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).