The Importance of Large-Diameter Trees to Forest Structural Heterogeneity

Large-diameter trees dominate the structure, dynamics and function of many temperate and tropical forests. However, their attendant contributions to forest heterogeneity are rarely addressed. We established the Wind River Forest Dynamics Plot, a 25.6 ha permanent plot within which we tagged and mapped all 30,973 woody stems ≥1 cm dbh, all 1,966 snags ≥10 cm dbh, and all shrub patches ≥2 m2. Basal area of the 26 woody species was 62.18 m2/ha, of which 61.60 m2/ha was trees and 0.58 m2/ha was tall shrubs. Large-diameter trees (≥100 cm dbh) comprised 1.5% of stems, 31.8% of basal area, and 17.6% of the heterogeneity of basal area, with basal area dominated by Tsuga heterophylla and Pseudotsuga menziesii. Small-diameter subpopulations of Pseudotsuga menziesii, Tsuga heterophylla and Thuja plicata, as well as all tree species combined, exhibited significant aggregation relative to the null model of complete spatial randomness (CSR) up to 9 m (P≤0.001). Patterns of large-diameter trees were either not different from CSR (Tsuga heterophylla), or exhibited slight aggregation (Pseudotsuga menziesii and Thuja plicata). Significant spatial repulsion between large-diameter and small-diameter Tsuga heterophylla suggests that large-diameter Tsuga heterophylla function as organizers of tree demography over decadal timescales through competitive interactions. Comparison among two forest dynamics plots suggests that forest structural diversity responds to intermediate-scale environmental heterogeneity and disturbances, similar to hypotheses about patterns of species richness, and richness- ecosystem function. Large mapped plots with detailed within-plot environmental spatial covariates will be required to test these hypotheses.

大径级林木主导了众多温带与热带森林的结构、动态与功能，但其对森林异质性的贡献却鲜有研究涉及。本研究建立了25.6公顷的永久样地——温德河森林动态样地（Wind River Forest Dynamics Plot），对样地内所有胸径（diameter at breast height, dbh）≥1厘米的30973株木本茎干、所有胸径≥10厘米的1966株枯立木（snags），以及面积≥2平方米的所有灌丛斑块进行了标记与测绘。26种木本植物的胸高断面积（basal area）总计为62.18平方米/公顷，其中林木贡献61.60平方米/公顷，高大灌丛贡献0.58平方米/公顷。胸径≥100厘米的大径级林木占茎干总数的1.5%、胸高断面积的31.8%，以及胸高断面积异质性的17.6%，其胸高断面积优势种为西部铁杉（Tsuga heterophylla）和花旗松（Pseudotsuga menziesii）。花旗松、西部铁杉与北美乔柏（Thuja plicata）的小径级亚种群，以及所有林木物种的合并种群，在9米尺度内均相较于完全空间随机性（complete spatial randomness, CSR）零模型呈现出显著的空间聚集格局（P≤0.001）。大径级林木的空间格局则要么与完全空间随机性无显著差异（西部铁杉），要么仅呈现微弱的空间聚集（花旗松与北美乔柏）。大径级与小径级西部铁杉之间存在显著的空间排斥效应，这表明大径级西部铁杉可通过种间竞争作用，在十年尺度上调控林木种群动态。对两处森林动态样地的对比分析表明，森林结构多样性响应中等尺度的环境异质性与生态干扰过程，这与物种丰富度格局以及丰富度-生态系统功能相关假说的结论一致。后续需依托带有详细样地内环境空间协变量的大型测绘样地，对上述假说开展验证。