Aim: To test the extent to which the vertical structure of tropical forests is determined by environment, forest structure or biogeographical history. Location: Pan-tropical. Methods: Using height ...and diameter data from 20,497 trees in 112 non-contiguous plots, asymptotic maximum height (H AM ) and height—diameter relationships were computed with nonlinear mixed effects (NLME) models to: (1) test for environmental and structural causes of differences among plots, and (2) test if there were continental differences once environment and structure were accounted for; persistence of differences may imply the importance of biogeography for vertical forest structure. NLME analyses for floristic subsets of data (only/excluding Fabaceae and only/excluding Dipterocarpaceae individuals) were used to examine whether family-level patterns revealed biogeographical explanations of cross-continental differences. Results: H AM and allometry were significantly different amongst continents. H AM was greatest in Asian forests (58.3 ± 7.5 m, 95% CI), followed by forests in Africa (45.1 ± 2.6 m), America (35.8 ± 6.0 m) and Australia (35.0 ± 7.4 m), and height—diameter relationships varied similarly; for a given diameter, stems were tallest in Asia, followed by Africa, America and Australia. Precipitation seasonality, basal area, stem density, solar radiation and wood density each explained some variation in allometry and H AM yet continental differences persisted even after these were accounted for. Analyses using floristic subsets showed that significant continental differences in H AM and allometry persisted in all cases. Main conclusions: Tree allometry and maximum height are altered by environmental conditions, forest structure and wood density. Yet, even after accounting for these, tropical forest architecture varies significantly from continent to continent. The greater stature of tropical forests in Asia is not directly determined by the dominance of the family Dipterocarpaceae, as on average non-dipterocarps are equally tall. We hypothesise that dominant large-statured families create conditions in which only tall species can compete, thus perpetuating a forest dominated by tall individuals from diverse families.
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Tropical tree height-diameter (H:D) 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 H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were: 1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap). 2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A). 3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass. Annual precipitation coefficient of variation (PV), dry season length (SD), and mean annual air temperature (TA) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. 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 H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D 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 D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account.
Forest degradation and deforestation is high on the international forest agenda, and in countries with a strong timber industry and dwindling forest resource such as Ghana, this poses severe threats ...to the sustainability of the industry as well as of the resource itself. To curb this, forest plantations are being established to supplement the rapidly declining timber resource base to meet the country's demand for timber. Concerns have been raised about the future timber productions from the plantations and natural forests due to poor management and widespread illegal logging. This study assesses the trends of the growing stock in the main production systems and recent development that has led to the current state of the forest resources in the high forest zone of Ghana. Analysis of national forest inventories data and timber harvesting records in Ghana highlights the trend of the growing stock in timber production areas and the increasing gap between timber demand and supply, which drives illegal logging. Current plantation establishment efforts are not sufficient to bridge the gap between demand and supply of timber, partly due to low establishment rates and lack of appropriate management of newly established plantations. Secure tenure and rights to on-farm trees appears to be a key condition to stimulate large scale planting of forest trees by farmers and other investors. Reform in the management practices is required to align timber harvesting levels to sustainable timber production in Ghanaian forests.
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Interspecific variation in water-induced fluctuations in stem girth demonstrates the mechanisms promoting coexistence in seasonally dry tropical forest. In addition, these fluctuations are a ...potential, but unevaluated, source of bias in measurements of annual tree growth rates. To examine diurnal and seasonal patterns of stem diameter change, tree girth was measured over 2 years (1997–1999), using dendrometer bands, for three species (
Celtis mildbraedii,
C. zenkeri and
Strombosia glaucescens) in semi-deciduous forest in Ghana. Soil matric potential was measured concurrently at 15
cm depth. In addition, measurements of all trees >20
cm dbh on three, 1
ha plots were made at the beginning and middle of the 1998/1999 dry season. During the severe 1997/1998 dry season, soil matric potential declined below −1.5
MPa and two species showed significant stem shrinkage. For the evergreen species,
C. mildbraedii, there was a significant positive effect of tree diameter on stem shrinkage, and shrinkage was greater in the second, compared to the first, half of the dry season. For the deciduous species,
C. zenkeri, shrinkage was reduced during the second half of the dry season, following leaf fall. During 1998/1999, soil matric potential, did not decline below −1.5
MPa, and rates of girth change remained positive for all species. There were no significant effects of size or phenology on the rate of girth change in the plot-based study. Deviations in annual increment calculated over successive monthly intervals indicate that a 10-fold difference in soil water availability between measurement occasions can lead to a 4% bias in estimates of annual growth. Measurements of forest plots should be made when inter-annual variation in soil water availability is low. In this forest, measurements should, therefore, be made during the wet season, contrary to published recommendations.
