How climate affects the fall phenology of the northern hardwood forest

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VanGyzen, James M.
Phenological phases are strongly correlated with their environment in attempts to maximize their potential nutrient growth and efficiency. Because of this adaptation changes in the environment can been seen through phenological observations of predetermined events. If these events or phenophases are recorded annually we may be able to infer changes in climate if these phenophases occur later or earlier than expected. This is especially important with the recent attention given to climate change scenarios attributable to global warming. Meteorological conditions such as light, temperature, and light intensity are influential in determining the timing of phenophases during both the Fall and Spring seasons. Fall phenological phases such as leaf senescence and abscission were recorded on Mt. Tecumseh, Waterville valley, New Hampshire and Mt. Starr King, Jefferson, New Hampshire. Twenty canopy trees were monitored for fall phenophases on each research site comprised of five common northern hardwood species: yellow birch (Betula alleghaniensis), paper birch (Betula papyrifera), mt. paper birch (Betula cordifolia), sugar maple (Acer saccharum), and American beech (Fagus grandifolia). Along with phenological observations, ambient air and soil temperatures were recorded for the development of an environmental lapse rate and thermal time estimation. Nonlinear and linear mixed effects models were used to detect species preferences of environmental factors in the mediation of fall phenophases. The factors considered were photoperiod, incoming solar radiation, accumulated degree days, day of year, elevation, and species type. It was hypothesized that the influence of light was greater than temperature on fall phenology. The influence of solar radiation and photoperiod were strong for leaf senescence phenophases for sugar maple and both leaf senescence and abscission phenophases for American beech. Temperature was a strong indication of both leaf senescence and abscission phenophases for paper birch, and leaf abscission phenophases for yellow birch and sugar maple. The day of year in which fall phenophases occurred were strongly affected by elevation with evidence suggesting the effect of species type on phenological progression as well. Hardwood species subjected to variable climates at high elevations such as mt. paper birch would reduce their chances of poor phenological timing by tracking temperature. However, climate warming is gradually extending the autumn season which may falsely lead this species to extend their fall phenophases later into the season. This response can lead to an increased chance of late autumn frosts and therefore canopy dieback. Under this scenario, this species would increase its survival by tracking light cues such as photoperiod or perceived incoming solar radiation. In contrast, species such sugar maple, American beech, and possibly yellow birch preferably track light when regulating their plant development phases. These species which on average occupy lower elevation sites than mt. paper birch would benefit by tracking temperature which could extend their growing season. Lower elevation sites do not experience the large environmental fluctuations as mid-high elevation sites do. Hardwood trees are highly sensitive to their environment, which is necessary to increasing their potential growth and therefore survival. Because of this, microclimatic differences between sites and within sites influence the progression of fall phenophases. Therefore, light and temperature are not the only factors which control fall phenology. Factors such as nitrogen, water availability, soil fertility, and CO2concentrations would be useful components to explain the variability in fall phenology trends.