Spring temperature change and its implication in the change of vegetation growth in North America from 1982 to 2006

Understanding how vegetation growth responds to climate change is a critical requirement for projecting future ecosystem dynamics. Parts of North America (NA) have experienced a spring cooling trend over the last three decades, but little is known about the response of vegetation growth to this change. Using observed climate data and satellite-derived Normalized Difference Vegetation Index (NDVI) data from 1982 to 2006, we investigated changes in spring (April-May) temperature trends and their impact on vegetation growth in NA. A piecewise linear regression approach shows that the trend in spring temperature is not continuous through the 25-year period. In the northwestern region of NA, spring temperature increased until the late 1980s or early 1990s, and stalled or decreased afterwards. In response, a spring vegetation greening trend, which was evident in this region during the 1980s, stalled or reversed recently. Conversely, an opposite phenomenon occurred in the northeastern region of NA due to different spring temperature trends. Additionally, the trends of summer vegetation growth vary between the periods before and after the turning point (TP) of spring temperature trends. This change cannot be fully explained by summer drought stress change alone and is partly explained by changes in the trends of spring temperature as well as those of summer temperature. As reported in previous studies, summer vegetation browning trends have occurred in the northwestern region of NA since the early 1990s, which is consistent with the spring and summer cooling trends in this region during this period.

了解植被生长如何对气候变化作出响应，对于预测未来生态系统动态至关重要。在过去的三十年里，北美洲部分地区经历了春季降温趋势，但人们对植被生长对这种变化的响应知之甚少。利用1982年至2006年观测到的气候数据以及由卫星得出的归一化植被指数（NDVI）数据，我们研究了北美洲春季（4月 - 5月）温度趋势的变化及其对植被生长的影响。分段线性回归方法表明，在25年期间，春季温度趋势并非是连续的。在北美洲西北地区，春季温度一直上升到20世纪80年代末或90年代初，之后停滞不前或下降。相应地，该地区在20世纪80年代明显的春季植被绿化趋势，近期停滞或逆转。相反，由于春季温度趋势不同，北美洲东北地区出现了相反的现象。此外，夏季植被生长趋势在春季温度趋势转折点前后的两个时期有所不同。这种变化不能仅仅用夏季干旱胁迫变化来完全解释，部分原因是春季温度趋势以及夏季温度趋势的变化。如先前研究报告所述，自20世纪90年代初以来，北美洲西北地区出现了夏季植被变黄的趋势，这与该地区在此期间的春季和夏季降温趋势是一致的。