It is a sobering statistic that most of the world’s tropical forests are not ‘primary’ – that is, those that have not suffered some alteration or disturbance from humans (previously logged, cleared for agriculture, burned, etc.).
Today I highlight a really cool paper that confirms this, plus adds some juicy (and disturbing – pun intended – detail). The paper by Phil Martin and colleagues just published in Proceedings of the Royal Society B came to my attention through various channels – not least of which was their citation of one of our previous papers ;-), as well as a blog post by Phil himself. I was so impressed with it that I made my first Faculty of 1000 Prime recommendation1 of the paper (which should appear shortly).
As we did in 2011 (to which Phil refers as our “soon-to-be-classic work” – thanks!), Martin and colleagues amassed a stunning number of papers investigating the species composition of disturbed and primary forests from around the tropics. Using meta-analysis, they matched disturbed and undisturbed sites, recording the following statistics:
- at least one measurement of above-ground biomass, below-ground biomass, soil carbon content, plant species richness and/or plant species community composition
- the time since last disturbance for secondary forests
- the type of disturbance prior to secondary succession, which included conversion to pasture, cropland or small-scale shifting agriculture.
What they found is thought-provoking. While the good news is that forests can be restored or re-generated to some degree, and therefore have some value for biodiversity conservation, Martin and colleagues show that the recovery is slow – around 50 years for tree species richness, and about 100 years for epiphyte richness.
While this appears ultimately to be good news, the story becomes more complicated when looking at species composition. The authors found no increase in the proportion of ‘primary forest’ species in the recovering forests over time, suggesting that initial extirpations of the most sensitive species were never rescued by seed dissemination or re-colonisation.
Carbon sequestration is an increasingly important consideration in tropical forest conservation beyond biodiversity concerns (think REDD, etc.), and the paper confirmed about 85% above-ground (biomass) carbon recovery after about 80 years. However, below-ground (soil) carbon showed no relationship with time since clearance. Even though well over half of tropical forest carbon is held above-ground (mainly in the woody biomass), there is still a very important component stored in the soils. Will it ever recover? Only (lots of) time will tell.
Permitted several more centuries of recovery, these statistics might change, but it does re-iterate the point that nothing can replace the biodiversity – or carbon – value of primary tropical forests.
1I have just been added to the Faculty of 1000 as of September 2013.