Fertilization increases sensitivity of canopy stomatal conductance and transpiration to throughfall reduction in an 8-year-old loblolly pine plantation

Abstract

Loblolly pine (Pinus taeda L.) is the most important plantation species in the southeastern United States and therefore reductions in productivity associated with climate change may have significant economic impacts. To better understand the potential impact of drought on loblolly pine productivity, we studied the combined effects of reduced water availability and soil fertility on transpiration and canopy stomatal conductance (GS) over one year in an 8-year-old loblolly pine plantation by applying a throughfall treatment (ambient versus an approximate 30% reduction in throughfall) and a fertilization treatment (no fertilization versus one-time fertilization with N, P, K and micronutrients). We hypothesized that stomatal behavior would regulate water loss under a throughfall reduction treatment and the response would be greater under fertilization, due to an increase in leaf area index. Significant interactions between treatments indicated lower average monthly transpiration per unit ground area (EG) and per unit leaf area (EL), lower whole-plant hydraulic conductance, and lower average monthly midday GS in response to throughfall reduction only in the fertilized treatment, most likely due to increases in the leaf-to-sapwood area ratio in addition to isohydric regulation of water loss. In the throughfall reduction treatment but not the ambient throughfall treatment, reference GS (at vapor pressure deficit=1kPa, GSref) was significantly related to soil relative extractable water (REW) to a 0.6m soil depth, and sensitivity of GSref to REW was greater when combined with fertilization. These results indicate that the influence of drought on canopy-level processes of loblolly pine will be greater with fertilization.