Warming in the forests

While looking for something else, I found a couple of papers in JXB and Forestry that caught my eye. The popular expectation is that more CO2 is good for plants, because that’s what they breathe. What gets lost is that you need water too for photosynthesis, and in climate hotter and that often means somewhere becomes drier. There’s also the scale of change, which is rapid. What does this mean for trees?

A forest under the hot sun.
Photo: Vinoth Chandar / Flickr

Growth response and acclimation of CO2 exchange characteristics to elevated temperatures in tropical tree seedlings is free in the Journal of Experimental Botany. Cheesman and Winter looked at how the seedlings of ten neo-tropical trees grew at high temperatures. The results were mixed. All the trees grew better in the high temperature conditions, which sounds like good news, but it’s clear from the experiments water is an issue. Hotter and more CO2 will not be enough. At the highest end of the tests, some trees suffered badly.

What caught my eye was not simply that many trees coped, but how they coped. A further experiment grew trees above and below the optimal conditions and though they produced similar biomass they coped with their climate in different ways. One the key points Cheesman and Winter point out is that these are experiments on saplings. The way they’re growing is laying out how they’ll cope in the future. The trees in the rainforest now are not juveniles. Their structure grew in a different climate, so they may not be able to cope with higher temperatures, even if saplings of the same species can. It raises the prospect of a generational die off in some forests and the ensuing battle for sunlight could shift the balance of species in favour of different trees with knock on effects for the rest of the ecosystem.

The other paper that I saw was The benefits and hazards of exploiting vegetative regeneration for forest conservation management in a warming world by Sjölund and Jump, free in December’s Forestry journal. Pollarding is where you chop off the upper branches of tree to promote growth. Coppicing is cutting to a stump so that new growth comes from the stump to form new trunks. It’s almost like resetting a tree. An example Sjölund and Jump give is Ash, Fraxinus excelsior, which has a natural life of around 200 years. Coppiced trees are known with lives measured in thousands of years. This becomes important when the range of trees shifts with the climate.

Sjölund and Jump point out that there’s the opportunity for species to move north and grow higher up, but that at the other end of the range the more southerly and lower trees come under much more stress. This matters because when a range changes you’re not simply exchanging like-for-like. The trees at the rear-edge are the relict populations and their genes can be very important sources of diversity in a species. Coppicing and pollarding are ancient European practices and this means that at the rear of the range there are many trees that depend on human intervention for their survival.

It’s not simply the global warming factor that made the two papers seem connected to me. Much more interesting is that they both show how trees are ongoing processes, not simply seed and then adult (or in the case of a sapling, short adult). Sjölund and Jump show that there are some serious consequences when interrupting the process of regrowth and from the other direction Cheesman and Winter show there are also potential consequences from the maturation of wild forest. Both papers show that any mitigation of the effects of warming are going to need active human management at a mind-boggling range of scales from the DNA scale in managing genetic diversity within species to areas of thousands of square miles managing diversity within the ecosystem as a whole.


Sunset @ Kabini by Vinoth Chandar / Flickr. This image licensed under a Creative Commons by licence.