Abstract
Quantifying the relationship between tree diameter and height is a key component of efforts to estimate biomass and carbon stocks in tropical forests. Although substantial site‐to‐site variation in height–diameter allometries has been documented, the time consuming nature of measuring all tree heights in an inventory plot means that most studies do not include height, or else use generic pan‐tropical or regional allometric equations to estimate height.Using a pan‐tropical dataset of 73 plots where at least 150 trees had in‐field ground‐based height measurements, we examined how the number of trees sampled affects the performance of locally derived height–diameter allometries, and evaluated the performance of different methods for sampling trees for height measurement.Using cross‐validation, we found that allometries constructed with just 20 locally measured values could often predict tree height with lower error than regional or climate‐based allometries (mean reduction in prediction error = 0.46 m). The predictive performance of locally derived allometries improved with sample size, but with diminishing returns in performance gains when more than 40 trees were sampled. Estimates of stand‐level biomass produced using local allometries to estimate tree height show no over‐ or under‐estimation bias when compared with biomass estimates using field measured heights. We evaluated five strategies to sample trees for height measurement, and found that sampling strategies that included measuring the heights of the ten largest diameter trees in a plot outperformed (in terms of resulting in local height–diameter models with low height prediction error) entirely random or diameter size‐class stratified approaches.Our results indicate that even limited sampling of heights can be used to refine height–diameter allometries. We recommend aiming for a conservative threshold of sampling 50 trees per location for height measurement, and including the ten trees with the largest diameter in this sample.
Highlights
Tropical forests play a key role in the global carbon cycle and are a major carbon pool, with ca. 285 Pg of carbon estimated to be stored in above- ground live biomass (Feldpausch et al, 2012)
Using cross-validation, we found that allometries constructed with just 20 locally measured values could often predict tree height with lower error than regional or climate-based allometries
The importance of tree height in allometric models used to estimate tropical tree biomass is widely recognised (Feldpausch et al, 2012), it is rare to measure the heights of all trees in a permanent sample plot, meaning that it is often necessary to use existing allometric models to estimate tree height (Chave et al, 2014; Feldpausch et al, 2012)
Summary
Tropical forests play a key role in the global carbon cycle and are a major carbon pool, with ca. 285 Pg of carbon estimated to be stored in above- ground live biomass (Feldpausch et al, 2012). For example estimates of global tropical forest biomass carbon stocks vary by 35.2 Pg depending on whether height is incorporated (Feldpausch et al, 2012), equivalent to c. 15 years of the global forest carbon sink (Pan et al, 2011) This has led to the incorporation of tree height in REDD+ carbon monitoring (Global Forests Observations Initiative, 2013). Improved plot-level knowledge of height–diameter relationships would help improve remote sensing-based estimates of local and global forest biomass.
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