Spatial scale in prescribed fire regimes: an understudied aspect in conservation with examples from the southeastern United States

The idea that not all fire regimes are created equal is a central theme in fire research and conservation. Fire frequency (i.e., temporal scale) is likely the most studied fire regime attribute as it relates to conservation of fire-adapted ecosystems. Generally, research converges on fire frequency as the primary filter in plant community assembly and structure, which is often critical to conservation goals. Thus, conservation success is commonly linked to fire frequency in fire regimes.The spatial scale of fire may also be vital to conservation outcomes, but this attribute is underrepresented in the primary literature. In our global, contemporary literature search, we found 37 published syntheses concerning the effects of prescribed fire in conservation over the last decade. In those syntheses, only 16% included studies that reported data-based inferences related to the spatial scale of the fire, whereas 73% included discussion of empirical studies on the temporal scale. Only one of the syntheses discussed studies that explicitly tested the effects of spatial extent, and none of those studies were experiments manipulating spatial scale. Further, understanding spatial-scale-dependent patterns may be relevant because two databases of fire-occurrence data from the United States indicated that spatial scale among lightning-ignited and prescribed fires may have been mismatched over the past few decades.Based on a rich ecological literature base that demonstrates pervasive scale-dependent effects in ecology, spatial-scale-dependent relationships among prescribed fire regimes and conservation outcomes are likely. Using examples from the southeastern United States, we explored the potential for scale-dependent ecological effects of fire. In particular, we highlighted the potential for spatial scale to (a) influence wildlife populations by manipulating the dispersion of habitat components, and (b) modulate plant community assembly and structure by affecting seed dispersal mechanics and spatial patterns in herbivory. Because spatial-scale-dependent outcomes are understudied but likely occurring, we encourage researchers to address the ecological effects of spatial scale in prescribed-fire regimes using comparative and manipulative approaches.

Dominant drivers of plant community assembly vary by soil type and time in reclaimed forests

Information on plant community assembly mechanisms is limited on forest reclamation sites after mining in the Canadian boreal forest. We assessed the change in plant community composition after Year 2 and Year 5 on species-rich forest floor mineral mix (FFMM) and species-poor peat mineral mix (PMM) reclamation soils by examining assembly mechanisms, i.e., seed bank, seed rain, biotic dispersal, vegetative expansion, and competition. Initial plant cover and diversity were greater on FFMM due to non-native species originating from the seed bank, which had 5× more seeds in the FFMM. By Year 5, both soil types had approximately 40% cover and 80 species richness due to the addition of wind and biotic-dispersed species and were characterized by a shift towards native species. Native forbs using vegetative reproduction expanded up to 2 m from FFMM into PMM. At Year 5 competition does not seem to have a large role in the structuring of the vegetation community. Overall, multiple factors were involved in structuring plant communities on reclamation sites, but we observed a general convergence between plant communities on different soil types in a relatively short period of time.

Increasing ground-layer plant taxonomic diversity masks declining phylogenetic diversity along a silvicultural disturbance gradient

Most plant diversity in temperate deciduous forests is found in the ground layer, but nearly all studies comparing plant community assembly using taxonomic, trait, and phylogenetic diversity indices are limited to woody plants. To examine the relationship between short-term ground-layer plant community assembly and disturbance severity, we leveraged a silvicultural experiment that applied a combination of harvest and site preparation treatments in a northern hardwood forest in Michigan, USA. We predicted that after two growing seasons, plant communities would be less sensitive to harvest treatments when compared with site preparation treatments that disturb the rhizosphere and modify rooting substrate. We also predicted that an increase in taxonomic diversity would accompany a decline in trait diversity and phylogenetic diversity. Instead, plant species composition responded similarly to harvest treatment and site preparation treatment. However, our measure of disturbance severity was positively correlated with both trait diversity and taxonomic diversity but negatively correlated with phylogenetic diversity, indicating that increasingly diverse traits and taxonomies along this disturbance severity gradient were comprised of more phylogenetically simple plant communities. Informed management decisions should therefore consider the underlying value of each diversity measure, as taxonomic diversity alone may not be the best metric for assessing plant community assembly.

Plant Community Assembly in Invaded Recipient Californian Grasslands and Putative Donor Grasslands in Spain

The introduction of exotic species to new regions offers opportunities to test fundamental questions in ecology, such as the context-dependency of community structure and assembly. Annual grasslands provide a model system of a major unidirectional introduction of plant species from Europe to North America. We compared the community structure of grasslands in two Mediterranean regions by surveying plots in Spain and in California with similar environmental and management conditions. All species found in Spanish grasslands were native to Spain, and over half of them (74 of 139 species) are known to have colonized California. In contrast, in California, over half of the species (52 of 95 species) were exotic species, all of them native to Spain. Nineteen species were found in multiple plots in both regions (i.e., shared species). The abundance of shared species in California was either similar to (13 species) or greater than (6 species) in Spain. In California, plants considered pests were more likely than non-pest species to have higher abundance. Co-occurring shared species tended to maintain their relative abundance in native and introduced communities, which indicates that pools of exotic species might assemble similarly at home and away. These findings provide interesting insights into community assembly in novel ecosystems. They also highlight an example of startling global and local floristic homogenization.