End of the first growth period in central Arizona-Phoenix 2001-2002 (Day of Year)

By studying vegetation leafing phenology and its coupling with climate along the urban-rural gradient in Phoenix metropolitan region, USA, we elucidated the degree of urbanization-induced transformations of phenology and primary productivity. In this study we used time-series of Normalized Difference Vegetation Index (NDVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and spatially interpolated rainfall. Our analyses were stratified by major land covers and dominant soil texture. We also assessed time scales at which NDVI responds most strongly to climatic factors. No distinctive patterns in phenology were found along the urban-rural gradient; however some important generalities were confirmed. Agricultural and urban developments introduce growth multimodality, which is not attributable to desert but customarily found in riparian ecosystems of the area. Despite the existence of summer flush of growth in the desert its signal is not detected by de-noised satellite data. Urban and agricultural vegetation is characterized by fast growth and senescence rates. While agriculture has the shortest growth length, most urban vegetation stays photosynthetically active for longer periods. Growth in the desert is controlled by precipitation accumulated for 2-5 months. Spatial patterns of NDVI are predicted by precipitation grids. Positive relationship between these two variables changes seasonally reaching the maximum near the peak of annual growth. Spring and summer NDVI grids are in better agreement with longer term accumulated precipitation, but the early autumn growth is correlated more with immediate rainfall. Spatial and temporal correlations of desert NDVI with temperature are negative confirming the role of temperature in stimulating water loss from the soil. Our results supported the hypothesis that coarse-textured soils limit evaporative losses of soil water and promote growth. Riparian NDVI are moderately positively correlated with temperature but only weakly with precipitation. NDVI dynamics in urban and agricultural land covers are completely unsynchronized with natural vegetation communities and decoupled with precipitation. They exhibit positive, yet low, correlation with temperature. Overall, urbanization adds a greater diversity of phenological patterns that are not determined by climatic variability. Instead, urban and agricultural vegetation dynamics is expected to be explained largely by socio-economic variables.本研究以美国凤凰城大都市区为研究区域，针对沿城乡梯度（urban-rural gradient）的植被返青物候及其与气候的耦合关系展开分析，阐明了城市化诱导的物候与初级生产力（primary productivity）的转变程度。本研究采用中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）获取的归一化差分植被指数（Normalized Difference Vegetation Index, NDVI）时间序列数据与空间插值降雨数据，按主要土地覆盖类型与优势土壤质地（soil texture）对分析过程进行分层处理，并评估了NDVI对气候因子响应最为显著的时间尺度。尽管沿城乡梯度未发现显著的物候模式，但本研究验证了若干重要共性规律：农业与城市开发引入了生长多峰性（multimodality），该现象并非荒漠生态系统所特有，而是该区域河岸生态系统（riparian ecosystems）的常见特征；尽管荒漠植被存在夏季生长旺季，但去噪卫星数据（de-noised satellite data）未能捕捉到该信号。城市与农业植被以快速生长与衰老速率为典型特征，其中农业植被的生长周期最短，而多数城市植被可维持更长时间的光合活性（photosynthetically active）。荒漠植被的生长主要受2-5个月累积降水量调控；NDVI的空间分布可通过降水格网数据预测，二者的正相关关系呈季节动态，在年度生长旺季附近达到峰值。春季与夏季的NDVI格网数据与长期累积降水量吻合度更高，而初秋生长则与即时降雨相关性更强。荒漠NDVI与温度的时空相关性为负值，证实了温度通过促进土壤水分蒸发损失（evaporative losses）发挥的调控作用。本研究结果验证了“质地较粗的土壤可抑制土壤水分蒸发损失并促进植被生长”的假说。河岸植被的NDVI与温度呈中等程度正相关，但与降水量相关性较弱。城市与农业土地覆盖下的NDVI动态与自然植被群落完全不同步，且与降水量解耦，仅与温度呈现微弱正相关。总体而言，城市化增加了更多样化的物候模式，此类模式并非由气候变异决定，反之城市与农业植被的动态变化在很大程度上可由社会经济变量（socio-economic variables）解释。