Climate change increases threat to plant diversity in tropical forests of Central America and southern Mexico.

Global biodiversity is negatively affected by anthropogenic climate change. As species distributions shift due to increasing temperatures and precipitation fluctuations, many species face the risk of extinction. In this study, we explore the expected trend for plant species distributions in Central America and southern Mexico under two alternative Representative Concentration Pathways (RCPs) portraying moderate (RCP4.5) and severe (RCP8.5) increases in greenhouse gas emissions, combined with two species dispersal assumptions (limited and unlimited), for the 2061-2080 climate forecast. Using an ensemble approach employing three techniques to generate species distribution models, we classified 1924 plant species from the region's (sub)tropical forests according to IUCN Red List categories. To infer the spatial and taxonomic distribution of species' vulnerability under each scenario, we calculated the proportion of species in a threat category (Vulnerable, Endangered, Critically Endangered) at a pixel resolution of 30 arc seconds and by family. Our results show a high proportion (58-67%) of threatened species among the four experimental scenarios, with the highest proportion under RCP8.5 and limited dispersal. Threatened species were concentrated in montane areas and avoided lowland areas where conditions are likely to be increasingly inhospitable. Annual precipitation and diurnal temperature range were the main drivers of species' relative vulnerability. Our approach identifies strategic montane areas and taxa of conservation concern that merit urgent inclusion in management plans to improve climatic resilience in the Mesoamerican biodiversity hotspot. Such information is necessary to develop policies that prioritize vulnerable elements and mitigate threats to biodiversity under climate change.

Floristic composition, structure and environmental characterization of Cyathea costaricensis population in a remnant cloud forest in Mexico / Composición florística, estructura y caracterización ambiental de una población de Cyathea costaricensis en un relicto de bosque mesófilo de montaña en México

Abstract Introduction: Tree ferns are significant components of temperate, tropical and subtropical forests, contributing to shape complex forest stand structures. Objectives: 1) to describe the population structure of Cyathea costaricensis in a remnant cloud forest of West-central Mexico; 2) to characterize and relate the floristic composition and the structure of the most important tree species associated to the C. costaricensis population and; 3) to describe the environment where C. costaricensis occurs. Methods: We estimated the Importance Value Index (IVI) to select the most important canopy-dominant species associated to C. costaricensis; we constructed height and Diameter at Breast Height (DBH) frequency distributions for those selected species according to IVI as well as for C. costaricensis population; we computed the asymmetry of the frequency distributions through the coefficient of skewness and the probability density function via the Kernel density estimation. We tested for differences between canopy-dominant tree species and C. costaricensis population structure by the non-parametric Wilcoxon rank sum test. Results: C. costaricensis individuals presented the smallest heights and intermediate DBH sizes as compared with the canopy-dominant species, with statistically significant differences for height but not for DBH according to the Wilcoxon test. Most of the tree fern individuals were located in uneven terrains and over the base slope of the terrain; canopy openness and Total Radiation Under the Canopy values were similar to those reported for Cyathea species elsewhere. Conclusions: We confirm the hypothesis of comparable structure between the canopy-dominant species and the C. costaricensis population only for DBH; on the contrary, for trunk height, there were statistically significant differences; the small heights of C. costaricensis suggest their coexistence in the understory through sheltering from the taller canopy-dominants. Mostly all individuals of C. costaricensis were confined to local environmental conditions, particularly to physiography.

Resumen Introducción: Los helechos arborescentes son componentes importantes de los bosques templados, tropicales y subtropicales, que contribuyen a dar forma a rodales de estructuras complejas. Objetivos: 1) Describir la estructura poblacional de Cyathea costaricensis en un remanente de bosque nuboso del centro-oeste de México; 2) Caracterizar y relacionar la composición florística y la estructura de las especies arbóreas más importantes asociadas a la población de C. costaricensis y; 3) Describir el ambiente donde se encuentra C. costaricensis. Métodos: Estimamos el Índice de Valor de Importancia (IVI) para seleccionar las especies dominantes más importantes del dosel asociadas a C. costaricensis; para las especies seleccionadas según IVI, construimos distribuciones de frecuencia de alturas y diámetros a la altura del pecho (DAP), así como para la población de C. costaricensis. Calculamos la asimetría de las distribuciones de frecuencia a través del coeficiente de asimetría y la función de densidad de probabilidad mediante la estimación de densidad de Kernel. Probamos las diferencias entre las especies de árboles dominantes en el dosel y la estructura de la población de C. costaricensis mediante la prueba no paramétrica de suma de rangos de Wilcoxon. Resultados: los individuos de C. costaricensis presentaron las menores alturas y tamaños intermedios de DAP en comparación con las especies dominantes del dosel, con diferencias estadísticamente significativas para la altura, pero no para el DAP según la prueba de Wilcoxon. La mayoría de los individuos de helechos arborescentes se ubicaron en terrenos irregulares y sobre la pendiente baja del terreno. Los valores de apertura del dosel y Radiación total bajo el dosel fueron similares a los reportados para las especies de Cyathea en otros lugares. Conclusiones: Confirmamos la hipótesis de que hay una estructura similar entre las especies dominantes del dosel y la población de C. costaricensis solo para el DAP; por el contrario, para la altura del fuste, hubo diferencias estadísticamente significativas; las pequeñas alturas de C. costaricensis sugieren su coexistencia en el sotobosque a través de la cobertura árboles dominantes del dosel. La mayoría de los individuos de C. costaricensis fueron encontrados confinados a condiciones ambientales locales, en particular a la fisiografía.

4200 years of pine-dominated upland forest dynamics in west-central Mexico: human or natural legacy?

The pine-dominated forests of west-central Mexico are internationally recognized for their high biodiversity, and some areas are protected through various conservation measures including prohibition of human activity. In this region, however, there is evidence for human settlement dating back to ca. AD 1200. It is therefore unclear whether the present forest composition and structure are part of a successional stage following use by indigenous human populations during the past, or due to natural processes, such as climate. We present a study reconstructing the vegetation dynamics of pine-dominated forest over the past 4200 years using paleoecological techniques. Results from fossil pollen and charcoal indicate that, in this region, pine-dominated forests are the native vegetation type and not anthropogenically derived secondary succession. The predominant driving mechanism for the expansion of pine-dominated forest appears to be intervals of aridity and naturally induced burning. A close association is noted between pine abundance and longer-term climatic trends, including intervals of aridity between ca. 4200 and 2500, 1200 and 850, and 500 and 200 cal yr BP and shorter-term trends. Evident periodicity occurs in pine and Poaceae abundance every 80 years. These short-term quasi-periodic oscillations have been recorded in a number of lake and ocean sediments in Mexico and are thought to be linked to solar forcing resulting in drought cycles that occur at approximately the same time intervals.