Abrupt increases in Amazona tree mortality due to drought–fire interactions
Abrupt increases in Amazonian tree mortality due to drought–fire interactions
- Paulo Monteiro Brandoa,b,c,1,2,
- Jennifer K. Balchd,1,
- Daniel C. Nepstade,1,
- Douglas C. Mortonf,
- Francis E. Putzg,
- Michael T. Coeb,
- Divino Silvérioa,h,
- Marcia N. Macedob,
- Eric A. Davidsonb,
- Caroline C. Nóbregaa,i,
- Ane Alencara, and
- Britaldo S. Soares-Filhoj
Edited by Stephen W. Pacala, Princeton University, Princeton, NJ, and approved March 18, 2014 (received for review March 22, 2013)
Significance
Climate change alone is unlikely to drive severe tropical forest degradation in the next few decades, but an alternative process associated with severe weather and forest fires is already operating in southeastern Amazonia. Recent droughts caused greatly elevated fire-induced tree mortality in a fire experiment and widespread regional forest fires that burned 5–12% of southeastern Amazon forests. These results suggest that feedbacks between fires and extreme climatic conditions could increase the likelihood of an Amazon forest “dieback” in the near-term. To secure the integrity of seasonally dry Amazon forests, efforts to end deforestation must be accompanied by initiatives that reduce the accidental spread of land management fires into neighboring forest reserves and effectively suppress forest fires when they start.
Abstract
Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW⋅m−1). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change.
Footnotes
1P.M.B., J.K.B., and D.C.N. contributed equally to the work.
- 2To whom correspondence should be addressed. E-mail: pmbrando@ipam.org.br.
Author contributions: P.M.B., J.K.B., D.C.N., F.E.P., M.T.C., and E.A.D. designed research; P.M.B., J.K.B., D.C.N., M.T.C., and D.S. performed research; D.C.M., F.E.P., and M.N.M. contributed new reagents/analytic tools; P.M.B., J.K.B., D.C.M., D.S., M.N.M., C.C.N., A.A., and B.S.S.-F. analyzed data; and P.M.B., D.C.N., and F.E.P. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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