Stochastic and deterministic processes regulate spatio-temporal variation in seed bank diversity

Question: Seed banks often serve as reservoirs of taxonomic and genetic diversity that buffer plant populations and influence post-disturbance vegetation trajectories; yet evaluating their importance requires understanding how their composition varies within and across spatial and temporal scales (α- and β-diversity). Shifts in seed bank diversity are strongly governed by the deterministic role of differential seed longevities. Nevertheless, spatio-temporal variability in propagule depletion from and recruitment into seed banks may alter both α- and β-diversity. Here, we ask to what degree deterministic seed exhaustion, stochasticity in recruitment and mortality, or both, shape α- and β-diversity? Location: Temperate hardwood forest stands of varying ages in northern Pennsylvania, USA. Methods: We surveyed the seed bank and herbaceous vegetation communities at 39 sites and examined whether the species richness, abundance and composition of either community differs among and within sites of increasing age ranging from 43 to 106 yr old. We explored how α-diversity (species richness) and abundance (percentage cover, seed density) varied across the chronosequence age using regression analyses. We analyzed differences in β -diversity (community composition) using permutational multivariate analyses (i.e. PERMANOVA, PERMDISP). Finally, we tested whether community composition of the herbaceous layer and seed bank communities exhibited nestedness, where nestedness refers to the degree to which less species-rich sites are a non-random subset of more species-rich sites. Results: We found seed bank α-diversity and abundance consistently declined across a gradient of increasingly older sites. Moreover, nestedness analyses indicated species composition at older sites represented a subset of the species found at younger sites characterized by species with persistent seeds. Nevertheless, seed bank communities demonstrated divergent compositional trajectories, whereby older sites were increasingly dissimilar, not only from younger sites, but also from each other. Conclusions: Our results suggest that even in assemblages strongly structured by deterministic forces, such as seed banks, minor stochastic differences in colonization and extinction events may increase β-diversity over time. Therefore, we argue deterministic and stochastic processes are complementary factors governing post-disturbance turnover in species assemblages, and suggest that seed banks contribute to species’ persistence and overall forest community diversity across space and time

Herbaceous layer vegetation recovery following site preparation with herbicides in northern hardwood forests

Some forestry practices have short-term negative impacts on understory plants. Some of these man-made disturbances have no natural analog. Resilience of understory plant communities to the novel disturbance created by application of herbicides is the focus of this dissertation. A long-term study of vegetation recovery following partial overstory removal and herbicide application is reported in chapter two. Vegetation recovered by survival in place, from the seed bank, vegetative reproduction, dispersal, and by being browsed by white-tailed deer early in the growing season allowing the plant to act like an ephemeral. The role of the seed bank in recovery of the vegetation was further investigated in chapter three and showed that the earliest species to recover from herbicide application are those species with seed banking capability. Seed banks were unaffected by herbicides, but herbaceous vegetation cover declined temporarily. Red to far-red ratio and percent visible sky nearly doubled following understory treatment with herbicides. Other than light changes, there were no environmental variable relationships following herbicide. A study of the influence of increased temperature and nitrogen on seed bank response was conducted, showing an interaction of temperature and increased nitrate on number of seed bank germinants. In all of the studies collectively, it was shown that the herbaceous layer was resilient to the novel disturbance pattern created by herbicide application, with recovery of non-target species occurring after 3-5 years as is shown in chapters two and three. Herbicide differs from other understory disturbances like fire and periodic flooding in that both above- and below-ground plant parts are killed if the herbicide contacts the foliage. Thus, the role of the seed bank in recovery may be more critical than it is in following other disturbance in terrestrial systems. Herbicide application is a one-time event, so the ecosystem has not evolved with that kind of disturbance in its history. It will continue to be an important tool for land managers in restoration of ecosystems as invasive plants invade more areas and as long as legacy effects from long-term herbivory exist on the landscape. Key Words: Accord®, Arsenal®, community similarity, diversity, glyphosate, interfering plants, imazapyr, nitrate, Oust XP®, recovery mechanisms, resilience, seed bank, shelterwood silviculture, sulfometuron methyl, vegetation management