Forest stand structure and functioning: Current knowledge and future challenges

Species diversity is a part of (forest) stand structure but tree diameter diversity and height diversity alone or combined are typically defined as stand structural diversity or complexity or attribute(s). There is increasing evidence that stand structural attributes determine forest functioning. Here, I provide a review of forest stand structure and functioning (e.g. aboveground biomass, carbon storage and productivity) in order to explore the current knowledge across worldwide forest ecosystems including (sub-) tropical, temperate and boreal forests, as well as agroforests and experimental plantations or forests. A total of 31 original studies were selected, based on the hypothesized relationships between forest stand structure and functioning, through the systematic literature search in the Web of Science and Google Scholar. Hypothesized studies on forest stand structure and functioning, as compared to species diversity, are under-represented in the highly skewed ecological literature. The synthesis of this review indicates that stand structural attributes often increase aboveground biomass or carbon storage through the positive plant-plant interactions under the niche complementarity effect. Whereas, the influences of stand structural attributes on stand productivity are either negative, positive or nonsignificant. Here, the negative influence is attributable to the asymmetric competition for light, competitive exclusion and selection effect, whereas the nonsignificant effect is attributable to the absolute superiority of specific trees on stand growth. This review highlights that there is no ubiquitous relationship between stand structure and forest functioning, but this relationship greatly depends on the environmental conditions, biotic interactions, stand age and disturbance intensities within a specific forest ecosystem. I anticipate that this review might encourage further studies on the multivariate relationships between stand structural attributes and forest functioning while considering for the effects of other abiotic and biotic factors of the forests.