Deer browsing and shrub competition set sapling recruitment height and interact with light to shape recruitment niches for temperate forest tree species
For temperate forests, Michigan, USA, we asked: (1) does evidence exist for a height-dependent sapling recruitment bottleneck caused by shrub layer competition and white-tailed deer (Odocoileus virginianus) browsing and, if so, (2) how do these factors interact with light availability in shaping species-specific sapling recruitment niches? The combination of deer browsing effects on tree population height structure and the effects of deer and shrubs on tree seedling height and mortality indicate a shrub-deer bottleneck for sapling recruitment. The maximum height of deer browsing and shrub canopies as well as height dependent seedling mortality rates revealed a critical height threshold of approximately 2 m, above which trees escape from shrub and deer effects (i.e. sapling recruitment). Species’ sapling recruitment niches differed across a harvest gap size (i.e. light availability) gradient, over which tree seedling height growth potential and shrub competition increased with gap size, but deer browsing probability decreased. Specific to forests with high deer populations, niche differences were related to species growth and survival properties, with species generally falling into three groups: High Light, Broad, and Nowhere. High Light species incur high mortality from shade, shrub competition, and deer browsing. However, recruitment can occur in recently formed large gaps, where seedlings growing within developing shrub canopies avoid competition via rapid height growth and are partially protected from browsing deer. Broad species have lower mortality in shade and grow slower than High Light species and, compared to other groups, exhibit height growth and mortality rates that are less-sensitive to deer browsing pressure. These properties allows sapling recruitment of Broad species over a wide range of gap sizes, including larger gaps, where, via advance regeneration, they can avoid competition with developing shrub layers following canopy disturbance. Nowhere species are similar to Broad species, except their height growth and mortality are sensitive to deer browsing pressure. Heavy browsing pressure in smaller gaps and an inability to maintain taller advance regeneration to contend with shrub competition when large gaps form leave Nowhere species without a sapling recruitment niche. In conclusion, we provide evidence that shrub competition, ungulate browsing, harvest gap size and their interactions constrain height-specific sapling recruitment among tree species in the temperate forests of Michigan. Similar mechanisms may operate in other forests with high ungulate populations. Given these drivers and current management regimes, increasing tree species diversity and resilience to future disturbances may hinge on changing and integrating forest and ungulate management paradigms.