Naturally diverse plant communities do not resist invasion by the strong competitor, Microstegium vimineum.

PREMISE: Theory predicts and empirical studies have shown that ecologically manipulated communities with high species diversity are resistant to invasion, but do these predictions and results hold true when applied to highly competitive invaders in natural communities? Few studies of diversity-mediated invasion resistance have measured both invasion resistance and invader impact in the same study. METHODS: We used a two-year field experiment to test: (1) diversity-mediated competitive resistance to patch expansion by the grass, Microstegium vimineum; and (2) the competitive effect of M. vimineum on resident plant diversity. We examined responses of M. vimineum to two native plant density-reduction treatments that had opposite effects on species diversity: (1) reducing species richness via the removal of rare species; and (2) reducing dominance by reducing the density of the dominant resident species. We examined the effects of M. vimineum reduction by pre-emergent herbicide on resident diversity in the second year of the study. RESULTS: Neither rare species removal nor dominant species reduction significantly increased M. vimineum density (relative growth rate). The pre-emergent herbicide dramatically reduced M. vimineum in year 2 of the study, but not most resident plants, which were perennials and indirectly benefited from the herbicide at a more productive site, presumably due to reduced competition from M. vimineum. CONCLUSIONS: Diversity-mediated resistance did not effectively deter invasion by a highly competitive invader. In the case of M. vimineum and at more productive sites, it would appear that nearly complete removal of this invader is necessary to preserve plant species diversity.

Prey exclusion combined with simulated fire increases subsequent prey-capture potential in the pale pitcher plant, Sarracenia alata.

PREMISE: The association of carnivory (an adaptation to nutrient-poor soils) with fire has been described as a paradox, given increases in nutrient availability that often accompany fire. The nutrients that increase in availability following fire, however, may not be the same as those provided by prey and may not reduce nutrient limitation if accompanied by even greater increases in light. METHODS: Using a factorial experiment in the field, we examined how simulated fire (clipping plus nitrogen-free fertilizer addition) and prey-derived nutrient availability (prey exclusion) interacted to influence carnivorous potential in Sarracenia alata and belowground competition with its neighbors (manipulated via trenching). We hypothesized that simulated fire combined with prey exclusion would (1) increase the potential for prey capture relative to shade avoidance, hereafter, relative prey-capture potential (RPCP), and/or (2) increase belowground competition with neighboring plants. RESULTS: Sarracenia alata increased RPCP in response to the combination of simulated fire and prey exclusion, despite increases in phosphorus and other nutrients associated with the simulated fire treatment, suggesting that prey capture potential increases in response to increased nitrogen limitation resulting from increases in light and/or phosphorus after fire. We found no evidence of belowground competition. CONCLUSIONS: The potential importance of carnivory in Sarracenia alata increases following fire. This result helps to explain the paradoxical association of carnivorous plants with fire by demonstrating the potential for prey-derived nutrient limitation to increase rather than decrease in response to increases in light and nutrients other than nitrogen following fire.

Inter- and intraspecific competition and shade avoidance in the carnivorous pale pitcher plant in a nutrient-poor savanna.

PREMISE OF THE STUDY: Ecologists generally agree that weak interspecific competition for light contributes to high plant species diversity in ecosystems with nutrient-poor soils. However, the role of competition for light in such ecosystems that are also maintained by fire is poorly understood. I quantified intra- and interspecific competition for light in a fire-maintained nutrient-poor pine savanna by contrasting the effects of conspecific and heterospecific neighbors of the pale pitcher plant, Sarracenia alata. METHODS: Accounting for initial neighbor abundance/aboveground production and initial transplant size, I measured growth and survival of small and large pitcher plant ramets of Sarracenia alata transplanted to the vicinity of natural, undisturbed mixtures of large pitcher plants and their heterospecific neighbors in the field. I tested competition for light and nutrients by clipping conspecific neighbors and by excluding prey from unclipped neighbors of transplants. I tested interspecific competition by uprooting heterospecific neighbors. KEY RESULTS: Plant survivorship increased when conspecific neighbors were clipped and/or starved but not when heterospecific neighbors were uprooted. Small pitcher plants benefited from clipping large conspecific neighbors, suggesting that competition for light was important. Large pitcher plants benefited from excluding prey from their neighbors, with no additional benefit of clipping, suggesting that competition for prey limited their survival. Transplants produced new pitchers that were taller with narrower openings (i.e., shade avoidance) when heterospecific neighbors were left intact but not when conspecifics were unclipped. CONCLUSIONS: Results demonstrate size-dependent intraspecific competition for light and nutrients and interspecific shade avoidance in Sarracenia alata, which could be important to understanding species coexistence in fire-maintained nutrient-poor ecosystems.

Do natural disturbances or the forestry practices that follow them convert forests to early-successional communities?

Stand-replacing natural disturbances in mature forests are traditionally seen as events that cause forests to revert to early stages of succession and maintain species diversity. In some cases, however, such transitions could be an artifact of salvage logging and may increase biotic homogenization. We present initial (two-year) results of a study of the effects of tornado damage and the combined effects of tornado damage and salvage logging on environmental conditions and ground cover plant communities in mixed oak-pine forests in north central Mississippi. Plots were established in salvage-logged areas, adjacent to plots established before the storm in unlogged areas, spanning a gradient of storm damage intensity. Vegetation change directly attributable to tornado damage was driven primarily by a reduction in canopy cover but was not consistent with a transition to an early stage of succession. Although we observed post-storm increases of several disturbance indicators (ruderals), we also observed significant increases in the abundance of a few species indicative of upland forests. Increases in flowering were just as likely to occur in species indicative of forests as in species indicative of open woodlands. Few species declined as a result of the tornado, resulting in a net increase in species richness. Ruderals were very abundant in salvage-logged areas, which contained significantly higher amounts of bare ground and greater variance in soil penetrability than did damaged areas that were not logged. In contrast to unlogged areas severely damaged by the tornado, most upland forest indicators were not abundant in logged areas. Several of the forest and open-woodland indicators that showed increased flowering in damaged areas were absent or sparse in logged areas. Species richness was lower in salvage-logged areas than in adjacent damaged areas but similar to that in undamaged areas. These results suggest that salvage logging prevented positive responses of several forest and open-woodland species to tornado damage. Anthropogenic disturbances such as salvage logging appear to differ fundamentally from stand-level canopy-reducing disturbances in their effects on ground cover vegetation in the forests studied here and are perhaps more appropriately viewed as contributing to biotic homogenization than as events that maintain diversity.