DOI: 10.6073/pasta/bc5d2c15df4fa81aeadcd59ed7580c91
title: Phenology of Woody Species at Harvard Forest since 1990

description: Starting in the spring of 1990 we have observed the timing of woody vegetation development during the growing season. For the first twelve years (1990-2001) we observed bud break (50% leaf emergence) and leaf development (75% final size) on two to five permanently tagged individuals of 33 woody species at 3-7 day intervals from April through June. 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. For most species both overstory and understory individuals are represented.
 Bud break is defined as when 50% of the buds on the individual have recognizable leaves emerging from them. Leaf development (75%) is defined as when at least 75% of the leaves on the individual have reached 75% of their final (mature) size. This point is used rather than "fully developed" because the leaves are functional but still developing rapidly during this period, which permits better estimation of a date between observations. Flowering and fruit development are also recorded if they occurred during the observation period.
 Through 2001 these observations documented substantial (up to three weeks difference) inter-annual variation in the timing of leaf emergence (50%) and leaf development (75% final size), but good relative consistency among species and among individuals within species during these twelve years.
 Therefore, starting in 2002 we maintained the same observation protocol, but reduced the number of species observed to nine, including striped maple (Acer pensylvanicum), red maple (A. rubrum), sugar maple (A. saccharum), yellow birch (Betula alleghaniensis), beech (Fagus grandifolia), white ash (Fraxinus americana), witch hazel (Hamamelis virginiana), white oak (Q. alba) and red oak (Q. rubra). This subset of important, representative species allows 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 percent leaf coloration and percent leaf fall begin in September and continue through leaf fall. Observations of leaf drop through 2001 exhibited less variation than those of leaf emergence, but similar relative consistency among species and among individuals within species. In 2002 we reduced the number of species observed in the fall to fourteen, including red maple (Acer rubrum), sugar maple (A. saccharum), striped maple (A. pensylvanicum), shadbush (Amelanchier laevis), yellow birch (Betula alleghaniensis), black birch (B. lenta), paper birch (B. papyrifera), beech (Fagus grandifolia), white ash (Fraxinus americana), black gum (Nyssa sylvatica), black cherry (Prunus serotina), white oak (Quercus alba), (Q. rubra) and black oak (Q. velutina).
 Although leaf fall continues to exhibit somewhat less variability than leaf emergence, very late leaf drop in 2002 and 2005 has expanded leaf drop variability significantly.