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Chave J.,CNRS Biological Evolution and Diversity Laboratory | Rejou-Mechain M.,CNRS Biological Evolution and Diversity Laboratory | Burquez A.,National Autonomous University of Mexico | Chidumayo E.,Makeni Savanna Research project | And 20 more authors.
Global Change Biology | Year: 2014

Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees = 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development. © 2014 John Wiley & Sons Ltd.

Although tree growth in southern African savannas is correlated with rainfall in the wet season, some studies have shown that tree growth is controlled more by rainfall in the dry season. If more rainfall occurred in the dry season in future climates, it would affect the growth of savanna trees, especially saplings that have shallower roots which limit access to subsoil water during the dry season when leaf flush and shoot extension occur. Recent paleobotanical evidence has revealed that there was relatively more precipitation in the dry season in eastern Africa in the Eocene than under the current climate. Saplings therefore can be expected to respond more to water addition during the dry season than mature trees that have more stored water and deeper roots that access subsoil water. Accordingly, I hypothesized that irrigation in the dry season should (i) advance the onset of the growing season, (ii) increase growth rates and (iii) alter the growth responses of saplings to climate factors. To test these hypotheses saplings of five savanna woody species were irrigated during the hot-dry season at a site in central Zambia and their monthly and annual growth rates compared to those of conspecifics growing under control conditions. Although the responses among the species were variable, all irrigated saplings had significantly higher monthly and annual growth rates than control plants. In addition, dry season watering significantly altered the climatic determinants of sapling growth by either strengthening the role of the same climatic factors that were important under control conditions or displacing them altogether. In conclusion, more precipitation during the hot-dry season is likely to have significant positive effects on sapling growth and consequently reduce the sapling-tree transition periods and promote future tree population recruitment in some southern African savanna tree species. © 2015 Ecological Society of Australia.

Chidumayo E.N.,Makeni Savanna Research Project
Forest Ecology and Management | Year: 2013

Because dry tropical forests cover 40% of tropical forest area and contain considerable biomass and carbon stocks, deforestation and forest degradation of these forests have global significance. However, in most developing countries with dry forests there is no quantitative information on past trends of forest degradation so that the development of historical reference levels is virtually impossible. This study assessed impacts of forest practices on forest degradation from 1990 to 2012 using data from destructive tree sampling and permanent sample plots in a miombo woodland landscape in central Zambia. Aboveground live wood biomass was estimated using allometric equations developed from 113 felled trees. Resprouting was a major source of regeneration on clear-cut subplots and the rate of aboveground wood biomass accumulation was much higher on re-growth sites than on old-growth sites. Changes in aboveground live wood biomass on uncut subplots were linked to biomass loss due to fire, harvesting and conversion to crop cultivation on the one hand and biomass increment due to growth of remaining trees on the other hand. Forest degradation was estimated to cause aboveground wood biomass loss of 0.3tha-1year-1 on the least impacted site to 4.0tha-1year-1 on the most impacted site. Control of fire and harvesting for charcoal production is recommended if miombo woodland is to be managed for biomass and carbon storage and sequestration. The study also found that negative trends in woody biomass accumulation rate and standing stocks are good indicators of forest degradation while positive trends in species diversity indices and standing wood biomass are useful indicators of forest recovery. However, the application of these indicators requires repeated inventories at permanent sample plots to establish reference levels against which changes in these indicator variables can be compared. © 2012 Elsevier B.V.

Seed predation and seedling mortality can act as strong demographic "bottlenecks" to sapling recruitment in African savanna woodlands. Fire also limits tree recruitment from saplings by suppressing their growth. I conducted field experiments with 13 woody plant species to assess the effects of seed burial on seedling emergence rates and effects of fire on seedling and sapling survival and growth rates over a period of 8 years at a savanna plot in central Zambia, southern Africa. Seed removal rates by small rodents varied among years and buried seeds had significantly higher emergence rates than seeds exposed to predators in most but not all the species. Annual burning reduced sapling growth in some species but in other species saplings experienced successive shoot die back even in the absence of fire. The findings show that for some woody species, seed predation is an important constraint to seedling recruitment but not for others and annual fires are important hindrances to demography and growth for some species but not others. Thus, demographic "bottlenecks" occur at different life history stages in different savanna woody species and these have the potential to alter woody tree competitive relationships and ultimately savanna structure. © 2012 Springer Science+Business Media Dordrecht.

