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Abstract

The effects of early growing season droughts on water and carbon balances in conifer forests are poorly understood. In this study, the response of canopy transpiration (Ec) and growth rates to reduced precipitation input during the early growing season was evaluated in a 70-year old temperate white pine (Pinus strobus L.) plantation forest, in Southern Ontario, Canada. In order to induce the drought, a 20 x 20?m throughfall exclusion setup was established. Throughfall was excluded from 1 April to 3 July 2009. During this period, 270?mm of rainfall occurred (27% of annual precipitation), of which more than 90% was excluded. Sapflow, stem growth, soil moisture and soil temperature were measured in both drought and reference plots. Prior to the induced drought, both plots showed similar soil water content, transpiration rates and tree diameters. The primary control on forest water loss was vapour pressure deficit, whereas soil moisture had an effect when it reached below 0.068?m3?m-3 during the growing season. The rainfall exclusion did not negatively affect Ec until early June, approximately 54?days after drought initiation. Ec was 27% less in the drought plot compared to the reference plot when evaluated at the end of the growing season in November. Tree growth estimates at the end of the growing season indicated a 17% decrease in growth in the drought plot as compared to the reference plot. Because climate predictions foresee changes in precipitation pattern, drought spells similar to this artificial short-term rainfall manipulation may be more frequent in the future. Hence, although overall precipitation may remain the same, the short-term deficit in water supply may have important implications for forest ecosystems. The findings of this rainfall manipulation will help quantify the impacts of spring and early summer water deficit on forest ecosystems and evaluate their potential responses to future climate regimes. Copyright (C) 2012 John Wiley & Sons, Ltd.