Microsite-Limited Recruitment Controls Fern Colonization of Post-Agricultural Forests

Abstract

Assessing the relative roles of dispersal limitation and environmental effects in population dynamics and community assembly is fundamental to understanding patterns of species distribution and diversity. In forests growing on abandoned agricultural lands, both legacies of vegetation disturbance and changes in the abiotic environment shape the diversity and composition of recovering communities. Here I specify how interactions among historical, environmental, and biological factors influence species distributions, focusing on three fern species with contrasting distributions across forests of different history in central New York, USA: Dryopteris carthusiana, Dryopteris intermedia, and Polystichum acrostichoides. Using population surveys, spore-trap and spore-bank studies, and a three-year field experiment, I compare demographic rates among species and between forest types to determine which life history stages limit colonization and which traits explain species distributions. Adult plants of all three species were larger and more likely to produce spores in post-agricultural forests than in adjacent, uncleared stands. Though lower population densities led to fewer spores in post-agricultural soils, spore availability still exceeded recruitment by four to five orders of magnitude. Sowing additional spores had relatively little effect, while microhabitat conditions had the greatest impact on establishment rates. Given similar microsites, the two forest types had equal rates of establishment, but some forest-floor features preferentially occupied by juvenile plants were less frequent in post-agricultural stands. The availability of suitable sites for establishment, created by small-scale heterogeneity on forest floors, thus limits both the growth of fern populations and the colonization of new habitats. In fact, reduced microtopographic variation in post-agricultural forests may represent a greater hindrance to plant establishment than changes in mean environmental conditions. Among the three fern species, establishment rates differed as species distributions would predict, with the strongest colonizer consistently having the highest rates and the slowest colonizer the lowest. Rather than random or trait-mediated dispersal, the different distributions of these species reflect life history traits that determine establishment rates and thus colonization ability. This case study demonstrates that ecological interactions based on the unique life histories of individual species can override dispersal in determining species distributions.

Effects of forest fragmentation on the seedling recruitment of a tropical herb: assessing seed vs. safe‐site limitation

“…Moreover, model comparison failed to support the notion that disperser behavior lead to different dispersal distances for seeds in fragments vs. continuous forests (Appendix B). We therefore find little support for the hypothesized increases in dispersal limitation following fragmentation, which contrasts sharply with work in other systems documenting limited immigration of seeds into forest fragments (Flinn andVellend 2005, Cordeiro et al 2009). Previous research at the study site has demonstrated that, despite having lower population densities in forest fragments (Stouffer et al 2006), H. acuminata's avian frugivores readily move among fragments (M. Anciães, unpublished data).…”

“…Finally, under identical conditions, seeds from fragments are not less likely to germinate than those from continuous forest (Bruna 1999), suggesting the effects of inbreeding on recruitment are limited. Therefore, we emphasize mechanisms that our previous empirical work suggests are most relevant and that have been shown to strongly influence seedling recruitment in other herbs: canopy cover (a surrogate for the amount of light reaching the forest floor) and the density of established conspecific plants (Flinn andVellend 2005, Schleuning et al 2009). To this end, we modified the basic inverse model to account for the following factors.…”

“…Seed limitation is strong for many plant species (Flinn andVellend 2005, Svenning andWright 2005). In our system the number of inflorescences, and therefore number of potential seeds produced, varied 20-fold across the study plots (Table 1).…”

Understory fern community structure, growth and spore production responses to a large-scale hurricane experiment in a Puerto Rico rainforest

“…In a Mexican rainforest, tree ferns in high-light forest edges produced more leaves than in adjacent understory habitat where light levels were nine times lower (Bernabe et al, 1999). Flinn (2007) also found that adult ferns were more likely to be larger and produce more spores in secondary forests that had developed on agricultural lands than in undisturbed forests. Pérez-García et al (2007) determined that tropical spore germination was correlated with light intensity and that increased recruitment seemed to require canopy opening.…”

Plasticity in response to phosphorus and light availability in four forest herbs

“…Baeten et al 2009). The number of studies that explore how the (a)biotic legacies of former agricultural practices influence the establishment, performance and persistence of forest herbs is gradually increasing (e.g., Verheyen and Hermy 2004;Vellend 2005;Fraterrigo et al 2006;Flinn 2007;Baeten et al 2009). …”