Data from: Importance of deep water uptake in tropical eucalypt forest

Climate models predict that the frequency, intensity and duration of drought events will increase in tropical regions. Although water withdrawal from deep soil layers is generally considered to be an efficient adaptation to drought, there is little information on the role played by deep roots in tropical forests. Tropical Eucalyptus plantations managed in short rotation cycles are simple forest ecosystems that may provide an insight into the water use by trees in tropical forests.
The contribution made by water withdrawn from deep soil layers to the water required for evapotranspiration was quantified daily from planting to harvesting age for a Eucalyptus grandis stand using a soil water transfer model coupled with an ecophysiological forest model (MAESPA). The model was parameterized using an extensive data set and validated using time series of the soil water content down to a depth of 10 m and water-table level, as well as evapotranspiration measured using eddy covariance.
Fast root growth after planting provided access to large quantities of water stored in deep soil layers over the first 2 years. Eucalyptus roots reached the water-table at a depth of 12 m after 2 years. Although the mean water withdrawal from depths of over 10 m amounted to only 5% of canopy transpiration from planting to a harvesting age of 5 years, the proportion of water taken up near the water-table was much higher during dry periods. The water-table rose from 18 to 12 m below-ground over 2 years after the harvest of the previous stand and then fell until harvesting age as evapotranspiration rates exceeded the annual rainfall.
Deep rooting is an efficient strategy to increase the amount of water available for the trees, allowing the uptake of transient gravitational water and possibly giving access to a deep water-table. Deep soil layers have an important buffer role for large amounts of water stored during the wet season that is taken up by trees during dry periods. Our study confirms that deep rooting could be a major mechanism explaining high transpiration rates throughout the year in many tropical forests.

气候模型预测，热带地区干旱事件的发生频率、强度与持续时长均将呈上升趋势。尽管学界普遍认为从深层土壤取水是植物应对干旱的有效适应策略，但目前针对热带森林深层根系作用的相关研究仍较为匮乏。采用短轮伐期经营的热带桉树林是结构相对简单的森林生态系统，可为探究热带森林树木的水分利用策略提供理想研究载体。本研究依托耦合了森林生理生态模型（MAESPA）的土壤水迁移模型（soil water transfer model），对巨桉（Eucalyptus grandis）人工林从定植到采伐期内，深层土壤取水对蒸散（evapotranspiration）需水量的贡献量进行了逐日量化分析。该模型通过大规模数据集完成参数化，并基于0至10米深度的土壤含水量、地下水位时间序列数据，以及通过涡度协方差法（eddy covariance）实测的蒸散量完成了模型验证。定植后快速生长的根系在造林前两年即可获取深层土壤中储存的大量水分。造林两年后，巨桉根系即可抵达12米深度的地下水位。尽管从定植到5年采伐期内，从10米以下土层取水量仅占冠层蒸腾量的5%，但在干旱时段，根系从地下水位附近取水的占比会显著升高。前一代林分采伐后的两年内，地下水位从地下18米回升至12米；此后由于蒸散速率超过年降雨量，地下水位持续下降直至采伐期。深层根系是提升树木可利用水分总量的高效策略，既能够获取暂时性的重力水，也可利用深层地下水资源。深层土壤对湿季储存的大量水分具备重要的缓冲调节作用，可在旱季为树木提供水分来源。本研究证实，深层根系是解释诸多热带森林全年维持高蒸腾速率的核心机制之一。