PNNL

Abstract

Carbon (C) cycling processes are particularly dynamic following disturbance, with initial responses often indicative of longer-term change. In northern Michigan, USA, we initiated the Forest Resilience Threshold Experiment (FoRTE) to identify the processes supporting C cycling stability across multiple levels (0%, 45%, 65%, and 85% gross leaf area index loss) of disturbance severity and in response to separate disturbance types preferentially targeting large or small diameter trees. Disturbance was implemented by stem girdling >3,600 trees below diameter at breast height (DBH), immediately and permanently disrupting the phloem. Weekly DBH measurements of >700 trees revealed small but significant increases in daily aboveground wood net primary production (ANPPw) in the 65% and 85% disturbance severity treatments that emerged six weeks after girdling. However, we observed minimal change in peak leaf area index and no significant differences in annual ANPPw among disturbance severities or between disturbance types, suggesting continued C fixation by phloem-disrupted trees sustained stand-scale wood production in the first growing season after disturbance. The cohort of early successional tree species contributed more to annual ANPPw than middle and late successional species in the 65% disturbance severity treatment, suggesting higher disturbance severities may favor early successional species. We conclude that ANPPw stability immediately following phloem disruption is dependent on the continued – but inevitably temporary – growth of girdled trees, emphasizing that mechanisms supporting stand-scale production are dynamic.