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Beal, Kimberley S. K.

Plant Biology

2009

MS

The ecotone from closed forest to treeless alpine tundra in montane regions is expected to be sensitive to climate change. Treelines are expected to shift to higher elevations with global warming, but are also determined by non-climatic factors such as slope and disturbance. We examined the sensitivity of treeline to regional climate change at the summits of three mountains in New England. Our objectives were to determine whether treelines had moved upslope, whether tree islands had expanded and whether new tree islands had established above treeline in relation to regional climate change. We addressed these objectives by replicating historical photographs to examine changes in vegetation cover at the treeline of three New England mountains, Mt. Mansfield of Vermont, the White Mountains of New Hampshire, and Katahdin of Maine.
The ages of the historical photographs ranged from 1875 to 1981 with an average age of 78. We overlaid and separately photo-interpreted each mountain-slope in the paired photographs. We found that tree-cover increased at 78 of 116 slopes: 19 of these 78 have new tree islands, while the remaining slopes have expanded tree islands. Treeline shifted to a higher elevation for 35 of 53 slopes with a closed-canopy treeline. Increase in total winter precipitation as well as steep south-facing slopes were correlated with increased tree-cover. Steep slopes alone and steep northwestern-facing slopes were negatively correlated with increased tree-cover. Older historical photographs showed more increase by 2007 than more recent photographs. The expansion and upward movement of treelines at our study sites across New England correspond to a period of regional warming, suggesting that changing climate (especially increased winter precipitation) may be the driver of observed patterns.