Dynamical mechanism for sharp orientation tuning in an integrate-and-fire model of a cortical hypercolumn

Orientation tuning in a ring of pulse-coupled integrate-and-fire (IF) neurons is analyzed in terms of spontaneous pattern formation. It is shown how the ring bifurcates from a synchronous state to a non-phase-locked state whose spike trains are characterized by clustered but irregular fluctuations of the interspike intervals (ISIs). The separation of these clusters in phase space results in a localized peak of activity as measured by the time-averaged firing rate of the neurons. This generates a sharp orientation tuning curve that can lock to a slowly rotating, weakly tuned external stimulus. Under certain conditions, the peak can slowly rotate even to a fixed external stimulus. The ring also exhibits hysteresis due to the subcritical nature of the bifurcation to sharp orientation tuning. Such behavior is shown to be consistent with a corresponding analog version of the IF model in the limit of slow synaptic interactions. For fast synapses, the deterministic fluctuations of the ISIs associated with the tuning curve can support a coefficient of variation of order unity.<br/

Can a fast­growing early­successional tree (Ochroma pyramidale, Malvaceae) accelerate forest succession?

Species-specific traits of trees affect ecosystem dynamics, defining forest structure and understorey development. Ochroma pyramidale is a fast-growing tree species, with life-history traits that include low wood density, short-lived large leaves and a narrow open thin crown. We evaluated forest succession in O. pyramidale-dominated secondary forests, diverse secondary forests, both 10–15 y since abandonment, and rain forests by comparing height, density and basal area of all trees (> 5 cm dbh). Furthermore, we compared species richness of understorey trees and shrubs, and basal area and density of trees of early- and late-successional species ( 15%) and leaf litter (> 10 cm) were both highest in O. pyramidale forests, which positively affected density of understorey trees and shrubs and negatively affected density of late-successional trees. In conclusion, O. pyramidale forests presented structural features similar to those of rain forest, but this constrained the establishment of understorey tree species, especially late-successional species, decreasing successional developmen

Tropical rain-forest matrix quality affects bat assemblage structure in secondary forest patches

We studied Phyllostomidae bat assemblage structure in patches of secondary forest dominated by the pioneer tree Ochroma pyramidale, largely (.85%) or partially (,35%) surrounded by a matrix of tropical rain forest, to test 3 hypotheses: the highest bat diversity and richness is observed in the matrix rain forest in comparison to secondary forest patches; the proportion of rain forest surrounding secondary forest positively affects bat diversity and richness; and canopy openness is an important structural variable negatively affecting bat abundance. Rain-forest control sites had the highest bat species diversity and richness, and contributed more to total diversity than did secondary forest. Bat diversity was similar between secondary forest patches largely enclosed by rain forest and their controls, but higher diversity, richness, and contribution to total diversity were recorded in largely enclosed patches compared to partially enclosed patches. Partially enclosed patches were dominated by 2 small, frugivorous understory bat species (Carollia sowelli and Carollia perspicillata), whereas largely enclosed patches were dominated by 2 large-bodied, canopy-dwelling, frugivorous bats (Artibeus lituratus and Artibeus jamaicensis), which primarily feed on figs, a tree species that is abundant in rain forest. Bat diversity, richness, and contribution to total diversity were positively correlated with the proportion of area with rain forest, and bat abundance was negatively correlated with canopy openness

The role of local knowledge in determining shade composition of multistrata coffee systems in Chiapas, Mexico

This research explores interactions between farmers' knowledge and socioeconomic circumstances and the floristic composition of multistrata coffee plantations in Chiapas, Mexico. Interviews with 24 individual farmers with accompanying vegetation transects and two community level participatory workshops were carried out. The frequency, density, dominance, utility and importance value for all tree species surveyed were obtained. Farmers were grouped by cluster analysis on the basis of their land area, time producing coffee and the age of their coffee farms but the dominant shade species in their coffee plantations was not influenced by socioeconomic status (p<0.05). A total of 74 shade species were recorded and classified as temporary, suitable, or unsuitable as shade species by farmers, based on attributes such as leaf phenology, foliage density, crown shape and the amount and timing of litter decomposition, as well as their overall impact on coffee yield. Principal component and cluster analysis using these attributes confirmed the consistency of the farmers' classification system. A group of preferred species was identified, but less than half the trees recorded on farms were of these species, showing that farmers retained a wide range of trees and shrubs in their plantations, taking into account not only commercial interests but also their contribution to ecosystem functions. Farmers harnessed the forces of secondary succession by retaining pioneers as temporary shade, knowing that they would naturally be succeeded, while at the same time promoting and tolerating other longer living native species that they considered more suitable as coffee shade. Managing diverse secondary succession instead of establishing monospecific shade was an efficient way for farmers to achieve acceptable coffee yields while contributing to biodiversity and landscape conservation that could allow them access to niche markets. � 2006 Springer

Restoration of Forest Ecosystems in Fragmented Landscapes of Temperate and Montane Tropical Latin America

Temperate and tropical montane forests in Latin America represent a major natural resource at both regional and national levels for a number of reasons - biological, climatic, economic, cultural. Native tree species in these forests share conservation problems because of deforestation, habitat degradation, overall biodiversity loss and integrity of landscape structure. However, literature on forest restoration research and practices in these ecosystems is scanty and dispersed. We integrate forest restoration experiences aimed at a variety of purposes that allow us to gain insight over several years under contrasting ecological, social and economic conditions in six study regions: the Argentinian Andes, the IX and X Regions in Chile (including northern Chiloe Island), and central Veracruz and the central and northern Highlands of Chiapas (Mexico). By comparing analogous conditions and highlighting differences among the study sites, current pitfalls can be identified and used to define a minimum set of elements to be considered in a protocol for restoration practices. The restoration studies reviewed here include a wide variety of ecological and socio-economic circumstances that allow the identification of broad guidelines, criteria and indicators for planning, implementing and monitoring ecological restoration programmes. We conclude with statements that suggest approaches, strategies and concrete actions that might be considered as lessons learned and inputs for best practice in forest restoration research and programmes conducted in other developing regions