Managing for diversity: harvest gap size drives complex light, vegetation, and deer herbivory impacts on tree seedlings

Abstract

Abstract. Many managed northern hardwood forests are characterized by low-diversity tree regeneration. Small harvest gaps, competition from shrub-herb vegetation, and browsing by white-tailed deer (Odocoileus virginianus) contribute to this pattern, but we know little about how these factors interact. With a stand-scale experiment, we examined the effects of gap size (0-3234 m 2 ), vegetation (weeded:unweeded), and deer (fenced:unfenced) on seedling growth and survival for 18 tree species. With increasing gap size and light, shrub-herb vegetation density increased, while deer browsing on seedlings in unweeded plots decreased. Fenced:weeded seedlings of all species increased in height up to 35-45% light, with optimal growth in large-group selection and patch cut harvest gaps. Height growth rank order among tree species changed between gap sizes, but growth varied little in small, low-light gaps. Instead, a low-light survival (i.e., shade tolerance) vs. high-light growth tradeoff we observed is likely more important for species sorting of gap sizes. Shrub-herb vegetation decreased seedling survival and growth, especially in larger harvest gaps, shifting gap size optima to smaller gaps, but had little effect on growth/survival rank order among species. In contrast, deer had strong impacts on growth rank order, especially in larger gaps where species differences in growth potential were trumped by differences in deer browsing pressure responses. However, contrary to their consistently negative main effects, vegetation and deer had two positive interacting effects: dense shrub-herb vegetation in large gaps protected seedlings of faster-growing species from browsing and deer browsing of shrub-herb vegetation modestly increased light and growth of short, suppressed, browsing-avoided species. In summary, harvest gap size-mediated light availability, shrub-herb vegetation, and deer herbivory had strong interacting effects on tree seedling interspecific performance ranks and intraspecific optimal gap sizes. For management, a broad range of harvest gap sizes and rapid establishment of tree regeneration (naturally or planted) to minimize shrub-herb competition should increase tree diversity in forests with few deer. However, with deer browsing pressure, a more limited set of lesser-browsed species are likely to recruit successfully regardless of gap size, except in large patch cut gaps, where recruitment of faster-growing, shade-intolerant species is possible.

Nutrition and mycorrhizae affect interspecific patterns of seedling growth on coarse wood and mineral soil substrates

“…Patterns of growth among seedling species varied similarly across substrates, as mineral soil generally supported taller seedlings than any species of CWD. This result is not particularly surprising considering that mineral soil is a nutrient-rich substrate (Harmon et al 1986), and because our experimental design controlled for many of the above-discussed factors that can negatively affect seedlings on the forest floor (Harmon and Franklin 1989, Packer and Clay 2000, O'Hanlon-Manners and Kotanen 2004, Walters et al 2016, Webster et al 2018). However, seedling height also varied considerably across CWD substrates, as seedlings grown on northern white cedar and paper birch were generally taller than seedlings grown on sugar maple, balsam fir, and yellow birch.…”

“…), and because our experimental design controlled for many of the above‐discussed factors that can negatively affect seedlings on the forest floor (Harmon and Franklin , Packer and Clay , O'Hanlon‐Manners and Kotanen , Walters et al. , Webster et al. ).…”

“…, O'Hanlon‐Manners and Kotanen , Walters et al. ). Smaller‐seeded seedlings established on CWD may also be at lower risk to drought stress, as CWD substrates are known for their high moisture‐holding capacity (Cornett et al.…”

“…Evidence to date suggests that mineral soil provides a higher quality nutrient resource environment than CWD (Cornett et al 1997, Mori et al 2004). However, due to their small initial stature, smaller-seeded seedlings may be more likely to face several hazards on mineral soil than on CWD, including greater risk of high water tables, pathogens, competing vegetation, and leaf smothering (Harmon and Franklin 1989, Packer and Clay 2000, Simard et al 2003, O'Hanlon-Manners and Kotanen 2004, Walters et al 2016. Smaller-seeded seedlings established on CWD may also be at lower risk to drought stress, as CWD substrates are known for their high moisture-holding capacity (Cornett et al 1997, Mori et al 2004.…”

Introduction of suitable species for planting in gaps of different size (case study: Loveh forest, Golestan, Iran)

“…increasing light by increasing the harvest gap size should increase the growth of shrub-herb vegetation and its competitive impacts on tree seedlings, but should also increase tree seedling growth, with responses varying among species (Walters et al 2016). The growth situation of Pinus tabulaeformis carrière seedlings in different habitats was in the order of big gap in shady slope > big gap in sunny slope > small gap in sunny slope > small gap in shady slope > understory in sunny slope > understory in shady slope (Han et al 2012).…”