Size-density data from pure even-aged stands of sycamore maple (Acer pseudoplatanus L.) growing at maximum density

For sycamore, data from unthinned long-term permanent plots were not available (Le Goff et al, 2011). The establishment of the maximum size–density relationship for sycamore relied only on temporary plots installed in spring 2006 in fully stocked stands with no sign of recent cleaning or thinning treatment and presenting natural mortality, in Lorraine and Alsace regions (NE France). In these plots live trees were generally inventoried at two dates for the purpose of this study, in years 2006 and 2007. Dead trees at plot installation were also inventoried by sorting them by “age” class (see Le Goff et al. 2011). In the sample plots, sycamore trees appeared often mixed with other tree species (most often ash and beech) and may be also over-topped by an upper layer of older trees reducing significantly the transmitted radiation. Selection criteria were then defined to ensure homogeneous growing conditions among selected plots: 1) stands relatively pure, sycamore trees representing more than 80% of the stand, in terms of number of trees and total basal area 2) minimum annual mortality rate of 1%, so that stands were close to maximum density 3) trees not completely covered by an upper layer. Moreover, the data from plot 5 of “Matstall” in 2007 were discarded, an over-topping tree being removed at the end of 2006. The selection criteria are notified for each plot in Table 2 of the related publication, but they also appear in the dataset. Inventories were extended for one or more years beyond the year 2007, depending on the plot (variable “Sel-data” = “extra” in the data set), allowing representing a part of the size-density trajectory of sycamore at maximum density, with an insight into the possible impacts of species mixture and upper layer cover on the trajectory. The following data appear for each inventoried plot in the sycamore dataset: “Plot” (name of plot location), “Dep” (department number), NoPlot (plot number), “Cover“ (upper layer cover: “none”, “half-covered gap”, “whole covered gap”), “Mixed” (other species present: “no”, “yes”), “Mixed-species” (species in mixture, if any), “Sel-data” (“yes”, if inventory year data were selected for maximum size-density line fitting in the related publication, “no” or “extra data” if not), “Year“ (year of inventory), “Age“ (stand age, in years), “Ntot” (total number of live trees per ha), “Nsyc%” (relative number of sycamore live trees, in %), “Gtot” (total basal area of live trees, m2 ha-1), “Gsyc%” (sycamore basal area, in %), “Cg” (quadratic mean girth at breast height of all live trees, cm), “Hg” (mean height of sycamore trees at the date of plot installation, m).

悬铃木（sycamore）尚无来自未间伐长期固定样地的相关数据（Le Goff等，2011）。本研究中悬铃木最大胸径-密度关系的构建，仅依托2006年春季在法国东北部洛林与阿尔萨斯地区设置的临时样地完成。该类样地所在林分充分郁闭，无近期抚育或疏伐痕迹，且存在自然枯亡现象。为满足本研究需求，此类样地内的活立木通常在2006年与2007年两个时间节点开展调查。样地设置时已枯亡的林木，亦按"年龄"等级分类开展调查（详见Le Goff等，2011）。在样地中，悬铃木常与其他树种混交（最常见的为白蜡与山毛榉），且可能被上层老龄林冠覆盖，导致接收到的透射辐射显著降低。随后制定了筛选标准，以确保入选样地的立地生长条件均一：1）林分相对纯正，按林木株数与总胸高断面积计算，悬铃木占比均超过80%；2）年枯亡率不低于1%，以保证林分接近最大密度；3）林木未被上层林冠完全覆盖。此外，2007年"Matstall"5号样地的数据被剔除，原因是该样地内的上层覆盖林木已于2006年末被移除。相关论文的表2中列出了每个样地对应的筛选标准，该信息亦收录于本数据集内。根据样地不同，调查工作延长至2007年之后的1年或多年（数据集内变量"Sel-data"取值为"extra"时即代表此类情况），借此得以呈现悬铃木在最大密度下的部分胸径-密度动态轨迹，并可探究树种混交与上层林冠覆盖对该轨迹的潜在影响。本悬铃木数据集包含每个调查样地的如下数据："Plot"（样地位置名称）、"Dep"（法国行政省编号）、"NoPlot"（样地编号）、"Cover"（上层林冠覆盖情况："none"、"half-covered gap"、"whole covered gap"）、"Mixed"（是否伴生其他树种："no"、"yes"）、"Mixed-species"（伴生树种，若有）、"Sel-data"（若调查年份数据用于相关论文中最大胸径-密度关系拟合则为"yes"，否则为"no"或"extra data"）、"Year"（调查年份）、"Age"（林分年龄，单位：年）、"Ntot"（每公顷活立木总株数）、"Nsyc%"（悬铃木活立木相对株数占比，单位：%）、"Gtot"（活立木总胸高断面积，单位：m²·ha⁻¹）、"Gsyc%"（悬铃木胸高断面积占比，单位：%）、"Cg"（所有活立木的胸高断面积平均胸径，单位：cm）、"Hg"（样地设置时悬铃木的平均树高，单位：m）。