Summary1. Tree height is a key variable for estimating tree biomass and investigating tree life history, but it is difficult to measure in forests with tall, dense canopies and wide crowns. The traditional method, which we refer to as the 'tangent method', involves measuring horizontal distance to the tree and angles from horizontal to the top and base of the tree, while standing at a distance of perhaps one tree height or greater. Laser rangefinders enable an alternative method, which we refer to as the 'sine method'; it involves measuring the distances to the top and base of the tree, and the angles from horizontal to these, and can be carried out from under the tree or from some distance away. 2. We quantified systematic and random errors of these two methods as applied by five technicians to a sizestratified sample of 74 trees between 5.7 and 39.2 m tall in a Neotropical moist forest in Panama. We measured actual heights using towers adjacent to these trees. 3. The tangent method produced unbiased height estimates, but random error was high, and in 6 of the 370 measurements, heights were overestimated by more than 100%. 4. The sine method was faster to learn, displayed less variation in heights among technicians, and had lower random error, but resulted in systematic underestimation by 20% on average. 5. We recommend the sine method for most applications in tropical forests. However, its underestimation, which is likely to vary with forest and instrument type, must be corrected if actual heights are needed.

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Summary1. Current thinking holds that wood density mediates a tradeoff between strength and economy of construction, with higher wood density providing higher strength but at higher cost.2. Yet the further away wood fibres are from the central axis of the trunk, the more they increase the strength of the trunk; thus, a fat trunk of low-density wood can achieve greater strength at lower construction cost than a thin trunk of high-density wood. 3. What then are the countervailing advantages of high wood density? 4. We hypothesize that high wood density is associated with lower maintenance costs due to lower trunk surface area, as surface area correlates with maintenance respiration. 5. This advantage would be particularly important to long-lived trees and could in part explain why they tend to have high wood density. 6. High wood density has also been associated with lower risk of trunk breakage, xylem implosion and pathogen invasion, but we argue that these relationships are not causal and instead reflect correlated selection on other traits of value to long-lived trees. 7. This revaluation of the costs and benefits of high wood density has important implications for understanding tree life-history evolution, functional diversity, forest carbon stocks and the impacts of global change.

The deviation from proportional scaling for modulus of rupture is so small that our central conclusion remains correct: for a given construction cost, trees with lower wood density have higher strength and higher resistance to bending.

Aim To develop and test a simple climate-based ecophysiological model of aboveground biomass -an approach that can be applied directly to predicting the effects of climate change on forest carbon stores.Location Humid lowland forests world-wide. MethodsWe developed a new approach to modelling the aboveground biomass of old-growth forest (AGBmax) based on the influences of temperature on gross primary productivity (GPP) and what we call total maintenance cost (TMC), which includes autotrophic respiration as well as leaf, stem and other plant construction required to maintain biomass. We parameterized the models with measured carbon fluxes and tested them by comparing predicted AGBmax with measured AGB for another 109 old-growth sites.Results Our models explained 57% of the variation in GPP across 95 sites and 79% of the variation in TMC across 17 sites. According to the best-fit models, the ratio of GPP to maintenance cost per unit biomass (MCB) peaks at 16.5°C, indicating that this is the air temperature leading to the highest possible AGBmax when temperatures are constant. Seasonal temperature variation generally reduces predicted AGBmax, and thus maritime temperate climates are predicted to have the highest AGBmax. The shift in temperatures from temperate maritime to tropical climates increases MCB more than GPP, and thus decreases AGBmax. Overall, our model explains exactly 50% of the variation in AGB among humid lowland oldgrowth forests. Main conclusionsTemperature plays an important role in explaining global variation in biomass among humid lowland old-growth forests, a role that can be understood in terms of the dual effects of temperature on GPP and TMC. Our simple model captures these influences, and could be an important tool for predicting the effects of climate change on forest carbon stores.

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Abstract:Inventories of the necromass of coarse woody debris typically involve measurements of density (e.g., kilograms per cubic metre) on a sample of logs, with densities of other logs estimated based on assignment to decay classes. Here, we compare two new devices for assessing density of woody debris, a spring penetrometer and a dynamic penetrometer, with the traditional decay classification and knife test in terms of the strength of the relationship with measured density and the consistency in measurements by four different people. Our evaluation was conducted in a diverse tropical forest and involved only a brief training period in each method. Classifications or scores from all four methods were only weakly correlated with measured density, and consistency among technicians in the measurement-density relationship was highest for the dynamic penetrometer. Therefore, we conclude that when training time is limited and the sampled logs can reasonably be assumed to be representative of all of the logs (e.g., an inventory of one site at one time), it is best to simply assume that the average density of the sampled logs is representative of nonsampled logs. For inventories involving multiple people, limited training, and cases where the sample average is likely to be unrepresentative, we recommend the dynamic penetrometer.Résumé : Les inventaires de nécromasse des débris ligneux grossiers nécessitent habituellement la mesure de la densité (p. ex. kilogrammes par mètre cube) d'un échantillon de billes alors que la densité des autres billes est estimée en les répartis-sant dans des classes de décomposition. Dans cet article, nous comparons deux nouveaux appareils pour évaluer la densité des débris ligneux : un pénétromètre à ressort et un pénétromètre dynamique, avec le test traditionnel du couteau et l'utilisation de classes décomposition sur la base de la robustesse de la relation avec la densité mesurée et de l'uniformité des mesures prises par quatre personnes différentes. Notre évaluation a été réalisée dans une forêt tropicale diverse et comportait seulement une brève période de formation pour chaque méthode. Les classements ou les résultats obtenus avec les quatre méthodes étaient seulement faiblement corrélés avec la densité mesurée. L'uniformité de la relation entre les mesures des techniciens et la densité était la plus élevée avec le pénétromètre dynamique. Par conséquent, nous concluons qu'il vaut mieux simplement assumer que la densité moyenne des billes échantillonnées est représentative des billes non échantillon-nées lorsque la durée de la formation est limitée et qu'on peut raisonnablement assumer que les billes échantillonnées sont représentatives de toutes les billes (p. ex. dans le cas d'un inventaire effectué à un seul endroit et à un seul moment). Lorsqu'un inventaire implique plusieurs personnes, que la durée de la formation est limitée et dans les cas où la moyenne des échantillons n'est probablement pas représentative, nous recommandons d'utiliser le pénétromètre dynamique.[Traduit par la ...