Community Composition is Different at Forest Edges, but Carbon Storage Remains the Same

by Stephen Johnson

Forest fragmentation is one of the leading ways that humans alter natural habitat. Forests are frequently fragmented as land is cleared piecemeal for the expansion of agriculture, logging, and human settlement. Often, rather than clearing an entire forest, fragments of forest are left embedded in a matrix of agricultural and other habitats. As an increasing percentage of the world’s forests are fragmented, it is crucial to understand how forest fragments function. Fragments are subject to a variety of influences, most notably edge effects. Edge effects occur at the edges of two habitats, and include altered microclimate, reduced biodiversity, and vegetation changes. These edge effects can bring about altered species communities, which in turn could affect the amount of carbon that can be sequestered near forest edges. As forest fragmentation continues, a greater percentage of forest will be exposed to edge effects, potentially inhibiting forests’ ability to act as carbon sinks. To understand these effects, Ziter et al. (2014) examined how tree species composition and carbon storage capacity change with proximity to forest edge in large and small fragments. Using tree measurements and allometric data in the literature, they determined how much carbon was stored, and which species were present. Using linear mixed models and multidimensional scaling, they found that community composition shifts with proximity to the forest edge. Despite this shift, however, carbon storage did not decrease closer to the edge. Continue reading

Carbon Storage Increases Continuously as Trees Grow

by Stephen Johnson

Though it has been assumed that the rate of carbon accumulation declines with the age of an individual tree, little empirical evidence has been produced to support this assumption. Understanding how carbon storage capacity changes throughout the life of the tree is important in modeling carbon dynamics in forests, which can be used to determine how forests will contribute to climate change mitigation plans. Net primary productivity is well known to decline in even-aged forests, as does mass gain per unit leaf area. However, few forests are completely even-aged, and many are subjected to selective logging that removes the largest trees. Proper modeling of the amount of carbon lost through this logging can be used to more accurately price carbon credits for the preservation of natural forests, aiding efforts to keep them intact. In order to determine how carbon storage rates change with tree age, Stephenson et al. (2014) collected data from long-term monitoring plots in tropical and temperate areas across the globe. By measuring the diameter of each tree and using allometric equations, the researchers determined how much carbon was being stored over time. They found that while stand productivity declined with age, individual tree carbon gain rate increased, with no signs of declines at any age. Continue reading

Large Forest Blocks are Essential for Biodiversity Protection and Carbon Storage

by Stephen Johnson

Habitat loss is the primary threat to the survival of most tropical biodiversity. Typically, this habitat loss is driven by deforestation for agricultural use. However, deforested landscapes are rarely homogenous fields with low diversity; most often, forest fragments are left embedded in a matrix of varying types of agriculture, from open field monocultures, to pastures and forest-mimicking shaded plantations. The process of fragmentation has a significant negative effect on the biodiversity present in the area; however, fragments are often able to support a variety of species, as are some types of agriculture, such as agroforestry. Less is known about the capacity of such landscapes to sequester and store carbon. What little has been done has focused on carbon in agroforestry systems, with promising, though mixed, results. Continue reading

Allometric and Structural Changes Reduce Carbon Storage in Forest Fragments

by Stephen Johnson

Contrary to the popular image of deforestation as a clear-cut resulting in the absolute destruction of forests, most deforestation in the tropics takes place piecemeal. As forest is logged or converted to agriculture, patches are often left standing, resulting in a fragmented system of forest patches in a mosaic composed primarily of agriculture. Forests in tropical areas are increasingly highly fragmented, which has significant impacts on biodiversity and environmental conditions within the fragments. However, little is known about the impact of fragmentation on the ability of the forest to store carbon. In an ever-more-fragmented, ever-more-carbon-saturated world, understanding how these remnant forests sequester carbon is critical. Osuri et al. (2014) examined the relationship between rainfall, fragmentation, and carbon storage in fragments and continuous forest in the Western Ghats of southern India. Using linear mixed models and regressions, they found that fragmented forests stored almost 40% less carbon than continuous forests, as a result of trees that were shorter, had less dense wood, and were shorter for a given trunk diameter. Fragmented forests also relied more on large trees to store carbon, while displaying signs of transitioning to a community of less-dense, lower-carbon species. Continue reading