Influence of artificial alteration of groundwater level on vessel lumen area and tree-ring width of Quercus robur

The response of tree-ring widths to groundwater-level decline varies among individual trees in forest stands. By contrast, vessel lumen area series contain common climatic signal of previous summer temperature. Drought events and the overuse of groundwater for water supply can cause significant declines of groundwater levels in naturally poorly drained forest stands. However, the documented growth reactions of trees to declines in groundwater level vary a lot and range from positive because of increased soil aeration to strongly negative because of drought stress. We analysed changes in tree-ring width and earlywood average vessel lumen area of Quercus robur from sites above an artesian aquifer, whose groundwater level declined in the late 1980s by about 5 m due to water pumping. Because we expected different responses of individual trees due to local microtopography and fine root distribution, we performed both site-level and tree-level analyses. Our results show that the site-level response of tree growth to groundwater-level pumping was not uniform. Individual trees were clustered into groups of drought-limited and water abundance-limited individuals. The response of trees to groundwater pumping differed between clusters-drought-sensitive trees responded negatively, whereas the growth of trees limited by water abundance remained stable or slightly increased. Inter-series correlation of drought-limited trees significantly increased in the period with the lowest groundwater level. In contrast to tree-ring widths, earlywood vessel lumen area series contained common temperature signal, with no imprint of groundwater-level alteration. Our results indicate that poorly drained forest ecosystems are characterized by a significantly individualistic response of radial growth to groundwater-level alteration. These individualistic responses could be, however, overshadowed in stand-level average tree-ring width chronologies.