Rate of senescing during the first growth period in 2001-2002

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，通过探究沿城乡梯度的植被展叶物候期及其与气候的耦合关系，阐明了城市化驱动的物候与初级生产力改变程度。本研究采用了来自中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）的归一化差分植被指数（Normalized Difference Vegetation Index, NDVI）时序数据，以及空间插值降水数据。分析按主要土地覆被类型与优势土壤质地进行分层，并评估了NDVI对气候因子响应最强的时间尺度。研究未在城乡梯度上发现显著的物候模式，但验证了若干重要共性结论。农业与城市开发会引入生长多峰性，这一现象并非荒漠生态系统所特有，而是该区域河岸生态系统的常见特征。尽管荒漠植被存在夏季生长高峰，但去噪卫星数据并未捕捉到该信号。城市与农业植被以快速生长与衰老速率为特征。尽管农业植被的生长时长最短，但多数城市植被可维持更长时间的光合活性。荒漠植被的生长受2-5个月累积降水量调控，NDVI的空间分布可通过降水网格进行预测。二者间的正相关关系随季节动态变化，在年度生长峰值附近达到最强。春夏季节的NDVI网格与长期累积降水吻合度更高，而初秋的植被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，证实了温度通过促进土壤水分蒸发发挥的调控作用。本研究结果支持了"质地粗糙的土壤可限制土壤水分蒸发损失并促进植被生长"这一假说。河岸植被NDVI与温度呈中等程度正相关，但与降水的相关性较弱。城市与农业土地覆被下的NDVI动态与自然植被群落完全不同步，且与降水解耦，仅与温度呈现微弱的正相关关系。总体而言，城市化增加了物候模式的多样性，这类模式并非由气候变异性决定，城市与农业植被的动态变化主要可通过社会经济变量进行解释。