Height-diameter allometry of tropical forest trees

Abstract

Tropical tree height-diameter (<i>H:D</i>) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent <i>H</i> and <i>D</i> measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were: <br><br>   1. to determine if <i>H:D</i> relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap).<br><br>   2. to ascertain if the <i>H:D</i> relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, <i>A</i>).<br><br>   3. to develop <i>H:D</i> allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass. <br><br> Annual precipitation coefficient of variation (<i>P</i><sub>V</sub>), dry season length (<i>S</i><sub>D</sub>), and mean annual air temperature (<i>T</i><sub>A</sub>) emerged as key drivers of variation in <i>H:D</i> relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high <i>A</i> being, on average, taller at any given <i>D</i>. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar <i>H:D</i> relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given <i>D</i> than their counterparts elsewhere. <br><br> The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level <i>H</i> across all plots to within a median –2.7 to 0.9% of the true value. Some of the plot-to-plot variability in <i>H:D</i> relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller <i>D</i>. Pantropical and continental-level models provided only poor estimates of <i>H</i>, especially when the roles of climate and stand structure in modulating <i>H:D</i> allometry were not simultaneously taken into account

Height-diameter relationships of tropical Atlantic moist forest trees in southeastern Brazil

“…For trees with 100 cm DBH, this difference reached 76 % when compared with trees in central Amazon. This result supports the need to use tree height as an independent variable in biomass allometric models used to compare different tropical forests, especially in sites where regional site specific biomass models are not available (Chave et al, 2005;Feldpausch et al, 2010;Nogueira et al, 2008). This is especially important in the case of the coastal Atlantic forest of southeastern Brazil due to various limitations resulting in vegetation suppression.…”

“…In tropical forests, there are few studies reporting height-diameter models and limited information about their performance (Batista et al, 2001;Fang and Bailey, 1998;Feldpausch et al, 2010). Here, we propose a performance test of the eleven linear and non-linear height-diameter models (Table 1), tested earlier by Fang and Bailey (1998) and Batista et al (2001) in other tropical forests.…”

“…However, height-diameter relationships vary within a geographic region (Peng et al, 2001) due to species composition and local environmental conditions. Hence, there is a growing consensus that the use of site specific height-diameter models fitted using field measures of tree height is an important alternative to reduce uncertainties in forest biomass estimation in tropical forests (Chave et al, 2005;Feldpausch et al, 2010;Nogueira et. al., 2008) and to study forest structure.…”

Guidelines for documenting and reporting tree allometric equations

“…Properly documented volume data, for example, are very useful to develop biomass expansion factors, which are used by most countries to assess national forest biomass (FAO 2010). In contrast, poorly documented raw data or missing metadata preclude validation or further development equations based on expanded datasets.…”

“…For example, it is not always obvious if a basic variable such as "tree height" refers to total tree height or to commercial tree height (e.g. Feldpausch et al 2010or Ribeiro et al 2011. Information about the height where the diameter measurement was taken is needed.…”