Deer browsing and shrub competition set sapling recruitment height and interact with light to shape recruitment niches for temperate forest tree species

• Published in: Forest ecology and management 2020-07, Vol.467, p.118134, Article 118134
• Main Authors: Walters, Michael B., Farinosi, Evan J., Willis, John L.
• Format: Article
• Language: English
• Subjects: Critical height threshold; Harvest gap size; Light availability; Regeneration niche; Sapling recruitment; Shrub competition; Size-dependent mortality; Temperate forests; Tree regeneration; Ungulate browsing
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2020.118134

•We found strong deer-browsing and shrub-competition bottlenecks to sapling recruitment.•Saplings escape deer and shrubs at 2 m tall, defining sapling recruitment height.•Light, shrubs, deer and species properties interact in defining sapling recruitment niches.•With high deer populations, several species have no recruitment niche.•Deer and forest management will require modification in many northern mesic forests. 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. Broadspecies have lower mortality in shade and grow slower thanHigh Lightspecies and, compared to other groups, exhibit height growth and mortality rates that are less-sensitive to deer browsing pressure.These properties allowssapling recruitment ofBroadspecies 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.Nowherespecies are similar toBroadspecies, except their height growth and mortality are sensitive to