Spatial aspects of vernal pool ecology and restoration
Solano County, California

Vernal pools in northern California

    Vernal pools are temporary wetlands with abrupt boundaries that form on relatively level sites underlain by an impervious hardpan soil layer (Zedler 1987, Holland and Jain 1988, Keeley and Zedler 1998). They are typical of Mediterranean climates and are characterized by winter and spring inundation of pools, followed by complete drying in summer.  Like vernal pools elsewhere in the world, California’s pools are important biological reservoirs for amphibian and aquatic invertebrate species (Witham et al. 1998), but they are distinct in that they support a unique, highly endemic, and largely annual flora (Stebbins 1976, Holland and Jain 1981).  Vernal pools were once widespread along the Pacific coast and Central Valley of California (Barbour et al. 1993), but due to urban expansion and agricultural development, most habitat has been destroyed; it is estimated that only 3-10 percent of pools remain (Leidy and White 1998) and many vernal pool endemics are now legally protected (Witham et al. 1998).

    This research focuses on the ecology and restoration of native vernal pool plant communities in central California.  These communities are particularly appropriate for hypothesis testing in community ecology because of their dramatic temporal and spatial variation in plant species composition.  Species composition is highly variable even under relatively similar physical conditions within study sites.  This observation suggests that chance occurrences, such as past colonization events, and biotic factors, such as competition and pollination may strongly influence plant community composition.  As spatially discrete habitats, vernal pools are especially well suited to field experiments that investigate factors influencing colonization and persistence of species in the formation of ecological communities.  Moreover, colonization of vernal pools is likely mediated by their spatial position in the landscape, as predicted by ecological theory.  Given that many vernal pool plants have limited dispersal capabilities, pools near a source of propagules should be colonized more frequently than those further away.

Project goals

    The overall objective of this research effort is to elucidate the relative importance of historical factors and pool spatial position in controlling plant community composition in vernal pools, and to investigate possible abiotic and biotic mechanisms for observed patterns in species composition during the process of colonization.  The role of historical factors and pool spatial position is examined via seed addition treatments and natural colonization of created vernal pools in a large field experiment, while mechanisms are investigated through an additional series of greenhouse experiments, field observations, and field experiments with vernal pool plant species.  The specific goals of the research are to 1) characterize plant communities in experimental vernal pools, 2) analyze effects of water depth, duration of inundation, and soil quality on plant species composition, 3) measure the magnitude and direction of interactions among five vernal pool plant species, 4) determine pollinator visitation and efficiency in experimental and naturally-occurring pools, and 5) quantify the contribution of jackrabbit seed dispersal to plant community composition. The outcomes of these experiments will contribute greatly to our understanding of the basic ecological forces that govern population establishment, persistence, and community composition, and will also highlight a novel approach to restoration of native ecological communities.

Conceptual background

    Efforts to understand the factors controlling the composition of local communities have played a central role in the theoretical and empirical development of ecology.  Particularly intriguing to ecologists is the question of how local communities form from regional species pools.  For a population to establish in a local community, individuals of a particular species must first arrive in a potential habitat.  Arrival may be constrained by dispersal ability of the organisms or stochastic events that transport propagules to a particular site.  Local community composition will ultimately depend on success of propagules during this colonization phase.  Ecological theory regarding the assembly of communities posits a crucial role for this “historical contingence” in structuring ecological communities.  In the context of community assembly theory, historical contingence refers to the identity of colonists, the order of species colonization, and the frequency of colonization attempts by a given species.  The extent to which these historical factors govern community composition remains a major gap in our knowledge, because it is exceedingly difficult to observe early stages of community formation.  In this study, I use an ecological restoration project that involves reconstructing a native plant community to test key hypotheses regarding the importance of colonization patterns on plant community composition.

Progress to date

    The purpose of this vernal pool field experiment and restoration project is to preserve and restore the natural vernal pool ecosystem at the Travis AFB Aero Club site.  The project’s long-term objectives are to preserve existing vernal pools at Travis AFB, and to construct vernal pools that exhibit similar structure and function to currently existing, protected vernal pools at the Aero Club.  The viability of this vernal pool ecosystem is particularly important to prevent extinction of the federally endangered plant, Lasthenia conjugens (Contra Costa goldfields: Asteraceae; hereafter “LASCON”), and to maintain and enhance vernal pool ecosystems in central California, which have declined to less than 10% of their original extent.

