Browsing by Author "Du Toit, Ben"
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- ItemAboveground biomass and carbon in a South African mistbelt forest and the relationships with tree species diversity and forest structures(MDPI, 2016) Mensah, Sylvanus; Veldtman, Ruan; Du Toit, Ben; Kakai, Romain Glele; Seifert, ThomasENGLISH ABSTRACT: Biomass and carbon stocks are key information criteria to understand the role of forests in regulating global climate. However, for a bio-rich continent like Africa, ground-based measurements for accurate estimation of carbon are scarce, and the variables affecting the forest carbon are not well understood. Here, we present the first biomass study conducted in South Africa Mistbelt forests. Using data from a non-destructive sampling of 59 trees of four species, we (1) evaluated the accuracy of multispecies aboveground biomass (AGB) models, using predictors such as diameter at breast height (DBH), total height (H) and wood density; (2) estimated the amount of biomass and carbon stored in the aboveground compartment of Mistbelt forests and (3) explored the variation of aboveground carbon (AGC) in relation to tree species diversity and structural variables. We found significant effects of species on wood density and AGB. Among the candidate models, the model that incorporated DBH and H as a compound variable (DBH2 × H) was the best fitting. AGB and AGC values were highly variable across all plots, with average values of 358.1 Mg·ha−1 and 179.0 Mg·C·ha−1, respectively. Few species contributed 80% of AGC stock, probably as a result of selection effect. Stand basal area, basal area of the ten most important species and basal area of the largest trees were the most influencing variables. Tree species richness was also positively correlated with AGC, but the basal area of smaller trees was not. These results enable insights into the role of biodiversity in maintaining carbon storage and the possibilities for sustainable strategies for timber harvesting without risk of significant biomass decline.
- ItemAgroforestry : an appropriate and sustainable response to a changing climate in Southern Africa?(MDPI, 2020-08-21) Sheppard, Jonathan P.; Reckziegel, Rafael Bohn; Borrass, Lars; Chirwa, Paxie W.; Cuaranhua, Claudio J.; Hassler, Sibylle K.; Hoffmeister, Svenja; Kestel, Florian; Maier, Rebekka; Mälicke, Mirko; Morhart, Christopher; Ndlovu, Nicholas P.; Veste, Maik; Funk, Roger; Lang, Friederike; Seifert, Thomas; Du Toit, Ben; Kahle, Hans-PeterENGLISH ABSTRACT: Agroforestry is often discussed as a strategy that can be used both for the adaptation to and the mitigation of climate change e ects. The climate of southern Africa is predicted to be severely a ected by such changes. With agriculture noted as the continent’s largest economic sector, issues such as food security and land degradation are in the forefront. In the light of such concerns we review the current literature to investigate if agroforestry systems (AFS) are a suitable response to the challenges besetting traditional agricultural caused by a changing climate. The benefits bestowed by AFS are multiple, o ering ecosystem services, influence over crop production and positive impacts on rural livelihoods through provisioning and income generation. Nevertheless, knowledge gaps remain. We identify outstanding questions requiring further investigation such as the interplay between trees and crops and their combination, with a discussion of potential benefits. Furthermore, we identify deficiencies in the institutional and policy frameworks that underlie the adoption and stimulus of AFS in the southern African region. We uphold the concept that AFS remains an appropriate and sustainable response for an increased resilience against a changing climate in southern Africa for the benefit of livelihoods and multiple environmental values.
- ItemImpact of climate trends and drought events on the growth of oaks (Quercus robur L. and Quercus petraea (Matt.) Liebl.) within and beyond their natural range(MDPI, 2018-02-28) Perkins, Diana; Uhl, Enno; Biber, Peter; Du Toit, Ben; Carraro, Vinicio; Rotzer, Thomas; Pretzsch, HansENGLISH ABSTRACT: Due to predicted climate change, it is important to know to what extent trees and forests will be impacted by chronic and episodic drought stress. As oaks play an important role in European forestry, this study focuses on the growth response of sessile oak (Quercus petraea (Matt.) Liebl.) and pedunculate oak (Quercus robur (L.)) under contrasting climatic conditions. Analyses cover both site conditions of their natural occurrence (Southern Germany and Northeast Italy) and site conditions beyond their natural range (South Africa). The sites beyond their natural range represent possible future climate conditions. Tree-ring series from three different sites were compared and analysed using dendrochronological methods. The long-term growth development of oak trees appears to be similar across the sites, yet the growth level over time is higher in the drier and warmer climate than in the temperate zone. When compared with previous growth periods, growth models reveal that oak trees grew more than expected during the last decades. A recent setback in growth can be observed, although growth is still higher than the model predicts. By focusing on the short-term reactions of the trees, distinct drought events and periods were discovered. In each climatic region, similar growth reactions developed after drought periods. A decline in growth rate occurred in the second or third year after the drought event. Oaks in South Africa are currently exposed to a warmer climate with more frequent drought events. This climatic condition is a future prediction also for Europe. In view of this climate change, we discuss the consequences of the long- and short- term growth behaviour of oaks grown in the climate of South Africa for a tree species selection that naturally occurs in Europe.