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Aboveground tropical tree biomass and carbon storage estimates commonly ignore tree height (H). We estimate the effect of incorporating H on tropics-wide forest biomass estimates in 327 plots across ...four continents using 42 656 H and diameter measurements and harvested trees from 20 sites to answer the following questions: 1. What is the best H-model form and geographic unit to include in biomass models to minimise site-level uncertainty in estimates of destructive biomass? 2. To what extent does including H estimates derived in (1) reduce uncertainty in biomass estimates across all 327 plots? 3. What effect does accounting for H have on plot- and continental-scale forest biomass estimates? The mean relative error in biomass estimates of destructively harvested trees when including H (mean 0.06), was half that when excluding H (mean 0.13). Power- and Weibull-H models provided the greatest reduction in uncertainty, with regional Weibull-H models preferred because they reduce uncertainty in smaller-diameter classes (≤40 cm D) that store about one-third of biomass per hectare in most forests. Propagating the relationships from destructively harvested tree biomass to each of the 327 plots from across the tropics shows that including H reduces errors from 41.8 Mg ha−1 (range 6.6 to 112.4) to 8.0 Mg ha−1 (−2.5 to 23.0). For all plots, aboveground live biomass was −52.2 Mg ha−1 (−82.0 to −20.3 bootstrapped 95% CI), or 13%, lower when including H estimates, with the greatest relative reductions in estimated biomass in forests of the Brazilian Shield, east Africa, and Australia, and relatively little change in the Guiana Shield, central Africa and southeast Asia. Appreciably different stand structure was observed among regions across the tropical continents, with some storing significantly more biomass in small diameter stems, which affects selection of the best height models to reduce uncertainty and biomass reductions due to H. After accounting for variation in H, total biomass per hectare is greatest in Australia, the Guiana Shield, Asia, central and east Africa, and lowest in east-central Amazonia, W. Africa, W. Amazonia, and the Brazilian Shield (descending order). Thus, if tropical forests span 1668 million km2 and store 285 Pg C (estimate including H), then applying our regional relationships implies that carbon storage is overestimated by 35 Pg C (31–39 bootstrapped 95% CI) if H is ignored, assuming that the sampled plots are an unbiased statistical representation of all tropical forest in terms of biomass and height factors. Our results show that tree H is an important allometric factor that needs to be included in future forest biomass estimates to reduce error in estimates of tropical carbon stocks and emissions due to deforestation.
Pantropical variability in tree crown allometry Loubota Panzou, Grace Jopaul; Fayolle, Adeline; Jucker, Tommaso ...
Global ecology and biogeography,
February 2021, Volume:
30, Issue:
2
Journal Article, Web Resource
Peer reviewed
Open access
Aim
Tree crowns determine light interception, carbon and water exchange. Thus, understanding the factors causing tree crown allometry to vary at the tree and stand level matters greatly for the ...development of future vegetation modelling and for the calibration of remote sensing products. Nevertheless, we know little about large‐scale variation and determinants in tropical tree crown allometry. In this study, we explored the continental variation in scaling exponents of site‐specific crown allometry and assessed their relationships with environmental and stand‐level variables in the tropics.
Location
Global tropics.
Time period
Early 21st century.
Major taxa studied
Woody plants.
Methods
Using a dataset of 87,737 trees distributed among 245 forest and savanna sites across the tropics, we fitted site‐specific allometric relationships between crown dimensions (crown depth, diameter and volume) and stem diameter using power‐law models. Stand‐level and environmental drivers of crown allometric relationships were assessed at pantropical and continental scales.
Results
The scaling exponents of allometric relationships between stem diameter and crown dimensions were higher in savannas than in forests. We identified that continental crown models were better than pantropical crown models and that continental differences in crown allometric relationships were driven by both stand‐level (wood density) and environmental (precipitation, cation exchange capacity and soil texture) variables for both tropical biomes. For a given diameter, forest trees from Asia and savanna trees from Australia had smaller crown dimensions than trees in Africa and America, with crown volumes for some Asian forest trees being smaller than those of trees in African forests.
