A British Ecological Society Special Interest Group
From Dan Bebber
Have you seen the headlines? Looks like we might be in for a Big Climate Event this year, with potentially disastrous consequences for tropical forests.
Back in 1997 I packed my bags and flew off to the Danum Valley Field Centre to study the effects of herbivory on tree seedling growth and survival, as part of my doctoral thesis (1,2). I remember hearing reports of mass flowering of the Dipterocarpaceae, the dominant tree family of the lowland forests of Borneo, and thinking this was good news because there would be plenty of seedlings to measure and monitor. I found out later that the mast fruiting was a warning of an impending disaster, namely the strongest El Niño event (by some measures) of the 20th Century. Mast fruiting of dipterocarps appears to be triggered by temperature fluctuations accompanying changing oceanic and atmospheric currents in the Pacific – perhaps by a dip in night time temperatures (3) just before the heat and drought, or by the droughts themselves (4). The previous big El Niño occurred in 1982-3, which was the first such event reported extensively in the scientific literature (5), partly because remote sensing satellites were able to monitor the impacts on forests (6).
The effects of the 1997-8 drought and heatwave on Southeast Asian and Amazonian forests are well documented (7, 8). At Danum, for several months the usual afternoon rains were absent, the Segama river nearly dried up, and the leeches virtually disappeared (hooray!). The forest litter crackled underfoot, and seedlings withered in the understorey. Occasionally we smelled smoke on the breeze, but the extensive fires that affected forests across the region fortunately didn’t threaten. By the time my fieldwork ended, the Pacific had swung back into cold ‘La Niña’ mode, the rains returned with a vengance (as did the leeches), clothing rotted on the line, and roads and bridges were washed away by flooding rivers.
There appeared to be a clear trend of increasing frequency and intensity of El Niño events over the 20th Century (9), and the impression of an increasingly violent Pacific Ocean was reinforced by certain climate models that projected increasing amplitude of such oscillations driven by anthropogenic global warming (10). However, since 1997 the Pacific has been relatively quiet, although climate projections in CMIP5 appear to confirm increasing frequency of El Niño, at least in the medium term (11,12,13). There was a moderate warming in 2009-10, but nothing close to 1982 or 1997 levels. Last year, a new statistical model was used to forecast a major El Niño in late 2014 (14), but nothing happened. This year once again, the NOAA Climate Prediction Center multiple model forecast is suggesting that a major El Niño could be emerging, hence the news headlines. The NINO3.4 temperature anomaly is currently at +1.1C and many models are projecting a continuing increase in temperatures, with some heading beyond +2.0C.
What will be the consequences for forests? The clearest pattern to emerge from several decades of research is that when an El Niño drought occurs, secondary forests burn and primary forests don’t (15,16). Primary forests are cool, moist and dark while secondary forests are hot, dry and light (17). Logging adds fuel to the understorey, and gaps allow sunlight and wind to dry the litter to tinder. Human population density and activity is greater in secondary forests, and people deliberately light fires to clear land for agriculture. The first fire may be small, but it kills smaller trees with thinner bark and these become fuel for the next fire (18). In the Amazon and Southeast Asia, the fire return interval has declined from millenia to centuries to decades (19). Droughts alone can increase tree mortality, even in primary forests, releasing carbon to the atmosphere and potentially increasing forest susceptibility to further drought and fire (20,21).
So, keep your eyes on the Pacific, and we’ll see what 2015 has in store for the remaining forests of Southeast Asia and the Amazon.
Ps. If you have any ‘burning issues’ related to forest ecology to discuss, why not write a short piece for our blog?
1. Bebber D, Brown N, Speight M (2002) Drought and root herbivory in understorey Parashorea Kurz (Dipterocarpaceae) seedlings in Borneo. Journal of Tropical Ecology, 18, 795–804.
2. Bebber DP, Brown ND, Speight MR (2004) Dipterocarp seedling population dynamics in Bornean primary lowland forest during the 1997-8 El Niño-Southern Oscillation. Journal of Tropical Ecology, 20, 11–19.
3. Appanah S (1993) Mass flowering of dipterocarp forests in the aseasonal tropics. Journal of Biosciences, 18, 457–474.
4. Sakai S, Harrison RD, Momose K et al. (2006) Irregular droughts trigger mass flowering in aseasonal tropical forests in asia. American Journal of Botany, 93, 1134–1139.
5. Woods P (1989) Effects of Logging, Drought, and Fire on Structure and Composition of Tropical Forests in Sabah, Malaysia. Biotropica, 21, 290–298.
6. Malingreau JP, Stephens G, Fellows L (1985) Remote Sensing of Forest Fires: Kalimantan and North Borneo in 1982-83. Ambio, 14, 314–321.
7. Wooster MJ, Perry GLW, Zoumas A (2012) Fire, drought and El Niño relationships on Borneo (Southeast Asia) in the pre-MODIS era (1980–2000). Biogeosciences, 9, 317–340.
8. Aragão LEOC, Malhi Y, Roman-Cuesta RM, Saatchi S, Anderson LO, Shimabukuro YE (2007) Spatial patterns and fire response of recent Amazonian droughts. Geophysical Research Letters, 34, L07701.
9. NOAA (1998) The Top 10 El Niño Events of the 20th Century. Available at http://www.ncdc.noaa.gov/oa/climate/research/1998/enso/10elnino.html
10. Timmermann A, Oberhuber J, Bacher A, Esch M, Latif M, Roeckner E (1999) Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature, 398, 694–697.
11. Fountalis I, Bracco A, Dovrolis C (2014) ENSO in CMIP5 simulations: network connectivity from the recent past to the twenty-third century. Climate Dynamics, 45, 511–538.
12. Cai W, Borlace S, Lengaigne M et al. (2014) Increasing frequency of extreme El Nino events due to greenhouse warming. Nature Climate Change, 4, 111–116.
13. Kim ST, Cai W, Jin F-F, Santoso A, Wu L, Guilyardi E, An S-I (2014) Response of El Nino sea surface temperature variability to greenhouse warming. Nature Climate Change, 4, 786–790.
14. Ludescher J, Gozolchiani A, Bogachev MI, Bunde A, Havlin S, Schellnhuber HJ (2014) Very early warning of next El Niño. Proceedings of the National Academy of Sciences, 111, 2064–2066.
15. Gutiérrez-Vélez VH, Uriarte M, DeFries R et al. (2014) Land cover change interacts with drought severity to change fire regimes in Western Amazonia. Ecological Applications, 24, 1323–1340.
16. Siegert F, Ruecker G, Hinrichs A, Hoffmann AA (2001) Increased damage from fires in logged forests during droughts caused by El Niño. Nature, 414, 437–440.
17. Uhl C, Kauffman JB (1990) Deforestation, fire susceptibility, and potential tree responses to fire in the eastern Amazon. Ecology, 71, 437–449.
18. Cochrane MA, Alencar A, Schulze MD, Souza CM, Nepstad DC, Lefebvre P, Davidson EA (1999) Positive Feedbacks in the Fire Dynamic of Closed Canopy Tropical Forests. Science, 284, 1832–1835.
19. Mouillot F, Field CB (2005) Fire history and the global carbon budget: a 1°× 1° fire history reconstruction for the 20th century. Global Change Biology, 11, 398–420.
20. Doughty CE, Metcalfe DB, Girardin C a. J et al. (2015) Drought impact on forest carbon dynamics and fluxes in Amazonia. Nature, 519, 78–82.
21. Slik JWF (2004) El Niño droughts and their effects on tree species composition and diversity in tropical rain forests. Oecologia, 141, 114–120.