Impact of severe drought and 40% throughfall reduction on water relations and transpiration of longleaf pine

Background/Question/Methods

Longleaf pine (Pinus palustris) forests were once a dominant forest ecosystem in the southeastern United States and there is increased interest in the restoration of longleaf pine forests for not only traditional forest products but also to provide a variety of ecosystem services and, more recently, as a species resistant to disturbances associated with climate change. Longleaf pine is considered drought resistant based on hydraulic traits; however, hydraulic traits that confer drought avoidance or tolerance during moderate drought may increase drought susceptibility during extreme drought, thereby challenging our understanding of drought-induced tree mortality under future climate conditions. The objective of this research was to examine the impact of 40% throughfall reduction treatment relative to ambient throughfall treatment in the context of a severe drought on leaf water relations and tree and stand-level water use of 11-year-old longleaf pine in Marion County, Georgia. Mean average monthly temperature in the region during the 2016 drought relative to the previous 30 years was 1.1-2.4^oC warmer for the months of June through October 2016 and rainfall during the same period ranked as the second driest on record. From mid-September to late November, there were 77 consecutive days with little or no precipitation.

Results/Conclusions

Drought reduced mean predawn leaf water potential (Ψ_L) from -1.0 MPa in July 2016 to -2.9 MPa in November 2016, and mean midday Ψ_L ranged from -1.7 to -3.6 MPa over the same period. Throughfall reduction reduced predawn Ψ_L on average from -1.8 MPa to -1.9 MPa, but midday Ψ_L was similar between treatments. Midday sap flux density during Ψ_L measurements was on average 41% lower in response to throughfall reduction. Whole-tree hydraulic conductance (K) was also reduced by throughfall reduction, from 1.0 to 0.7 mol m^-2 s^-1 MPa^-1, and midday sap flux density and K were reduced to zero in both treatments during the drought. Monthly transpiration was 46 mm month^-1 at the beginning of the drought and declined to 1 mm month^-1 in October. These results indicate a significant impact of severe and prolonged drought on hydraulic characteristics of longleaf pine. This experimental platform will be used to empirically define drought adaptive capacity, drought resilience and recovery time to return to pre-stress condition in longleaf pine.