Department of Forest Science, School of Natural Resources, University of Energy and Natural Resources, Sunyani, Ghana.
Department of Agroforestry, Faculty of Renewable Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
Sci Rep. 2023 Sep 9;13(1):14897. doi: 10.1038/s41598-023-42219-6.
Allometric models which are used to describe the structure of trees in agroforestry systems are usually extrapolated from models developed for trees in forest ecosystems. This makes quantitative assessment of the functions of shade trees in agroforestry systems challenging since increased availability of light and space in these systems may induce structural differences from those growing under forest conditions. We addressed this issue by providing species-specific allometric information on the structural characteristics of associated shade trees on cocoa agroforestry systems and assessed if allometries conformed to theoretical predictions. At the plot level, stand and soil characteristics affecting tree structural characteristics were assessed. The study was conducted in cocoa agroforestry systems at Suhum, Ghana. The height-diameter at breast height (H-DBH) allometry had the best fits (R = 53-89%), followed by the crown area (CA)-DBH allometry (R = 27-87%) and then the CA-H allometry (R = 22-73%). In general, the scaling exponents of the CA-DBH, H-CA and H-DBH allometries conformed to the metabolic scaling theory (MST). However, both the CA-DBH and H-DBH allometries diverged from the geometric similarity model. Though forest tree species had similar crown areas as fruit trees, they were slenderer than fruit trees. Tree slenderness coefficients were positively correlated with soil P, Ca, Cu and the ratios (Ca + Mg):K, (Ca + Mg):(K + Na) and Ca:Mg, but not C:N while DBH and H were correlated with soil P and C:N ratio. Our results show that critical soil nutrients and their ratios affects shade tree structural attributes (e.g. slenderness and CA), which possibly restrict variations in species-specific allometries to a narrow range on cocoa systems. Furthermore, shade tree species richness and density are better predictors of relative canopy projection area (a proxy for shade intensity) than tree species diversity. In conclusion, the results have implications for shade tree species selection, monitoring of woody biomass and maintenance of biodiversity.
在农林业系统中，用于描述树木结构的异速生长模型通常是从森林生态系统中开发的模型推断而来。这使得定量评估农林业系统中遮荫树的功能具有挑战性，因为这些系统中光照和空间的增加可能会导致与在森林条件下生长的结构差异。我们通过提供与可可农林业系统相关遮荫树的结构特征的特定物种异速生长信息来解决这个问题，并评估了这些异速生长是否符合理论预测。在斑块水平上，评估了影响树木结构特征的林分和土壤特征。该研究在加纳苏胡姆的可可农林业系统中进行。胸径（DBH）-高度（H）的异速生长模型具有最佳拟合度（R=53-89%），其次是冠幅（CA）-DBH 异速生长模型（R=27-87%），然后是 CA-H 异速生长模型（R=22-73%）。总体而言，CA-DBH、H-CA 和 H-DBH 异速生长的标度指数符合代谢比例理论（MST）。然而，CA-DBH 和 H-DBH 异速生长模型都与几何相似性模型有偏差。尽管森林树种的树冠面积与果树相似，但它们比果树更细长。树木细长系数与土壤 P、Ca、Cu 以及比值（Ca+Mg）：K、（Ca+Mg）：（K+Na）和 Ca：Mg 呈正相关，但与 C：N 无关，而 DBH 和 H 与土壤 P 和 C：N 比值有关。我们的结果表明，关键土壤养分及其比值会影响遮荫树的结构属性（例如，细长度和 CA），这可能会将物种特异性异速生长限制在可可系统的一个狭窄范围内。此外，遮荫树种的丰富度和密度比树种多样性更好地预测相对树冠投影面积（遮荫强度的替代物）。总之，这些结果对遮荫树种的选择、木质生物质的监测和生物多样性的维持具有重要意义。
