Tree rings reveal drought avoidance through groundwater usage and historic groundwater variations in sandy temperate forests

Temperate forests are thought to infrequently experience water stress, but water stress may be more common than assumed where sandy soils exist and may increase under climate change with projected increases in the frequency and intensity of droughts. Thus, it is important to monitor temperate forests for water stress signals so that we may understand how these forests respond to water limitation currently and in the future. During drought, trees may avoid the negative consequences of water limitation by accessing shallow groundwater to maintain transpiration and functioning. We established a hydroecological observatory in sandy Wisconsin forests to investigate the interactions among groundwater, tree growth, and drought stress. Along a depth to groundwater gradient (1-9m), we observed higher tree growth and frequent groundwater use in trees in areas of shallow groundwater (1-3m) when compared to trees in areas of deeper groundwater (4m+). Further, we related depth to groundwater and tree growth to reconstruct groundwater depth histories in this environment since the 1930s. Climatic fluctuations are captured in both the tree growth and reconstructed groundwater depth times series. Our data suggest that even during historic droughts, trees in areas of shallow groundwater continued to use groundwater as tree growth remained consistently higher than other trees in the landscape which lacked access to groundwater. Our research is providing an understanding of groundwater-tree interactions in temperate forests which may help guide sustainable water and forest management decisions.