Agroecosystem Research, University of Bayreuth, BayCEER Bayreuth, Germany.
Agroecosystem Research, University of Bayreuth, BayCEER Bayreuth, Germany ; Computational Hydrosystems, Helmholtz Center for Environmental Research (UFZ) Leipzig, Germany.
Front Plant Sci. 2014 Oct 7;5:530. doi: 10.3389/fpls.2014.00530. eCollection 2014.
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.
半干旱生态系统对全球净初级生产力 (GPP) 的贡献约为 40%，尽管水是限制碳吸收的主要因素。蒸散 (ET) 占水分损失的 95%，此外，植被还可以通过改变土壤水分分布来减轻干旱的影响。因此，土壤和植被之间碳和水通量的分配对于深入了解植被对碳和水循环的影响至关重要。然而，草原类型生态系统中草本植被的可能影响经常被忽视。因此，我们旨在量化林下植被对地中海栎草原水分平衡和生产力的影响。根据通量和稳定氧同位素测量结果对蒸散和净生态系统 CO2 交换 (NEE) 进行了分配，同时还估计了降雨入渗。林下植被对总生态系统蒸散和总初级生产力的贡献很大，分别达到 43%和 51%。它达到了与栓皮栎树相似的水分利用效率 (WUE；水分损失与碳收益的比值)。林下植被抑制了土壤蒸发 (E)，尽管在湿润期 E 较大，但在水分受限时期并未降低 WUE。林下植被强烈增加了土壤水分入渗，特别是在大雨之后。与此同时，林下植被本身容易受到干旱的影响，这导致树木下的林下植被比开阔地区更早衰老，因为它们竞争水分。因此，有益的林下植被效应占主导地位，有助于提高生态系统的恢复力。与此同时，林下植被对干旱的脆弱性表明，未来地中海盆地的气候变化情景威胁到林下植被的发展。这反过来又很可能会减少林下植被的有益效应，如渗透和地下水补给，从而降低生态系统对干旱的恢复力。
