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Fire effects on the coexistence of trees and grasses in savannas and the resulting outcome on organic matter budget

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Factors affecting the tree/grass coexistence in savannas and consequences on the organic matter budgets were explored in this review In savannas water and nutrients are commonly limiting resources and consequently the tree and grasses are interacting. However this interaction is only apparent since the tree/grass competitors are coexisting in savanna vegetations which are characterized by a wide range of physiognomic types spanning herbaceous to woodland savannas. The tree/grass coexistence is evident in climatic and edaphic savannas whereas in humidic and mesic savannas the fire factor controls tree density and its competitor ability depending upon the accumulation of organic matter. In order to explain the tree/grass coexistence several hypotheses have been proposed An exclusion solution due to competitor superiority of grasses has been proposed for dry savannas. As the water supply increases the grasses coexist by a spatial partitioning of the soil water resources. This equilibrium solution occurs by niche separation. Without ruling out these possibilities there exist savannas where trees and grasses are competing by limiting resources. The competitive abilities of trees and grasses differed in response to environmental features; thus coexistence is associated with niche difference. In disturbance-prone savanna the mechanisms of the tree/grass coexistence for the competing species is based on a reduction in tree-grass competition due to control of tree density. Under savanna protection grasses are competitively excluded by trees as the life-span of individuals is affected by the competitive interactions between trees and trees as well as trees and grasses. The causal mechanisms allowing tree/grass coexistence in savannas were explored on the basis of the ecological hypothesis of system equilibrium or stability. The changes in the proportion of the tree/grass components affect the organic matter budget of the system. As a result of protection of the savannas vegetation is able to sequester significant quantities of carbon. The calculated sequestering pool of carbon in both the restored forest vegetation and soil would be 5.99 Pg C for a restoration time of 51 years. KEY WORDS / Neotropical Savannas / Fire/ Interactions /

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