UGent-Woodlab, Laboratory of Wood Technology, Department of Environment, Ghent University, Coupure Links 653, 9000 Gent, Belgium.
Service of Wood Biology, Royal Museum for Central Africa, Leuvensesteenweg 13, 3080 Tervuren, Belgium.
Ann Bot. 2024 May 10;133(5-6):905-916. doi: 10.1093/aob/mcad079.
Heartwood plays an important role in maintaining the structural integrity of trees. Although its formation has long been thought to be driven solely by internal ageing processes, more recent hypotheses suggest that heartwood formation acts as a regulator of the tree water balance by modulating the quantity of sapwood. Testing both hypotheses would shed light on the potential ecophysiological nature of heartwood formation, a very common process in trees.
We measured quantities of heartwood and sapwood, xylem conduits and the width and number of growth rings on 406 stems of Pericopsis elata with ages ranging from 2 to 237 years. A subset of 17 trees with similar ages but varying growth rate were sampled in a shaded (slower-growth) site and a sun-exposed (faster-growth) site. We used regression analysis and structural equation modelling to investigate the dynamics and drivers of heartwood formation.
We found a positive effect of growth rate on the probability of heartwood occurrence, suggesting an earlier heartwood onset in faster-growing stems. After this onset age, heartwood area increased with stem diameter and age. Despite the similar heartwood production per unit stem diameter increment, shaded trees produced heartwood faster than sun-exposed trees. Tree age and hydraulics showed similar direct effects on heartwood and sapwood area of sun-exposed trees, suggesting their mutual role in driving the heartwood dynamics of sun-exposed trees. However, for shaded trees, only tree hydraulics showed a direct effect, suggesting its prominent role over age in driving the heartwood dynamics in limited growing conditions. The positive relationship between growth rate and maximum stomatal conductance supported this conclusion.
Heartwood area increases as the tree ages, but at a slower rate in trees where water demand is balanced by a sufficient water supply. Our findings suggest that heartwood formation is not only a structural process but also functional.
心材对于维持树木的结构完整性起着重要作用。尽管长期以来人们一直认为心材的形成完全是由内部老化过程驱动的，但最近的假设表明，心材的形成通过调节边材的数量来调节树木的水分平衡，从而起到调节作用。检验这两种假说将有助于揭示心材形成的潜在生态生理学性质，这是树木中非常常见的过程。
我们测量了 406 株 Pericopsis elata 树干的边材和心材、木质部导管以及生长轮的宽度和数量，这些树干的年龄从 2 年到 237 年不等。在一个遮荫（生长较慢）和阳光充足（生长较快）的地点，选择了 17 棵具有相似年龄但生长速度不同的树木作为样本。我们使用回归分析和结构方程模型来研究心材形成的动态和驱动因素。
我们发现生长速度对心材出现的概率有积极影响，这表明在生长较快的树干中，心材的出现较早。在心材出现年龄之后，心材面积随着茎直径和年龄的增加而增加。尽管每单位茎直径增量的心材产量相似，但遮荫树木的心材生成速度快于阳光充足的树木。树木年龄和水力在阳光充足树木的心材和边材面积上表现出相似的直接影响，这表明它们在心材动态的驱动中相互作用。然而，对于遮荫树木，只有树木水力对心材有直接影响，这表明在有限的生长条件下，年龄对驱动心材动态的作用更为突出。生长速度与最大气孔导度之间的正相关关系支持了这一结论。
心材面积随着树木年龄的增长而增加，但在水分需求通过充足的供水得到平衡的树木中，增加的速度较慢。我们的研究结果表明，心材的形成不仅是一个结构过程，也是一个功能过程。
