Length of the first growth period in 2001 (Number of days)

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都会区为研究区域，通过探究沿城乡梯度分布的植被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，以及空间插值降水数据。分析过程按主要土地覆盖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。研究未发现城乡梯度上存在显著的物候格局，但验证了若干重要共性结论。农业与城市开发引入了生长多峰性，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峰值，但去噪卫星遥感数据未能捕捉到该信号。城市与农业植被以快速生长与衰老速率为特征：农业植被的生长周期最短，而多数城市植被的光合活性持续时长更长。荒漠植被的生长受2-5个月累积降水量调控。NDVI的空间分布格局可通过降水网格数据进行预测，二者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。春季与夏季的NDVI网格数据与长期累积降水的吻合度更高，而初秋的生长状况则与即时降水的相关性更强。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而对植被生长产生的抑制作用。研究结果支持了“质地粗糙的土壤会限制土壤水蒸发损失并促进植被生长”这一假说。河岸植被NDVI与温度呈中度正相关，但与降水的相关性较弱。城市与农业土地覆盖类型下的NDVI动态与自然植被群落完全不同步，且与降水解耦；它们与温度呈微弱的正相关关系。总体而言，城市化增加了不受气候变率调控的物候格局多样性，城市与农业植被的动态变化应主要由社会经济变量予以解释。