Resource Heterogeneity Relationship with Understory Vegetation in Post-fire Xeric Jack Pine Forests

Abstract

Xeric jack pine forests in northern boreal ecosystems are becoming more vulnerable with the increasing fire frequencies in this region. Understory vegetation is the most diverse aboveground component of this seemingly homogeneous system consisting of only jack pine in the overstory. Understanding the relationship between understory vegetation, fire severity, and resource availability is therefore necessary for managing these forests. In this study, we used spatial statistics to examine the relationship between understory vegetation and resource conditions as affected by two different pre-fire ages ( 60 years) and two fire severities (non-stand replacing fire, NRF, and stand replacing fire, SRF) in xeric jack pine forests in northern Alberta, Canada, four growing seasons post-fire. Using the same framework, we also tested the relationship between resource heterogeneity and species richness commonly known as the resource heterogeneity hypothesis. Spatial heterogeneity, that is, variability, scale of spatial dependency, and patchiness in plant functional groups (cover and richness), microsite conditions (forest floor and soil properties), and light availability were characterized using semivariograms, and their interrelationships were analyzed using spatial autoregressive models. Our findings indicated a stronger fire severity effect on the spatial properties of vegetation composition and microsite conditions than the pre-fire stand age effect. The spatial variability of microsite properties was greater in the NRF sites compared to the SRF sites, but varied in between pre-fire stand ages. Positive relationships were detected between species richness and variability of microsite properties in the NRF sites, whereas such relationships were negative in the SRF sites. Partial Mantel tests indicated a possible dependency of species richness on forest floor variability in the NRF and older sites, and on soil variability in the SRF and younger sites. Overall, findings from this study suggest that spatial heterogeneity in post-fire resource conditions influences the spatial assemblages of understory vegetation in xeric jack pine forests, and a change in fire severity might significantly alter the microsite variability and therefore the mechanisms that drive spatial patterns in understory vegetation.