Developing crown width model for mixed forests using soil, climate, and stand factors

The tree crown is a useful measure of tree vigor and is highly relevant to
a tree's environmental adaptability. Crown allometry depends on
environmental and stand conditions. Several studies have focused on the
effects of climate change and competitive intensity on the crown, but the
regulatory role of soil resources and diversity on crown allometry and
carbon allocation has been neglected. Data from 20,994 trees in 232 mixed
forests collected between 2011 and 2019 was located near four major
mountain ranges in northeast China. The proposed crown width model
includes the stand developmental stage, soil, climate, competition
intensity, species mixture, species diversity, structural diversity, and
their interactions. We observed that the cross-species allometric scaling
exponent does not conform to the universal scaling law. Our results showed
that crown width increased with increasing soil bulk density, quadratic
mean diameter, and coefficient of diameter variation but decreased with
increasing de Martonne aridity index, basal area, Simpson index, and
species mixture. The interaction of quadratic mean diameter and soil bulk
density had a significant negative effect on crown width. The influence of
a particular factor within the interaction term on crown width was
modulated by the gradients of other factors. Furthermore, soil bulk
density contributed more to crown width modeling than the aridity index,
and structural diversity had a greater effect on crown width than species
diversity. Synthesis. Our results provide new insights into the
environmental variability of crown allometry in mixed forests under global
change, which is critical for improving regional and global estimates of
forest biomass and carbon stocks.

树冠是评估树木活力的有效指标，且与树木的环境适应性密切相关。冠层异速生长关系取决于环境与林分条件。已有多项研究探讨了气候变化与竞争强度对树冠的影响，但土壤资源与多样性对冠层异速生长及碳分配的调控作用仍未得到足够关注。
本数据集采集了2011至2019年间，中国东北四大山脉周边232个混交林内共20994株树木的相关数据。
本研究提出的冠幅模型纳入了林分发育阶段、土壤、气候、竞争强度、树种组成、物种多样性、结构多样性及其交互项。
研究发现，跨物种异速生长缩放指数并不符合通用缩放法则。
结果表明，冠幅随土壤容重、二次平均胸径以及直径变异系数的升高而增大，却随德马顿干旱指数、断面积、辛普森指数以及树种混交度的增加而降低。
二次平均胸径与土壤容重的交互项对冠幅存在显著负向影响。
交互项中某一因子对冠幅的影响，会随其他因子的梯度变化而被调控。
此外，相较于干旱指数，土壤容重在冠幅模型构建中的贡献度更高；而结构多样性对冠幅的影响也大于物种多样性。
综上，本研究结果为全球变化背景下混交林冠层异速生长的环境变异规律提供了新认知，这对提升区域及全球森林生物量与碳储量的估算精度至关重要。