Dynamics of the Tropical Tree Poulsenia armata in the Fragmented Forest of Los Tuxtlas

Lowland tropical forests are of special concern for they are the most diverse and the most ecologically complex of all terrestrial ecosystems (Myers 1984), and their destruction would significantly reduce global biological diversity (Turner & Corlett 1996). Forests around the globe are experiencing unprecedented rates of deforestation as a result of burgeoning human populations, with tropical countries having the highest human population growth rates (Laurance 1999). By 1990, almost 24% of the worldwide lowland tropical rainforest was lost, with considerable variation among regions at the level of disturbance (Turner & Corlett 1996). Areas experiencing a dramatic loss of forest cover include the African forests of Madagascar (Smith 1997), Nigeria and the Ivory Coast (Boahene 1998), which possess the highest rates of deforestation. The Asian forests of New Guinea still retain large tracts of original forest but are experiencing rapid conversion due to an intensification of human land use (Myers 1994). In the Neotropics, rapid forest deforestation occurs mainly in Central America, the Caribbean and the southeastern Amazon forest (Laurance 1999).;Habitat loss inevitably leads to forest fragmentation (Laurance & Bierregaard 1997), resulting in the reduction of a continuous forest into several small isolated remnant patches (Fahrig 2003). Island biogeography theory (MacArthur & Wilson 1967) predicts that smaller and more isolated habitats will experience reduced species richness and abundance. Additionally, habitat fragmentation leads to the creation of edges between the forest and a surrounding matrix composed mainly of human-made vegetation (e.g. pastures and agricultural fields) (Fahrig 2003). Reduced humidity, elevated temperatures, low relative humidity, low soil moisture, increased wind turbulence and increased luminosity have been described as potential microclimatic conditions affecting fragmented plant populations (Kapos 1989, Laurance 1991). However, vulnerability of the remaining plant species to forest fragmentation depends on several factors that range from differences in plant life history to modification of plant-animal interactions.;Forest fragmentation may affect plant populations through the modification of plantanimal interaction such as pollination and seed dispersal. Several studies have documented reduced fruit production (Alvarez-Buylla et al. 1996) and decreased genetic diversity (Young et al. 1996, Aguilar et al. 2006, Kramer et al. 2008) in forest fragments, due to reduced pollination (Aizen & Feinsinger 1994, Aguirre et al. 2011) or changes in abiotic conditions. Additionally, many tropical frugivores are at risk of becoming locally extinct due to habitat loss and fragmentation (Peres 2001, Wright & Duber 2001). Decreased abundance of frugivore species may reduce seed dispersal in forest fragments, leading to a magnification of the negative results of limited dispersal (e.g. density dependence mortality) and reducing seedling recruitment (Santos & Telleria 1994, Cordeiro & Howe 2001, 2003). But not all plant and animal species are negatively affected by forest fragmentation.;Forest fragmentation could have positive effects for certain species. Many studies have documented changes in patterns of regeneration of forest fragments with increased colonization of weedy species (Janzen 1983), such as lianas and vines (Laurance 1991, Laurance & Bierregaard 1997, Laurance et al. 1997) and early-successional tree species, as altered microclimatic conditions favor germination and recruitment of these plant species (Laurance et al. 1998b). Moreover, as human impact progresses, small mammals may remain unaffected or even increase in abundance due to an ecological release in the absence of top competitors (Terborgh et al. 2001, Wright 2003, Dirzo et al. 2007). This in turn, could lead to an increase in seed removal rates for small-seeded plants, due to a competitive release of the unpreferred game.;In this context, I documented the dynamics of a late-succesional tropical tree, Poulsenia armata (Moraceae), in the severely fragmented forest of Los Tuxtlas, Veracruz, Mexico. This is an area of great relevance as it represents the northern most limit of tropical rainforest in the New World and is a highly diverse hotspot combining elements of tropical and temperate origins (Dirzo & Miranda 1990b). While many studies have documented the effects of forest fragmentation on the fauna of Los Tuxtlas (Estrada & Coates-Estrada 1988, Dirzo & Miranda 1990b, Estrada et al. 1993a, Estrada et al. 1994, Estrada & Coates-Estrada 1996, Estrada et al. 1997, Estrada & Coates-Estrada 2002, Cristobal-Azkarate & Arroyo-Rodriguez 2007, Dirzo et al. 2007, Arroyo-Rodriguez et al. 2008, Aguirre et al. 2011), far fewer studies have addressed the implications on the flora (Arroyo-Rodriguez & Mandujano 2006b, Arroyo-Rodriguez et al. 2007, Arroyo-Rodriguez et al. 2009). I examined the impact of forest fragmentation on the dynamics of P. armata by comparing reproductive success and subsequent offspring performance (Chapter 2) in a continuous forest and forest fragments at Los Tuxtlas. I considered whether trees in fragmented forest traded quality for quantity (lower mass and higher C-to-Nratios than continuous forest counterparts), resulting in lower germination success and offspring fitness. I also observed seedling and juvenile recruitment patterns (Chapter 3) and examined seed predation and fruit removal (Chapter 4) of P. armata in a continuous forest and forest fragments. Reduced juvenile recruitment may affect the regeneration of P. armata, jeopardizing persistence of fragmented populations of this widespread rainforest tree. Moreover, differences in seed predation and fruit removal likely reflect differences in mammal communities in forest fragments. Finally, to test if forest fragmentation resulted in declines in populations, I compared population growth rates of P. armata in the continuous and fragmented forests with the use on Integral Population Models (Chapter 5). Results of this study will significantly contribute to the knowledge of the effects of forest fragmentation on plants populations at Los Tuxtlas forest.