64. Longer Growing Season Decreases Peak Summer Vegetation Productivity in North American Ecosystems

During the last decades, snow-dominated Northern American (NA) ecosystems have experienced prolonged spring warming and earlier spring onsets. Recent studies showed that these changes have adverse effects on plant productivity due to water stress. In this study, we analyze relationships between longer non-frozen period, peak summer vegetation greenness index and a drought index by using the three decades of optical satellite data, microwave Freeze-Thaw record and climatic data. Results show that longer non-frozen period caused decline in summer soil moisture availability. This suggests a mechanism of ‘longer non-frozen period-summer drought’ which has led to widespread decline in peak summer vegetation greenness across large portions of NA’s mid- and high-latitude ecosystems. This mechanism may be exacerbated in the future under accelerated spring warming and associated longer growing season, and depending on the future precipitation pattern generally larger degrees of dryness during the peak of the northern growing seasons can be expected.