Assessing the impact of large-scale water table modifications on riparian trees : a case study from Australia

Mining below groundwater tables is increasing globally, yet little is known of how associated large-scale modification of water tables impact functioning of surrounding ecosystems. We used measurements of foliage density (ρF) and sapwood-related sap flow (QS) to assess effects of depth to groundwater on Eucalyptus victrix, a tree species that is common in riparian zones in central and northern parts of Australia. Foliage density (ρF) varied with season and among sites. Of itself, ρF provided a partial indicator of how trees responded to falling (more than 10 m) and rising (more than 9 m) water tables. Assessment of QS was highly informative. Across all sites, QS was least (90–130 l m−2 sapwood h−1) where groundwater was naturally deep (30 m) or had fallen substantially over the past 4 years (from 8 to 19 m). Fastest rates of QS (>245 l m−2 sapwood h−1) were recorded where groundwater had risen to a depth similar to a site where depth to groundwater remained stable at 6–7 m. Our analyses of daytime and night-time QS emphasize that water use by E. victrix is highly plastic and opportunistic. We discuss how empirical analysis of QS, coupled with a sound understanding of local hydrogeology, can help assess responses in ecosystem function to large-scale modification of groundwater levels – an important issue globally, as well as in Australia.