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

1. 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. 2. The contribution made by water withdrawn from deep soil layers to the water required for evapotranspiration was quantified daily from planting to harvesting age for a E. grandis stand using a soil water transfer model coupled with an ecophysiological forest model (MAESPA). The model was parameterized using an extensive dataset 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. 3. Fast root growth after planting provided access to large quantities of water stored in deep soil layers over the first two 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 m 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. 4. 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.

1. 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，从深层土层汲取水分是植物应对干旱的高效适应策略，但目前关于热带森林中深根所发挥的作用的相关研究仍较为匮乏。采用短轮伐期经营模式的热带桉树人工林属于结构相对简单的森林生态系统，可为阐明热带森林树木的水分利用策略提供研究载体。2. 本研究以一片巨桉（E. grandis）人工林为研究对象，通过耦合土壤水分运移模型与森林生态生理模型（MAESPA），逐日量化了从种植到采伐龄期内，深层土壤汲取的水分对树木蒸散发所需水分的贡献占比。研究团队利用大规模实测数据集对模型进行参数化，并基于0至10米深度的土壤含水量时间序列、地下水位数据，以及通过涡度相关法测得的蒸散发数据对模型进行了验证。3. 造林后快速生长的根系在种植前两年即可获取深层土层中储存的大量水分。巨桉根系在种植两年后即可抵达12米深度的地下水位。尽管从种植到5年采伐龄期内，从10米以下土层汲取的水分平均仅占冠层蒸腾需水量的5%，但在干旱时段，靠近地下水位处汲取的水分占比会显著升高。在前一茬林分采伐后的两年内，地下水位从地下18米回升至12米，随后由于蒸散发速率超过年降雨量，地下水位持续下降直至采伐期。4. 深根是提升树木可利用水分总量的高效策略，不仅能够获取暂时性的重力水，还可能获取深层地下水源。深层土层可作为重要的水分缓冲库，储存湿季的大量水分，供树木在旱季汲取利用。本研究证实，深根或许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。