Mediterranean mountainous areas of shallow soil often display a mosaic of tree clumps surrounded by grass. The combined role and dynamics of water extracted from the underlying rock, and the competition between adjacent patches of trees and grass, has not been investigated. We quantified the role rock water plays in the seasonal dynamics of evapotranspiration (ET), over a patchy landscape in the context of current and past seasonal climate changes, and land‐cover change strategies. Soil water budget suggests deep water uptake by roots of trees (0.8–0.9 mm/d), penetrating into the fractured basalt, subsidized grass transpiration in spring through hydraulic redistribution. However, in summer trees used all the rock water absorbed (0.79 mm/d). A 15‐year data set shows that, with increasing seasonal drought‐severity (potential ET/precipitation) to >1.04, the vertical water flux through the bottom of the thin soil layer transitions from drainage to uptake in support of ET. A hypothetical grass‐covered landscape, with no access to deep water, would require 0.68–0.85 mm/d more than is available, forcing shortened growing season and/or reduced leaf area. Long‐term decreasing winter precipitation and increasing spring potential ET suggest drying climate, so far with stable vegetation mosaic but progressively earlier peak of grass leaf area. Intervention policies to increase water yield by reducing tree cover will curtail grass access to rock moisture, while attempting to increase tree‐related products (including carbon sequestration) by increasing forest cover will limit water availability per tree leaf area. Both changes may further reduce ecosystem stability.
Seasonal changes in grass cover impact the generation of surface runoff due to the effects of grass roots on soil hydrologic properties and processes (i.e., infiltration). Using a rainfall simulator in a grass field site, we broadly investigated the influence of different initial conditions of soil moisture and grass growth stages on rainfall–runoff transformations. To parameterize the stages of grass growth, we used the height of the vegetation hveg, which is related to the leaf area index. Surprisingly, typical characteristics of runoff formation (peak flow and time to peak flow) were conditioned mainly by hveg. The runoff coefficient decreased about 40% when grass reached its maximum growth and was inversely and significantly related to the height of grass in general. Using the rainfall simulator experiments, we estimated the saturated soil hydraulic conductivity ks, a key parameter of infiltration models. We found strong relationships between ks and hveg when the Philip infiltration model was used, and we proposed a linear relationship between ks and hveg, making ks vary in time with grass growth (i.e., hveg). We compared predictions of hydrologic models at plot scale using ks varying with grass growth with predictions using a constant ks, as hydrological models commonly assume. Neglecting ks variability with grass growth can lead to errors up to 100% in surface runoff predictions at an event time scale and up to 87% at a monthly time scale. Ecohydrological models for runoff predictions should take into account the influence of grass growth dynamics on soil infiltration parameters.
<p>Mediterranean mountainous areas of shallow soil often display a mosaic of tree clumps surrounded by grass. During dry seasons, evapotranspiration (<em>ET</em>) cannot be met by soil moisture. However, the combined role and dynamics of water extracted from the underlying rock, and the competition between adjacent patches of trees and grass, has not been investigated. We quantified the role rock water plays in the seasonal dynamics of evapotranspiration, and its components, over a patchy landscape in the context of current and past seasonal climate changes, and land-cover change strategies. Soil water budget, using precipitation (<em>P</em>), <em>ET</em>, and soil moisture changes (&#916;<em>S</em>; ~17 cm soil layer), suggests deep water uptake by roots of trees (<em>f<sub>d</sub></em>; 0.8 &#8211; 0.9 mm/d), penetrating into the fractured basalt below clumps and the surrounding pasture, subsidized grass transpiration in spring through hydraulic redistribution. However, in summer trees used all the deep water absorbed (0.79 mm/d; <em>f<sub>d</sub></em> > tree transpiration). A 15-year dataset shows that, with increasing seasonal drought-severity (potential <em>ET</em>/<em>P</em>) to >1.34, the vertical water flux through the bottom of the thin soil layer transitions from drainage to uptake in support of <em>ET</em>. A hypothetical grass-covered landscape, with no access to deep water, would require 0.68 &#8211; 0.85 mm/d more than is available from <em>P</em> and &#916;<em>S</em>, forcing shortened growing season and/or lower leaf area. In summer, <em>ET</em> in such a landscape would be half that of the existing mosaic, with consequences to energy balance. The vegetation mosaic may represent trending equilibrium, as long-term decreasing winter precipitation and increasing spring potential evaporation suggest drying climate. Intervention policies to increase water yield by reducing tree cover will curtail grass access to rock moisture, while attempting to increase tree-related products by increasing forest cover will limit water availability per tree leaf area. Both changes may further reduce ecosystem stability.</p><p>&#160;</p>
<p>Fire, harvesting and beetles attacks are important disturbances for the forested ecosystems. The aim of this study is to examine the impact of the disturbances on water and carbon fluxes using a eddy&#8208;covariance (EC) &#8211; based tower in a wild-olive forest.</p><p>The study has been performed at the Orroli site, Sardinia (Italy), which is an experimental site for the FLUXMED project of the Water Joint Programming Initiative. From 2003, a 10 m micrometeorological tower equipped with eddy-covariance system has been used to measuring water, carbon and energy surface fluxes, as well as key state variables (e.g. leaf and soil skin temperature, radiations, air humidity and wind velocity).</p><p>The landscape is covered by patchy vegetation: wild olives trees in clumps and herbaceous species, drying to bare soil in late spring. The climate is Mediterranean maritime with long droughts from May to October, and rainy period is concentrated in the autumn and winter months. In this ecosystem water uptake by olive&#8217;s roots, from underlying substrate to the shallow soil layer, allow woody vegetation and grass to remain physiologically active during dry conditions.</p><p>In summer 2017, which was a very dry season, an extended fire affected the forested area, impacting the north &#8211; west footprint of the tower, with consequences also to the close trees due to beetle attack, probably related to the sensitive conditions of the trees after the drought.</p><p>We compared pre-disturbance with post-disturbance land surface fluxes. Both fire and beetle attack, altered the partitioning of available energy to lE and H, evapotranspiration (ET) and carbon assimilation. Results show a reduction of evapotranspiration and carbon assimilation during the growing season. Differently, in autumn and winter the difference between pre-disturbance and post-disturbance was negligible due to low physiological activities of vegetation.</p>
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