FOREST BIOPHYSICAL PARAMETERS (SNF)

The purpose of the SNF study was to improve our understanding of the relationship between remotely sensed observations and important biophysical parameters in the boreal forest. A key element of the experiment was the development of methodologies to measure forest stand characteristics to determine values of importance to both remote sensing and ecology. Parameters studied were biomass, leaf area index, above ground net primary productivity, bark area index and ground coverage by vegetation. Thirty two quaking aspen and thirty one black spruce sites were studied. Sites were chosen in uniform stands of aspen or spruce. Aspen stands were chosen to represent the full range of age and stem density of essentially pure aspen, of nearly complete canopy closure, and greater than two meters in height. Spruce stands ranged from very sparse stands on bog sites, to dense, closed stands on more productive peatlands. Diameter breast height (dbh), height of the tree and height of the first live branch were measured. For each plot, a two meter diameter subplot was defined at the center of each plot. Within this subplot, the percent of ground coverage by plants under one meter in height was determined by species. For the aspen sites, a visual estimation of the percent coverage of the canopy, subcanopy and understory vegetation was made in each plot. Dimension analysis of sampled trees were used to develop equations linking the convenience measurements taken at each site and the biophysical characteristics of interest (for example, LAI or biomass). Fifteen mountain maple and fifteen beaked hazelnut trees were also sampled and leaf area determined. These data were used to determine understory leaf area. The total above-ground biomass was estimated as the sum of the branch and bole biomass for a set of sacrificed trees. Total branch biomass was the sum of the estimated biomass of the sampled and unsampled branches. Total biomass is the sum of the branch and bole biomass. Net primary productivity was estimated from the average radial growth over five years measured from the segments cut from the boles and the terminal growth measured as the height increase of the tree. The models were used to back project five years and determine biomass at that time. The change in biomass over that time was used to determine the productivity. Measurements of the sacrificed trees were used to develop relationships between the biophysical parameters (biomass, leaf area index, bark area index and net primary productivity) and the measurements made at each site (diameter at breast height, tree height, crown depth and stem density). These relationships were then used to estimate biophysical characteristics for the aspen and spruce study sites that are provided in this data set. Biomass density was highest in stands of older, larger Aspen trees and decreased in younger stands with smaller, denser stems. LAI remains relatively constant once a full canopy is established with aspen's shade intolerance generally preventing development of LAI greater than two to three. Biomass density and projected LAI were much more variable for spruce than aspen. Spruce LAI and biomass density have a tight, nearly linear relationship. Stand attributes are often determined by site characteristics. However, differences between maximum LAI for aspen and spruce may also be related to differences in the leaf distribution within the canopy.本SNF研究旨在加深对北方森林（Boreal Forest）内遥感观测（Remotely Sensed Observations）与重要生物物理参数（Biophysical Parameters）之间关联的认知。 本实验的核心环节为开发森林林分特征的测量方法，以获取对遥感与生态学均具有重要价值的参数值。 本次研究的参数包括生物量（Biomass）、叶面积指数（Leaf Area Index, LAI）、地上净初级生产力（Above-ground Net Primary Productivity）、树皮面积指数（Bark Area Index）以及植被地面覆盖率（Ground Coverage by Vegetation）。 共选取32个美洲山杨（Quaking Aspen）样地与31个黑云杉（Black Spruce）样地开展研究。 所有样地均选自纯山杨林或纯黑云杉林的均匀林分。 山杨林样地需覆盖几乎纯林、林分郁闭度（Canopy Closure）接近100%且树高大于2米的全龄级与全茎密度范围。 黑云杉林样地涵盖了沼泽生境下的极稀疏林分，至生产力更高的泥炭地上的致密郁闭林分。 本研究测定了胸径（Diameter at Breast Height, DBH）、树高以及首活枝高度（Height of the First Live Branch）。 每个样地的中心均设置一个直径2米的副样地（Subplot）。 在该副样地内，按物种统计高度1米以下植物的地面覆盖百分比。 针对山杨样地，研究人员对每个样地的冠层（Canopy）、亚冠层（Subcanopy）与林下植被（Understory Vegetation）的覆盖百分比进行目视估算。 通过对采样树木的维度分析（Dimension Analysis），建立起样地便捷测量指标与目标生物物理参数（如叶面积指数LAI、生物量）之间的关联方程。 本研究同时采集了15株山枫（Mountain Maple）与15株榛树（Beaked Hazelnut）样本，并测定其叶面积。 上述数据用于计算林下叶面积（Understory Leaf Area）。 对一批伐倒木，其总地上生物量（Above-ground Biomass）估算为枝生物量（Branch Biomass）与干生物量（Bole Biomass）之和。 总枝生物量为采样枝条与未采样枝条的估算生物量之和。 总生物量为枝生物量与干生物量之和。 地上净初级生产力通过以下方式估算：从树干截取的样本段测得的5年平均径向生长量（Radial Growth），以及以树高增量表征的顶端生长量（Terminal Growth）。 利用所建模型反推5年前的生物量。 基于该时段内的生物量变化量计算净初级生产力。 基于伐倒木的测量数据，建立生物物理参数（生物量、叶面积指数LAI、树皮面积指数与净初级生产力）与样地测量指标（胸径DBH、树高、冠层深度（Crown Depth）与茎密度（Stem Density））之间的关联关系。 随后利用上述关联关系，估算本数据集所包含的山杨与黑云杉样地的生物物理特征。 生物量密度（Biomass Density）在老龄、大径级山杨林分中最高，而在茎径更小、密度更高的幼龄林分中更低。 山杨因不耐阴（Shade Intolerance），当林分形成完整冠层后，叶面积指数LAI基本维持稳定，通常不会超过2~3。 黑云杉林分的生物量密度与预测LAI的变异性远高于山杨林。 黑云杉的LAI与生物量密度之间存在紧密的近乎线性的关联。 林分特征通常由生境条件决定。 但山杨与黑云杉的最大LAI差异，也可能与冠层内叶片分布的差异有关。