基于树轮资料重建森林净初级生产力的研究进展

森林净初级生产力(NPP)反映了森林植被固定和转换光合产物的能力,表示了森林碳汇功能强度,也是评价森林植被的演替状况以及陆地生态系统承载力的主要指标。基于遥感、清查资料等方法估算NPP已经取得了一些进展,但传统的研究方法受限于观测(调查)年份,难以有效获取长时间尺度的区域森林种群或群落年际NPP。树轮资料较为有效地反映了历史时期森林植被的逐年生长状况,从而在估算高精度且长时间尺度区域森林种群及群落NPP中具有较大的优势。本文对利用树轮资料重建区域森林NPP的两种主要方法进行了总结,第一种方法主要是依据树轮资料提供的立木逐年生长量进行生物量以及NPP的估算;第二种方法则是利用树轮指数与其他植被指数的相关性间接反演过去时间段区域森林群落NPP的变化。上述两种估算NPP的方法均存在较多的限制性,未来利用树轮资料估算NPP的时空精度仍有待提高。

Advances in study of reconstruction of regional forest net primary productivity based on tree rings

Forest net primary productivity (NPP) shows the capacity of forest vegetation to convert or fix CO₂ into compounds. As a reflection of forest carbon sequestration capacity, NPP is one of the main indicators of forest succession and carrying capacity of terrestrial ecosystems. Methods based on remote sensing or inventory data are limited by observation time, which makes the estimation of NPP over a long time period very difficult. However, tree ring data effectively reflect the history of forest growth year by year at decadal and centennial scales, and thus have great advantages in retrieving NPP of forest populations or communities in the past. It can be used to carry out long-term, dynamic, and continuous estimation and prediction of forest biomass, NPP, and dynamic status of carbon cycle. This article summarizes the main scientific research for reconstruction of regional NPP based on tree ring data. The results show that current studies mainly focus on two types of methods for reconstructing the NPP of forests: the first is to obtain forest growth volume in historical periods directly by measuring the radial growth and diameter at breast height of trees; the second is using correlation between vegetation index and tree ring index to infer the evolution of regional NPP in the past. The first approach can be further divided into two applications. Combined with survey data from sampling plots, NPP of the ecosystem can be evaluated by calculating total biomass and productivity of all single trees on the plot. In addition, models that contain tree ring data as an argument that can refer to the vegetation growth can be used to derive NPP of the ecosystems. Measuring forest annual biomass with tree ring data at plot level generates high precision results and the method is suitable for reflecting NPP change of small-area forest stands. For large-scale applications, however, as a result of widely spaced sampling sites this method produces low precision estimates of NPP. The second method is capable of dealing with high heterogeneity in NPP and vegetation indices, and has irreplaceable advantage in estimating forest biomass and NPP at large scales. In some areas, however, the method has limitations because of low correlation between tree ring index and vegetation indices. Accuracy of the results is also limited by the accuracy of remote sensing vegetation indices. Most of the results show that climate factors have conspicuous correlation with NPP in most places. Since the industrial revolution, the warming of the global climate and increasing CO₂ concentration led to the increase of NPP of forest in most areas, especially in high latitudes in the northern hemisphere. These results are beneficial to ecosystem carbon sink and carbon balance evaluation in the climate change scenarios. However, assumptions and limiting factors are involved in the two methods. The spatiotemporal accuracy of NPP estimation using tree rings can still be improved in future research.