Abstract:

The meteorological drought has happened much more frequently since 20 century in middle latitudes, whereas the impact of it on the carbon cycle and ecosystem productivity remains unclear. Forest is the largest carbon sink in terrestrial ecosystems and plays an irreplaceable part in coping with the climate change. As the first step of the carbon storage in terrestrial ecosystems, net primary productivity (NPP) plays an important role in carbon balance as well as regional productivity. The better the understanding of how drought affect NPP is, the more helpful for improving the resistibility and stability of an ecosystem, which can also help provide a theoretical basis for research of ecosystem carbon sink capacity of Hubei Province. Here, the droughts that occurred in Hubei Province from 2001 to 2010 have been reconstructed by standardized precipitation index (SPI), and standardized anomaly index (SAI) has been extended to evaluate the anomalies of NPP. Estimation of NPP value in Hubei Province has been made using CASA model, then the impact of drought has been accessed by SPI in this study. The results are as follows. (1) Annual NPP of forest during 2001-2010 of Hubei Province changed from 7102 Tg to 9345 Tg with an average of 8062 Tg. Spatial distribution of NPP was not obvious with the average of 97934 g·m-2·a-1 and simultaneously seasonal change of NPP presented a single peak curve with the highest value in July and August and lowest in January and December; (2) The more intense the drought was, the lower the NPP value, and NPP value dropped by 2609% in mild drought, 3267% in moderate drought and 3828% in severe drought; (3) In the years when drought happened broadly in forest of Hubei Province, the impact of drought on NPP seemed more clear and the interannual variation of NPP value and drought areas presented strong negative correlation; (4) The variation of NPP anomaly proportion along with the drought intensity presented a linear trend. NPP value will get lower in half of the forest when SPI value is lower than -04 which means that the forest is sensitive to mild drought; and when SPI value is lower than -16, more than 80% of forest will suffer a low productivity. The result showed that the impact of drought on NPP was obvious, while there are still questions that remain unclear and deserve further investigation. (1) Vegetation growth has significant seasonal variation and drought intensity at different time scales has different impact on the vegetation in different growth stage. Our tentative study showed that the forests in Hubei Province were sensitive to drought of three and six months while the linear trend of the impact of drought of twelve months was not so obvious. So the studies of response of forest to different time scales would be worth exploring in future researches; (2) Meteorological factors such as precipitation influence the vegetation growth by changing the moisture of soil, so there is a time lag in the response of vegetation to climate factors, and vegetation is also affected by the cumulative effects of the previous climate conditions. In addition, there is a special difference of how the vegetation growth response to the meteorological factors, which should be investigated further; (3) Besides the drought conditions, natural factors like radiation and temperature and some human factors are also important to NPP, which cannot be eliminated in this study. So it is necessary to analyze how these factors influence the vegetation growth especially when some natural disasters and extreme weather happen so that the relationship of plant and climate can be better understood