Chronic and acute precipitation manipulations in eastern deciduous forests: Build wide, measure deep and be patient
Because long-lived forest trees that exist today may need to cope with new hydrologic regimes associated with climate warming and atmospheric changes, there is interest in gathering response data from manipulations to inform processes-based predictive models. Successful forest drought manipulations have been accomplished in eastern US forests using understory throughfall interception canopies. Variation in the level of canopy interception (% ground cover) and the duration of a manipulation are effective means of introducing soil water deficits. We have conducted both chronic (±33 percent) and acute (-100 percent) precipitation change manipulations to evaluate the response of upland-oak forests trees to drought across a broad range of drought conditions. These manipulations were supported by investments in vegetation response variables, soil water content and potential measurements with depth, and rooting distributions.
Chronic manipulations were conducted through the manipulation of intact 80 x 80 m eastern deciduous forest plots dominated by Quercus sp. and Acer rubrum trees over a 13-year period. Although transient physiological drought effects were evident in the large trees (especially when overlapping a historically dry year), single-year reductions in growth and subsequent canopy production were not observed. Such insensitivity was attributed to a disconnect between early season growth activities and late-season development of drought. Subsequent multi-year acute manipulations were conducted on dominant canopy trees (Quercus prinus; Liriodendron tulipifera) in attempt to find a threshold for catastrophic drought response. From 2003 to 2005, 100% rainfall reductions were conducted on replicate large canopy trees, and they still showed limited growth reductions. Elimination of lateral root water sources for the acute treatment trees, via trenching, during the 2004 growing-season, enabled the conclusion that deep rooting was a key mechanism for large-tree resilience to severe drought.
A few lessons-learned conclusions for new manipulations include: 1) build large plots to accommodate the “space” occupied by target trees, 2) quantify available soil water and tree water status throughout the study, 3) operate these studies through multiple annual cycles to overlap manipulations with uncharacteristically wet and dry years. Projected levels of warming and elevated atmospheric CO2 will take forest trees into climate conditions that they have never experienced. Conversely water stress is bounded nicely in the historical record by years of abundant versus minimal precipitation. If long-term monitoring (e.g., AmeriFlux sites) is executed with appropriate measurement investments and diligence, a patient science community may capture data on forest response. Manipulations simply speed that process.