Shifting Sands in Florida Scrub Gaps and Roadsides: Dynamic Microsites for Herbs

Abstract

Florida scrub grows on relict beach ridges with xeric sands, which are constantly transported by wind and water. These sand movements are likely affect microsites occupied by Florida scrub herbs and their demography. We analyzed temporal variation in sand accretion and erosion in natural post-fire gaps within the shrub matrix and along sand roads at Archbold Biological Station in five time intervals: 1 and 2 wk and 1, 3 and 6 mo. Also, in three experiments, we tested effects of sand movements on three herbaceous scrub species: Balduina angustifolia, Paronychia chartacea and Polygonella basiramia. In two pot experiments we studied: (1) seedling emergence from seeds buried to 0, 5 and 20 mm sand depths and (2): seedling survival with (a) frequent but shallow burial by sand, (b) alternating sand addition and removal and (c) infrequent deep sand addition vs. (d) control. Finally, in a field experiment, we studied plant rosette and flowering stem growth with (a) sand addition, (b) sand removal, (c) alternating sand addition and removal, (d) controls protected from spontaneous sand movements and (e) non-manipulated controls. Sand accretion and erosion fluctuated over time with sand erosion greater than accretion. With increasing time intervals, the range of oscillations increased. Sand erosion was significantly greater and more variable along roads than in natural gaps. In the pot experiments, significantly more seedlings emerged from shallower depths. Seedling survival with experimental complete burial was lower than in other treatments. In the field experiment, plants responded to sand removal by significant elongation of flowering stems. The three studied species differed in seedling emergence and growth responses to treatments. The demographic responses of the species to sand movements indicate that mobile sands create constantly shifting arrays of microsites that can influence post-dispersal seed germination, survival and growth of Florida scrub herbs. Roadside habitats have more dynamic patterns of sand movements than natural gaps and may alter selection regimes important for demographic variation of endemic Florida scrub plants.