Long-term effects of herbivory on tree growth are not consistent with browsing preferences

Abstract

Herbivores affect plants via direct consumption, but also indirectly via changes in the vegetation. These indirect effects may only manifest after decades of exposure and as a result, their impact is rarely accounted for. To better understand the direct and indirect impacts of a large herbivore, moose (Alces alces), on forests, we measured tree growth in areas that were both subjected to and excluded from herbivory for over 80 years. Growth data were gathered from five tree species, ranging from low to high palatability. We found that at small sizes Betula papyrifera, a preferred species, benefited from herbivore exclusion. However, larger individuals grew more when exposed to herbivory, a response we attribute to lower competition in heavily browsed conditions. Populus tremuloides, a highly preferred tree, did not show any differences between levels of herbivory. Abies balsamea, a preferred winter browsed, was only marginally affected by browsing at smaller sizes. The two non-preferred species, Picea, did not show differential growth between herbivory levels. We conclude that herbivores can impact forests through both direct and indirect effects, that these effects are size specific, and that effects vary among species in ways not always predicted by consumption patterns.

The natural ‘exclosure effect’ and tree regeneration following post‐windstorm salvage logging

“…Structural differences between salvage and blowdown areas remained consistent with those reported by Fraver et al (2017) 8 years earlier at this same site. For example, we observed CWM during which saplings escape browsing via girth and height growth (Zonnevylle et al, 2022). This elevated CWM, as well as greater CWM volumes, clearly distinguishes blowdown from salvage and control treatments.…”

“…(2017), we note that the mean CWM height had apparently decreased from 0.61 m to 0.50 m over the 9 years between inventories. While elevated CWM following blowdown diminishes over time, it may provide an exclosure effect for a critical period (years or decades) during which saplings escape browsing via girth and height growth (Zonnevylle et al., 2022). This elevated CWM, as well as greater CWM volumes, clearly distinguishes blowdown from salvage and control treatments.…”

“…As we anticipate a future with more frequent and intense climate‐related disturbances, situations for which salvage logging is considered will increase in tandem (Lindenmayer et al., 2008). Numerous studies have shown that salvage influences the trajectory of forest regeneration (D'Amato et al., 2011; Kleinman et al., 2017; Santoro & D'Amato, 2019), with desirable outcomes in some situations (Royo et al., 2016; Zonnevylle et al., 2022), and undesirable outcomes in others (Taeroe et al., 2019). Results from our study system indicate that salvage logging simplified CWM structure and altered regeneration structure and composition relative to blowdown conditions.…”