TERRA Teaching and Research Center, Université de Liège, Gembloux Agro-Bio Tech, Central African Forests, 2, Passage des Déportés, 5030, Gembloux, Belgium.
UPR Forêts et Sociétés, CIRAD, Montpellier, France.
Ecol Appl. 2018 Jul;28(5):1273-1281. doi: 10.1002/eap.1726. Epub 2018 May 10.
Although the importance of large trees regarding biodiversity and carbon stock in old-growth forests is undeniable, their annual contribution to biomass production and carbon uptake remains poorly studied at the stand level. To clarify the role of large trees in biomass production, we used data of tree growth, mortality, and recruitment monitored during 20 yr in 10 4-ha plots in a species-rich tropical forest (Central African Republic). Using a random block design, three different silvicultural treatments, control, logged, and logged + thinned, were applied in the 10 plots. Annual biomass gains and losses were analyzed in relation to the relative biomass abundance of large trees and by tree size classes using a spatial bootstrap procedure. Although large trees had high individual growth rates and constituted a substantial amount of biomass, stand-level biomass production decreased with the abundance of large trees in all treatments and plots. The contribution of large trees to annual stand-level biomass production appeared limited in comparison to that of small trees. This pattern did not only originate from differences in abundance of small vs. large trees or differences in initial biomass stocks among tree size classes, but also from a reduced relative growth rate of large trees and a relatively constant mortality rate among tree size classes. In a context in which large trees are increasingly gaining attention as being a valuable and a key structural characteristic of natural forests, the present study brought key insights to better gauge the relatively limited role of large trees in annual stand-level biomass production. In terms of carbon uptake, these results suggest, as already demonstrated, a low net carbon uptake of old-growth forests in comparison to that of logged forests. Tropical forests that reach a successional stage with relatively high density of large trees progressively cease to be carbon sinks as large trees contribute sparsely or even negatively to the carbon uptake at the stand level.
尽管大树在古老森林的生物多样性和碳储量方面的重要性不可否认，但它们在林分水平上对生物量生产和碳吸收的年度贡献仍未得到充分研究。为了阐明大树在生物量生产中的作用，我们利用在中非共和国物种丰富的热带森林中监测的 20 年树木生长、死亡和更新数据，在 10 个 4 公顷的样地中进行了研究。使用随机块设计，在 10 个样地中应用了三种不同的造林处理，即对照、采伐和采伐+疏伐。使用空间自举程序，根据大树的相对生物量丰度和树木大小类，分析了年度生物量增益和损失。尽管大树个体生长速度高，构成了大量的生物量，但在所有处理和样地中，林分水平的生物量生产随着大树的丰度增加而减少。与小树相比，大树对年度林分水平生物量生产的贡献似乎有限。这种模式不仅源于小树与大树之间的丰度差异或树木大小类之间的初始生物量库存差异，还源于大树相对生长率的降低和树木大小类之间相对稳定的死亡率。在大树作为天然林有价值和关键结构特征的重要性日益受到关注的背景下，本研究为更好地评估大树在年度林分水平生物量生产中的相对有限作用提供了关键见解。就碳吸收而言，这些结果表明，与采伐林相比，古老森林的净碳吸收较低。随着密度较高的大树逐渐达到演替阶段，热带森林将不再是碳汇，因为大树对林分水平的碳吸收贡献很少甚至为负。
