| Global climate change has critical impact on terrestrial ecosystems that are dominated by vegetation,and alpine timberline are fragile to climate change.Based on the observed meteorological data of 14 meteorological stations around Taibai Mountain from 2000 to 2015 and the MODIS NDVI(16d,250m×250m resolution)dataset of the corresponding period,we studied the response of the Larix chinensis' s NDVI to climate change in this paper.The temperature threshold suitable for the growth of Larix chinensis was discussed.Based on the data set of MODIS EVI(8d,250m×250m resolution),the start(SOS),end(EOS),and length(LOS)of growth period of Larix chinensis were extracted,the variations of the key phenological parameters and the temporal and spatial responses to the climatic changes at the beginning and ending stage of the phenology were analyzed to reveal the patterns and mechanisms behind response of Larix chinensis to climate change.The research results and progress achieved are as follows:(1)The NDVI of Larix chinensis is sensitive to temperature.The annual maximum NDVI was correlated with the mean temperature from previous August to current July,and the suitable temperature range for the growth of Larix chinensis was-0.4~0.8°C.On the annually scale,the average NDVI of Larix chinensis was positively correlated with the average temperature(96.30%),and the percentage of the pixel number with significant positive correlation(p<0.05)is 31.42%;On the monthly scale,the response of Larix chinensis region mean NDVI to mean air temperature was mostly positive,with the characteristics of synchronism and hysteretic,and the lag time was around 1-3 months.The response pattern of monthly mean NDVI to monthly extreme high temperature is as the same as the monthly mean temperature.The annual maximum NDVI is concentrated in July and August.The results of spatial analysis revealed that the correlation analysis between annual maximum NDVI and the mean air temperature from previous August to current July showed that the number of negative pixel was much more than that of the positive pixel.Furthermore,when the annual mean temperature from previous August to current July was within the range of-0.4~0.8°C,the response of maximum NDVI to air temperature reached the peak.On the other hand,if the temperature was lower than-0.4°C or higher than 0.8°C,the percentage of negative pixel number would increase and the growth of Larix chinensis would be inhibited somehow.Thus,the optimum temperature threshold for the growth of Larix chinensis is-0.4~0.8°C.(2)There is spatial and temporal difference in response of Larix chinensis NDVI to precipitation.Precipitation in May was positively related to the vast majority of NDVI in August(92.99%)at the north slope,but the precipitation in June was negatively related to September NDVI(98.76%).The southern slope is just the opposite.The lag time of NDVI response to precipitation is about 3-6 months,and the precipitation in May is positively correlated with NDVI in August(p<0.05).The precipitation in June is negatively correlated with NDVI in September(p<0.05).The northernnd southern slopes have obvious turning points.The number of pixels related to precipitation positively accounted for 92.99% in May and August NDVI at the northern slope,of which the number of significant positive correlation pixels reached 39.38%,while the positively related pixel number in the southern slope was 77.56%,only 8.93% is with significant positive correlation;The number of pixels negatively related to precipitation in June and September NDVI accounted for 98.76% from the northern slope,of which 37.11% were significant.And 81.26% were negatively related in the southern slope,of which only 9.59% were significant.In summary,the May precipitation is conducive to its August NDVI growth in the northern slope,and the precipitation in June will have an inhibitory effect on September NDVI,while the southern slope is just the opposite.This phenomenon indicates that the precipitation in May and June has a spatio-temporal difference in the growth of the southern and northern aspects of the forest.(3)The annual maximum NDVI of Larix chinensis is affected by the combined effects of climatic factors such as temperature and precipitation.The order of the most important factors is March temperature > December evapotranspiration > May precipitation>January extreme high temperature>July temperature.The partial least-squares regression was used to compare the annual maximum NDVI of Larix chinensis with various climatic factors to the most significant climatic factors(March,July,January extreme high temperature,May precipitation,and December evapotranspiration).For the PLS analysis,the rankings of VIP values for each factor were March temperature(1.17)> December evapotranspiration(1.10)> May precipitation(0.97)> January extreme high temperature(0.96)> July temperature(0.88).Except for precipitation in May,which had a positive effect on the maximum NDVI of Larix chinensis,all other factors had a negative effect on the maximum NDVI of Larix chinensis.(4)The growth period of Larix chinensis is about 168 days.It begins to grow at about 120 days and ends in 288 days.From 2000 to 2015,the key phenological parameters were spatially expressed as follows: the SOS has advanced at a rate of 0.65 days decade-1,the EOS has delayed at a rate of 0.35 days decade-1,and the LOS has prolongated at a rate of 0.99 days decade-1.In the Larix chinensis distributional area,78.78% of the SOS began between the 104 th and the 152 th day,with an average of 120 days.81.12% of the EOS ended at 264-312 days,with an average of 288 days.The 78.86% of LOS is 144-192 days,and the average growth period is 168 days.From 2000 to 2015,the key phenological parameters(SOS,EOS and LOS)of Larix chinensis disributional area on the spatial pixel scale demonstrated: SOS of Larix chinensis showed an advance trend,and the rate of change was-0.65d/ 10a;EOS showed a delayed trend with a rate of change of 0.35d/10a;The average extension of the whole LOS was 0.99d/10 a.(5)The phenology of Larix chinensis in advance was mainly affected by temperature and precipitation,and it was influenced by various climatic factors of the end of the period delayed.Spatial correlations results between temperature,precipitation,sunlight,wind speed,potential evapotranspiration,and the SOS and EOS of Larix chinensis zone at the same periodindicated that the majority of pixels in the SOS were negatively correlated with the temperature and precipitation over the same period,and negatively related pixels accounted for 73.11% and 61.25% respectively,indicating that temperature and precipitation are the important factors affecting the advance phylogenetic about SOS.At the final stage,most of the pixels were positively correlated with temperature(74.91%),precipitation(72.87%),sunshine(78.14%),wind speed(68.60%)and potential evapotranspiration(64.74%)during the same period,indicating that the five climatic factors play an important role in delaying the ending stage of phenology. |
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