Changes In Growing Season And Gross Primary Productivity Of Vegetation In The Hengduan Mountains And The Main Drivers

The Hengduan Mountains are situated in the transitional zone between the Tibetan alpine region and subtropical areas,and are characterized by a distinct vertical ridge and valley terrain that creates a clear"channel-barrier"effect.This results in a diverse and unique flora,a distinct transitional nature,and a unique pattern of vegetation distribution,with significant implications for the function of vegetation biodiversity.The examination of changes in vegetation distribution and conversion,growing seasons,and productivity in the context of climate change is critical to understanding,developing,and conserving mountain ecosystems.This paper utilizes landcover data,remote sensing data,GPP data,climate data,and topographic data from 2000 to 2019 to study the distribution and conversion of vegetation types,changes in vegetation growing seasons,and resulting productivity changes in the Hengduan Mountains.Transfer matrix,trend analysis,and correlation analysis were employed to examine spatial and temporal changes in precipitation and temperature,vegetation type distribution and conversion,as well as characteristics of vegetation growing seasons and primary productivity in the Hengduan Mountains.The key findings of the study are:(1)The Hengduan Mountains experienced an increasing trend in annual mean,maximum,and minimum temperatures between 2000 and 2019,with the maximum temperature warming at a rate of 0.24°C/10a,while annual precipitation showed a fluctuating pattern with a general trend of"dry warming"and a non-significant decreasing trend.In terms of spatial distribution,the distribution of mean,maximum,and minimum temperatures showed a gradual decrease from south to north,while annual precipitation decreased from southeast,southwest to northwest.(2)The evergreen coniferous forest and scrub-grassland mosaic type were the most widely distributed vegetation types in the Hengduan Mountains.The areas with changing vegetation cover were mainly concentrated in the river valleys and lower elevation areas in the south,where the grassland and artificial vegetation types were largely replaced by forest types,especially the evergreen coniferous forest,leading to a gradual improvement in vegetation cover.This indicates that ecological protection policies such as mountain reforestation,afforestation,and reforestation have played a crucial and positive role in this region.(3)The average Start of Season(SOS)for all vegetation types in the Hengduan Mountains ranged from 115 to 142 days,while the average End of Season(EOS)ranged from286 to 304 days,and the average Length of Season(LOS)ranged from 145 to 182 days.The SOS and LOS of all vegetation types showed different degrees of advancement and lengthening,while the interannual variation of EOS was less significant than that of SOS,and there was no consistent pattern of variation.The relationship between SOS and precipitation was mostly negative for all vegetation types,with artificial vegetation showing the most significant correlation.The positive and negative correlations between vegetation EOS and maximum and minimum temperatures,respectively,were reversed,indicating that higher minimum temperatures and lower maximum temperatures lead to a later vegetation EOS,with forest types being less sensitive.The growing season of each vegetation type was most influenced by elevation,followed by slope,and least influenced by slope orientation.(4)The Gross Primary Productivity(GPP)in the Hengduan Mountains exhibited regional differences from south to north and from southeast to northwest,with a multi-year average of 930.13 g C/(m~2·a)from 2000 to 2019.The GPP growth rate was highest for the forest type,followed by the shrub type,and the slowest for the grassland type.However,among the types with an increasing GPP trend,the proportion of pixels was highest for the grassland type,followed by the artificial vegetation type,and the lowest for the forest type.Changes in maximum and minimum temperatures had opposite effects on GPP change,with higher maximum temperature promoting GPP growth,while higher minimum temperature inhibiting it.Reduced precipitation was not a major limiting factor for GPP growth in the Hengduan Mountains.Regarding elevation variation,the multi-year mean GPP and change trends of each vegetation type increased and then decreased with increasing elevation.Slope variation was not as significant as elevation variation,and the multi-year mean GPP and change trends of the other eight vegetation types decreased with increasing slope,as well as other vegetation types.The slope direction variation of the multi-year mean GPP and change trends of each vegetation type varied,with the most significant weakening trend of GPP observed on the eastern slope.The relationship between vegetation GPP and growing season in the Hengduan Mountains was found to be close,except for sparse grassland.Among the other eight vegetation types,GPP was mainly negatively correlated with SOS and positively correlated with LOS,with forest types being the most sensitive.The GPP of forest types was mainly negatively correlated with EOS,while grassland types and artificial vegetation were mainly positively correlated.