Carbon economy of subtropical forest

Abstract

Compared to tropical and temperate forests, subtropical forests havereceived little attention in physiological and ecological studies until now, and the contribution of this ecosystem type to the global carbon cycle has not been fully assessed. In this chapter we discuss results on the carbon balance of subtropical forests at different spatial and temporal scales, analyze the potential limitation of seasonal low temperatures and water deficits on physiological processes of sub-11 tropical trees, and characterize the uniqueness of subtropical forest ecosystems in terms of carbon economy. Results from multiple techniques and scales were included in the carbon balance assessment. The largest two regions with subtropical forests are located in Asia and South America. The net ecosystem carbon gain of subtropical forests in these two regions, which have annual precipitations larger than 800 mm, is probably neither strongly limited by soil water availability nor byseasonal low temperatures. Relatively low evapotranspiration in the winter/dry season and high soil water-holding capacity help maintain good water availability for trees in most subtropical forests. High solar radiation, light penetration and low ecosystem respiration in winter may compensate for the negative effects of low temperatures on gross photosynthesis. Therefore, subtropical forests in many areas can assimilate carbon in excess of respiration throughout the year and they are, probably, among the largest terrestrial carbon sinks across terrestrial ecosystems worldwide. In addition, because leaf and ecosystem respiration respond to temperature changes to a larger extent compared to ecosystem carbon assimilation, a negative relationship between net ecosystem carbon gain and mean annual temperature was found in Asian subtropical and tropical forests. This relationship suggests that global warming may weaken the carbon sink strength of these forest ecosystems. These results indicate the important contribution of subtropical foreststo the global carbon cycle and the potentially negative response of these forests to global warming. We hope this information will promote additional physiological and ecological research and conservation in subtropical forests.