March 22, 2012

Tropical Forests Rejoice!

Filed under: Adaptation, Plants

When was the last time you heard good news about our tropical forests? Well, that’s just too long.

All we ever seem to hear about the tropical forests is that they are being destroyed, their destruction will exacerbate global warming, and on and on. You will even discover that some scientists think global warming destroyed the first tropical forests that evolved on our planet bringing rise to the dinosaurs! So it’s high time for some good news and World Climate Report is at your service!

A recent article in Landscape Ecology caught our eye with the title “Has global environmental change caused monsoon rainforests to expand in the Australian monsoon tropics?” Is someone really suggesting that global environmental change is causing rainforests to expand? We knew we would really like this one!

The article was produced by a team of three scientists with various agencies in Australia, and the work was supported financially by Australian Research Council and Kakadu National Park. Bowman et al. note that changes have been occurring in the northern Australian home of the rainforest resulting in an expansion of the forest. The expansion is well-documented and a subject of great interest. In trying to explain the expansion, Bowman et al. report “Average yearly rainfall has shown an increasing trend in northern Australia over the last century. Between 1910 and 1995 total annual rainfall in the Northern Territory rose by 15–18%, with the increasing trend considerably steeper for the second half of the 20th century. There was also an almost 20% increase in the number of rain days between 1910 and 1995”. They suggest “the observed expansion of rainforest boundaries throughout northern Australia may be driven by a wetting trend.” While it is tempting to speculate that the buildup of carbon dioxide (CO2) may have been indirectly beneficial to the rainforest by causing the increase in rainfall, we wonder whether the elevated CO2 may have had more of a direct impact on the rainforest?

Bowman et al. help us out here:

“[A]tmospheric CO2 concentration has been rapidly increasing for the last two centuries, with the increase dramatically accelerating in the 20th century. Numerous controlled experiments have frequently shown an increase in tree growth rates under elevated CO2 due to the CO2 ‘fertilisation effect’, including species of trees from Australian monsoon rainforest. Elevated atmospheric CO2 has the potential to shift forest-savanna boundaries as it advantages trees and shrubs (C3 photosynthetic pathway) over (predominantly C4) tropical grasses with the consequence of changing the strongly contrasting fuel types between rainforest and savanna (grass vs. leaf litter). Thus, fire regimes may also change because woody plants can suppress grassy fuels, reversing the well known positive feedback between fire and grass cover. Elevated CO2 may also indirectly increase growth by improving plant water status. Thus, rising atmospheric CO2 may be the primary driver of the widespread woody vegetation expansion.”

Bowman et al. go on to explain:

“There is emerging evidence of profound changes to the structure and function of tropical rainforests, both in northern Australia and elsewhere. The expansion of northern Australian monsoon rainforests parallels reports of expansion of tropical rainforest on the east coast of Australia and increased tree growth and biomass accumulation in tropical rainforests elsewhere in the world”.

Can you believe this talk about rainforests expanding throughout the world? They conclude:

“We consider it most likely that the expansion of rainforest patches is related to global climate change via increased rainfall and/or the CO2 ‘fertiliser effect’.”


But not all tropical trees grow in rainforests where precipitation is plentiful; there are tropical dry forests where the trees are forced to cope with periods of low rainfall and even extended periods of drought. Such a forest exists in southern Mexico where large amounts of rainfall occur during the summer but only a few inches of rain occur from November to May. One of the common trees there is Mimosa acantholoba which is a deciduous tree that forms distinctive annual growth rings. Most of these trees are indeed in southern Mexico, but it is found in forests extending south into Ecuador (Figure 1).

Figure 1. Geographical range of Mimosa acantholoba.

An article appeared recently in the journal Trees with a title suggesting that these tropical dry forest plants have improved their water use efficiency and their responses to drought – sounded interesting to us. We definitely became interested when we saw “CO2” appearing six times in the abstract, including the first and last sentences.

The three authors are from Mexico and Austria; they acknowledge financial support for their work from agencies in both Mexico and Austria. Brienen et al. begin noting that “Understanding the responses of tropical trees to increasing [CO2] and climate change is important as tropical forests play an important role in carbon and hydrological cycles.” They define several key terms including water use efficiency which is the ratio of photosynthesis to transpiration; the ratio is basically the amount of water used per carbon gain by the trees. As fate would have it, a carbon isotope signal is preserved in each annual growth ring, and that signal is strongly related to the water use efficiency of each year.

The key finding of the work is that water use efficiency has shown “an increase of nearly 40% over the past four decades”. The explanation for this increase is that rising levels of atmospheric CO2 are allowing for a reduction in the stomatal conductance. At higher levels of CO2, the plants relax a bit, the CO2 is easier to extract from the atmosphere, the pores in the leaves through which transpiration takes place are smaller in diameter, and far less water escapes from the plants. The trees grow at the same rate with far less water! They did observe increasing wet season temperatures and an increase in cloud cover, but neither seemed to explain the increase in water use efficiency.

The facts seem inescapable—extra atmospheric CO2 is our gift to the trees growing throughout tropical regions!


Brienen, R.J.W., W. Wanek, and P. Hietz. 2011. Stable carbon isotopes in tree rings indicate improved water use efficiency and drought responses of a tropical dry forest tree species. Trees, 25, 103-113.

Bowman, D.M.J.S., B.P. Murphy, and D.S. Banfai. 2010. Has global environmental change caused monsoon rainforests to expand in the Australian monsoon tropics? Landscape Ecology, 25, 1247–1260.

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