Here we present some preliminary results of water fluxes acquired with weighing lysimeters and sap flow sensors in a tree-grass ecosystem in Majadas (Spain).

Tree-grass” ecosystems. Mixed tree-grass systems are widely distributed (~16-35% of global land-surface) vegetation formations such as tropical and Mediterranean savannas, the “waldsteppe” in Eurasia and culturally influenced vegetation types such as agro-forestry systems or grazed open-forests in Europe (Hanan & Hill 2011). Semi-arid tree-grass systems are considered one of the major contributors to the interannual variability of the global carbon cycle (Poulter et al., 2014).Despite their wide distribution, Earth observation systems, and associated land-surface modeling development have been so far poorly adapted to the key structural and functional characteristics of tree-grass ecosystems. As consequence a significant uncertainty and bias in the assessments of energy, carbon, water and biogeochemical dynamics is often observed (Hanan & Hill 2011; Beringer et al. 2011). Nutrient (N, P) imbalance. Human induced CO2 and N fertilization leads to a stoichiometric imbalance, which confers an important role to P availability and leads to shifts in C-N-P ratios and balances (Peñuelas et al. 2012). N/P imbalances are particularly important in water-limited ecosystems (Sardans et al., 2012), where the synergistic effect of water and nutrient (N and P) availability/imbalance could impact ecosystem functioning, structure, allocation patterns and the nutrient and carbon cycling, and ultimately how the ecosystem will respond to extreme drought events. Hence it is important to study the effects of N and P imbalances under different water regimes, in particular in mixed tree-grass at ecosystem scale. MaNiP project offers an original experimental design integrating cutting-edge approaches (including eddy covariance, lysimiters and hyperspectral; remote sensing) to study the combined effect of nutrient and water limiting factors on fundamental ecosystem, plant and soil processes.
Soil variables (soil tension and volumetric soil water content; SWC) at different soil depths (line 10 cm and dashed line for 40 cm), and climatic variables ( daily Precipitation) are presented to describe the seasonal time course of daily evapotranspiration (ET) measured with the lysimeter. Data allow us to get more insight into non-well known processes that drive water flow upward and downward and determining water budgets in soils.

In the following plot, we show the agreement between the diurnal time course of ET aquired with two lisimeters and an eddy covariance tower. Sap velocity are also overlapped to investigate the contribution of the tree and grass transpiration on the ecosystem ET.

Thanks to the group of the Max Planck Institute for Biogeochemistry, Department Biogeochemical Integration for the first results in this study. The figures above show clearly that the methods match perfectly together!