A study evaluating the response of canopy transpiration (Ec) and growth rates to reduced water input, was conducted in a managed 70-year old planted temperate white pine (Pinus strobus L.) forest, in Southern Ontario, Canada from January to December 2009. In order to induce the drought, a 20m x 20m throughfall exclusion setup was established using interlocking aluminum troughs at a 3-inch slope. Throughfall was excluded from April 1st until July 3rd. During this period, 270mm of rainfall occurred (27% of annual precipitation) of which 90% was excluded. Sap flow velocity, soil moisture and soil temperature (at multiple depths) were measured continuously in both reference and drought plots. Dendrometer bands were also installed on all instrumented trees. Prior to enforced drought, adjacent plots showed slight variability in soil moisture while tree diameter and soil temperature did not show significant variability. Daily values of Ec from each plot ranged from 0 to 1.6 mm d-1 over the growing season (March-November) for the drought and reference plot respectively. The impact of the rainfall exclusion did not affect Ec until early June, 60 days after the drought was in place. Normalized values of Ec showed a 20% decrease from the drought trees compared to the reference. Cumulative growth rates between the two plots showed a net decrease in the drought trees of 42% from the reference and earlier termination of growth. However, the growing season Ec values were 174 mm y-l and 171 mm y-l for the drought and reference plot respectively. Currently, the effects of extreme drought
events on carbon and water balances in conifer forests are poorly understood, due
to their sporadic occurrence in natural ecosystems. The findings of this study help
to establish the impacts drought may have on these ecosystems and evaluate their
potential responses under predicted future climate regimes.