Chidumayo E.N.,Makeni Savanna Research Project | Gumbo D.J.,Center for International Forestry Research
Energy for Sustainable Development | Year: 2013

Charcoal production in tropical regions of the world is often perceived to have devastating ecological and environmental effects and governments, public forestry institutions and non government organizations have been particularly concerned about these charcoal-related impacts. The most commonly cited impact is deforestation, i.e., the clearance of forest or woodland. At a small spatial scale this may indeed be the case but on a larger landscape scale charcoal production most frequently results only in forest degradation. Much of the charcoal in tropical countries is commonly made in traditional earth and pit kilns with a wood-to-charcoal conversion rate of about 20% and in 2009 the contribution of charcoal production to deforestation in tropical countries with the highest rates of deforestation is estimated at less than 7%. A large proportion of the area utilized for charcoal production has the potential for rapid forest recovery especially with good post-harvest management. There are conflicting reports on the effects of deforestation on catchment hydrology with the majority of small catchment studies indicating increased runoff and low evapotranspiration while studies of large basins have shown no such changes. Emissions of greenhouse gases from charcoal production in tropical ecosystems in 2009 are estimated at 71.2. million. t for carbon dioxide and 1.3. million. t for methane. The failure of past charcoal policies to address environmental impacts and achieve sustainability can be attributed to erroneous assumptions and predictions by national and international organizations regarding wood-based fuels. Possible ways of enhancing charcoal policies' legitimacy and therefore effective implementation are multi-stakeholder participation and demonstration of coherence with globally recognized principles, goals and relevant international regimes, such as the Millennium Development Goals (MDGs). In this way charcoal production can significantly contribute to poverty reduction and environmental sustainability. © 2012 International Energy Initiative.

This study aimed at developing allometric models from destructive sample field data for estimating both aboveground and belowground tree biomass and assessing changes in root biomass after old-growth Brachystegia-Julbernardia (miombo) woodland clearing in central Zambia. Logarithmic linear models were selected for estimating tree biomass because they gave the most accurate (low mean error) predictions. On average aboveground and belowground biomass in regrowth woodland represented 29% and 41%, respectively, of the biomass in old-growth woodland. The root:shoot ratios were 0.54 and 0.77 in old-growth and regrowth woodland, respectively. Ten years after clear-cutting old-growth woodland, root biomass loss was about 60% of the original biomass. The main cause of post clearing root biomass loss was fire which at the study sites occurred annually or biannually. Control of fire in cleared sites should be encouraged in forest management for carbon storage and sequestration in miombo woodland of southern Africa. Copyright © 2013 Foundation for Environmental Conservation.

Chidumayo E.N.,Makeni Savanna Research Project
Biodiversity and Conservation | Year: 2016

Schinziophytonrautanenii is a keystone tree species whose fruits are eaten by wildlife and livestock, nuts are used to extract oil for human use and its wood is used for making curios that are sold to tourists. The species occurs in southern African countries of Angola, Namibia, Botswana, Mozambique, Zambia, Zimbabwe and the Democratic Republic of Congo but little is known about its population and conservation status. The objectives of this study were to (i) assess factors determining the distribution and abundance of the species in Zambia, (ii) evaluate its regeneration potential and population status and (iii) assess whether the main determinants of forest and species stand structures also favour S. rautanenii. The study is based on three forest surveys conducted in 2004, 2005–2008 and 2015. In Zambia S. rautanenii abundance and population structure are controlled by soil, climate and human factors. The species is most abundant in southwest Zambia where population recruitment is good but is rare and recruitment is poor elsewhere in the country. Fire appears to be a major cause of tree damage and possibly death. The elephant is a major disperser of S. rautanenii nuts and the low recruitment levels in the country may partly be attributed to the decline in the elephant population from about 250,000 in 1960 to 28,000 in 2008. Different conservation strategies will be required for areas where the species is rare and for those where the species is abundant. Whatever the strategies, it will be important to address issues of fire management, fruit harvesting and the role of wildlife in the conservation of S. rautanenii in Zambia. © 2016 Springer Science+Business Media Dordrecht

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