Ecological Importance of Large-Diameter Trees in a Temperate Mixed-Conifer Forest

Large-diameter trees dominate the structure, dynamics and function of many temperate and tropical forests. Although both scaling theory and competition theory make predictions about the relative composition and spatial patterns of large-diameter trees compared to smaller diameter trees, these predictions are rarely tested. We established a 25.6 ha permanent plot within which we tagged and mapped all trees ≥1 cm dbh, all snags ≥10 cm dbh, and all shrub patches ≥2 m2. We sampled downed woody debris, litter, and duff with line intercept transects. Aboveground live biomass of the 23 woody species was 507.9 Mg/ha, of which 503.8 Mg/ha was trees (SD = 114.3 Mg/ha) and 4.1 Mg/ha was shrubs. Aboveground live and dead biomass was 652.0 Mg/ha. Large-diameter trees comprised 1.4% of individuals but 49.4% of biomass, with biomass dominated by Abies concolor and Pinus lambertiana (93.0% of tree biomass). The large-diameter component dominated the biomass of snags (59.5%) and contributed significantly to that of woody debris (36.6%). Traditional scaling theory was not a good model for either the relationship between tree radii and tree abundance or tree biomass. Spatial patterning of large-diameter trees of the three most abundant species differed from that of small-diameter conspecifics. For A. concolor and P. lambertiana, as well as all trees pooled, large-diameter and small-diameter trees were spatially segregated through inter-tree distances大径级树木主导了诸多温带与热带森林的结构、动态与功能。尽管缩放理论（scaling theory）与竞争理论（competition theory）均对大径级树木相较于小径级树木的相对组成及空间格局做出了预测，但此类预测极少得到验证。我们设立了一块面积为25.6公顷的永久样地，对其中所有胸径（dbh, diameter at breast height）≥1厘米的树木、所有胸径≥10厘米的枯立木（snags），以及所有面积≥2平方米的灌丛斑块进行了标记与定位。我们采用样线截距法（line intercept transects）对倒木残体、枯落物及腐殖质层进行了采样。23种木本植物的地上活生物量为507.9 Mg/ha，其中树木生物量为503.8 Mg/ha（标准差SD=114.3 Mg/ha），灌丛生物量为4.1 Mg/ha。地上活生物量与死生物量总计为652.0 Mg/ha。大径级树木仅占个体总数的1.4%，却贡献了49.4%的生物量，其生物量以白冷杉（Abies concolor）和糖松（Pinus lambertiana）为主，占树木总生物量的93.0%。大径级组分主导了枯立木的生物量（59.5%），并对倒木残体的生物量贡献显著（36.6%）。传统缩放理论既无法很好地拟合树木半径与树木丰度的关系，也无法很好地拟合树木半径与生物量的关系。三种优势木本植物的大径级个体空间格局与同种小径级个体存在差异。就白冷杉（A. concolor）、糖松（P. lambertiana）以及所有合并统计的树木而言，大径级与小径级树木在树间距离维度上呈现空间隔离格局。