Shrub Seedling Recruitment in Unburned Californian Coastal Sage Scrub and Adjacent Grassland

Abstract

Shrub species of southern Californian coastal sage scrub recruit seedlings immediately after fire but also recruit in unburned coastal sage scrub stands and annual grasslands. We examined the influence of natural disturbances other than fire on shrub seedling establishment, especially the role of small-scale disturbance. Our goals were to determine: (1) whether seedling recruitment is associated with gaps in grassland, scrub, and grassland-scrub ecotones; (2) the identity of gap-creating agents; (3) the influences of biotic (vertebrates and ants) and abiotic (late spring moisture) disturbances on seedling establishment; and (4) whether disturbance effects vary spatially, both among and within (gap vs. nongap microsites) the three zones. We censused natural seedlings and vegetation gaps monthly in belt transects for two growing seasons and also measured seed banks at three 16-yr-old sites of adjacent grassland and coastal sage scrub. We initiated factorial experiments at one site to determine effects of animals, gaps, and late spring moisture on seed and seedling numbers. Gap-creating agents varied among the three zones, but seedlings were associated with gaps in each. Animals, especially small mammals, were important in creation and maintenance of gaps. Animal effects varied among but not within zones. Small mammals positively affected seedling recruitment in the grassland (germination on pocket gopher mounds) and negatively affected seed numbers of species with larger and multipleseeded propagules in the ecotone and scrub; they created and/or maintained gaps in all three zones. Late spring moisture did not play an important role in shrub seedling recruitment in any zone. Because coastal sage shrub seedlings lose transpiring surfaces during drought, they may be less vulnerable than evergreen mediterranean shrubland species to the variability in both annual and seasonal extent of rainfall that is typical of mediterranean climates.

Spatio-Temporal Variation in Contrasting Effects of Resident Vegetation on Establishment, Growth and Reproduction of Dry Grassland Plants: Implications for Seed Addition Experiments

“…Such open sites might originate e.g. from disturbances by animals [65], [66], from specific site conditions (e.g., on steeper slopes [32]) or from temporal collapse of whole vegetation canopy (e.g., due to drought [27], [67]). However, in case that such open sites are only transient, they need not to be always exploited by colonizing species.…”

Altered water and nitrogen input shifts succession in a southern California coastal sage community

“…Increasing fire frequency, atmospheric nitrogen (N) deposition, and drought have all been cited as possible causes of conversions, and all are expected to become increasingly problematic in the future (Diaz-Delgado et al 2002, Syphard et al 2006, Wood et al 2006, Seager et al 2007, Stow et al 2008, Klausmeyer and Shaw 2009, Fenn et al 2010. Observational studies indicate the relative abundance of grassland and shrub species is influenced by fire frequency, with shrubs increasing in importance in undisturbed grasslands and grasses expanding in recently burned shrublands (Callaway and Davis 1993, DeSimone and Zedler 1999, Keeley 2002. Most of the dominant CSS shrubs, including Artemisia californica, Salvia mellifera, and Eriogonum fasciculatum, crown-sprout following fire.…”

Survival of <i>Sarcopoterium Spinosum</i> seedlings growing on terra rossa soil

“…Accordingly, even after shrub control and establishment of a dense herbaceous cover (Henkin et al, 1998b) S. spinosum reestablish (Perevolotsky et al, 2001), as has been found also for other shrubs (DeSimone and Zedler, 1999). Accordingly, even after shrub control and establishment of a dense herbaceous cover (Henkin et al, 1998b) S. spinosum reestablish (Perevolotsky et al, 2001), as has been found also for other shrubs (DeSimone and Zedler, 1999).…”

“…The dominance and persistence of the species has elicited research interest for more than 70 years (Eig, 1938;Seifan and Kadmon, 2006), but the reasons for its dominance have not been fully clarified. The life history of the shrub in Mediterranean-type ecosystems is dominated by two distinct phases: one associated with fires and the first years of post-fire development; and a second, longer, period between fires, when growth, mortality, and seedling recruitment set the stage for the next fire event (DeSimone et al, 1999). Seed germination at the beginning of the wet season can be quite profuse but, despite the dominance of the species in many sites, very few seedlings survive the first summer.…”