Does compensatory growth offset declines from dying trees?: An experimental approach to unraveling mechanisms underlying forest production resistance to disturbance
Abstract
As disturbances increase globally, the capacity of forests to sustain carbon (C) sequestration is in question. Disturbances, which vary in severity and originate from multiple sources, may erode the net C balance of forests by reducing net primary production (NPP), or rate of carbon accumulated in plant biomass. How much disturbance an ecosystem can withstand before NPP declines and the mechanisms that determine NPP resistance -- or decline -- following disturbance are unknown, but this knowledge is critical to models and managers that aim to forecast and sustain the C sink.
We implemented a large-scale disturbance manipulation to characterize the timing and magnitude of compensatory NPP across a gradient of disturbance severity and type. The Forest Resilience Threshold Experiment (FoRTE) was initiated in Spring 2019 at the University of Michigan Biological Station (UMBS). The gradient of disturbance severity spanned 0%, 45%, 65%, and 85% gross defoliation, and included the manipulation of a variety of deciduous forest types common to the region. Two disturbance types were implemented at each disturbance severity: a top down, canopy-focused disturbance and a bottom up, subcanopy-focused disturbance. To capture the fine-scale timing of immediate compensatory growth, we collected weekly stem diameter measurements and, from these data, derived NPP. Hemispherical NDVI images were used to track shifts in leaf area index (LAI) and infer changes in canopy nitrogen. Preliminary results suggest significant shifts in LAI are already underway, particularly in oak-dominated ecosystems. Differences in the relative growth rates of live and dying trees are emerging across the severity gradient, with a possible growth stimulation at 45% and 65% defoliation, but a growth suppression at 85% defoliation. In the first few months following disturbance, the compensatory growth of healthy trees and continued wood production by senescent individuals suggests NPP is overall sustained, but initial signs of decline are surfacing at the highest disturbance severity level.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13K2609G
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0793 Biogeochemistry;
- CRYOSPHERE;
- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1813 Eco-hydrology;
- HYDROLOGY