Secondary forests, which are areas that were previously cleared of old-growth cover, now comprise the majority of the forested areas in the United States, Europe, and elsewhere. A heavily debated issue is to what extent secondary forests are able to contribute to the preservation of biodiversity. In an article published in PLOS ONE, a group of researchers from the Smithsonian Tropical Research Institute led by Michiel van Breugel evaluated the biodiversity preservation potential of secondary forests. However, while they found that secondary forests can provide suitable homes for broad arrays of species, the true biodiversity potential of these forests is often limited by cycles of reclearing.
According to the researchers, in tropical regions like Panama’s Agua Salud, where the study was conducted, young secondary forests comprise more land area than do mature forests. Their study examined patterns of how plant species and plant diversity are established over time, both within small areas called “patches” as well as across the larger scale of landscapes. This allowed for a comparison between the processes occurring at both levels.
Agua Salud landscape, with a recently slashed and burned secondary forest on the foreground. The forest patch was about 15 years old. Author: Jake Slusser
This capacity to compare across areas of varying size and ages was a plus given, “local niche-based processes do not predict landscape scale patterns of diversity and composition nor how these change during succession,” states the study.
The Agua Salud Project is located in the watershed of the Panama Canal. Twenty-three years ago, when the project was established, its purpose was to quantify what the forests in the canal’s watershed provided ecologically, socially, and economically. At that time, deforestation was compromising the canal’s use by contributing to erosion and flooding. Now, the Panamanian government has outlawed deforestation in much of the watershed area. Agua Salud comprises a patchwork of various land uses, which allows for the study and comparison of mature forest remnants, crop land, pastures, and secondary forests of different ages.
Sapling / regeneration of canopy tree (Xylopia frutenscens) in understory of few years old secondary forest. Photo: Michiel van Breugel
On the 45 randomly chosen plots aged two to 32 years of age, the researchers counted seedlings, saplings, juveniles, and adult trees along with the number of shrubs, trees, lianas, and palms. After some heavy number crunching, they were able to determine what species predominated in plots cleared within the past seven years (i.e., two to seven years), and what species composition looked like for plots that had been re-establishing themselves for a longer period of time (i.e., 18 to 32 years). They also examined abilities of individual species to reproduce within a plot.
The researchers found that the process of succession follows a predictable course, starting with small, quickly growing species and ending with those that take longer to become established. Diversity also increases with time.
Agua Salud landscape. Photo printed under a CC BY license with permission from Christian Ziegler.
“The first group of species consists mostly of small seeded, well dispersed (by wind, bats, birds) small tree and shrub species that grow very fast once established, reproduce within a few years and have a short life span,” van Breugel told mongabay.com. “The second group of species is much more diverse. It may take them more time to arrive at different sites because they are less efficiently dispersed, depend for their dispersal on animal species that are more forest bound. They may also grow slower and therefore it may take them longer to grow.”
Findings also indicated that while secondary forests are capable of having high biodiversity, the typical re-clearing cycle of less than 32 years limits their capacity. Many species can not become fully established as a result of secondary forests’ continuous use, affecting the abilities of secondary forests to preserve surrounding mature forests species diversity and composition.
Canopy of ~10 year old secondary forest in Agua Salud. Photo by Michiel van Breugel.
At Agua Salud. no imminent threat of deforestation exists. This has made it possible for research to be directed towards how to build a forest and allows for the analysis of how plant species thrive across patches of various ages, light, soil composition, and plant species. Chazdon emphasized how unique this is.
“In the context of the Agua Salud landscape, second-growth forests in this region are clearly following a successional trajectory towards convergence of species composition with old-growth forests,” she said. “They are not impoverished in any way, and as far as I know they are not in imminent danger of being cleared. They are undergoing succession and are not fully formed after 32 years. Yes, they have limited tree biodiversity in their current state. Yes, if all of the old-growth forest in this region were cleared and only these young second-growth stands remained, the landscape would indeed become very impoverished. But it does not follow that the value of these forests should be discounted because the tree species in them are not yet reproductively mature, especially if old-growth trees are establishing as seedlings and saplings. This is good news, not bad news!”
Just 21 percent of the world’s old-growth forests remain, and every minute, the equivalent of thirty-six football fields are lost. If the prospect of establishing secondary forests could be viewed as a solution for re-establishing old-growth forests, wouldn’t the world breathe a bit easier?
Citations:
- Van Breugel, M., Hall, J.S., Craven, D., Bailon, M., Hernandez, A., Abbene M., van Breugel, P. 2013 Succession of ephemeral secondary forests and their limited role for the conservation of floristic diversity in a human-modified tropical landscape. PLOS ONE.
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