Persistence and ubiquity of vertically transmitted Neotyphodium endophytes in grass populations is puzzling because infected plants do not consistently exhibit increased fitness. Using an annual grass population model, we show that the problems for matching endophyte infection and mutualism are likely to arise from difficulties in detecting small mutualistic effects, variability in endophyte transmission efficiency and an apparent prevalence of non-equilibrium in the dynamics of infection. Although endophytes would ultimately persist only if the infection confers some fitness increase to the host plants, such an increase can be very small, as long as the transmission efficiency is sufficiently high. In addition, imperfect transmission limits effectively the equilibrium infection level if the infected plants exhibit small or large reproductive advantage. Under frequent natural conditions, the equilibrium infection level is very sensitive to small changes in transmission efficiency and host reproductive advantage, while convergence to such an equilibrium is slow. As a consequence, seed immigration and environmental fluctuation are likely to keep local infection levels away from equilibrium. Transient dynamics analysis suggests that, when driven by environmental fluctuation, infection frequency increases would often be larger than decreases. By contrast, when due to immigration, overrepresentation of infected individuals tends to vanish faster than equivalent overrepresentation of noninfected individuals.

Summary1 Theory and empirical evidence suggest that community invasibility is influenced by propagule pressure, physical stress and biotic resistance from resident species. We studied patterns of exotic and native species richness across the Flooding Pampas of Argentina, and tested for exotic richness correlates with major environmental gradients, species pool size, and native richness, among and within different grassland habitat types. 2 Native and exotic richness were positively correlated across grassland types, increasing from lowland meadows and halophyte steppes, through humid to mesophyte prairies in more elevated topographic positions. Species pool size was positively correlated with local richness of native and exotic plants, being larger for mesophyte and humid prairies. Localities in the more stressful meadow and halophyte steppe habitats contained smaller fractions of their landscape species pools. 3 Native and exotic species numbers decreased along a gradient of increasing soil salinity and decreasing soil depth, and displayed a unimodal relationship with soil organic carbon. When covarying habitat factors were held constant, exotic and native richness residuals were still positively correlated across sites. Within grassland habitat types, exotic and native species richness were positively associated in meadows and halophyte steppes but showed no consistent relationship in the least stressful, prairie habitat types. 4 Functional group composition differed widely between native and exotic species pools. Patterns suggesting biotic resistance to invasion emerged only within humid prairies, where exotic richness decreased with increasing richness of native warm-season grasses. This negative relationship was observed for other descriptors of invasion such as richness and cover of annual cool-season forbs, the commonest group of exotics. 5 Our results support the view that ecological factors correlated with differences in invasion success change with the range of environmental heterogeneity encompassed by the analysis. Within narrow habitat ranges, invasion resistance may be associated with either physical stress or resident native diversity. Biotic resistance through native richness, however, appeared to be effective only at intermediate locations along a stress/ fertility gradient. 6 We show that certain functional groups, not just total native richness, may be critical to community resistance to invasion. Identifying such native species groups is important for directing management and conservation efforts.

Here, we show a unique crop response to intraspecific interference, whereby neighboring sunflower plants in a row avoid each other by growing toward a more favorable light environment and collectively increase production per unit land area. In high-density stands, a given plant inclined toward one side of the interrow space, and the immediate neighbors inclined in the opposite direction. This process started early as an incipient inclination of pioneer plants, and the arrangement propagated gradually as a “wave” of alternate inclination that persisted until maturity. Measurements and experimental manipulation of light spectral composition indicate that these responses are mediated by changes in the red/far-red ratio of the light, which is perceived by phytochrome. Cellular automata simulations reproduced the patterns of stem inclination in field experiments, supporting the proposition of self-organization of stand structure. Under high crop population densities (10 and 14 plants per m2), as yet unachievable in commercial farms with current hybrids due to lodging and diseases, self-organized crops yielded between 19 and 47% more oil than crops forced to remain erect.

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We examined the demographic responses of Fagus grandifolia to Hurricane Kate (1985) in an old-growth Southern mixed-hardwood forest in northern Florida. Matrix population models were used to contrast pre-and post-hurricane population trends (1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992) with predictions of four hypotheses of the persistence of this shade-tolerant species in hurricane-frequented forests (resistance, recovery, release, and complementation). Although growth and mortality schedules changed as a result of the hurricane, the asymptotic population growth rate did not change significantly and did not depart significantly from ϭ 1 (stability) in either the pre-or the posthurricane periods. Long-term trends simulated for varying hurricane frequencies also projected stability in population size. These results supported the resistance hypothesis. Elasticity analysis showed that, while recruitment had a minor influence, survival of medium-sized trees was the most critical character influencing population growth. Similar patterns of life history sensitivity are common among other trees, suggesting that traits related to survival of large understory individuals would often be under high selection pressure. Among these traits, those associated with hurricane resistance could also be advantageous in forests subjected to light to moderate disturbance. Thus, resistance to canopy disruption could have remained linked to shade tolerance during expansions and contractions of the distribution of F. grandifolia.

We examined the demographic responses of Fagus grandifolia to Hurricane Kate (1985) in an old‐growth Southern mixed‐hardwood forest in northern Florida. Matrix population models were used to contrast pre‐ and post‐hurricane population trends (1978–1992) with predictions of four hypotheses of the persistence of this shade‐tolerant species in hurricane‐frequented forests (resistance, recovery, release, and complementation). Although growth and mortality schedules changed as a result of the hurricane, the asymptotic population growth rate did not change significantly and did not depart significantly from λ = 1 (stability) in either the pre‐ or the posthurricane periods. Long‐term trends simulated for varying hurricane frequencies also projected stability in population size. These results supported the resistance hypothesis. Elasticity analysis showed that, while recruitment had a minor influence, survival of medium‐sized trees was the most critical character influencing population growth. Similar patterns of life history sensitivity are common among other trees, suggesting that traits related to survival of large understory individuals would often be under high selection pressure. Among these traits, those associated with hurricane resistance could also be advantageous in forests subjected to light to moderate disturbance. Thus, resistance to canopy disruption could have remained linked to shade tolerance during expansions and contractions of the distribution of F. grandifolia.

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Abstract. The first 10 yr of old‐field successional dynamics on the Argentine Inland Pampa were studied on a series of adjacent plots established consecutively between 1978 and 1989. We examined differences in species abundance patterns among plots in order to detect the spatial and temporal variability of succession. Perennial grasses steadily increased in cover and replaced the dominant annual species after 5 yr. Pioneer dicots persisted in older seral stages with 20 — 23 species/plot. Overall, exotic species (mostly the grasses Lolium multiflorum and Cynodon dactylon) contributed much to the plant cover in these communities. Native grasses comprised 45 % of total cover at years 7 — 10 of succession, but occurred with less than 7 species/plot. Substantial variation was found in the successional pathway, which reflected the particular sequence from annual forbs to short‐lived and perennial grasses in the various plots. The course of succession was apparently influenced by a 2‐yr period of unusually high rainfall. Deyeuxia viridiflavescens, a native perennial grass virtually absent before the wet period, spread over the study area and dominated seral communities for 3 yr, irrespective of plot age. Climatic conditions thus affected the successional turnover of life forms by increasing the rate of colonization by perennial grasses. We further point out the constraints imposed on secondary succession by the life histories of ‘available’ species.