Abiotic stress, competition, and the distribution of the native annual grass Vulpia microstachys in a mosaic environment

Abstract

We analyzed how abiotic stress and competition interact to control the abundance and performance of the native annual grass Vulpia microstachys (Lonard and Gould; Poaceae) in a heterogeneous environment. At our study site, V. microstachys grows in nonserpentine grasslands dominated by tall invasive grasses, serpentine meadows dominated by short native forbs, and rocky serpentine slopes with a sparse native herb cover. We hypothesized that these three intermixed habitats acted as a gradient of increasing abiotic stress and decreasing aboveground competition, respectively. We further expected that the abundance and performance of V. microstachys would be highest in serpentine meadows, where neither aboveground competition nor abiotic stress were maximal. Soil and biotic variables showed roughly the expected patterns, but V. microstachys did not show the predicted peak in the middle of the gradient. Emergence, seedling survival, and abundance of V. microstachys were highest, and growth and seed production of survivors were lowest, on rocky serpentine slopes. Field experiments revealed that removal of competitors enhanced all demographic parameters, but only in the more productive habitats. An interaction between seed source and habitat, affecting emergence and survival, indicated ecotypic adaptation to the rocky serpentine slope habitat. We conclude that individual variation caused by local adaptation and phenotypic plasticity allows V. microstachys to survive in widely different habitats, none of which are optimal, resulting in considerable variation in demography.

Soil calcium and plant disease in serpentine ecosystems: a test of the pathogen refuge hypothesis

“…Although calcium availability undoubtedly represents a force critical in driving ecological and evolutionary dynamics in serpentine plant communities, the speciWc mechanism(s) by which calcium exerts its eVects remain unresolved. While the hypothesis that a tradeoV between competitive ability and tolerance of serpentine conditions (especially low calcium) has received the most support (Kruckeberg 1954;Proctor and Woodell 1975;Rice 1989;Jurjavcic et al 2002), few other mechanistic possibilities have been investigated in great detail (Brady et al 2005). One such alternative is the hypothesis that low calcium serpentine soils may provide a refuge from pathogen attack (Kruckeberg 1992).…”

Influence of multiple factors on plant local adaptation: soil type and folivore effects in Ruellia nudiflora (Acanthaceae)

“…It is interesting to note that the comparison of results for survival and fecundity reveals that local adaptation may arise for one fitness component (i.e., survival, in the present instance) but not for another (fecundity), suggesting that different life history strategies could be selected for in order for plants to adapt to local conditions at each site (Sambatti and Rice 2006). Accordingly, differences in results may be related to contrasting evolutionary strategies used across plant ontogenetic stages (Jurjavcic et al 2002), which may lead to local adaptation at one stage (i.e., in terms of survival during early stages), but not in another (reproduction as adults). Based on results from this study, it appears that survival plays an important role for plant fitness during early life stages for which local adaptation is expressed.…”

A California grasslands alkali specialist, Hemizonia pungens ssp. pungens, prefers non‐alkali soil

“…Lolium is a known invader of vernal pools and serpentine areas (Kruckeberg 1954;Linhart 1988), and there is evidence of ecotypic differentiation in this species (Dawson et al 2007) and other species (Linhart 1988;Jurjavcic et al 2002) that can result in increased tolerance to harsh areas or to competition. Some of the Lolium collected from inside the pools may have been an ecotype that was more adapted to competition with Hemizonia in alkali conditions than Lolium found outside the pools.…”

“…Since then, there have been few experimental examinations of these patterns and no formal tests of the refuge hypothesis. Jurjavcic et al (2002) found conflicting experimental patterns of competition and tolerance on serpentine soils in Vulpia microstachys, which is not a serpentine specialist. Hart (1980) found results suggestive of a refuge effect in serpentine outcrops, but did not directly test this effect.…”