Long-term growth, mortality and regeneration of trees in permanent vegetation plots in the Pacific Northwest, 1910 to present

A network of more than 130 permanent vegetation plots provides long-term information on patterns and rates of forest succession in most of the major forest zones of the Pacific Northwest. The plot network extends from the coast to the Cascades in western Oregon and Washington and east to ponderosa pine forests in the Oregon Cascades.

Most of the permanent plots were established during two intervals: from 1910 to 1948, and from 1970 to 1989. The earlier plots were established by U.S. Forest Service researchers to quantify timber growth in young stands of important commercial species and to help answer other applied forestry questions. The more recent period of plot establishment began under the Coniferous Forest Biome program of the International Biological Program during the 1970s, and continued under the Long-term Ecological Research program. A broader set of objectives motivated plot establishment since 1970, especially quantification of composition, structure, and population and ecosystem dynamics of natural forests.

Plots have one of three spatial arrangements: (1) contiguous rectangles subjectively placed within an area of homogeneous forest; (2) circular plots subjectively placed within an area of homogeneous forest; and (3) circular plots systematically located on long transects to sample an entire watershed, ridge, or reserve. Rectangular study areas are mostly 1.0 ha or 0.4 ha (1.0 ac) in size (slope-corrected). Circular plots are 0.1 ha (0.247 ac), not corrected for slope. The tree stratum is the focus of work in closed-forest study areas. All trees larger than a minimum diameter (5 cm for most areas) are permanently tagged.

Plots are censused every 5 or 6 years. Attributes measured or assessed at each census include tree diameter, tree vigor, and the condition of the crown and stem. The same attributes are recorded for trees (ingrowth) that have exceeded the minimum diameter since the previous census. In many plots tree locations are surveyed to provide a plot-specific x-y location. A mortality assessment is done for trees that have died since the previous census. The assessment characterizes rooting, stem, and crown condition, obvious signs of distress or disturbance, and the apparent predisposing and proximate causes of tree death.

由130余个永久性植被样地构成的监测网络，可为太平洋西北区域绝大多数主要森林带的森林演替格局与演替速率提供长期观测数据。该样地网络的覆盖范围从俄勒冈州与华盛顿州西部的海岸延伸至喀斯喀特山脉，东至俄勒冈州喀斯喀特山脉内的黄松（Ponderosa pine）林。

绝大多数永久性样地的布设分为两个阶段：1910年至1948年，以及1970年至1989年。早期样地由美国林务局（U.S. Forest Service）的研究人员布设，旨在量化重要商用针叶树种幼林的木材生长量，并解答其他应用林学相关问题。1970年代的样地布设工作，始于国际生物学计划（International Biological Program）下设的针叶林生物群系（Coniferous Forest Biome）项目，并在后续由长期生态研究（Long-term Ecological Research）计划持续推进。1970年以来布设的样地，其监测目标更为多元，核心目标之一是量化天然林的群落组成、群落结构、种群动态与生态系统动态。

样地的空间布设方式分为三类：（1）在均质森林区域内主观布设的连续矩形样地；（2）在均质森林区域内主观布设的圆形样地；（3）沿长样带系统布设的圆形样地，用于对完整流域、山脊或保护区进行抽样调查。矩形样地的面积多为1.0公顷或0.4公顷（1.0英亩），并经过坡度校正。圆形样地面积为0.1公顷（0.247英亩），未进行坡度校正。封闭森林样地的监测重点为乔木层。所有胸径超过最小阈值（多数区域为5厘米）的乔木均会被永久标记。

所有样地每5至6年开展一次清查。每次清查中测定或评估的指标包括乔木胸径、树势、冠层与茎干状况。对于上次清查后胸径已超过最小阈值的新增乔木（Ingrowth），同样记录上述指标。多数样地会对乔木位置进行测绘，以获取样地专属的x-y坐标。针对上次清查后死亡的乔木需开展死亡评估：评估内容包括根系、茎干与冠层状况、明显的胁迫或干扰迹象，以及导致树木死亡的潜在诱因与直接成因。