Main conclusions
Our results provide new insight into geographical variability, with large continental differences in tropical tree crown allometry that were driven by stand‐level and environmental variables. They have implications for the assessment of ecosystem function and for the monitoring of woody biomass by remote sensing techniques in the global tropics.
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Through interpretations of remote-sensing data and/or theoretical propositions, the idea that forest and savanna represent "alternative stable states" is gaining increasing acceptance. Filling an ...observational gap, we present detailed stratified floristic and structural analyses for forest and savanna stands located mostly within zones of transition (where both vegetation types occur in close proximity) in Africa, South America and Australia. Woody plant leaf area index variation was related to tree canopy cover in a similar way for both savanna and forest with substantial overlap between the two vegetation types. As total woody plant canopy cover increased, so did the relative contribution of middle and lower strata of woody vegetation. Herbaceous layer cover declined as woody cover increased. This pattern of understorey grasses and herbs progressively replaced by shrubs as the canopy closes over was found for both savanna and forests and on all continents. Thus, once subordinate woody canopy layers are taken into account, a less marked transition in woody plant cover across the savanna-forest-species discontinuum is observed compared to that inferred when trees of a basal diameter > 0.1 m are considered in isolation. This is especially the case for shrub-dominated savannas and in taller savannas approaching canopy closure. An increased contribution of forest species to the total subordinate cover is also observed as savanna stand canopy closure occurs. Despite similarities in canopy-cover characteristics, woody vegetation in Africa and Australia attained greater heights and stored a greater amount of above-ground biomass than in South America. Up to three times as much above-ground biomass is stored in forests compared to savannas under equivalent climatic conditions. Savanna-forest transition zones were also found to typically occur at higher precipitation regimes for South America than for Africa. Nevertheless, consistent across all three continents coexistence was found to be confined to a well-defined edaphic-climate envelope with soil and climate the key determinants of the relative location of forest and savanna stands. Moreover, when considered in conjunction with the appropriate water availability metrics, it emerges that soil exchangeable cations exert considerable control on woody canopy-cover extent as measured in our pan-continental (forest + savanna) data set. Taken together these observations do not lend support to the notion of alternate stable states mediated through fire feedbacks as the prime force shaping the distribution of the two dominant vegetation types of the tropical lands.
Forest degradation and deforestation is high on the international forest agenda, and in countries with a strong timber industry and dwindling forest resource such as Ghana, this poses severe threats ...to the sustainability of the industry as well as of the resource itself. To curb this, forest plantations are being established to supplement the rapidly declining timber resource base to meet the country's demand for timber. Concerns have been raised about the future timber productions from the plantations and natural forests due to poor management and widespread illegal logging. This study assesses the trends of the growing stock in the main production systems and recent development that has led to the current state of the forest resources in the high forest zone of Ghana. Analysis of national forest inventories data and timber harvesting records in Ghana highlights the trend of the growing stock in timber production areas and the increasing gap between timber demand and supply, which drives illegal logging. Current plantation establishment efforts are not sufficient to bridge the gap between demand and supply of timber, partly due to low establishment rates and lack of appropriate management of newly established plantations. Secure tenure and rights to on-farm trees appears to be a key condition to stimulate large scale planting of forest trees by farmers and other investors. Reform in the management practices is required to align timber harvesting levels to sustainable timber production in Ghanaian forests. Keywords: annual allowable cut, degradation, forest reserve, growing stock, over-exploitation La degradation forestiere et la deforestation figurent hautement dans l'agenda de la foret international, et dans les pays possedant une industrie du bois forte et des ressources forestieres diminuant, comme le Ghana, tout cela posant des menaces severes a la durabilite de l'industrie et des ressources elles-memes. Pour parer a cela, des plantations forestieres sont en train d'etre etablies pour supplementer la base de ressource de bois rapidement declinante pour faire face a la demande de bois du pays. Le futur de la production de bois des plantations et des forets naturelles occasionne des soucis dus a une pauvre gestion et a une coupe illegale courante et repandue. Cette etude evalue les