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Veenendaal E.M.,Wageningen University | Torello-Raventos M.,James Cook University | Feldpausch T.R.,University of Leeds | Domingues T.F.,University of Edinburgh | And 49 more authors.
Biogeosciences | Year: 2015

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. © Author(s) 2015. Source

Galbraith D.,University of Leeds | Galbraith D.,University of Oxford | Malhi Y.,University of Oxford | Affum-Baffoe K.,Forestry Commission | And 13 more authors.
Plant Ecology and Diversity | Year: 2013

Background: The woody biomass residence time (τw) of an ecosystem is an important variable for accurately simulating its biomass stocks. Methods and results: We reviewed published data from 177 forest plots across the tropics and found a six-fold variation (23-129 years) in τw across our dataset, with a median τw of ca. 50 years. This value is similar to the median default value across 21 vegetation models for tropical forests, although the range of values used in models is large (20 to 200 years).Conclusions: The notion of a constant τw across all tropical forests may be of limited utility, given the large observed variation in τw. We found that while there was little relationship between climate variables and τw, there was evidence that edaphic factors exerted a strong influence on τw. In both the Neotropics and the Paleotropics, τw was highest in heavily weathered soils, suggesting that low soil fertility and/or non-limiting soil physical conditions exert a critical influence on τw. There is considerable uncertainty in how τw will be affected by global environmental change, especially by increased atmospheric CO2. Even small changes in τw could significantly reduce the future tropical forest carbon sink predicted by many vegetation models. © 2013 Copyright 2013 Botanical Society of Scotland and Taylor & Francis. Source

Fauset S.,University of Leeds | Baker T.R.,University of Leeds | Lewis S.L.,University of Leeds | Lewis S.L.,University College London | And 5 more authors.
Ecology Letters | Year: 2012

The future of tropical forests under global environmental change is uncertain, with biodiversity and carbon stocks at risk if precipitation regimes alter. Here, we assess changes in plant functional composition and biomass in 19 plots from a variety of forest types during two decades of long-term drought in Ghana. We find a consistent increase in dry forest, deciduous, canopy species with intermediate light demand and a concomitant decrease in wet forest, evergreen, sub-canopy and shade-tolerant species. These changes in composition are accompanied by an increase in above-ground biomass. Our results indicate that by altering composition in favour of drought-tolerant species, the biomass stocks of these forests may be more resilient to longer term drought than short-term studies of severe individual droughts suggest. © 2012 Blackwell Publishing Ltd/CNRS. Source

Oduro K.A.,Forestry Research Institute of Ghana | Oduro K.A.,Wageningen University | Mohren G.M.J.,Wageningen University | Affum-Baffoe K.,Resource Management Support Center | Kyereh B.,Kwame Nkrumah University Of Science And Technology
International Forestry Review | Year: 2014

SUMMARY 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. Source

Feldpausch T.R.,University of Leeds | Lloyd J.,University of Leeds | Lloyd J.,James Cook University | Lewis S.L.,University of Leeds | And 89 more authors.
Biogeosciences | Year: 2012

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. © 2012 Author(s). Source

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