    As part of the vernal pool mitigation requirements for Travis AFB (Collinge 1999), 256 pools were constructed at the Aero Club in November 1999.  Because this project’s ultimate goal is to reconstruct viable vernal pool communities, in addition to LASCON, the constructed pools were seeded with four other species found in vernal pools at the study site, including Eryngium vaseyi (Button celery, Apiaceae), Deschampsia danthonioides (Annual hairgrass, Poaceae), Layia chrysanthemoides (Tidy tips, Asteraceae), and Plagiobothrys stipitatus (Popcorn flower, Boraginaceae).  Seeds of the five native species were collected at the study site in May-June 1999.  In August-December 1999, seeds of each species were counted and stored in labeled coin envelopes under cool, dry conditions until planting (N=19,200 LASCON seeds and 6,400 seeds of each of the other four species for a total of 44,800 seeds).  Seeds were collected from the site again in May-June 2000, counted, and distributed in the vernal pool basins in December 2000 according to the seed addition treatments (Table 1).

    In December 1999, I randomly assigned one of four seed addition treatments (Table 1) to each of the 256 constructed pools  (Fig. 1).  Pools in the control treatment were left unseeded to assess the rate of natural colonization of created vernal pools.  I chose one hundred seeds for the LASCON treatment, and one hundred seeds of each species for the group treatments, because natural colonization of vernal pools would likely involve a relatively small number of propagules.  All seeds for each treatment were distributed in a marked, 0.5 m x 0.5 m square plot within the 5-10 cm depth zone of each pool basin (Figs. 2, 3), so that their fates could be easily determined over the five-year study duration.  For control plots, marked quadrats were placed in control pools at the same depth zone as the seeded plots.

Table 1.  Seed inoculation treatments for created vernal pools.
 
Treatment Year 1 Year 2 Year 3 Year 4 Year 5
Control No seeds No seeds No seeds No seeds No seeds
LASCON 100 seeds One half of the replicates will receive no seeds, one half  will receive 100 seeds One half of the replicates will receive no seeds, one half  will receive 100 seeds No seeds No seeds
Group 1: (LASCON+DESDAN+ERYVAS) 100 seeds of each species 100 seeds each of Group 2 species No seeds No seeds No seeds
Group 2: LASCON+PLASTI+LAYCHR) 100 seeds of each species 100 seeds each of Group 1 species No seeds No seeds No seeds

Figure 1.  Constructed (blue) and natural (green) vernal    Figure 2.  Planting seeds in constructed pool basins.        Figure 3.  Lasthenia conjugens in constructed pool
pools at Travis AFB Aero Club site.                                                                                                                                            during the second growing season of the experiment.

    The 256 vernal pool basins constructed as part of the mitigation for damage to vernal pools at the Travis AFB Aero Club appeared to function very well during the first year after construction.  Perhaps most importantly, the vernal pool basins were inundated with water for a similar period of time during the wet season as were the reference pools.  Ecological studies of vernal pools have suggested that duration of inundation is crucial to maintain populations of native vernal pool species and to prevent the invasion of vernal pools by exotic grasses and forbs.  Second, the five vernal pool species that were seeded into constructed pools germinated, grew, and reproduced.  This further suggests that environmental conditions at the site are appropriate for these species, and bodes well for future increases in population sizes of these species.  Third, several native vernal pool species germinated in constructed pool basins from seeds dormant in the seed bank.  Given that it may have been at least 40 years since this site supported vernal pools, this suggests that vernal pool plant seeds may persist in the seed bank and then germinate when appropriate environmental conditions are reached.

    With continued seeding, monitoring, and management over the next four years, it is likely that the constructed vernal pool basins will support increasing densities of native vernal pool plants, and that the vernal pool mitigation project will achieve its ultimate goals to preserve existing vernal pools at Travis AFB, and to construct vernal pools that exhibit similar structure and function to currently existing, protected vernal pools at the Aero Club.  The viability of this vernal pool ecosystem is particularly important to prevent extinction of Lasthenia conjugens and to maintain and enhance vernal pool ecosystems in central California.
 

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