Andrew R. Dyer

Effects of competition on resource availability and growth of a California bunchgrass

In California, little is known about the sensitivity of native bunchgrasses to competition or to changes in resource availability. We investigated the effect of nonnative annual vegetation on resource availability and growth of a native bunchgrass, Nassella pulchra, in a pair of factorial field experiments that incorporated effects of both interspecific and intraspecific competition as well as variation in soil depth. Plots of differing target densities and neighborhoods were used to assess changes in aboveground (light) and be- lowground (water) resource availability over multiple seasons in two sites with differing soil depth. N. pulchra grown without interspecific competitors grew larger and produced more culms at all planting densities compared to plants in plots with interspecific com- petitors. Intraspecific competition significantly influenced growth only in the absence of interspecific competition. Reproductive effort, as measured by flowering culm production, was more sensitive than vegetative growth to both forms of competition. Light availability and variability at the soil surface was greatly reduced by the nonnative annual neighborhood. As expected, soil moisture was rapidly depleted by annuals to 30 cm in all plots. In deep- soil plots, soil moisture was reduced at 60-150 cm depths only when annual vegetation was removed, and depletion was correlated with N. pulchra basal area. This result suggests that the interspecific neighborhood reduced root growth in N. pulchra and its subsequent ability to use deep moisture. Within Californias inland grasslands, nonnative annual veg- etation has changed seasonal patterns of resource availability. We conclude that (1) increased competition for light during the spring, when growth of annuals is most rapid, suppresses growth and reproduction of N. pulchra; (2) by suppressing bunchgrass growth, annual grasses reduce access to belowground resources by competitive interference; and (3) the loss of perennial grasses in California grasslands and the general dominance by nonnative annual species results in the relative underutilization of deep soil resources. These con- clusions suggest that the dominance of California grasslands by nonnative annual vegetation has shifted the primary limiting resource from soil moisture to light and the timing of resource limitation from summer to winter and spring.

DENSITY DEPENDENCE IN AN ANNUAL PLANT COMMUNITY: VARIATION AMONG LIFE HISTORY STAGES

Most studies of density-dependent demography in plants consider the density only of the single focal species being studied. However, density-dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments. We constructed seminatural communities of desert annuals composed of all the constituent species in the same relative proportions as found in the natural habitat. These experimental communities were planted at a range of densities that extended far above and below mean natural field density. We compared among physical environments (irrigation treatments), among communities from different physical environments, and among growth forms (dicot and graminoid) to search for generalizations about the magnitude and direction of density dependence. Strong evidence of community-level density dependence was detected at all three life history stages studied in these desert annuals: emergence, survival, and final size. However, both the direction and degree of consistency of this density dependence varied considerably among the stages. The strongest and most consistent competitive effects were experienced at the emergence stage, where the mechanism is most likely a form of interference competition rather than exploitation competition. At the survival stage, the magnitude of effects was highly variable among physical environments and source communities, but negative effects were relatively rare, with either positive or no significant effects of increasing density. Thus, exploitation competition was also unimportant at the survival stage. In contrast, for growth, exploitation competition appeared to be the primary mechanism of interaction influencing growth. This variation in mechanism, direction, and magnitude of interactions among life history stages suggests that current models of plant community structure that are based largely on exploitation competition as it influences growth (with mortality a simple function of growth) are inadequate for even this simple annual plant community. We also compared growth forms and found that graminoids were superior competitors to dicots at the emergence and survival stages; they also had higher emergence and survival, regardless of density. Consistent with this result, grasses are always the numerical dominants in the source communities. In contrast, the two growth forms did not differ in competitive ability for growth, and dicots were consistently larger individuals, independent of density, even though grasses were also usually the biomass dominants in the source communities. These results suggest the importance of nontrophic mechanisms of interaction in controlling community structure and again emphasize the importance of constructing and testing models that incorporate multiple mechanisms of interactions.

INTRASPECIFIC AND DIFFUSE COMPETITION: THE RESPONSE OF NASSELLA PULCHRA IN A CALIFORNIA GRASSLAND

In inland California grasslands, the high densities of alien annual species have altered the growing environment for native perennial grasses. Using variable-density plots, we measured the influence of intraspecific competition (conspecifics only) and diffuse competition (mixed-composition neighborhoods that include conspecifics) on growth and survival of Nassella pulchra, purple needlegrass. We assessed the effects of intraspecific and diffuse competition in weeded plots and unweeded plots, respectively, across a density gradient of N. pulchra plants (16-356 plants/M2). We used summer fire and spring sheep grazing to reduce diffuse competition in unweeded plots. The potential effect of rooting volume on competitive interactions was explored by establishing plots on two sites of different soil depth. Diffuse competition had an overriding influence on N. pulchra growth in all treatments. Intraspecific competitive effects were apparent only in the absence of diffuse competition. The effects of grazing and soil depth on growth were only short-lived interactions with the burning treatment. Burning was a longer-lived interaction, but only in weeded plots. Plant mortality was significantly increased by diffuse competition. Overall, N. pulchra survival was greatest in weeded plots, in grazed plots, and in deeper soil plots. The growth of N. pulchra individuals was negatively affected by alien annual species in all treatment combinations. Our data indicate that recruitment of N. pulchra within inland California grasslands is reduced by the adverse environment created by high densities of alien annual species. Successful attempts to increase populations of N. pulchra through management of the grassland community must involve significant modification of the biotic environment.

