Global warming is forcing many species to shift their distributions upward, causing consequent changes in the compositions of species that occur at specific locations. This prediction remains largely untested for tropical trees. Here we show, using a database of nearly 200 Andean forest plot inventories spread across more than 33.5° latitude (from 26.8° S to 7.1° N) and 3,000-m elevation (from 360 to 3,360 m above sea level), that tropical and subtropical tree communities are experiencing directional shifts in composition towards having greater relative abundances of species from lower, warmer elevations. Although this phenomenon of 'thermophilization' is widespread throughout the Andes, the rates of compositional change are not uniform across elevations. The observed heterogeneity in thermophilization rates is probably because of different warming rates and/or the presence of specialized tree communities at ecotones (that is, at the transitions between distinct habitats, such as at the timberline or at the base of the cloud forest). Understanding the factors that determine the directions and rates of compositional changes will enable us to better predict, and potentially mitigate, the effects of climate change on tropical forests.

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The dry forest of southern Ecuador and northern Perú (called the Tumbesian region) is known for its high diversity, endemism, and healthy conservation state. Nevertheless, the forest is exposed to many threats linked with human activities. To understand the effects of these threats, which have not been appropriately assessed, we pose two questions: (a) What are the diversity and structural situations of the forest? (b) Are anthropogenic activities affecting the composition and structure of the forest? The assessed factors were species richness, diversity, species similarity, abundance, and density. Forest information was obtained from 72 plots (total area 25.92 ha) randomly placed to cover a wide range of stand densities (from 200 to 1100 m a.s.l.). After constructing linear mixed models and selecting the most influential one, we determined the individual influences of 12 predictors. The human pressure index (HPI) was the most negative predictor of forest health, and annual precipitation was the most important abiotic predictor of good health conditions. Livestock grazing did not significantly change the diversity and structure of mature forest. The mean annual temperature and stoniness influenced only the basal area and number of individuals, respectively. The species composition in our study area was not affected by the HPI, but was strongly predicted by annual precipitation.

ResumenEl presente trabajo es un estudio de la flora de los páramos de pajonal seriales de distintos macizos montañosos de los Andes del Sur de Ecuador (provincias de Azuay y Loja) comprendidos entre los 2850 m y 3635 m. Fueron identificadas 43 familias de plantas vasculares, 120 géneros y 216 especies. La flora local es comparada con la de otros páramos de pajonal andinos; la riqueza florística (géneros y especies) es analizada por tramos altitudinales y se establece la flora característica de cada tramo del territorio estudiado. De forma complementaria, discutimos las influencias biogeográficas de la flora de los páramos de pajonal.Palabras clave: Andes, Biodiversidad, Biogeografía, Páramo, Riqueza florística. AbstractA study of the flora of the bunchgrass paramo with antropic origin of different mountain ranges from the Andean areas in southern Ecuador (provinces of Azuay and Loja) was conducted. The study areas range between the 2850 m and 3635 m of altitude. The local flora is composed by 43 families, 120 genera and 216 species of vascular plants; this particular flora is compared with that of other Andean bunchgrass paramos, the floristic richness of genus and species is studied by altitudinal levels (100 m), and the characteristic flora of each level of the area is established. Finally the biogeographic aspects of the flora are discussed.