Interspecific variation in tree seedling height growth responses to harvest gap size: Implications for increasing tree diversity in mesic northern temperate forest

Background/Question/Methods

Managing forests for increased tree species diversity may increase, among other values, wildlife habitat diversity and ecological and economic resilience to disturbance.  Tree species diversity in mesic northern temperate forest may be declining, and several factors may be responsible, including white-tailed deer (Odicoileus virginianus) herbivory, catastrophic forest health epidemics and harvest legacy effects (e.g., reduction of seed source trees and of coarse wood seedling establishment substrates, single tree selection favoring few shade tolerant species). To help find ways to increase tree diversity we established a project in Michigan, USA that examines the relative impacts of local seed availability, seedling establishment substrates, light (harvest gap size) and soil resources, non-tree vegetation competition and deer herbivory on natural tree seedling dynamics, and resource and deer herbivory effects on planted seedlings.  Here, we focus on third-year planted seedling performance results.

Results/Conclusions

Among 45 gaps and forest understory sites, light ranged from 2.5% of full light in understory to > 30% in multi-tree gaps, and soil water and mineral N covaried positively with light.  By the third growing season following harvest, vegetation (mostly non-tree) density in un-weeded natural seedling plots was positively related to light and water availability such that, regardless of gap size, light availability beneath vegetation was nearly as low as in the forest understory in the wetter, but not drier, gaps. For planted seedlings in weeded plots, height growth varied with shade tolerance classifications. Intolerants (e.g. Betula papyrifera, Prunus pensylvanica), generally showed strong height responses to light and soil mineral N concentration with most seedlings in > 20 % full light exceeding 150 cm height. Tolerants (e.g. Tsuga canadensis, Acer saccharum) generally showed weak height growth responses to light and soil N. Intermediate species (e.g. Quercus rubra, Acer rubrum, Ulmus americana) responded intermediately. Virtually no seedlings of intermediate or tolerant species exceeded 150 cm height which is both the mean canopy height for non-tree vegetation and the maximum reach of browsing deer. Thus, intolerant species may be able to rapidly escape browsing deer and competing vegetation via rapid height growth in larger and perhaps drier gaps and tolerant species may persist, if avoided by deer, regardless of gap size as either advanced regeneration or gap origin seedlings beneath competing vegetation. In contrast, browse preferred intermediate species may be particularly difficult to restore in the face of competing vegetation and browsing deer given their lesser shade tolerance and slow vertical growth rates.