近二十年來，科學(xué)家們注意到北極的苔原棲息地發(fā)生了巨大的變化。腳踝那么高的草和樹籬已經(jīng)騰出地兒，讓位給了長到腰或脖子那么高的大片灌木叢。苔原變灌木，對像北美馴鹿這樣的動物提出了挑戰(zhàn)，后者一般只適應(yīng)較低高度的北美植被。和低緯度地區(qū)的樹木一樣，每個生長季節(jié)里，凍原上的灌木會在它們的莖周圍形成同心環(huán)。一個研究小組利用了這些像年輪一樣的同心環(huán)的特點，收集了不同土壤條件下的數(shù)百株灌木莖。研究小組發(fā)現(xiàn)，無論土壤是否肥沃，或降雨量如何變化，迄今為止決定一年內(nèi)灌木生長情況的最大因素是六月的氣溫。六月氣溫越高，則灌木生長越旺盛。

Rings around the rosy

For nearly two decades, scientists have noted dramatic changes in Arctic tundra habitat. Ankle-high grasses and sedges have given way to a sea of woody shrubs growing to waist- or neck-deep heights. This shrubification of the tundra challenges animals like caribou that are adapted to low-stature arctic vegetation. Like trees at lower latitudes, shrubs in the tundra form concentric rings around their stems each growing season. A research team used these annual rings to their advantage, collecting hundreds of shrub stems across a range of soil conditions. The team's research found that regardless of soil fertility or rainfall amounts, the single variable that was by far the strongest determinant of how much a shrub grew in a given year was the temperature in June. A warmer June means faster shrub growth.

Image credit: Daniel Ackerman

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