Longleaf pine (Pinus palustris) forests are considered drought resilient forest ecosystems. Soil respiration (Rs) is often the largest component of ecosystem respiration, but the effects of drought on Rs and net ecosystem productivity of longleaf forests is not well understood. The objective of this study was to explore the influence of drought, imposed by a 40% reduction in throughfall (TR40), relative to ambient throughfall treatment (TR0) on Rs and heterotrophic respiration (Rh). The study was conducted in a longleaf pine plantation located in the Chattahoochee Fall Line Region in Georgia. Soil respiration, soil temperature to 15 cm, and soil moisture to a 20 cm depth were measured approximately every three weeks at randomly selected locations within each plot from July 2016 to November 2017. On average, 40% of the random sample locations in TR40 plots were located below exclusion troughs. Because the site experienced a severe drought in 2016, we explored treatment effects and relationships during the drought (2016) and post-drought (2017). Rh was estimated using the root exclusion method in 2017.
Results/Conclusions
During the severe drought, soil moisture and Rs declined to near zero in both treatments, and throughfall treatment had no significant effect on Rs or soil moisture. Following the drought in 2017, TR40 decreased Rs and soil moisture from June through November, with the exception of one measurement date when soil moisture was low in both treatments. In both treatments in 2016, soil moisture explained the majority (>50%) of the variability in Rs, and soil temperature combined with distance to nearest tree accounted for <10%. In 2017, soil temperature explained the majority (40-56%) of the variability in Rs followed by soil moisture, distance to nearest tree, and surrounding tree basal area. Rh was not affected by treatment, and the ratio Rh:Rs ranged from 69% to 92% depending on treatment and date. In August, Rh:Rs was increased from 69% in TR0 to 85% in TR40, because of lower root respiration (Ra) in response to TR40. Results suggest that sensitivity of Rs to throughfall reduction in a longleaf pine plantation on sandy, xeric soils was mainly due to reductions in Ra rather than Rh.