O papel do fogo e de recursos do solo na estrutura e nas dinâmicas de transição de floresta-savana

Author(s)

Paulo Negri Bernardino

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Biologia
• Defense date: 2016-10-14

Examining board members

Rafael Silva Oliveira; Marina Hirota; Flaviana Maluf de Souza

• Advisor: Rafael Silva Oliveira; Vinícius de Lima Dantas

Abstract

Forests and savannas have been suggested to be alternative vegetation states under certain precipitation regimes and soil conditions. A series of feedbacks are responsible for maintaining each state, including a fire-vegetation feedback, which is of remarkable importance to maintain the savanna state in wet tropical regions worldwide. In closed canopy forests, the very shaded understory inhibits the growth of shade-intolerant C4 grasses while creating a moist microclimate, which prevents the occurrence of frequent and intense fires, permitting the persistence of fire-susceptible species. In open canopy savannas, the high light availability in the system promotes high levels of C4 grass biomass, which is fuel for fire, favoring the occurrence of frequent and intense fires, which prevent canopy closure and promote the persistence of fire-tolerant species. Besides fire, soil properties and some disturbances frequently affect forests and savannas, and their transition dynamics as well. Soil resource availability, for instance, can exert an effect on tree growth rate. This effect affects the time required for the community to reach a closure level that will result in the suppression of shade-intolerant species and fire inhibition. This work aimed to evaluate how the factors influencing forest-savanna transitions (i.e. fire activity, resources availability and their interactions) affect their dynamics. To evaluate such influences, we analyzed variations of the vegetation functional traits at the plant communities distributed along a forest-savanna gradient, and compared them between landscapes with distinct fire regimes and soil resource availability. We found strong evidence that distinct fire activity is inducing differences in savanna structure, and that soil resources may have an indirect role, by modulating fire through C4 grass productivity. Our results also indicated that different fire activity may be provoking distinction in the relationship of community densification and functional thresholds between savannas and forests. At the landscape that burned more often, the functional threshold occurred at more open communities. The magnitude of the functional traits change between forests and savannas was also larger in sites with more frequent fire activity and more marked differences in soil resources of each vegetation state. The landscape with longer fire-free intervals and little difference in soil resources between forests and savannas presented shorter magnitudes, suggesting that both fire activity and soil properties are acting together to smoothen the transition. Our study evidenced the importance of both fire and soil to forest-savanna transitions at the landscape scale. Fire acts directly and strongly, resulting in more open savannas and higher differences in the functional traits between forests and savannas, where it occurs more frequently. More marked differences in soil resource availability between forests and savannas within a landscape result in more accentuated differences in the functional traits of the communities at each state. Each one alone may affect community structure and dynamics, and their interactions may accentuate or mitigate the transition dynamic (AU)

• FAPESP's process: 14/06100-0 - Ecological processes driving biome boundary transitions in a multi- scale perspective
• Grantee: Paulo Negri Bernardino
• Support Opportunities: Scholarships in Brazil - Master