Stand-replacing disturbance drastically alters forest structure and function. However, forest disturbances have largely transitioned away from severe, stand-replacing events, to more moderate disturbances such as partial harvests, wind, insects, pathogens, ice storms, and age-related senescence that result in only partial canopy defoliation, species loss, and limited mortality. Unlike stand-replacing disturbances, less studied moderate disturbances result in variable structural outcomes rather than completing resetting forest succession to bare ground, thus contributing to a gradient of disturbance intensities across the landscape. These variable structural outcomes, however, are not well understood, though likely have significant effects on ecosystem function.
The Forest Resilience and Threshold Experiment (FoRTE) is a replicated, manipulative experiment at the University of Michigan Biological Station (UMBS) in northern, lower Michigan that explicitly controls for disturbance severity. In spring of 2019, >6,700 trees were stem-girdled across 8 ha, at 45, 65, and 85% severity levels—based on estimated loss of leaf area. Using terrestrial lidar (2018-2020), hemispherical imaging (2018-2020), and aerial observations from the National Ecological Observatory Network (NEON) Airborne Observation Platform (2019), we assess structural change among control and treatment plots across the FoRTE project.
Results/Conclusions
Analysis of canopy structural traits derived from terrestrial lidar shows minimal changes in canopy structure from pre- to post-disturbance—2018 to 2019. However, previous work indicates that significant structural change does not occur till 2-3 years post stem-girdling. Both 2020 terrestrial lidar data as well as 2019 NEON AOP data, have yet to be collected and/or analyzed, but may yield more informative assessment of structural change based on current understanding of the progression of disturbance and structural change following stem-girdling. Hemispherical imaging data does show changes at the highest severity disturbance (i.e. 85%) with significant divergence in greenness, as indicated by the normalized difference vegetation index (NDVI), from control plots in 2019. However, significant changes were not evident until the end of the growing season (early August) and were not evident between control and 45% disturbance or control and 65% disturbance plots.