Can savannas become forests? A coupled analysis of nutrient stocks and fire thresholds in central Brazil
- 1.1k Downloads
The effects of fire ensure that large areas of the seasonal tropics are maintained as savannas. The advance of forests into these areas depends on shifts in species composition and the presence of sufficient nutrients. Predicting such transitions, however, is difficult due to a poor understanding of the nutrient stocks required for different combinations of species to resist and suppress fires.
We compare the amounts of nutrients required by congeneric savanna and forest trees to reach two thresholds of establishment and maintenance: that of fire resistance, after which individual trees are large enough to survive fires, and that of fire suppression, after which the collective tree canopy is dense enough to minimize understory growth, thereby arresting the spread of fire. We further calculate the arboreal and soil nutrient stocks of savannas, to determine if these are sufficient to support the expansion of forests following initial establishment.
Forest species require a larger nutrient supply to resist fires than savanna species, which are better able to reach a fire-resistant size under nutrient limitation. However, forest species require a lower nutrient supply to attain closed canopies and suppress fires; therefore, the ingression of forest trees into savannas facilitates the transition to forest. Savannas have sufficient N, K, and Mg, but require additional P and Ca to build high-biomass forests and allow full forest expansion following establishment.
Tradeoffs between nutrient requirements and adaptations to fire reinforce savanna and forest as alternate stable states, explaining the long-term persistence of vegetation mosaics in the seasonal tropics. Low-fertility limits the advance of forests into savannas, but the ingression of forest species favors the formation of non-flammable states, increasing fertility and promoting forest expansion.
KeywordsCerrado Ecosystem dynamics Fire Forest expansion Nutrient cycling Soil-plant interactions Succession Tradeoffs Tropics
We thank the staff of RECOR-IBGE, CIPLAN, and Embrapa Cerrados, for the research infrastructure and logistic support. We also thank Ricardo Haidar, Gabriel Damasco, Daniel Marra, Gabriel Ribeiro, and Artur Paiva, for help with field work and species identification, and Timothy Doane and three anonimous reviewers for valuable comments on the manuscript. This research is based upon work supported by the National Science Foundation Grant No. DEB-0542912 (W. H.), AW Mellon Foundation (W. H.), National Science Foundation Grant No. EAR-BE-332051 (L. S.,M. H., F. M.-W., A. F.), and the J. G. Boswell Endowed Chair in Soil Science.
- Allen S, Grimshaw H, Parkinson J, Quarmby C (1974) Chemical analysis of ecological materials. Blackwell Scientific, OxfordGoogle Scholar
- Behling H, Pillar V, Bauermann S (2005) Late Quaternary grassland (Campos), gallery forest, fire and climate dynamics, studied by pollen, charcoal and multivariate analysis of the Sao Francisco de Assis core in western Rio Grande do Sul (southern Brazil). Rev Palaeobot Palynol 133:235–248CrossRefGoogle Scholar
- Bremner JM, Mulvaney CS (1982) Nitrogen total. In: Page AL, Miller RH, Keeney DR (eds) Methods of soil analysis: Chemical and microbiological properties, Vol. 2, 2nd edn. American Society of Agronomy, Madison, pp 595–624Google Scholar
- Brown S, Gillespie AJR, Lugo AE (1989) Biomass estimation methods for tropical forests with applications to forest inventory data. For Sci 35:881–902Google Scholar
- Durigan G, Ratter JA (2006) Successional changes in cerrado and cerrado/forest ecotonal vegetation in Western São Paulo State, Brazil, 1962–2000. Edinburgh Journal of Botany 63:119. Cambridge University PressGoogle Scholar
- Furley PA (1999) The nature and diversity of neotropical savanna vegetation with particular reference to the Brazilian cerrados. Glob Ecol Biogeogr 8:223–241Google Scholar
- Furley PA, Proctor J, Ratter JA (1992) Nature and dynamics of forest-savanna boundaries. Chapman & Hall, London, p 616Google Scholar
- Gottsberger G, Silberbauer-Gottsberger I (2006) Other vegetation types in the Cerrado region and transitional forms. In: Gottsberger G, Silberbauer-Gottsberger I (eds) Life in the cerrado: A South American tropical seasonal ecosystem. Vol I Origin, structure, dynamics and plant use. Reta Verlag, Ulm, pp 62–75Google Scholar
- Haidar RF (2008) Fitiossociologia, diversidade e sua relação com variáveis ambientais em florestas estacionais do bioma Cerrado no planalto central e nordeste do Brasil. Dissertation, Universidade de BrasiliaGoogle Scholar
- Kellman M (1984) Synergistic relationships between fire and low soil fertility in Neotropical Savannas: A hypothesis. Oecologia 16:158–160Google Scholar
- Mendonça RC, Felfili JM, Walter BMT, Silva-Júnior MC, Rezende AB, Filgueiras TS, Nogueira PE, Fagg CW (2008) Flora Vascular do Bioma Cerrado: Checklist com 12.356 espécies. In: Sano SM, Almeida SP, Ribeiro JF (eds) Cerrado: Ecologia e Flora, vol 2. Brasília, Embrapa Cerrados, pp 213–228Google Scholar
- Ribeiro JF, Haridasan M (1984) Comparação fitossociológica de um cerrado denso e um cerradão em solos distróficos no Distrito Federal. In: Congresso nacional de botanica 35, Manaus, AM, BrazilGoogle Scholar
- Rice C, Moorman T, Beare M (1996) Role of microbial biomass carbon and nitrogen in soil quality. SSSA Spec Publ 49:203–216Google Scholar
- Rossatto DR, Hoffmann WA, Silva LCR, Haridasan M, Sternberg LSL, and Franco AC (2013) Seasonal variation in leaf traits between congeneric savanna and forest trees in Central Brazil: implications for forest expansion into savanna. Trees. doi: 10.1007/s00468-013-0864-2
- Sankaran M, Hanan NP, Scholes RJ, Ratnam J, Augustine DJ, Cade BS, Gignoux J, Higgins SI, Le Roux X, Ludwig F, Ardo J, Banyikwa F, Bronn A, Bucini G, Caylor KK, Coughenour MB, Diouf A, Ekaya W, Feral CJ, February EC, Frost PGH, Hiernaux P, Hrabar H, Metzger KL, Prins HHT, Ringrose S, Sea W, Tews J, Worden J, Zambatis N (2005) Determinants of woody cover in African savannas. Nature 438:846–849CrossRefPubMedGoogle Scholar
- Silva Júnior MC (2005) Fitossociologia e estrutura diamétrica na mata de galleria do Pitoco, na Reserva Ecologica do IBGE, DF. Cerne 11:147–158Google Scholar
- Silva JSO, Haridasan M (2007) Acúmulo de biomassa aérea e concentração de nutrientes em Melinis minutiflora P. Beauv. e gramíneas nativas do cerrado. Rev Bras Bot 30:337–344Google Scholar