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Harvard Forest Symposium Abstract 2005

  • Title: Woody Species Phenology, Prospect Hill Tract, Harvard Forest - 2004
  • Primary Author: John O'Keefe (Harvard Forest)
  • Abstract:

    2004 was the fifteenth year in our ongoing investigation of the timing of woody vegetation development during the growing season. However in 2002 the scope of the study was changed significantly. For the first twelve years we observed bud break, leaf development, flowering, and fruit development on three or more individuals of 33 woody species at 3-7 day intervals from April through June. These observations documented substantial (up to three weeks difference) interannual variation in the timing of spring development, but good relative consistency among species and among individuals within species during these twelve years.


    Therefore, starting in 2002 we maintained the same observation schedule, but reduced the number of species observed to eight, including red maple (Acer rubrum), sugar maple (A. saccharum), striped maple (A. pensylvanicum), yellow birch (Betula alleghaniensis), white ash (Fraxinus americana), witch hazel (Hamamelis virginiana), red oak (Quercus rubra), and white oak (Q. alba). This subset of important, representative species should allow us to continue to characterize leaf development each spring, and document inter-annual variability while reducing the resources required for the study significantly.


    We have also recorded fall phenology since 1991, with the exception of 1992. Weekly observations of leaf coloration and leaf fall begin in September and continue through leaf fall. All individuals are located within 1.5 km of the Harvard Forest headquarters at elevations between 335 and 365 m, in habitats ranging from closed forest, through forest-swamp margins, to dry, open fields.


    The winter of 2003-2004 was colder than normal (thanks to a bitterly cold January) with below normal precipitation. Spring turned mild with a very wet April. By summer the pattern was slightly cooler than normal and moist and September was wet and mild. The first frost at Harvard Forest didn’t occur until October 6th, ten days later than the mean first frost date since 1990, but still a week earlier than the first frost in the anomalous fall of 2002.


    For most species initial bud break in 2004 was slightly earlier than the mean (Table 1/Figure 1), putting 2004 in the group of neither very early nor very late years. Leaf development then progressed steadily with 75% leaf development also occurring earlier than the mean. Fall coloration and leaf fall in 2004 were somewhat later than the mean, but a week ahead of the extremely late timing of these events in 2002. Averaging the four species in Figure 1, 50% leaf fall occurred seven days earlier than in 2002, but still one day later than the prior eleven-year mean, which includes the very late fall of 2002.


    The extreme lateness observed in 2002 expanded the variability observed in leaf senescence significantly, so that it more closely resembled the variability observed in leaf emergence over the course of this study, and called into question our previous assumption of considerably less variability in the timing of fall events. These observations point out the need for long-term data sets and emphasize the importance of temperature in regulating these events.


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