The devil is in the details: Identifying suitable microhabitat of an endemic herb

Background/Question/Methods

Eryngium cuneifolium is a gap specialist herb endemic to south central Florida rosemary scrub. Demographic patterns of E. cuneifolium at Archbold Biological Station are strongly driven by time-since-fire and life history stage and population models predict local extinction after 15 years without fire.  Long term demographic data at a second site (Royce Ranch) document continued persistence and high recruitment after 20 years post-fire. We hypothesize these differences are due to microhabitat variation between sites. Specifically we asked 1) What microhabitat characteristics are associated with E. cuneifolium abundance? 2) How do gap microhabitats vary with E. cuneifolium abundance? and 3) How does the microhabitat influence E. cuneifolium demography? We sampled microhabitat in sixteen 10x10 m quadrats in 2013 at Royce Ranch by randomly selecting up to 10 points, 10 gap points and 12 E. cuneifolium plants in each quadrat (N=343). In 50 cm diameter circular plots at each point, we recorded E. cuneifolium abundance, percent ground cover (bare sand, litter, lichen, herbs, and woody plants), and distance, height and species of the nearest dominant plant. AIC information criterion model selection was used to identify the most informative variables explaining abundance.

Results/Conclusions

Gaps accounted for a mean of 22.1% (±16.1SD; range 2.0-55.6%) of quadrats and gap area was positively correlated with E. cuneifolium abundance (R²=0.52) and 2013 recruitment (R²=0.46).  Top AIC models predicting abundance included distance to nearest neighbor and the interaction between neighbor height and species within the site (20.8% deviance) and in gaps (23.6%). Abundance of E. cuneifolium was positively associated with distance and negatively associated with height of Florida rosemary (Ceratiola ericoides). Abundance decreased with oak height but was positively associated with palm height. Increased distance from both palms and oaks increased E. cuneifolium abundance. These microhabitat variables and interactions describe gap features favoring E. cuneifolium abundance. Examination of individual E. cuneifolium vital rates in relation to microhabitat may add additional insight into how gap dynamics influence E. cuneifolium dynamics and persistence. Larger scale dynamic models linking fire to microhabitat variability to species demography may add insight into what factors promote population persistence.