William B. Batista

Neotyphodium endophyte infection frequency in annual grass populations: relative importance of mutualism and transmission efficiency

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 non-infected individuals.

Old-field successional dynamics on the Inland Pampa, Argentina

. 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.

The Use of Satellite Imagery in Quantitative Phytogeography: A Case Study of Patagonia (Argentina)

Ecological surveys of arid zones, both with basic and applied phytogeographical objectives, are very difficult because of their magnitude and the low density of their renewable natural resources. The lack of adequate knowledge of the heterogeneity and the functioning of the vegetation is common in these regions. This leads to increasing damage of the environment as a consequence of the inbalance between supply and demand in the use of their natural resources (Soriano 1986).

Woody Plant Invasions in Pampa Grasslands: A Biogeographical and Community Assembly Perspective

Grasslands provide a number of ecosystem services that have been essential to human well-being since pre-historical times (Gibson 2009). Yet they also represent the most endangered terrestrial biome due to conversion into agricultural systems (Sala 2001). The grassland biome covers 15 million km2 (11 %) of the Earth’s surface with non-woody vegetation, excluding deserts and savannas (Sala 2001). Natural grasslands encompass vast regions including the North American Great Plains, the Eurasian steppes of Russia, China and Mongolia, and the South American Pampas (Gibson 2009).

Light-mediated self-organization of sunflower stands increases oil yield in the field

Significance Self-organization in biological systems spans from molecular to ecosystem levels. In plant populations and communities, only a primitive mode of self-organization has been described, which involves changes in demography (survival of individuals affected by the availability of resources). Here, we show that sunflower plants cultivated in high-density stands perceive light signals from their immediate neighbors, adopt alternate positions of their single stem along the crop row, and collectively increase production per unit land area. This process is a case where a communicative sensory network leads to self-organization in plants, without changes in demography. Agronomic and genetic adaptations will be necessary to capture the increased oil production of self-organized stands. 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.

Relating new information to a previous vegetation classification: a case of discriminant coordinates analysis

Discriminant coordinates analysis is an adequate technique for analyzing the linear relationships between a number of new variates (i.e. environmental or functional attributes) and a set of vegetational attributes already summarized in the form of a classification. It displays the principal differences among classes in relation to the new variates considered. The procedure and its rationale are equivalent to a special case of principal components analysis. A case study on radiometer satellite data is presented. Two discriminant coordinates displayed the main differences in the seasonal dynamics of the NDVI (an index of standing green biomass) among broad phytogeo-graphic units in the Patagonia region. The first coordinate can be interpreted as an index of height and convexity of the NDVI seasonal curve. It suggests that the principal difference among regions was the total seasonal growth. The second coordinate represents a contrast that discriminated between two already detected patterns of seasonal NDVI curve.

From pattern to process: estimating expansion rates of a forest tree species in a protected palm savanna

We assessed the possible influences of dominant tree density (Butia yatay palm trees) and fire on the expansion of a riparian tree population (Myrcianthes cisplatensis) over El Palmar National Park, a protected savanna in Argentina. Our approach is based on Skellam’s model of population expansion, which predicts that populations with density-independent reproduction and random dispersal will exhibit Gaussian-shaped expansion fronts. Using Poisson regression, we fitted Gaussian curves to Myrcianthes density data collected at varying distances from a riparian forest, within four environmental conditions resulting from combinations of palm density (dense and sparse) and fire history (burned and unburned). Based on the estimated parameters, we derived statistics appropriate to compare attained expansion velocity, mean squared effective dispersal distance, and density-independent population growth among environmental conditions. We also analyzed the effects of palm density, fire history, and distance from the riparian forest on local maximum size of Myrcianthes individuals. Gaussian curves fitted the data reasonably well and slightly better than two alternative front models. Palm density and fire history interacted to control Myrcianthes spread, making unburned dense palm savannas the preferential avenue for Myrcianthes population expansion across the landscape. Limitation of Myrcianthes expansion by fire appeared to result from low survival of small individuals to fire, whereas facilitation of Myrcianthes expansion by palm trees may have resulted from increased population growth. Our results stress the interactive role of fire regime and local biotic influences in determining propagule pressure and tree establishment at the forefront, and the overall vulnerability of savannas to colonization by forest species.

Climatic and land‐use drivers along a latitudinal gradient: species diversity in temperate grasslands on agricultural soils

Questions Do remaining mesophytic grasslands on soils with agricultural potential respond to a latitudinal gradient? Are climatic or land use factors the principal drivers of regional and local diversity of these grasslands? Location The mesophytic grasslands of the Argentine Pampas, between 32°S and 38°S, along 600km within the Mesopotamic, Rolling and Flooding Pampas of the Rio de la Plata grasslands. Methods The species presence and cover was recorded in each of 96 remnant grasslands on soils suitable for cropping, grouped in five sampling locations. In each region, confidence intervals for gamma diversity and for the slope of species/area curves were estimated. Unconstrained ordination was applied to detect the principal gradients in floristic composition and correlation analysis to identify their main drivers. Mantel test was used to evaluate the correlation between floristic similarity and geographical distance, and with Euclidean distance in fragmentation and climatic variables. To rank the climatic and land use factors that account for local species richness and percent number of exotics, we applied correlation analysis and regression models. Results Community composition (73% native species) was mainly related to climatic variables determined by latitude, with a slight influence of fragmentation variables. Regional species richness responded to latitude: gamma diversity decreased linearly with increasing distance from the Equator, and so did the rate of species accumulation with expanding area. Alpha diversity did not vary in a systematic way with latitude, being associated with landscape fragmentation and mean annual precipitation. Relative importance of C4 grasses and mean percent exotics showed opposite latitudinal tendencies, the former decreasing and the latter increasing towards the South. Conclusions Latitude was a strong determinant of regional diversity and community composition, but a partial driver for local species richness mainly influenced by landscape fragmentation. While climatic and geographical drivers determined gradual latitudinal turnover in regional species pools, within regions land-use history, stochastic processes and biotic interactions were also important. All these patterns need to be considered together when delimiting grassland nature reserves for conservation of the most diverse ecosystems of this region, threatened by agricultural expansion and intensification. This article is protected by copyright. All rights reserved.