Whether annual evapotranspiration of native ecosystems is increasing or decreasing
with time as CO2 concentrations are rising, the climate is warming and rainfall experiences booms and busts, remains an unanswered question in the field of global change
biology. To answer this question, we measured evapotranspiration and carbon dioxide exchange over and under an oak savanna and over an annual grassland in the
Mediterranean climate of California, USA, from 2001 through 2019 with the eddy
covariance method; during this 19-year period, CO2 rose 40 ppm, air temperature increased by 1°C and annual rainfall ranged between 133 and 890 mm/year. No temporal
trend in evapotranspiration or water use efficiency was observed over this time duration. Many competing positive and negative feedbacks among stomatal sensitivity to
carbon dioxide concentrations, soil moisture, and vapor pressure deficit, the impact
of temperature on saturation vapor pressure and access to groundwater muted the
response of evapotranspiration to its changing world when integrated to the ecosystem
scale and annual time steps. At the intra-annual time scale, we found that plants transmit information on soil moisture status through their influence on the vapor pressure
deficit of the atmospheric boundary layer. The inter-annual variations in evaporative
water use by the savanna and annual grassland were relatively decoupled from the
booms and busts in rainfall. Instead, variations in length of growing season and access
to groundwater explained much of this year-to-year variation in annual evapotranspiration. The access of groundwater by the oak savanna may make these ecosystems more
robust in a warmer world, than was previously thought. This is a scale emergent property that needs better consideration in coupled climate-ecosystem models.
