Abstract
Surface fires burn extensive areas of tropical forests each year, altering resource availability, biotic interactions, and, ultimately, plant diversity. In transitional forest between the Brazilian cerrado (savanna) and high stature Amazon forest, we took advantage of a long-term fire experiment to establish a factorial study of the interactions between fire, nutrient availability, and herbivory on early plant regeneration. Overall, five annual burns reduced the number and diversity of regenerating stems. Community composition changed substantially after repeated fires, and species common in the cerrado became more abundant. The number of recruits and their diversity were reduced in the burned area, but burned plots closed to herbivores with nitrogen additions had a 14 % increase in recruitment. Diversity of recruits also increased up to 50 % in burned plots when nitrogen was added. Phosphorus additions were related to an increase in species evenness in burned plots open to herbivores. Herbivory reduced seedling survival overall and increased diversity in burned plots when nutrients were added. This last result supports our hypothesis that positive relationships between herbivore presence and diversity would be strongest in treatments that favor herbivory--in this case herbivory was higher in burned plots which were initially lower in diversity. Regenerating seedlings in less diverse plots were likely more apparent to herbivores, enabling increased herbivory and a stronger signal of negative density dependence. In contrast, herbivores generally decreased diversity in more species rich unburned plots. Although this study documents complex interactions between repeated burns, nutrients, and herbivory, it is clear that fire initiates a shift in the factors that are most important in determining the diversity and number of recruits. This change may have long-lasting effects as the forest progresses through succession.
Highlights
Tropical forests are extremely rich ecosystems in terms of their biodiversity and carbon stocks (Strassburg et al 2010)
Fire is naturally important in this region as it helps to maintain the cerrado ecosystem, but, with increasing demands for agricultural and pasture land, fire is used as a tool to remove unwanted biomass, and it often degrades forests via escaped fires (Nepstad et al 1999). This has increased fire frequencies in the southern Amazon above historical levels, during drought years (Bush et al 2008). These human impacts and a lack of protected areas combine to place the seasonally-dry forest of the southern Amazon among the most vulnerable of the Amazonian ecoregions (SoaresFilho et al 2006), because predictions for this area suggest temperature will increase and precipitation will decrease—a combination that will result in increased fire potential (Malhi et al 2008)
Among the many factors hypothesized to affect plant diversity, in this work we investigate the roles of disturbance, resource availability, and herbivory (Janzen 1970; Connell 1971; reviewed in Carson et al 2008)
Summary
Tropical forests are extremely rich ecosystems in terms of their biodiversity and carbon stocks (Strassburg et al 2010). Fire is naturally important in this region as it helps to maintain the cerrado ecosystem, but, with increasing demands for agricultural and pasture land, fire is used as a tool to remove unwanted biomass, and it often degrades forests via escaped fires (Nepstad et al 1999). This has increased fire frequencies in the southern Amazon above historical levels, during drought years (Bush et al 2008). The capacity for forest recovery after fire is an important topic for research and conservation in this area
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