I tend to be pessimistic about the possibilities for ecosystem responses to global environmental changes that indicate how much we humans have screwed up the Earth, but this paper by Chave and colleagues perhaps provides some rays of sunlight. And not bad rays of sunlight, like the kinds that cause DNA damage in human skin and that destroy understory species when large old-growth trees are felled for lumber, but rather rays of hope about the adaptability of intact tropical forest communities.
Two important points should be made as background, before launching into the methods and data. First, forest ecosystems, which cover about 4 billion hectares across all latitudes, serve as major carbon sinks for the planet. The largest area of forests, about 40% of the worldwide total, is located in low latitudes, with more than half of these in tropical America (Dixon et al., 1994). Long-term monitoring of mature humid tropical forest plots in South America confirmed the contributions of these ecosystems to the global carbon balance, with 38 of 50 neotropical sites exhibiting biomass gains, with tree growth exceeding losses due to tree deaths (Phillips et al., 1998). Second, forest fragments are much more vulnerable to alterations in tree community composition than are intact forests, with loss of many large-seeded, slow-growing, and old-growth species, and a shift in tree size distributions (Laurance et al., 2006). Fragmented forest habitats are especially sensitive to disturbances from selective logging, fires, and other anthropogenic causes of tree mortality, and trees on the edges of such fragments are often subjected to abnormal drought conditions, and to increased wind damage.
Walking Palm, Socratea exorrhiza, Costa Rica
Photo: Jim F. Bleak, via Wikipedia
Chave and colleagues addressed questions about shifts in plant species and changes in aboveground biomass in tropical forests by repeatedly measuring over two million trees, in ten large (16-52 hectares each) plots. These plots were located on three different continents (America, Asia, and Africa), and differed from the study sites used by Laurance and colleagues (2006), in that they represented undisturbed large samples, rather than small habitat fragments. Chave and colleagues included both taxonomic identification and comparative species traits (e.g. seed size, growth rate, canopy species) in their repeated forest inventories. The researchers made over 5 million stem-diameter measurements between 1985 and 2005, and calculated “aboveground biomass” for each free-standing stem that was > 1 cm diameter at breast height. Wood density was taken into account in the biomass calculations, and tree species were grouped into quartiles according to their positions on a slow-growth/low-mortality to fast-growth/high-mortality axis.
Ceylon Ironwood, Mesua spp., an abundant canopy species that underwent an unexplained die-off at the Sinharaja, Sri Lanka study site.
Photo: Master Gardener website
Four of the ten plots showed a significant increase in aboveground biomass during the study period, while only one, in Sinharaja (Sri Lanka) showed a large decline in aboveground biomass, due to high mortality of the shade-tolerant Ceylon Ironwood. For three of the plots, measurements were made at intervals, and revealed fluctuations in aboveground biomass that probably reflect disturbances such as droughts and El Niño weather patterns. When slow-growing tree species were considered separately, there was a significant increase in biomass for five of the plots. Moreover, species with high wood density and large seed size increased in biomass at all study sites except for Sinharaja. In the discussion, the researchers emphasized that their data do not support the hypothesis that fast-growing species are increasing in dominance in low latitude tropical forests. The biomass increase for slow-growing species observed by this group may reflect both recovery from unknown past disturbances, and adaptive physiological changes in response to the changing environment. However, fragmented forest communities, and those subject to serious human interference, may not survive the challenges of global environmental changes in nitrogen deposition, atmospheric carbon dioxide concentration, temperature, and drought frequency.
Citation: Chave J, Condit R, Muller-Landau HC, Thomas SC, Ashton PS, et al. (2008) Assessing evidence for a pervasive alteration in tropical tree communities. PLoS Biol 6(3):e45.
Dixon, R.K., Brown, S., Houghton,R.A., Solomon, A.M., Trexler, M.C., and Wisniewski, J. (1994). Carbon pools and flux of global forest ecosystems. Science 263, 185-190.
Laurance, W.F., Nascimento, H.E.M., Laurance, S.G., et al. (2006). Rapid decay of tree-community composition in Amazonian forest fragments. Proc. Natl. Acad. Sci. USA 103(50), 19010-19014.
Phillips, O.L., Malhi, Y., Higuchi, N., et al. (1998). Changes in the carbon balance of tropical forests: evidence from long-term plots. Science 282, 439-442.
Chave, J., Condit, R., Muller-Landau, H.C., Thomas, S.C., Ashton, P.S., Bunyavejchewin, S., Co, L.L., Dattaraja, H.S., Davies, S.J., Esufali, S., Ewango, C.E., Feeley, K.J., Foster, R.B., Gunatilleke, N., Gunatilleke, S., Hall, P., Hart, T.B., HernÃ¡ndez, C., Hubbell, S.P., Itoh, A., Kiratiprayoon, S., LaFrankie, J.V., Loo de Lao, S., Makana, J., Noor, M.N., Kassim, A.R., Samper, C., Sukumar, R., Suresh, H.S., Tan, S., Thompson, J., Tongco, M.D., Valencia, R., Vallejo, M., Villa, G., Yamakura, T., Zimmerman, J.K., Losos, E.C. (2008). Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities. PLoS Biology, 6(3), e45. DOI: 10.1371/journal.pbio.0060045