Abstract
Implications of chronic (�33 percent) and acute (-100 percent) precipitation change were evaluated for trees of upland-oak forests of the eastern United States. Chronic manipulations have been conducted since 1993, and acute manipulations of dominant canopy trees (Quercus prinus; Liriodendron tulipifera) were initiated in 2003. Through 12 years of chronic manipulations tree growth remained unaffected by natural or induced rainfall deficits even though severe drought conditions dramatically reduced canopy function in some years. The resilience of canopy trees to chronic-change was the result of a disconnect between tree growth phenology and late-season drought occurrence. Acute precipitation exclusion from the largest canopy trees also produced limited growth reductions from 2003 through 2005. Elimination of lateral root water sources for the acute treatment trees, via trenching midway through the 2004 growing-season, forced the conclusion that deep rooting was a key mechanism for large-tree resilience to severe drought.
