Scarification and gap size have interacting effects on northern temperate seedling establishment

Abstract

a b s t r a c tAfter decades focused on promoting economically valuable species, management of northern temperate forests has increasingly become focused on promoting tree species diversity. Unfortunately, many formerly common species that could contribute to diversity including yellow birch (Betula alleghaniensis Britton.), paper birch (Betula papyrifera Marsh), eastern hemlock (Tsuga canadensis L.), and eastern white pine (Pinus strobus L.) are now uncommon in the seedling layer, raising concerns about our ability to use these species to increase diversity. In this study, two related seed addition experiments conducted in 45 variably-sized harvest gaps (unharvested to 6500 m 2 ) in adjacent mesic northern hardwood stands, Emmet County, Michigan, USA were used to investigate mechanisms potentially limiting seedling recruitment. The first experiment examined the influence of light (i.e. harvest gap size), competing vegetation, and deer browsing on seedling survival for three years in a 2 Â 2 factorial, where subplots were unfenced or fenced to exclude deer, unclipped or clipped to control competing vegetation, and located across the gradient of gap sizes. The second experiment explored the influence of scarification, light, and competing vegetation on germination and subsequent survival for 2 years in a 2 Â 2 factorial, in subplots that were unscarified or scarified to remove litter, unclipped or clipped to control competing vegetation, and located across the gradient of gap sizes. Eastern hemlock, paper birch, and yellow birch, all smallerseeded species, were 12, 17, and 95 times more abundant in scarified plots compared to unscarified plots. In contrast, white pine, the largest-seeded species, was unaffected by scarification and had low overall germination. Shade tolerant hemlock and shade intolerant paper birch germinated at higher densities in lower light, smaller harvest gap environments, while both mid-tolerant species, white pine and yellow birch, were unaffected by light. Each species' initial survival significantly increased with increasing light availability, and with the exception of yellow birch, each species also survived at a significantly higher rate with increasing light availability. Paper birch and hemlock third year survival also increased with increasing light. By the end of the third growing season, only paper birch survival was negatively impacted by competition from vegetation and no species were affected by exposure to deer browse pressure. At the conclusion of the study, large group selection gaps (24-50 m diameter) contained the highest density of each species except white pine, suggesting that large group selection gaps may provide the best opportunity for reestablishing this particular group of species in the seedling layer.

Evaluating the ecological impacts of salvage logging: can natural and anthropogenic disturbances promote coexistence?

“…Similarly, Willis et al. () demonstrated interactions between canopy openness and scarification were critically important for tree establishment and coexistence in harvest gaps. These results suggest that a historical disturbance regime (i.e., windthrow) can promote coexistence, but only in combination with additional disturbances to the understory vegetation and soil (here, scarification caused by salvaging; see also Oliver and Larson , Roberts , ).…”

“…In sharp contrast, increasing canopy windthrow severity combined with scarification resulting from salvage logging enhanced the establishment of hardwood species of high economic value and promoted species coexistence. Similarly, Willis et al (2015) demonstrated interactions between canopy openness and scarification were critically important for tree establishment and tAble 3. Results of repeated measure analyses of covariance sapling richness, basal area, stem density, and diversity (H′).…”

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

“…Seedlings are often found over a range of substrates in the forest understory. In managed northern hardwood forest in the Lake States region, hardwood litter is the most common seedling establishment substrate (Willis et al 2015). Compared to other substrate types, hardwood litter may present a more imposing physical barrier for root radicle penetration, and resource availability may be more variable and limited as the most stable supply of moisture and mineral nutrients is contained in the underlying humus and mineral soil layers (Houle 1992, Prescott et al 2000.…”

“…), northern white cedar (Thuja occidentalis L.), and eastern hemlock (Tsuga canadensis L.), have less reserve energy available to allocate toward root development (Moles and Westoby 2004). Consequently, in northern hardwood forests, smaller-seeded species seedling establishment is often confined to mineral soil and heavily decayed, large pieces of coarse woody debris (CWD), where resources are more easily accessed and less variable (Cornett et al 2001, Shields et al 2007, Marx and Walters 2008, Bolton and D'Amato 2011, Willis et al 2015.…”

Effects of Gap Size and Locations on the Regeneration of Castanopsis Kawakamii in a Subtropical Natural Forest, China

“…Sapkota and Odén (2009) studied the effect of gap characteristics on woody species in a dry Shorea robusta forest, and the results indicated that Terminalia alata seedling density was significantly and positively correlated with gap area. Large gap size supplies a convenient environment for plant establishment, as noted in a study of seedling establishment in a northern temperate forest, where highest density of plant species in the seedling layer was found in large gap size (Willis et al 2015). The seedling and shrub stages of C. kawakamii preferred the environment in large gap size over small gap size.…”