Abstract:

Carbon use efficiency (CUE), which is defined as the ratio of net carbon gain to gross carbon assimilation, can be used to assess not only the capacity of forests to transfer carbon from the atmosphere to the terrestrial biomass but also to determine the impact of respiration on productivity in forests. CUE is an important parameter for comparing carbon cycle variability among ecosystems. Understanding such controls on CUE can be helpful in determining whether the terrestrial ecosystem is a carbon source or sink. Forest CUE under different environmental regimes and global change scenarios has recently received increasing attention. This paper introduces the calculation methods of plant CUE and the corresponding measurement techniques, and reviews the research progress in the effects of important factors on forest CUE. The main findings are as follows: (1) Some studies proposed that CUE is constant among forests with a possible appropriate universal value of 0.50. However, it is doubtful whether this conservative CUE assumption regardless of ecosystem types is globally applicable. CUE can vary with ecosystems, forest types, species, and ontogeny of plant development. Forest ecosystems have a lower CUE than shrub and herbaceous ecosystems. CUE is significantly higher in deciduous than in mixed and evergreen forests. Tropical forests often have lower CUE than temperate forests. CUE is known to depend on successional stage and stand age. (2) Forest CUE is related to temperature, precipitation, and geographical factors. A parabolic relationship between CUE and annual mean temperature is founded at a global scale. Acclimation of the respiration to temperature contributes to high carbon-use efficiency in seasonally dry vegetation. The CUE decreases with enhanced precipitation and remains unchanged in areas where water availability is in surplus. CUE of plants grown at low light level is low. (3) The elevated CO₂ may increase whole-plant respiration, causing CUE to decline. The potential for elevated CO₂ to affect CUE may depend on tree age or genotype. (4) Plants grown on the barren soil, and under low temperature and drought conditions, may have larger changes in CUE than plants grown under near-optimal conditions. Forest managements such as irrigation, fertilization, and selective logging can affect ecosystem CUE. (5) CUE varies widely with the changing seasons within a year. The maximum of CUE in temperate forests usually occurs in spring. The future research should be focused on: (1) exploring the spatial variations in forest CUE and their driving mechanism from tissues, individual plant, community, to ecosystem scales; (2) analyzing the processes and mechanism in CUE of different vegetation types at temporal scales by combining the plant eco-physiology and biology with eddy covariance technique and modeling approaches; and (3) evaluating the response and adaption of forest CUE to climate change by synergistic experiments of multi-factors.

Key words: carbon use efficiency, climatic factor, forest management, forest vegetation, measurement technique, soil nutrition