Contributions of secondary forest and nitrogen dynamics to terrestrial carbon uptake

T K Richardson,A K Jain,X Yang

doi:10.5194/bg-7-3041-2010

Abstract

Abstract. We use a terrestrial carbon-nitrogen cycle component of the Integrated Science Assessment Model (ISAM) to investigate the impacts of nitrogen dynamics on regrowing secondary forests over the 20th century. We further examine what the impacts of nitrogen deposition and land use change history are on terrestrial carbon uptake since preindustrial time. Our results suggest that global total net land use emissions for the 1990s associated with changes in cropland, pastureland, and wood harvest are 1.22 GtC/yr. Without considering the secondary forest regrowth, the estimated net global total land use emissions are 1.58 GtC/yr or about 0.36 GtC/yr higher than if secondary forest regrowth is considered. Results also show that without considering the nitrogen dynamics and deposition, the estimated global total secondary forest sink for the 1990s is 0.90 GtC/yr or about 0.54 GtC/yr higher than estimates that include the impacts of nitrogen dynamics and deposition. Nitrogen deposition alone is responsible for about 0.13 GtC/yr of the total secondary forest sink. While nitrogen is not a limiting nutrient in the intact primary forests in tropical regions, our study suggests that nitrogen becomes a limiting nutrient for regrowing secondary forests of the tropical regions, in particular Latin America and Tropical Africa. This is because land use change activities, especially wood harvest, removes large amounts of nitrogen from the system when slash is burnt or wood is removed for harvest. However, our model results show that carbon uptake is enhanced in the tropical secondary forests of the Indian region. We argue that this may be due to enhanced nitrogen mineralization and increased nitrogen availability following land use change in the Indian tropical forest ecosystems. Results also demonstrate that there is a significant amount of carbon accumulating in the Northern Hemisphere where most land use changes and forest regrowth has occurred in recent decades. This study indicates the significance of secondary forests to terrestrial carbon sinks, the importance of nitrogen dynamics to the magnitude of secondary forests carbon uptake, and therefore the need to include both primary and secondary forests and nitrogen dynamics in terrestrial ecosystem models.

Highlights

Human activities have significantly altered the Earth’s vegetation cover in nearly every part of the world
The land use emissions associa2t5ed with changes in cropland, pastureland, and wood harvest are calculated by subtracting C fluxes based on experiment S2 from C fluxes based on experiment S1 (Table 1)
Total global land use emissions for the 1990s are estimated by Integrated Science Assessment Model (ISAM) to be 1.22 GtC/yr, which falls within the range of values estimated by IPCC AR4 (0.5–2.7GtC/yr, median value of 1.6 GtC/yr)

Summary

Introduction

Human activities have significantly altered the Earth’s vegetation cover in nearly every part of the world. Such changes have the potential to alter regional and global climate through changes in the biophysical characteristics of the Earth’s surface, such as albedo and surface roughness and in the biogeochemical cycles of terrestrial ecosystems, such as the global carbon (C) and nitrogen (N) cycles. C stocks in forest ecosystems have increased through reforestation, afforestation and forest regrowth on abandoned land (Houghton et al, 1999). Recent studies suggest that forest regrowth is one of the important causes of the net carbon sink in terrestrial biosphere over the past few decades (Canadell et al, 2007; Houghton et al, 2005; Denman et al, 2007). Shevilakova et al (2009) show that, on a global scale, secondary forests were a substantial sink of C during the 1990s, accumulating 0.35–0.6 GtC/yr

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