Abstract
Abstract. Forest’s net primary productivity (NPP) is a key index in studying interactions of climate and vegetation, and accurate prediction of NPP is essential to understand the forests’ response to climate change. The magnitude and trends of forest NPP not only depend on climate factors (e.g., temperature and precipitation), but also on the succession stages (i.e., forest stand age). Although forest stand age plays a significant role on NPP, it is usually ignored by remote sensing-based models. In this study, we used remote sensing data and meteorological data to estimate forest NPP in China based on CASA model, and then employed field observations to inversely estimate the parameter of maximum light-use efficiency (εmax) of forests in different stand ages. We further developed functions to describe the relationship between maximum light-use efficiency (εmax) and forest stand age, and estimated forest age-dependent NPP based on these functions. The results showed that εmax has changed according to forest types and the forest stand age. For deciduous broadleaf forest, the average εmax of young, middle-aged and mature forest are 0.68, 0.65 and 0.60 gC MJ-1. For evergreen broadleaf forest, the average εmax of young, middle-aged and mature forests are 1.05, 1.01 and 0.99 gC MJ-1. For evergreen needleleaf forest, the average εmax of young, middle-aged and mature forests are 0.72, 0.57 and 0.52 gC MJ-1.The NPP of young and middle-aged forests were underestimated based on a constant εmax. Young forests and middle-aged forests had higher εmax, and they were more sensitive to trends and fluctuations of climate change, so they led to greater annual fluctuations of NPP. These findings confirm the importance of considering forest stand age to the estimation of NPP and they are significant to study the response of forests to climate change.
Highlights
The vegetation is the main body of the biosphere, and it is significant to regulate the global carbon balance (Piao et al, 2001)
The evergreen broadleaf forest has the largest deviation in Net Primary Production (NPP), and the young forest for the forest types have higher deviation of NPP than the middle-aged forest and the mature forest
The original Carnegie-Ames-Stanford Approach (CASA) model based on the fixed maximum light-use efficiency does not consider the effect of the forest stand ages among forest types, and it will lead to the deviation in the estimate of NPP
Summary
The vegetation is the main body of the biosphere, and it is significant to regulate the global carbon balance (Piao et al, 2001). The accuracy of the estimated NPP based on the CASA model has been mostly affected by the maximum light-use efficiency (εmax), and it exists differences among forest types and the forest stand ages (Zhu et al, 2006; Hui et al, 2012 ). The maximum light-use efficiency could influence the carbon sink of the ecosystem, and reflect the forest ecosystem productivity (Zhou et al, 2010). It is essential to determine the maximum light-use efficiency based on the forest stand ages among forest types (Potter et al, 1993; Raymond et al, 1994; Field et al, 1995, 1998). We calculated the maximum light-use efficiency of forests in the forest stand ages based on the estimated NPP and the field observation NPP. The relationship between the maximum light-use efficiency and the forest stand ages could be developed to reflect the impact of change in the forest stand ages on the maximum light-use efficiency, and the estimated age-dependent NPP could be calculated based on the relationship
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