Defying the decline: Carbon storage resistance to moderate disturbance in a temperate forest

Carbon (C) storage rates in many forests are sustained, or decline only briefly, following disturbances that cause partial canopy defoliation. The mechanisms supporting such functional resistance to moderate forest disturbance are largely unknown. We used a large-scale experiment, in which >6,700 canopy dominant Populus (aspen) and Betula (birch) trees were stem girdled within a 39 ha area, to identify mechanisms sustaining C storage through partial canopy defoliation. The Forest Accelerated Succession ExperimenT (FASET) in northern Michigan, USA employs a suite of C cycling measurements within paired treatment and control meteorological flux tower footprints. We observed peak declines in leaf area index (LAI) of 44 % three years following disturbance without an associated reduction in C storage, quantified as net ecosystem production and (NEP) net primary production (NPP). The treatment forest resisted C storage declines by sustaining canopy light-use efficiency and light absorption during this period of low LAI. In the year following peak defoliation, complete reallocation of nitrogen (N) in the treatment forest from the foliage of senescent early successional aspen and birch to ungirdled later successional species facilitated the full recovery of total LAI to pre-disturbance levels. Sustained canopy physiological competency and light absorption following moderate disturbance coincided with upper canopy gap formation and a rise in structural complexity as the canopy became more multi-layered. Stability of canopy processes central to sustaining C storage during defoliation suggests a rapid shift in structure-function occurred in which the photosynthetic contribution of subcanopy vegetation increased to compensate for defoliation of canopy dominant trees. We conclude that C storage resistance to moderate disturbance depends not only on replacement of lost leaf area, but also on shifts in forest structure that sustain physiological functioning of the canopy during defoliation. Identification of disturbance thresholds among forested ecosystems and the underlying mechanisms supporting functional resistance will improve models simulating C cycling responses to moderate disturbance.