Hemlock Mapped Tree Plot at Harvard Forest since 1990

Most of the central New England landscape was cleared for agriculture in the
mid-19th century and then naturally reforested into "secondary forests" with the
abandonment of agricultural land. Some sites, often poorly drained, remained
forested, but were usually subjected to intensive fuelwood cutting or logging and are
termed "primary forests." The Hemlock Woodlot was never cleared for agriculture, but
has a history of cutting and natural disturbance. The hemlock woodlot is located in
the center of Harvard Forest's Prospect Hill Tract, adjacent to a spruce-blackgum
swamp. Soils are moist and rocky, with a thick organic layer. Hemlock dominates tree
species composition (62% by basal area), with hardwoods and scattered large white
pine comprising the remainder. Most of the trees are 100-150 years old, with a few
hemlock trees up to 230 years old. While the site was never cleared for agriculture,
it was logged several times and chestnut blight removed a chestnut-dominated
overstory in the 1910s. The 0.72 ha stem-mapped plot is at the center of a 4-ha
hemlock-dominated forest. This plot serves as a major reference site and is part of a
network of hemlock forests that are being intensively sampled as the hemlock woolly
adelgid arrives.

新英格兰中部的绝大多数区域于19世纪中期被开垦为农田，后随农业用地被弃置，经自然演替形成次生林（secondary forests）。部分因排水条件不佳得以保留原生植被的区域，却通常遭受过高强度薪柴砍伐或林木采伐，这类森林被称为原生林（primary forests）。铁杉林地块（Hemlock Woodlot）从未被开垦为农田，但存在人为砍伐与自然干扰的历史。该地块位于哈佛大学森林（Harvard Forest）展望山片区（Prospect Hill Tract）的中心地带，毗邻云杉-紫树沼泽（spruce-blackgum swamp）。其土壤湿润多石，且发育有深厚的有机质层。林分组成以铁杉占绝对优势（按断面积计占比62%），剩余组分包含阔叶树种与零星分布的大型白松。多数林木的树龄介于100至150年之间，仅少量铁杉个体树龄可达230年。尽管该地块未被开垦为农田，但曾经历多次采伐，且1910年代爆发的栗疫病彻底清除了原本以栗树为优势种的上层林冠。这片面积0.72公顷的定位测树样地（stem-mapped plot）坐落于一片4公顷铁杉优势林的核心区域，作为核心参考样地之一，它隶属于一个铁杉林监测网络——随着铁杉球蚜（hemlock woolly adelgid）的入侵，该网络内的铁杉林正被开展高强度采样监测。