Data from: Groundwater salinisation intensifies drought impacts in forests and reduces refuge capacity

1. Shallow groundwater aquifers regularly support drought refuges for water-dependent ecosystems. However, many aquifers are impacted by over-extraction and pollution, potentially degrading their ability to support groundwater-fed drought refuges. 2. We investigated the response of groundwater-connected riverine forests to a drought considered equivalent in intensity to those predicted under severe climate change for 2030. The drought's impact was investigated in an area where shallow groundwater resources are heavily exploited and polluted by salinization. 3. We used remotely sensed vegetation productivity (enhanced vegetation index) data from a long-term data set (2000–2011) at 475 riverine forest sites in the Campaspe catchment, south-eastern Australia. Generalized additive mixed models and boosted regression trees were used to model the relationship between groundwater and other environmental covariates with forest change during drought. 4. Models explained up to 44% of the variation in forest change during drought. Forests underwent the greatest declines in areas of high salinity (>6000 μS cm−1) associated with shallow groundwater depths (0–5 m). Conversely, forests in areas of lowest salinity (<2000 μS cm−1) and groundwater depths of more than 7·5 m showed the least decline during drought. 5. In landscapes where groundwater quality is not compromised, previous studies have shown that shallow groundwater provides important drought refuges and refugia. Here, we show that when groundwater salinization has occurred, forests connected to shallow groundwater are more vulnerable to drought. In effect, salinization reduces the capacity of groundwater-connected habitats to function as drought refuges. 6. Synthesis and applications. Currently, there is an emphasis on managing environmental flows to support freshwater ecosystems and associated forests under water stress. However, delivery of environmental water is restricted to areas within a linear stream network and there is often limited capacity to deliver environmental flows during drought. Alternatively, a focus on drought refuges and refugia and processes important for maintaining groundwater quality (e.g. catchment revegetation to reduce shallow groundwater salinization) may better allow drought effects to be managed across a catchment, without directly focusing on highly contested surface water resources.

1. 浅层地下水含水层（shallow groundwater aquifers）可为依赖水源的生态系统持续提供干旱避难所。然而，多数含水层正遭受过度开采与污染，可能降低其支撑依赖地下水型干旱避难所的能力。
2. 本研究针对与地下水连通的河岸森林（groundwater-connected riverine forests），探究其对一场强度等同于2030年极端气候变化情景下预测的干旱事件的响应。研究区域内浅层地下水资源被过度开发，且遭受盐渍化污染。
3. 本研究使用澳大利亚东南部坎帕斯普流域（Campaspe catchment）内475个河岸森林样点的遥感植被生产力数据——增强型植被指数（enhanced vegetation index, EVI），数据源自2000–2011年的长期数据集。研究采用广义可加混合模型（generalized additive mixed models, GAMMs）与提升回归树（boosted regression trees, BRTs），构建干旱期间地下水与其他环境协变量和森林变化之间的关系模型。
4. 模型可解释干旱期间森林变化中高达44%的变异。在与0–5米浅层地下水埋深相关的高盐度（>6000 μS cm⁻¹）区域，森林出现最为显著的衰退；反之，在低盐度（<2000 μS cm⁻¹）且地下水埋深超过7.5米的区域，森林在干旱期间的衰退程度最轻。
5. 在地下水水质未受损害的景观中，已有研究证实浅层地下水可为生态系统提供关键的干旱避难所。本研究表明，当发生地下水盐渍化时，与浅层地下水连通的森林对干旱的脆弱性显著提升。简言之，盐渍化会降低与地下水连通的生境充当干旱避难所的能力。
6. 综合与应用。当前，学界普遍重视通过管理环境流量（environmental flows），以支撑水分胁迫下的淡水生态系统及相关森林。然而，环境水的输送仅局限于线性河流网络覆盖的区域，且干旱期间环境流量的输送能力往往有限。相较之下，聚焦干旱避难所与庇护生境，以及维持地下水水质的关键过程（例如通过流域植被恢复降低浅层地下水盐渍化程度），可在无需直接关注争议频发的地表水的前提下，更好地实现全流域尺度的干旱影响管控。