Permanent Plot Vegetation Sampling, 2003 Tree Data BES ID 415-

Permanent plots are a powerful tool for understanding systems over the long term because of the rigorous quantification of patterns in a spatially refined way and because of the comparability of numerous variables measured in the same place. In 1998, 7 plots were surveyed into 3 forests in the study region of the Baltimore Ecosystem Study (BES): Oregon Ridge Park (4 plots), Hillsdale Park (1 plot), and Leakin Park (2 plots). In 2003, the same seven plots were re-surveyed and one in Hillsdale Park was added, for a total of eight plots. Plots were surveyed in order to correct for slope and to achieve an accuracy of plot side length to within 0.5 cm. Plots are 1600 m2 with the exception of the Hillsdale plots which are 900 m2. The Hillsdale plots are smaller to fit within the confines of the forest patch. Sites were selected 1) to represent urban and non-urban forests, 2) away from obvious habitat boundaries or edges, 3) with consistent drainage lines within the plot, and 4) with at least 80 % continuous tree canopy. All vegetation layers were sampled in order to characterize the structure and composition of the plant community. A variety of sampling methods were used for the different layers but all layers were quantified with a high level of detail to minimize variation and to best characterize the plot.

固定样地（permanent plots）是长期生态系统研究的有力工具，其核心优势在于能够以空间精细化的方式对生态格局开展严格量化，且可对同一地点所测定的多项变量进行跨期对比。1998年，研究团队在巴尔的摩生态系统研究（Baltimore Ecosystem Study, BES）的研究区域内，针对3片森林布设并完成了7块固定样地的调查：俄勒冈岭公园（Oregon Ridge Park）4块、希尔斯代尔公园（Hillsdale Park）1块、利金公园（Leakin Park）2块。2003年，研究人员对原有的7块固定样地进行了重测，并在希尔斯代尔公园新增1块样地，样地总数增至8块。本次样地调查过程中，研究人员对坡度进行了校正，并将样地边长的测量精度控制在0.5厘米以内。除希尔斯代尔公园内的固定样地外，其余样地的面积均为1600平方米，希尔斯代尔样地面积为900平方米。希尔斯代尔样地面积较小，以适配该森林斑块的空间范围。样地选址遵循以下四项原则：1）能够代表城市与非城市森林生态系统；2）远离明显的生境边界或边缘地带；3）样地内排水状况一致；4）连续树冠覆盖度不低于80%。研究人员对所有植被层开展了采样，以全面表征植物群落的结构与组成。针对不同植被层，研究团队采用了多样化的采样方法，但所有植被层均通过高精度量化手段进行数据采集，以尽可能降低采样误差，最大化样地特征刻画的准确性。