Examining Patterns of Carbon Assimilation and Allocation to Defense Processes in a Restored Southern Pine Forest

Abstract:

Southern pine forests provide many important ecosystem services, including biodiversity, carbon sequestration, and softwood timber production, which is a vital component of local economies in the American South. However, all southern pine forests are sensitive to damage from infestations of bark beetles and drought events, which can lead to declines in productivity that may cause mortality in extreme cases, and which may increase in frequency in the future due to ongoing climate change. This study explores how southern pine management for restored, old-growth like conditions, in contrast with management for timber production, affects stand scale drought response and tree resistance to bark beetle herbivory by leveraging a suite of data from a new eddy covariance flux monitoring site in a 65-year-old restored loblolly (Pinus taeda) and shortleaf (Pinus echinata) pine forest situated in the Crossett Experimental Forest (Arkansas, USA). The sensitivity of ecosystem scale fluxes of CO₂ and H₂O to drought is interpreted through a synthesis with other long-running Ameriflux sites located in southern pine forests. The effects of the management regime on resin production, which is the pine trees’ main defense against beetle attacks, are assessed by comparing monthly resin flow observations collected over the course of the 2013 growing season in the restored stand and in a co-located stand of even-age planted loblolly pines managed for timber production. Results show that loblolly in the uneven-aged stand consistently produced much larger amounts of resin than loblolly in the even-aged stand, and shortleaf pines were the lowest producers throughout the growing season. No significant relationship between resin flow and diameter at breast height was observed within or across species and sites; thus, species and management effects are independent of their effect on tree size.