Structural complexity and primary production resistance are coupled in a temperate forest

Abstract

The capacity of forests to resist structural change and retain material legacies–the biotic and abiotic resources that persist through disturbance–is crucial to sustaining ecosystem function after disturbance. However, the role of forest structure as both a material legacy and feature supporting carbon (C) cycling stability following disturbance has not been widely investigated. We used a large-scale disturbance manipulation to ask whether legacies of lidar-derived canopy structures drive 3-year primary production responses to disturbance. As part of the Forest Resilience Threshold Experiment (FoRTE) in northern Michigan, USA we simulated phloem-disrupting disturbances producing a range of severities and affecting canopy trees of different sizes. We quantified the legacies of forest structure using two approaches: one measuring the change in structure and primary production from pre-to post-disturbance and the second estimating resistance as log transformed ratios of control and treatment values. We found that total aboveground wood net primary production (ANPPw) was similar across disturbance severities as legacy trees rapidly increased rates of primary production. Experiment-wide, the disturbance had limited effects on change in mean structural complexity values; however, high variance underscored large differences in the magnitude and direction of complexity's response at the plot-scale. Plot-scale structural complexity, but not vegetation area index (VAI), resistance strongly predicted ANPPw resistance while temporal VAI and structural complexity changes did not. We conclude that the presence of material legacies in the form of forest structure may affect primary production stability following disturbance and that how legacies are quantified may affect the interpretation of disturbance response.

Dynamic subcanopy leaf traits drive resistance of net primary production across a disturbance severity gradient

“…This traits-production relationship held across a wide range of disturbance severity levels from 0 to 85% gross defoliation, suggesting a conserved ecological mechanism in forests experiencing partial to near total stem mortality. Our findings reinforce the critical role of material legacies, or the biotic and abiotic resources remaining after disturbance (Niedermaier et al, 2022). Although prior work attributed the stimulation of subcanopy production following disturbance to greater light availability (Muscolo et al, 2014;Stuart-Haëntjens et al, 2015;Kennard et al, 2020), our 4-year analysis demonstrates that multiple leaf functional traits responded to disturbance before deterioration of the upper canopy.…”

“…Although a number of studies have examined community trait responses to severe disturbances such as fire (Pausas et al, 2004;Huerta et al, 2021;Pellegrini et al, 2021), the timing and magnitude of plant functional trait change across more extensive gradients of severity, as well as how these shifts may relate to production, remains unclear (Diaz et al, 2007;Biswas and Mallik, 2010;Herben et al, 2018). Recent work from FoRTE has shown that growth of surviving subcanopy and canopy trees was able to fully compensate for reductions in canopy area up to 3 years following disturbance initiation (Grigri et al, 2020;Niedermaier et al, 2022). This stimulated subcanopy growth (i.e., increased carbon uptake of this stratum) coupled with declines in soil respiration (i.e., carbon loss from the system) that were proportional to disturbance severity and sustained through time (Mathes et al, 2023) enabled the forest to maintain remarkable NPP stability at even the highest levels of disturbance.…”

“…No significant differences from control were found for total NPP in each year. Total NPP analysis and statistical comparisons are detailed in Niedermaier et al (2022). Subcanopy community weighted mean leaf-level red edge normalized difference vegetation index (reNDVI) and aboveground wood net primary production (ANPP w ) in FoRTE, 2019-2021.…”

“…Aboveground wood net primary production (ANPP w ) was computed from repeated measurements of stem diameter for a subset of ungirdled subcanopy trees (1-8 cm DBH), detailed by Grigri et al (2020) and Niedermaier et al (2022). Stems of this size class were censused in one quarter of each subplot (0.025 ha) in 2019 to obtain species abundances and diameter distributions.…”

Revealing Three-Dimensional Variations in Fuel Structures in Subtropical Forests through Backpack Laser Scanning

“…To describe the vertical change in forest fuel density, we calculated the VAI from the ground surface to the top of the canopy of each plot. The VAI describes the amount of vegetation area per unit area in the vertical profiles of a stand [29,50], which allows for the quantification of the density distribution and relative position of fuel within the stand using point cloud data [29]. We took 1 cm as the basic unit of stratification in the vertical direction and then counted the vegetation area of each layer.…”