Does Existing System Complexity Convey Resistance?: Canopy Structural Change During the First Two Years of the Forest Resistance Threshold Experiment (FoRTE)

Forest disturbance regimes are changing, with lower severity disturbances supplanting stand-replacing disturbances as the most frequent and widespread disturbance events. Disentangling the influence of the level of disturbance severity on forest structure has proven difficult given that most forest disturbance studies necessarily rely on naturally occurring disturbances.

In 2018 we initiated the Forest Resistance Threshold Experiment (FoRTE), a modelling and field experiment that tests the effects of varying disturbance severity on carbon cycling dynamics in a temperate forest in northern, lower Michigan. Here, we address how forest structural response from baseline conditions (2018) through years one and two (2019-20) following disturbance affect canopy light interception. We hypothesized that canopy structural change would be most prominent at greater disturbance severities, becoming more pronounced as the experiment progresses, but the degree of structural change as assessed using the canopy structural traits framework, would be mediated by the existing structural and biological complexity of each stand—with more complex stands showing greater structural resistance. We also hypothesized that canopy light interception, measured as the fraction of absorbed photosynthetically available radiation (faPAR), would decline concomitantly with increasing disturbance.

We measured canopy structural complexity using a terrestrial based laser scanner and characterized canopy light interception using a handheld ceptometer that measures the above- and below-canopy light environment. We find that for 2018, pre-disturbance, there are no statistically significant differences in light interception or in canopy structural traits. In 2019, we observe a notable decrease in light interception with increasing disturbance severity. This decrease is non-linear, and most notable at the 85% disturbance severity level. There are no statistically significant differences in canopy structural traits in 2019 despite reductions in leaf area for 65 and 85% severity stands, Pre-disturbance structural and biological complexity appear to have little impact after one year and effects on structural resistance are mixed. Further data will be added for the 2020 field season.