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.19世纪中叶，新英格兰中部的绝大多数区域均被开垦为农田，后续随着农田弃耕，自然演替形成了‘次生林’。部分区域因排水条件普遍较差而得以保留林地，但通常经历过高强度薪柴砍伐或林木采伐，这类林地被称为‘原始林’。铁杉林块（Hemlock Woodlot）从未被开垦为农田，但曾有过采伐与自然干扰的历史。该林块坐落于哈佛森林（Harvard Forest）展望山片区（Prospect Hill Tract）的中心区域，毗邻一片云杉-紫树沼泽。其土壤湿润且多岩，伴有厚层有机质层。铁杉在林木物种组成中占主导地位（按胸高断面积（basal area）计占比62%），其余组分则为阔叶树与零星分布的大型白松。绝大多数林木的树龄为100至150年，少量铁杉的树龄可达230年。尽管该地块从未被开垦为农田，但曾经历过多次采伐，且在20世纪10年代，板栗疫病（chestnut blight）摧毁了原本以栗树为主的上层林冠。这片面积0.72公顷的茎干测绘样地（stem-mapped plot），坐落于一片4公顷铁杉主导林分的核心区域。该样地是核心参考样地之一，同时隶属于一个铁杉林监测网络——随着铁杉球蚜（hemlock woolly adelgid）入侵该区域，该网络内的铁杉林正被实施高强度采样监测。