Seed aging, delayed germination and reduced competitive ability in Bromus tectorum

In annual plants, increased competitive advantage has often been attributed to rapid germination and early establishment. In contrast, many annual species exhibit some degree of delayed germination (i.e., seed dormancy) that results in the formation of age structure within the seed population. Delayed germination can be an effective bet-hedging strategy in variable or unpredictable environments as a seed bank can buffer against years with reproductive failures and reduce the probability of local extinction. However, there has been little consideration of the direct effects of aging within the seed pool although the potential demographic costs of such a strategy (e.g., mortality in the seed bank or delayed reproduction) are well known. We used aged (4 year-old) and freshly produced seed from meadow steppe and sagebrush steppe populations of an annual grass (Bromus tectorum)to investigate the importance of seed age on seedling vigor and competitive ability. Aged seed from the meadow steppe population exhibited delays in germination that reduced plant growth and final biomass when the plants were grown with competition. Aged seed from the sagebrush steppe population did not exhibit delays in germination. By including a treatment that experimentally delayed the germination of freshly produced meadow steppe seed, we also examined the effects of delayed germination alone. A comparison of results from this delay treatment with those from the aged seed treatment suggested that the reduced competitive ability of meadow steppe plants produced from aged seed, although largely a result of the temporal delay in germination, was partly due to reduced seed vigor. Together these results indicate that physiological costs associated with seed age may affect aboveground competitive interactions and, in turn, the relative fitness of older cohorts in the soil seed bank.

Maternal and sibling factors induce dormancy in dimorphic seed pairs of Aegilops triuncialis

Germination and emergence are stimulated by environmental cues, but strongly influenced by maternal controls. However, traits related to seed dispersal may have important influences on germination as well. For example, the “sibling rivalry” hypothesis suggests that germination may be inhibited when sibling seeds remain within a single dispersal unit. These two influences on germination suggest different, and possibly conflicting, evolutionary strategies for optimizing individual fitness. Using an invasive annual grass that produces dispersal units with dimorphic seeds, I found significant reductions in seedling emergence that suggested the presence of both strong maternal and sibling influences on the germination of the smaller seed of dimorphic pairs. Both influences were capable of nearly complete germination suppression of the small seed, but there was no strong evidence for a hierarchy among the two factors. The maternal effect is consistent with a bet-hedging strategy for survival in variable environments where resource availability can be unpredictable, but the sibling effect likely represents a mechanism for reducing competition between closely related individuals, particularly under conditions of resource limitation.

When do Genetic Considerations Require Special Approaches to Ecological Restoration

Conservation biology cannot be concerned solely with preserving what remains. With many habitat types reduced to as little as 1% (e.g., tallgrass prairie east of the Mississippi River, USA) or even 0.1% (e.g., Central Valley riparian forest, California, USA) of their original area (see references in Noss et al. 1995), protecting what is left often represents “too little, too late.” Even habitat types that remain relatively common often occur in isolated patches that are too small for long-term conservation of viable populations of all organisms, particularly those of large carnivores and ungulates (Schonewald-Cox 1983). For these reasons, effective preservation of biodiversity may require investment in ecological restoration to increase the size as well as the connectivity of available habitat (Jordan et al. 1988). Restoration will be especially vital for restoring native diversity to many of the world’s most fertile and productive communities, where habitat destruction resulting from human activities has been concentrated (Janzen 1988).

Evidence of spatial genetic structure in a California bunchgrass population

Abstract We investigated the scale of genetic variation of purple needlegrass (Nassella pulchra), a species commonly used in California for grassland restoration. Common garden and field data revealed evidence of genetic differentiation between two intermixed microhabitats characterized by differences in soil depth and community composition. We assessed the genetic variation within a single population using randomly amplified polymorphic DNA (RAPD) data collected from clusters of five individuals in 40 locations. We found no evidence for genetic structure at the whole population level. At smaller spatial scales, however, we found strong evidence that genetic subdivision of the population occurs at the level of the maternal neighborhood. We suggest that the interaction between widespread pollen dispersal and restricted seed dispersal may be the primary factor generating these results; panmictic pollen dispersal will make detection of genetic patterning difficult at larger spatial scales while limited seed dispersal will generate local genetic structure. As a result, the detection of population genetic structure will depend on the spatial scale of analysis. Local selection gradients related to topography and soil depth are also likely to play a role in structuring local genetic variation. Since N. pulchra is widely used in California in grassland and woodland habitat restoration, we suggest that, as a general rule, care should be exercised in transferring germplasm for the purposes of conservation when little is known about the within-population genetic subdivision of a plant species.

SYNTHESIS: The role of adaptive trans-generational plasticity in biological invasions of plants: Trans-generational plasticity and invasions

High‐impact biological invasions often involve establishment and spread in disturbed, high‐resource patches followed by establishment and spread in biotically or abiotically stressful areas. Evolutionary change may be required for the second phase of invasion (establishment and spread in stressful areas) to occur. When species have low genetic diversity and short selection history, within‐generation phenotypic plasticity is often cited as the mechanism through which spread across multiple habitat types can occur. We show that trans‐generational plasticity (TGP) can result in pre‐adapted progeny that exhibit traits associated with increased fitness both in high‐resource patches and in stressful conditions. In the invasive sedge, Cyperus esculentus, maternal plants growing in nutrient‐poor patches can place disproportional number of propagules into nutrient‐rich patches. Using the invasive annual grass, Aegilops triuncialis, we show that maternal response to soil conditions can confer greater stress tolerance in seedlings in the form of greater photosynthetic efficiency. We also show TGP for a phenological shift in a low resource environment that results in greater stress tolerance in progeny. These lines of evidence suggest that the maternal environment can have profound effects on offspring success and that TGP may play a significant role in some plant invasions.