March 18, 2014
When land that was once forest is abandoned after being cleared for timber or agricultural purposes, what emerges is known as a second-growth forest. Such "secondary forests" currently account for the majority of woodland in Europe and the U.S. and increasingly in the tropics as old-growth forests are lost. Ecologists have long suspected that remnant trees left over from original forests--often found in clusters known as “tree islands”--aid and accelerate the growth and development of secondary forests by altering soil chemistry, producing seeds and attracting small animals that disperse the seeds.
The Osa Peninsula in Costa Rica. Photo by Rhett A. Butler / mongabay.com
After monitoring the biodiversity of the plots, the researchers discovered that the areas without remnant trees housed 40 fewer species. Chazdon attributes this to the “ecological memory” of the left over trees.
“[Remnant trees] link the forests of the future with the forests of the past,” she told mongabay.com. “During succession, because of their height and the biological resources they provide, they attract birds and bats that arrive from patches of old-growth forests in the landscape. So they support biodiversity outside of old-growth forests and also provide a focal point for the dispersal of old-growth tree species.”
According to Chazdon, there is a worrying trend happening on the Osa Peninsula where farmers are removing forests to make way for pineapple cultivation, an important part of Costa Rica’s rural economy.
“This is concerning because we see a reduced potential for natural forest regeneration in this region, when pineapple ceases to become the preferred land use [and the land is left to revert back to forest],” Chazdon said.
Rainforest in the Osa Peninsula of Costa Rica. Photo by Rhett A. Butler / mongabay.com
The negative effects of pineapple cultivation on the landscape can last longer than cattle farming and ultimately make it more difficult for the land to be reforested. Unlike land used as a plantation, once the cattle are removed forests can quickly and easily return.
“We now know that forests can grow on former pastures with no intervention other than removing cattle, even when they have few or no remnant trees,” Chazdon added. “But pineapple is likely to be another story, and will likely require costly interventions to reforest. Pineapple production involves tilling soil (ploughing), levelling soil, clearing all forms of remnant vegetation, heavy use of pesticides, and often involves very large areas that are isolated from forest fragments. All of these factors would limit forest regeneration if the farm is abandoned. So restoring forest on a former pineapple plantation also requires restoring soil biota and planting trees.”
- Manette E. Sandor and Robin L. Chazdon (2014) Remnant Trees Affect Species Composition but Not Structure of Tropical Second-Growth Forest. PLoS ONE 9 (1): e83284.doi:10.1371/journal.pone.0083284: Januery 2014, Vol. 9.
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