Variation in soil moisture in relation to rainfall, vegetation, gaps, and time-since-fire in Florida scrub

Abstract

Florida scrub is a pyrogenic shrubland ecosystem occurring on well-drained sands derived from contemporary and relictual beach dunes. Despite average annual precipitation > 1300 mm, Florida scrub is dominated by xeromorphic plants. We monitored spatio-temporal variation in soil moisture to determine if the distribution of Florida scrub communities reflects patterns in soil moisture variation. Using frequency domain reflectometry, we measured soil moisture at 24 sampling stations (3 depths per station) in 3 Florida scrub communities (rosemary scrub, scrubby flatwoods, and oak-hickory scrub) at Archbold Biological Station for 3 y (October 1998-September 2001). Stations were arrayed to sample 2 microhabitats (gaps, shrubs) and 2 burn histories. Soil moisture closely tracked cumulative rainfall across widely varying precipitation in the 3 y studied. Soil moisture changed through time and differed significantly among habitats; it was generally highest in scrubby flatwoods, particularly during the wettest periods, and lowest in oak-hickory scrub. Soil moisture was generally greater at deeper depths, in more recently burned sites, and in gaps. Burn effects were particularly pronounced in rosemary scrub, where lack of resprouting dominants after fire maintains more distinct, larger gaps. Burn and gap effects were small in absolute terms, but burned sites and gaps consistently had greater soil moisture than unburned and matrix sites. These small differences may be critical to the germination, establishment, and growth of narrowly endemic plants, particularly in Florida rosemary scrub. However, factors such as competition for nutrients, cryptobiotic soil crusts, litter accumulation, gap size, and allelopathy may be more important in influencing distributions of endemic scrub plants.