Interregional Crown Width Models for Individual Trees Growing in Pure and Mixed Stands in Austria

Abstract

Crown width is a functional trait that is commonly used to improve the estimation of above-ground biomass of forests and is often included as a predictor variable in forest growth models. Most of the existing crown width models reflect the relationship between crown width, tree size and competition variables, but do not consider the effect of species mixture. In this study, we developed crown width models for individual-tree of the major tree species growing in Austria. Because these models should be applicable for mixed and pure stands and should also take into account the characteristics of different sites, the relationship between crown width, site variables and species composition was investigated. For that purpose, we used data from a sub-sample of the Austrian National Forest Inventory, which comprises crown width measurements of about 8900 trees from 1508 sample plots. Because of the hierarchical structure of the data set (i.e., trees nested within the plot) which destroys the independencies between observations, linear mixed-effects models were used. The species composition of the stand was included via the species-specific relative proportions of basal area. To describe the interregional variability of crown width, dummy variables were introduced, which account for region-specific differences. Site characteristics were incorporated through the altitude, slope and aspect of the site. For Norway spruce, silver fir, Scots pine, European larch, European beech, oak species and ash/maple species it was possible to develop crown width models, which reflect the effects of site characteristics and species composition of the stand. The crown widths of shade-tolerant species reacted mainly positively to admixture, whereas light-demanding species reacted with decreasing crown widths. Coniferous species were not as strongly affected by mixture as broadleaf species.

Predicting crown width using nonlinear mixed-effects models accounting for competition in multi-species secondary forests

“…CW-D models are commonly used to predict the crown width of trees ( Buchacher & Ledermann, 2020 ). The models proposed in the current study explain the greatest portion of crown width variation and fully confirm that crown width and DBH are strongly correlated.…”

“…However, the development of crown width is influenced by numerous factors. In many instances, such CW-D relationships are complex nonlinear processes that are difficult to describe using ordinary parameter models, which may overestimate crown width for crowded stands and underestimate crown width for sparse stands ( Sharma et al, 2017 ; Buchacher & Ledermann, 2020 ). Notably, this is the case regardless of competition inside the stand ( Raptis et al, 2018 ).…”

“…In tall, closed-canopy and dense stands, especially multi-species mixed forests, CW is less accurate and often far more difficult and time consuming to measure. Therefore, allometric models of CW based on a few trees are commonly used to predict the CW of remaining trees, thus reducing the cost of data acquisition ( Fu et al, 2013 ; Buchacher & Ledermann, 2020 ).…”

“…Thus, tree crown growth is significantly affected by competition, which directly or indirectly affects tree survival and population dynamics, as well as the CW of individual trees ( Aakala, Berninger & Starr, 2018 ; Wang et al, 2020 ). Models of CW are often a function of commonly measured stand and tree attributes, however, evidence has shown that crown widths are often over-predicted in dense stands and under-predicted in sparse stands when CW is modelled using tree size alone, and competition effects need to be considered for more accurate CW models ( Buchacher & Ledermann, 2020 ).…”

The Potential Influence of Tree Crown Structure on the Ginkgo Harvest

“…Research has shown that the crown structure of fruit trees is a major factor affecting fruit harvesting [19][20][21]. A suitable crown structure is conducive to vibration of the tree during mechanical harvesting, which improves harvesting efficiency.…”