Diversity patterns of herbaceous angiosperms along gradients of elevation and forest use intensity in Central Veracruz, Mexico

Abstract

Terrestrial herbs are an important element in tropical forests; however, there is a lack of research on their diversity patterns and how they respond to different forest use intensities. Studying the richness and distributional patterns of this group along elevational gradients is important in order to understand the general processes that influence this distribution. Previous investigations have been done on elevational gradients and the effect of land use intensity on plant diversity, however, relevant research on herbaceous angiosperms is still scarce compared to other groups of vascular plants, such as trees. In order to bridge this gap, I studied herbaceous angiosperm distributions along gradients of elevation and disturbance. Therefore, the study of an elevational gradient proposed in this work provides an opportunity to analyze distributional patterns of herbs in a tropical area with contrasting environmental conditions. It is important to notice that this is the first study of its class on Mexico. First, we analyzed species richness and floristic composition of herbaceous angiosperms at eight elevational sites (Chapter two). We compared those patterns with respect to land use intensities at Cofre de Perote, central Veracruz, Mexico. We established an elevational transect (40 to 3,520 m) where we recorded the occurrence of terrestrial angiosperm herbs within a total of 135 plots of 20 x 20 m. We compared species richness and floristic composition between the different elevational belts and degrees of forest disturbance. We recorded a total of 264 herb species, 31 endemic to Mexico and three classified as threatened. The total number of species represents 5.7% of species of the Veracruz´ herbaceous angiosperm flora. The elevational belts with highest species richness were 2,500 m (76) and 1,500 m (52). In most cases, secondary forests showed the highest species richness along the elevational gradient, whereas old-growth forests had fewer species. The observed species richness, including endemic elements, highlights the importance of plant conservation in areas threatened by land use changes. Additionally, we suggest that environmental heterogeneity formed by mature, disturbed and secondary forests is acceptable (and unavoidable) and can even increase species richness. Second, we described the influence of elevation and forest use intensity on alpha, beta, and gamma diversity along gradients of elevation (50 m to 3500 m) and human forest use intensity (Chapter three). We analyzed species richness and floristic composition in six vegetation belts at different elevations and in different habitats. We found some general elevational trends, such as the increase in α- diversity and βb-diversity and a decline in βw-diversity. Also, γ-diversity follows a hump-shaped pattern with a peak between 2500 m and 3000 m and a decrease at the extremes of the elevational gradient. There was no effect of forest use intensity on α-diversity, only β-diversity increased depending on the change of habitat with the highest values when old-growth forest are transformed into secondary forests. Therefore, a loss of a specific forest area is compensated by the occurrence of a similar assemblage at other areas of the same location. The observed high β-diversity, including endemic elements, suggests that a mix of different habitats is needed for reaching high γ-richness of terrestrial herbs. Third, we described the deforestation and fragmentation patterns of the study area during three different time periods (1993, 2000 and 2014; Chapter four). We quantified the net change in forest area and the loss of original forest area. Our results showed a reduction in the area of the original vegetation by about 57%. The annual net forest change rate for the first period (1993-2000) was -0.44%; and increased significantly to 0.11% for the second period (2000-2014). The old-growth forests of Central Veracruz have suffered high rates of land use change throughout the last twenty years with a reduction of this process during the last decade. In the first period, forest patches increased in irregularity, isolation and size with a reduction in the dimension of the fragments that represented the destruction of natural vegetation, whereas the second period experienced a slightly recovering of forest cover (0.11%). This chapter provides a thorough analysis of the forest transformations in Central Veracruz that will increase the consciousness of stakeholders for proper planning and management to maintain biological integrity of the area. Finally, I synthesize the principal findings of this thesis and highlight the implications for conservation. Also, I suggest potential topics be studied in the future with the data obtained, related with remaining forest and alpha and beta diversity patterns of endemic herbs (Chapter five).