Savannas cover approximately 30% of the earth land area, represent substantial carbon storage, and are home of 20% of a rapidly growing human population. Under scenarios of future climate change, and increasing atmospheric CO2 is thus important to understand the demographic processes determining tree cover. Mean annual precipitation is considered a critical factor settings limits to potential tree cover, while fire prevents this potential from being reached by preventing tree recruitment. We measured tree, mortality, recruitment and growth during 1989-1991 in Brazilian Cerrado (Neotropical savanna) protected from fire, in 50 (0.01 ha) subplots of a 20 m x 250 m transect in the Ecological Reserve of Instituto Brasileiro de Geografia Estatística, Brasília, DF, central Brazil. All plants with diameter at ground level (dgl) ≥ 5 cm were tagged, mapped, and identified.
Results/Conclusions
Mortality rate (≈ 2.5%yr-1) declined from 5 cm to about 18 cm dgl. At larger diameters, mortality was fairly constant. Overall, recruitment exceeded mortality, indicating a net increase in stem density. When analyzed by species, four of 25 species had greater recruitment than mortality. Across species, recruitment rate was positively correlated with maximum stem diameter and relative growth rate, perhaps because larger trees have access to soil deep water, and require longer times for reproduction and recruitment events. Published data of annual recruitment rate of trees (> 9 cm diameter) in sites ranging from 1000 mm – 1580 mm mean annual precipitation (MAP) was significantly higher in forest sites than in savanna sites (3.0%yr-1 vs. 1.7%yr-1, t = 3.07, P = 0.037). Using only sites between 1400 mm – 1600 mm of MAP the difference was not statistically significant (3.0%yr-1 vs 2.2%yr-1, t = 1.875, P = 0.201). We found also a relationship between mean annual precipitation and recruitment rate for trees ≥ 9 cm diameter (r2 = 0.46, P < 0.05). These results suggest that mesic savannas in cerrado (MAP> 1400 mm) are controlled by fire disturbance since herbivory by larger mammals is insignificant.
