Shrub Seedling Recruitment In Unburned Californian Coastal Sage Scrub And Adjacent Grassland - Statistical Data Included

Ecology, Sept, 1999 by Sandra A. DeSimone, Paul H. Zedler

INTRODUCTION

Shrub recruitment and population dynamics in mediterranean-type ecosystems are dominated by two distinct phases, one associated with fire and the first years of post-fire development, and a second, longer period between fires when differential growth, mortality, and seedling recruitment set the stage for the next occurrence of fire (Keeley 1987, 1995, Zedler and Zammit 1989). The post-fire phase is well-studied (Christensen and Muller 1975, Malanson and O'Leary 1982, Keeley 1987, 1995, Moreno and Oechel 1994); however, much less is known about population dynamics between recurring fires. Seedling recruitment in the absence of burning is common among shrub species of southern Californian coastal sage scrub. Shrub species recruit seedlings not only in mature coastal sage scrub stands (Malanson and Westman 1985) but also colonize grasslands (Oberbauer 1978, Hobbs 1986, Freudenberger et al. 1987, Callaway and Davis 1993, Zedler 1995). This research addresses the processes that promote shrub seedling recruitment in the absence of fire in the semiarid southern California shrub-grassland mosaic.

We examined the influence of small-scale, natural disturbances during shrub seedling establishment, 16 yr after fire. We defined disturbance as "any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment" (White and Pickett 1985:7). Disturbances affect community structure and dynamics at various spatial scales (Pickett et al. 1989). Factors such as fire, landslides, and precipitation variability usually act at relatively large spatial scales, while disturbances such as herbivory, burrowing, and fall of tree limbs often impact communities at much smaller spatial scales (Pickett and White 1985).

Others have shown that small-scale disturbances in grasslands can play an important role in population and community dynamics (Hobbs and Hobbs 1987, Coffin and Lauenroth 1988, Peart 1989, Reichman et al. 1993). Animals regulate early stages of woody seedling establishment in grasslands by suppressing invasion through herbivory (McCarthy and Facelli 1990, Weltzin et al. 1997) or, alternately, by creation of colonization microsites (Cox 1986, Brown and Archer 1987). We believed that small disturbances might also be important in coastal sage scrub and along the coastal sage/grassland boundary in periods between fires for at least two reasons. First, disturbance may create gaps for colonization. Denslow (1980) observed that under conditions of low productivity, such as in habitats with low total moisture availability, environmental differences between large and small disturbance-created gaps may be so slight that a single species may be a successful colonizer across a gradient of disturbance sizes. Thus, shrub seedling recruitment between fires in a grassland-shrubland mosaic may be associated with open areas much smaller than those created by fire, such as gaps produced by drought-induced shrub death in scrub (S. DeSimone, personal observation) or animal foraging in grasslands (Laycock 1958, Cox 1986). Second, disturbance may change resource availability. Consumption of herbaceous species by animals may release resources for shrubs and, thus, promote shrub seedling recruitment. Baccharis pilularis invasion of northern California grassland is associated with herbivory on grassland species under the canopy of invading shrubs (Hobbs and Mooney 1986). A similar process may operate in the coastal sage scrub-grassland ecotone, where 1-2 m wide "bare zones" devoid of annual herbaceous vegetation are often observed. Bartholomew (1970) noted the presence of shrub seedlings in these bare zones and hypothesized that secondary chemicals from shrubs protect these seedlings from mammalian herbivores.

Abiotic disturbances may also influence recruitment. Unusually high and late spring rainfall, a relatively large-scale disturbance (see White 1979), is associated with colonization of grasslands by the evergreen shrub, Baccharis pilularis, in northern California (Williams and Hobbs 1989) and could be necessary for coastal sage shrub seedling survival over the dry summer.

Our goals were to determine: (1) whether seedling recruitment is associated with gaps in mature 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.

METHODS

Study species

Research was conducted at the National Audubon Society's Starr Ranch Sanctuary in Orange County, California. The climate is mediterranean, with an average precipitation of [less than]40 cm annually that falls from November through April, with most rainfall concentrated in January-March.

Study species were four common subshrub dominants that recruit seedlings both after and between fires and co-occur in coastal sage scrub stands throughout semiarid southern California: Artemisia californica, Salvia apiana, Eriogonum fasciculatum, and Lotus scoparius. The four species are all small-seeded, low ([less than]2 m), and facultatively drought deciduous (Malanson and Westman 1985). Seedlings may have some protection from herbivores. Secondary chemicals (terpenes) have been identified in leaves of A. californica and S. apiana (Muller 1966). However, harvester ants (Pogonomyrmex spp) have been observed removing seeds of A. californica (S. Eliason, personal communication), E. fasciculatum (T. St. John, personal communication), and L. scoparius (M. Young, personal communication). There is some evidence that seedlings may benefit from conditions in vegetation gaps. Light-stimulated germination has been observed for A. californica and E. fasciculatum (Keeley 1991, Eliason 1995). Heat-stimulated germination is associated with "hard-seeded" species L. scoparius and S. apiana (Christensen and Muller 1975, Keeley 1991) and may be promoted by solar radiation in gaps (Christensen and Muller 1975). Shade intolerance was observed in A. californica seedlings that suffer mortality when overtopped by evergreen chaparral shrub species (Gray 1983). First year A. californica seedlings are inferior competitors for soil moisture against annual 'grasses (Eliason 1995).