Abstract
Semi-arid ecosystems contribute about 40% to global net primary production (GPP) even though water is a major factor limiting carbon uptake. Evapotranspiration (ET) accounts for up to 95% of the water loss and in addition, vegetation can also mitigate drought effects by altering soil water distribution. Hence, partitioning of carbon and water fluxes between the soil and vegetation components is crucial to gain mechanistic understanding of vegetation effects on carbon and water cycling. However, the possible impact of herbaceous vegetation in savanna type ecosystems is often overlooked. Therefore, we aimed at quantifying understory vegetation effects on the water balance and productivity of a Mediterranean oak savanna. ET and net ecosystem CO2 exchange (NEE) were partitioned based on flux and stable oxygen isotope measurements and also rain infiltration was estimated. The understory vegetation contributed importantly to total ecosystem ET and GPP with a maximum of 43 and 51%, respectively. It reached water-use efficiencies (WUE; ratio of carbon gain by water loss) similar to cork-oak trees. The understory vegetation inhibited soil evaporation (E) and, although E was large during wet periods, it did not diminish WUE during water-limited times. The understory strongly increased soil water infiltration, specifically following major rain events. At the same time, the understory itself was vulnerable to drought, which led to an earlier senescence of the understory growing under trees as compared to open areas, due to competition for water. Thus, beneficial understory effects are dominant and contribute to the resilience of this ecosystem. At the same time the vulnerability of the understory to drought suggests that future climate change scenarios for the Mediterranean basin threaten understory development. This in turn will very likely diminish beneficial understory effects like infiltration and ground water recharge and therefore ecosystem resilience to drought.
Highlights
Semi-arid ecosystems contribute about 40% to global net primary productivity (Wang et al, 2012) and in these ecosystems water and carbon dioxide cycles are tightly coupled via ecosystem water use efficiency (David et al, 2004; Pereira et al, 2006)
We focused on disentangling the inter-seasonal impact of understory vegetation effects on: (i) the ecosystem water and carbon fluxes, (ii) soil evaporation and (iii) the influence of vegetation on rain infiltration
We hypothesized that in savanna type ecosystems the herbaceous understory layer, despite its ephemeral life form, plays an important role in the water and carbon balances and for ecosystem resilience toward drought. This shall be discussed with respect to the contribution of the understory to total ecosystem ET and productivity, as well as influence on unproductive soil water loss, i.e., evaporation (E) and soil water distribution
Summary
Semi-arid ecosystems contribute about 40% to global net primary productivity (Wang et al, 2012) and in these ecosystems water and carbon dioxide cycles are tightly coupled via ecosystem water use efficiency (David et al, 2004; Pereira et al, 2006). ET has two distinct components: plant transpiration (T) and unproductive loss of water during soil evaporation (E) Due to their open bi-layered structure, savanna-type ecosystems are suitable to study the effect of water scarcity and the coupling between hydrological and biogeochemical processes of different plant layers (woody vs herbaceous species) and the soil. They cover large areas world-wide and in Europe they are the predominant land cover type on the southern Iberian Peninsula, covering about 1.5 Mio ha (Bugalho et al, 2011). The herbaceous understory vegetation has a strong www.frontiersin.org
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