Testing the Amazon savannization hypothesis: fire effects on invasion of a neotropical forest by native cerrado and exotic pasture grasses

Divino V. Silvrio Paulo M. Brando Jennifer K. Balch Francis E. Putz Daniel C. Nepstad Claudinei Oliveira-Santos Mercedes M. C. Bustamante 0 Woods Hole Research Center , 149 Woods Hole Road, Falmouth, MA 02450 , USA 1 Instituto de Pesquisa Ambiental da Amazonia , Forest Ecology Bele m, Bele m, Para , Brazil 2 Departamento de Ecologia Bras lia, Universidade de Bras lia , Brasilia, Distrito Federal , Brazil 3 Department of Biology, University of Florida , Gainesville, FL 32611-8525 , USA 4 Department of Geography, The Pennsylvania State University , Walker Building, University Park, PA 16802 , USA 5 Department of Global Ecology, Carnegie Institution , Stanford, CA 94305-4101 , USA Testing the Amazon savannization hypothesis: fire effects on invasion of a neotropical forest by native cerrado and exotic pasture grasses Articles on similar topics can be found in the following collections ecology (543 articles) environmental science (287 articles) plant science (94 articles) Receive free email alerts when new articles cite this article - sign up in the box at the top right-hand corner of the article or click here - Supplementary data References Subject collections Email alerting service rstb.royalsocietypublishing.org Research Cite this article: Silverio DV, Brando PM, Balch JK, Putz FE, Nepstad DC, Oliveira-Santos C, Bustamante MMC. 2013 Testing the Amazon savannization hypothesis: fire effects on invasion of a neotropical forest by native cerrado and exotic pasture grasses. Phil Trans R Soc B 368: 20120427. http://dx.doi.org/10.1098/rstb.2012.0427 One contribution of 18 to a Theme Issue Ecology, economy, and management of an agroindustrial frontier landscape in the southeast Amazon. Subject Areas: ecology, environmental science, plant science Author for correspondence: Divino V. Silverio e-mail: Electronic supplementary material is available at http://dx.doi.org/10.1098/rstb.2012.0427 or via http://rstb.royalsocietypublishing.org. Testing the Amazon savannization hypothesis: fire effects on invasion of a neotropical forest by native cerrado and exotic pasture grasses Divino V. Silverio1,2, Paulo M. Brando2,3,4, Jennifer K. Balch5, Francis E. Putz6, Daniel C. Nepstad2, Claudinei Oliveira-Santos2 and Mercedes M. C. Bustamante1 Changes in climate and land use that interact synergistically to increase fire frequencies and intensities in tropical regions are predicted to drive forests to new grass-dominated stable states. To reveal the mechanisms for such a transition, we established 50 ha plots in a transitional forest in the southwestern Brazilian Amazon to different fire treatments (unburned, burned annually (B1yr) or at 3-year intervals (B3yr)). Over an 8-year period since the commencement of these treatments, we documented: (i) the annual rate of pasture and native grass invasion in response to increasing fire frequency; (ii) the establishment of Brachiaria decumbens (an African C4 grass) as a function of decreasing canopy cover and (iii) the effects of grass fine fuel on fire intensity. Grasses invaded approximately 200 m from the edge into the interiors of burned plots (B1yr: 4.31 ha; B3yr: 4.96 ha) but invaded less than 10 m into the unburned plot (0.33 ha). The probability of B. decumbens establishment increased with seed availability and decreased with leaf area index. Fine fuel loads along the forest edge were more than three times higher in grass-dominated areas, which resulted in especially intense fires. Our results indicate that synergies between fires and invasive C4 grasses jeopardize the future of tropical forests. 1. Introduction In many parts of the world, tropical forest savannah boundaries shift in response to changing climate and disturbance regimes [1,2]. In the southern Amazon Basin, forest is predicted to retreat owing to climate change [3,4] and land-use practices [5,6], which may also facilitate grass invasion and increase both the frequencies and intensities of wildfires. Although some dynamic global vegetation models predict a late-century Amazonian forest dieback [7,8], an integrated view of how this process will occur is still lacking. One potent driver could be the increased forest edge invasion of exotic grasses that accompanies pasture expansion [5]. In addition, fires reportedly interact with grass invasion through a positive feedback cycle, which causes a decline in tree cover, facilitates grass invasion and increases the likelihood of future fires [9 11]. Grass fire cycles are important on many forest frontiers owing to the combination of increased ignitions, drier forest edges and proliferation of flammable species. For example, several studies document that establishment of invasive grasses, which benefits from reduced tree cover [6,12], increases fine fuel loads, fire intensities [13,14] and grass expansion [6,12 14]. Moreover, frequently burned forests lose carbon storage capacity and may remain in a & 2013 The Author(s) Published by the Royal Society. All rights reserved. BARE SEED control LAI m2 m2 degraded, low-carbon forest state in which they are susceptible to recurrent fires [15]. There is a wealth of knowledge about forest savannah boundary dynamics around the world and about how introduced grasses alter vegetation and help create new fire cycles [1,12,13,16]. However, the mechanisms by which grasses expand at the expense of forests are less well understood (but see [11]). In the southeastern Amazon, agricultural expansion [17], selective logging [18] and other land uses may accelerate the rate of reduction of forest cover and enhance the fire grass cycle by: (i) reducing tree cover and exposing vulnerable forest edges [10,19 21]; (ii) introducing propagules of exotic pasture grasses [11,22]; and (iii) increasing ignitions associated with land management practices [18,23]. Although the importance of fire grass feedbacks for tropical forests and savannahs is well established [6,9 14], little is known about the mechanisms by which grasses invade forests. In particular, the species composition, extent and fire-related traits that facilitate this process along the substantial amount of degraded forest edges created annually by deforestation need to be documented. In this study, we experimentally evaluated how two fire frequencies (an annual and triennial burn) and the resulting differences in canopy cover interact with grass invasion to change fire behaviour. Specifically, we tested the predictions that: (i) native cerrado (Brazilian savannah) and exotic pasture grass invasion of forests from the edge increases with increases in fire frequency and associated increases in light availability; (ii) the establishment of the exotic grass Brachiaria decumbens from seeds experimentally sown in the forest interior (250 m from forest edges) increases with canopy openness; and (iii) the presence of exotic grasses increases forest fire intensity. 2. Material and methods (a) Study area In 2004, we established a (...truncated)