courants de croissance du stock dans les systemes de production principaux et un developpement recent qui a conduit a l'etat actuel des ressources forestieres dans la zone de haute foret du Ghana. L'analyse des donnees des inventaires des forets nationales et des rapports d'exploitation du bois au Ghana met l'accent sur le courant du stock en croissance dans la production de bois et le hiatus s'elargissant entre la demande et l'offre du bois, moteur de la coupe illegale. Les efforts actuels des etablissements de plantations ne sont pas suffisants pour rapprocher l'offre de la demande de bois, du en partie au fait du faible taux d'etablissement et a une penurie de gestion appropriee dans les plantations nouvellement etablies. Un droit a la propriete sur et les droits sur les arbres dans les exploitations apparaissent comme une condition-cle pour stimuler des plantations d'arbres forestiers a grande echelle par exploitants et autres investisseurs. Une reforme dans les pratiques de gestion est necessaire pour aligner les niveaux de recolte du bois a une production de bois durable dans les forets du Ghana. La degradation forestal y la deforestation son altamente prioritarias en la agenda international forestal. En pafses con una fuerte industria de la madera y recursos forestales cada vez mas escasos, como Ghana, esto plantea graves amenazas a la sostenibilidad de la industria, asf como la del propio recurso. Para poner freno a esto se estan estableciendo plantaciones forestales como complemento a la cada vez mas escasa base de recursos maderables, para satisfacer la demanda de madera del pais. Existen inquietudes acerca de la futura production de madera de las plantaciones y bosques naturales debido a la mala gestion y la tala ilegal generalizada. Este estudio evalua las tendencias de las existencias de madera en pie en los principales sistemas de produccion y las novedades recientes que han llevado a la situacion actual de los recursos forestales en la zona de monte alto de Ghana. Un analisis de datos de los inventarios forestales nacionales y los registros de aprovechamiento de madera en Ghana destaca la tendencia de las existencias de madera en pie en las areas de production de madera y la brecha creciente entre la demanda y la oferta de madera, que impulsa la tala ilegal. Los esfuerzos actuales para el establecimiento de plantaciones no son suficientes para cerrar la brecha entre la demanda y la oferta de madera, en parte debido a las bajas tasas de arraigo y la falta de una gestion adecuada de las plantaciones recien establecidas. Asegurar la tenencia y los derechos de aprovechamiento de los arboles en las explotaciones agricolas parece ser una condition fundamental para incentivar la plantation a gran escala de arboles maderables por los agricultores y otros inversores. Es necesaria una reforma en las practicas de gestion para equilibrar los niveles de aprovechamiento de madera con la produccion sostenible de madera en los bosques de Ghana.
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Aim To test the extent to which the vertical structure of tropical forests is determined by environment, forest structure or biogeographical history. Location Pan-tropical. Methods Using height and ...diameter data from 20,497 trees in 112 non-contiguous plots, asymptotic maximum height (H AM) and height–diameter relationships were computed with nonlinear mixed effects (NLME) models to: (1) test for environmental and structural causes of differences among plots, and (2) test if there were continental differences once environment and structure were accounted for; persistence of differences may imply the importance of biogeography for vertical forest structure. NLME analyses for floristic subsets of data (only/excluding Fabaceae and only/excluding Dipterocarpaceae individuals) were used to examine whether family-level patterns revealed biogeographical explanations of cross-continental differences. Results H AM and allometry were significantly different amongst continents. H AM was greatest in Asian forests (58.3 ± 7.5¿m, 95% CI), followed by forests in Africa (45.1 ± 2.6¿m), America (35.8 ± 6.0¿m) and Australia (35.0 ± 7.4¿m), and height–diameter relationships varied similarly; for a given diameter, stems were tallest in Asia, followed by Africa, America and Australia. Precipitation seasonality, basal area, stem density, solar radiation and wood density each explained some variation in allometry and H AM yet continental differences persisted even after these were accounted for. Analyses using floristic subsets showed that significant continental differences in H AM and allometry persisted in all cases. Main conclusions Tree allometry and maximum height are altered by environmental conditions, forest structure and wood density. Yet, even after accounting for these, tropical forest architecture varies significantly from continent to continent. The greater stature of tropical forests in Asia is not directly determined by the dominance of the family Dipterocarpaceae, as on average non-dipterocarps are equally tall. We hypothesise that dominant large-statured families create conditions in which only tall species can compete, thus perpetuating a forest dominated by tall individuals from diverse families.
Full text
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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