Establishing baseline phenology trends for northern hardwood trees along an elevation gradient

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Votta, Kim
There is ample evidence that climate change has notable effects on the phenology of vegetation, effects that are variable at different elevations and across species. Two years of climate and phenology records were collected and analyzed to establish baseline phenology trends and temperature thresholds for four northern hardwood tree species. Seventeen deciduous trees were observed along an elevation transect spanning ~4 km on 1,220 m Mount Tecumseh in New Hampshire. Spring and autumn phenology observations (e.g. budburst, first leaf color) were conducted weekly to twice weekly throughout the growing (budburst through leaf elongation) and senescence (first leaf color and drop through last leaf color and drop) seasons. Microclimate measurements were obtained via dataloggers measuring ambient air temperature placed at regular intervals along the elevation transect. Phenology and microclimate data were analyzed in combination with a larger meteorological dataset obtained from the Lorenz Weather Station atop the summit ridge of Mount Tecumseh. Spring comes later and autumn sooner at higher elevations on Mount Tecumseh. This trend in phenology is in accord with the noted delay in spring and the advance in autumn calculated along the elevation gradient (1.03 days for every 30 m increase in elevation in spring and 1.46 days in autumn.) Mid8elevation trees did not always follow this trend. Threshold temperatures for specific phenological stages were not established as the spring phenophases and the autumn leaf fall phenophases showed wide variation in accumulated degree8days from one year to the next. Only leaf color in autumn indicated a possible threshold in accumulated chilling degree8days for triggering that particular phenology. A comparison among the Mount Tecumseh study and two other regional studies of northern hardwood trees and their phenology was conducted. Results on Mount Tecumseh confirmed the order of progression of spring and autumn phenology for several northern hardwood tree species as evidenced at Hubbard Brook Experimental Forest in Thornton, NH. There is a marked advance in spring phenology and a delay in autumn, evidenced by comparison between Mount Tecumseh and Mount Mansfield in Underhill, VT. Overall, phenology progression patterns among species between the two most recent studies were similar. Continued efforts are necessary to ensure the ongoing support of phenology monitoring sites. Long8term data records will be necessary to clarify seasonal trends in phenology and assess the ongoing and future impacts of climate change on the northeastern forest.