PS 46-205 - The microclimatic and physical influence of overstory leaf litter on understory wildflower phenology

Abstract: The aboveground activity of forest understory herbaceous species is constrained by risks of frost damage early in the growing season and limited light after tree canopy closure, making phenological timing critical. These species rely on soil temperature and moisture to cue phenological events, such as spring leaf out, which is highly dependent on microclimate and varies greatly within a site. Past studies have found that because of differences in quality and quantity, leaf litter species mediate soil temperature and moisture to varying degrees. Due to the direct impact on soil temperature and moisture, leaf litter quality and quantity should also have a large, yet overlooked impact on herbaceous phenology. In a litter manipulation study, we tested the impact of tree leaf litter microclimate on understory herb phenology. We monitored herb phenology, soil temperature, and soil moisture across replicated litter removal, litter addition, and control plots. Replicate plots had similar herbaceous cover but different overstory canopy composition. Due to greater microclimatic buffering and physical blocking, we hypothesized that thicker leaf litter layers lead to delayed wildflower emergence. Conversely, due to increased soil temperature variation, we hypothesized that species would emerge earlier in control and leaf litter removal plots.

We found that soil temperature varied the greatest across seasons within the leaf litter removal treatments. During the dormant season, soil temperatures in the litter-removal plots were, on average, 1.5 ± 0.07 and 0.73 ± 0.09 ºC colder than litter-addition treatments at the surface and ten centimeters below, respectively. Conversely, during the growing season, the leaf litter addition treatment had the lowest temperatures, likely owing to moisture insulation and decreased solar warming. Soil temperature also varied by tree leaf litter identity, with recalcitrant leaves (e.g., oak) having greater microclimatic buffering effects than faster decomposing leaves (e.g., maple). Spring ephemeral and spring blooming species emerged and leafed out, on average, 5 ± 1.08 days later and 4 ± 0.84 days earlier in litter-addition and litter-removal treatments, respectively, compared to control plots. Interestingly, we did not find significant differences in flowering phenology. Our findings provide new insight on wildflower phenological cues at the microclimatic scale and help explain within-site variation of understory phenology due to differences in leaf litter depth and canopy tree species.