Abstract
Planting forests is an important practice for climate change mitigation, especially in the tropics where the carbon (C) sequestration potential is high. Successful implementation of this mitigation practice requires knowledge of the role of species identity and diversity on carbon accrual of plantations. Despite this need, solid data on the long‐term development of forest plantations are still very scarce. Monospecific and two species mixture plots of a 77‐year‐old tree diversity experiment in Yangambi in the Congo basin were fully inventoried. We calculated above‐ground C stocks using allometric equations, and soil C stocks by analyzing soil samples at multiple depths. Linear mixed effects models were used to analyze the effect of taxonomic and functional identity and diversity on the aboveground and soil carbon stocks. A high variability in aboveground C stocks across tree species combinations was observed. Apart from a species identity effect, the proportion of planted species in the total stand basal area (BApl) and effective species richness were identified as compositional parameters with a significant effect on the aboveground carbon (AGC), with BApl being more important. Both AGC and BApl were coupled to the functional identity of the planted species; the planting of short‐lived pioneers led to low AGC. We found no clear benefits, but also no drawbacks, for AGC of two species mixture plots over monospecific plots or including nitrogen fixing species in the plantation scheme. However, the latter was the only compositional parameter with a significant positive effect on the soil carbon stock up to 1 m depth. We conclude that the different plantation configurations gave rise to a wide range in carbon stocks. This was predominantly caused by large differences in AGC sequestration over the past 77 years. Altogether, short‐lived pioneer species had a low BApl resulting in low carbon sequestration, while partial shade tolerant species achieved the highest AGC stocks. Tolerating spontaneous ingrowth during the plantation development can further increase the AGC stock, given that the appropriate functional type is planted.
Highlights
Forest ecosystems contain 45% of the terrestrial carbon (C) stock and are directly interacting with the atmosphere through C sequestration, deforestation and forest degradation (Bonan 2008)
Both aboveground carbon (AGC) and BApl were coupled to the functional identity of the planted species; the planting of short-lived pioneers led to low AGC
Including N-fixers in the plantation scheme was the only significant, positive, effect on SOC stocks (r2 1⁄4 0.11 and p 1⁄4 0.03). Both the average and the coefficient of variation of the AGC stocks were considerably higher than SOC stocks
Summary
Forest ecosystems contain 45% of the terrestrial carbon (C) stock and are directly interacting with the atmosphere through C sequestration, deforestation and forest degradation (Bonan 2008) The latter two processes give rise to 95% of the C emissions in the tropics, which equals nearly 10% of the global fossil fuel emissions This stresses the importance of carbon offset projects in the tropics, such as afforestation or reforestation, as currently traded in the voluntary carbon market or, to a lesser extent, financed by the Clean Development Mechanism (Canadell and Raupach 2008, Jindal et al 2008, Cerbu et al 2011) Implementation of such programs frequently results in plantations of fast growing monocultures of non-native species (Cossalter and Pye-Smith 2003). This study is the first to bring results on this experiment, where we analyze the long-term effects of tree species identity and admixture effects on carbon sequestration, which is highly relevant for the development of carbon plantations in the tropics
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