Abstract:

Net Ecosystem Productivity (NEP) of vegetation is a crucial component of the carbon cycle and a significant indicator of an ecosystem's carbon budget. This study analyzes the spatial and temporal distribution characteristics of vegetation carbon sources and sinks in the Beijing-Tianjin-Hebei region, based on MOD17A3 and meteorological data, combined with a soil respiration model. The relationships between vegetation NEP and meteorological factors, the Normalized Difference Vegetation Index (NDVI), and land-use change were explored using trend analysis, correlation analysis, and other methods. The results showed that: 1) From 2000 to 2022, NEP in the Beijing-Tianjin-Hebei region showed a fluctuating upward trend, with an annual growth rate of 5.65 g·m⁻² and an average annual NEP of 108 g·m⁻². The area of carbon sinks increased gradually, reaching a maximum of 95.0% in 2022, and the spatial pattern of NEP was characterized by a “high in the north and low in the south”, which was consistent with the regional elevation and obvious spatial heterogeneity. 2) Over the past 20 years, 98.3% of the area of Beijing-Tianjin-Hebei was characterized by an upward trend in vegetation NEP, and 85.9% of the area was characterized by a significant increase in NEP, which was more concentrated in Chengde, Zhangjiakou, and Beijing. The area with a downward trend in NEP accounted for only 1.72%. Chengde had the highest percentage of area with a significant increase in NEP of 98.2%, whereas Handan had the highest percentage of area with a significant decrease of 1.03%. 3) Vegetation NEP in most areas of the Beijing-Tianjin-Hebei region was positively correlated with precipitation and air temperature, and the mean values of correlation with precipitation and air temperature were 0.500 and 0.160, respectively. The percentage of the area where NEP was significantly positively correlated with precipitation and air temperature was 78.2% and 13.9%, respectively, and precipitation was the key meteorological factor influencing the changes in NEP in the Beijing-Tianjin-Hebei region. The average correlation coefficient between NDVI and NEP was 0.430, and the area with a very significant positive correlation accounted for 58.1%, and the area with a high positive correlation was concentrated in the mountainous areas of northwest Beijing-Tianjin-Hebei. The land use change results showed a significant increase in the area of forested land in Beijing-Tianjin-Hebei during the last 20 years, with increases of 31.4%, 24.0%, and 11.9%, respectively, over the three study periods, which is an important factor driving the rise in vegetation NEP in the region. This study provides a reference basis for the accurate assessment of vegetation carbon sources/sinks in the Beijing-Tianjin-Hebei region and the realization of the “double carbon” goal.

Key words: Net Ecosystem Productivity (NEP), Beijing-Tianjin-Hebei region, carbon sink/source, meteorological element, temporal-spatial variation, soil heterotrophic respiration

摘要：

植被净生态系统生产力（Net Ecosystem Productivity，NEP）在碳循环中起着关键作用，是生态系统碳预算的重要指标。基于MOD17A3数据、气象数据，结合土壤呼吸模型，对京津冀地区植被碳源/汇时空分布特征展开研究，并利用趋势分析、相关性分析等方法探究植被NEP与气象要素、植被归一化指数（Normalized Difference Vegetation Index，NDVI）及土地利用变化的关系。结果表明，1）2000－2022年京津冀地区NEP呈波动上升趋势，年增长率5.65 g·m⁻²，年均NEP为108 g·m⁻²，碳汇区面积也逐步增加，2022年占比达到最大值95.0%；NEP空间格局呈“北高南低”特征，与区域高程一致，并且有明显的空间异质性。2）近20年来，京津冀植被NEP呈上升趋势的面积占98.3%，且显著上升的区域占比为85.9%，在承德、张家口和北京较为集中，NEP为下降趋势的面积仅占1.72%；承德植被NEP呈显著上升的区域占比最多，为98.2%，而邯郸显著下降的区域占比最多，为1.03%。3）京津冀大部分地区植被NEP都与降水、气温呈正相关，与降水、气温的相关性均值分别为0.500、0.160，NEP与降水、气温呈显著正相关的面积占比分别为78.2%、13.9%，降水是影响京津冀地区NEP变化的关键气象因子；NDVI与NEP的平均相关系数为0.430，呈极显著正相关的面积占比为58.1%，其中正相关性较高的区域集中在京津冀西北部山区。土地利用变化结果显示近20年间京津冀林地面积大幅增加，3个研究时段内分别增加了31.4%、24.0%和11.9%，是驱动该地区植被NEP上升的重要因子。研究结果为京津冀地区植被碳源/汇的准确评估及“双碳”目标的实现提供了参考依据。

关键词: 净生态系统生产力（NEP）, 京津冀地区, 碳源/汇, 气象要素, 时空变化, 土壤异养呼吸