Logged tropical forests may never fully recover
Study reveals impact on soil nutrients.
By Nick Carne
Continually logging and re-growing tropical forests is reducing the levels of vital nutrients in the soil, which may limit future forest growth and recovery, according to new research.
The first landscape-scale study of how leaf function changes in response to logging revealed that trees of recovering forests have tougher leaves, with lower concentrations of the essential nutrients phosphorus and nitrogen than trees of old-growth forests.
This suggests, the researchers say, that multiple cycles of logging and recovery irreversibly remove as much as 30% of available phosphorus from the forest system and are pushing the nutrient content towards ecological limits.
"We see that as the logged forests start recovering, they're actually diverging from the old growth forests in terms of their leaf chemistry and possibly also species composition, as the amount of available nutrients goes down," says Tom Swinfield from the University of Cambridge, UK, the first author of the paper in the journal Global Change Biology.
"At the moment the trees can cope, but the fact that they're changing indicates phosphorus levels in the soil are dropping. This could affect the speed at which forests recover from future disturbances."
Swinfield and colleagues made their findings after creating high-definition images of a forest landscape in Kalimantan, Borneo, using LIDAR-guided imaging spectroscopy from an aircraft.
They took hundreds of measurements across the light spectrum, then combined this information with nutrient measurements from 700 individual trees in the forest. This allowed them to map the concentrations of nutrients in the leaves over an area containing repeatedly logged forest and old-growth forest and compare them.
Selective logging is carried out extensively across millions of hectares of forest in the tropics, so that degraded forests are now more widespread than old-growth forests.
"Phosphorus limitation is a really serious global issue: it's one of the areas where humans are using a vital resource beyond sustainable levels," says Cambridge’s David Coomes, the project leader.