- ItemPotential for utilization of wood ash on coastal arenosols with limited buffer capacity in KwaZulu-Natal and its effect on eucalypt stand nutrition and growth(Italian Society of Silviculture and Forest Ecology, 2017) Scheepers, Gerhardus Petrus; Du Toit, BenENGLISH ABSTRACT: A field trial was established to test the effects of various wood ash and fertilizer application rates on the nutrition and early growth of a clonal Eucalyptus grandis × urophylla stand near Richards Bay, KwaZulu-Natal province, South Africa. The trial consisted of wood ash treatments of 0, 0.3, 0.6 and 1.2 t ha-1, combined with fertilizer treatments of no fertilizer (control), 150 g tree-1 of conventional ammonium sulphate fertilizer or 320 g tree-1 of controlled release fertilizer mixture containing N, P and a balanced suite of several plant nutrients. The experiment was conducted on a young sandy soil of aeolian origin with a very low buffer capacity. Ash application rates were chosen after a pilot study was conducted to test the effect of CaCO3 on the soil reaction. At 4 and 8 months after treatment, soil heavy metal concentrations for cadmium (Cd), mercury (Hg), chromium (Cr) and lead (Pb) levels were substantially lower than toxic levels. Foliar heavy metal concentrations (for the same elements) were less than 1mg kg-1 at both time intervals. The wood ash induced a temporary liming effect up to 8 months after application. Foliar nutrient assessments revealed sub-optimal nutrient concentrations for phosphorous (P), potassium (K) and zinc (Zn) at 4 months and K at 8 months of age. The positive growth responses (expressed as a biomass index) at 8 months, ranged between 13% and 683% relative to the untreated control. At 21 months, the growth response to ash and fertilizer combinations ranged from -0.5% to 50% relative to the control. This research demonstrated that 1.2 t ha-1 of wood ash can safely be disposed of on a typical, poorly buffered Zululand coastal sand with little environmental risk and minimal growth suppression, provided that it is balanced with an appropriate NPS plus trace element fertilizer mixture.
- ItemUse of δ13C as water stress indicator and potential silvicultural decision support tool in Pinus radiata stand management in South Africa(Italian Society of Silviculture and Forest Ecology, 2019-01-24) Fischer, Phillip M.; Du Toit, BenENGLISH ABSTRACT: In this study, the carbon isotope ratio in tree rings was investigated as a potential measure of water availability and drought stress in Pinus radiata stands in South Africa. An understanding of water availability and its variation in space is fundamental to the implementation of increasingly site-specific management regimes that have the potential to improve stand productivity. Fourteen plantation compartments, situated on water shedding (convex) terrain were identified where reliable weather data existed and a water balance model could be run. This output was used to derive water stress indicators: (a) relative canopy conductance (gc/gcmax) and (b) the ratio of actual to potential evapotranspiration (ETa/ETp). The water stress indicators (calculated per year of growth) were related to δ13C values in five tree rings formed in the five years before mid-rotation thinning took place. The water balance model used adequately described soil water availability throughout each growing season and indicated that most severe stand water stress occurred during the summer months of the study period (November to April). The ETa/ETp ratio for this period as well as the relative canopy conductance proved to be good measures of water stress. The 5-year averages of the ETa/ETp ratios (taken over the driest 6 month period) ranged from 0.17 to 0.32 (winter rainfall zone) and 0.44 to 0.70 (all-year rainfall zone). The 5-year averages of ETa/ETp ratios could be accurately predicted (p< 0.0001; adjusted r2 = 0.83) with multiple regression using δ13C values in whole-wood samples (i.e., earlywood and latewood) and the site index of stands (where site index is the average height of the dominant 20% trees in the stand at base age 20). The δ13C values in tree rings across the planted range of P. radiata in South Africa can therefore be used to identify broad categories of water availability for purposes of increasingly sitespecific silvicultural management.
- ItemUse of δ¹³C as water stress indicator and potential silvicultural decision support tool in Pinus radiata stand management in South Africa(Italian Society of Silviculture and Forest Ecology, 2018-11-15) Fischer, Phillip M.; Du Toit, BenENGLISH ABSTRACT: In this study, the carbon isotope ratio in tree rings was investigated as a potential measure of water availability and drought stress in Pinus radiata stands in South Africa. An understanding of water availability and its variation in space is fundamental to the implementation of increasingly site-specific management regimes that have the potential to improve stand productivity. Fourteen plantation compartments, situated on water shedding (convex) terrain were identified where reliable weather data existed and a water balance model could be run. This output was used to derive water stress indicators: (a) relative canopy conductance (gc/gcmax) and (b) the ratio of actual to potential evapotranspiration (ETa/ETp). The water stress indicators (calculated per year of growth) were related to δ¹³C values in five tree rings formed in the five years before mid-rotation thinning took place. The water balance model used adequately described soil water availability throughout each growing season and indicated that most severe stand water stress occurred during the summer months of the study period (November to April). The ETa/ETp ratio for this period as well as the relative canopy conductance proved to be good measures of water stress. The 5-year averages of the ETa/ETp ratios (taken over the driest 6 month period) ranged from 0.17 to 0.32 (winter rainfall zone) and 0.44 to 0.70 (all-year rainfall zone). The 5-year averages of ETa/ETp ratios could be accurately predicted (p< 0.0001; adjusted r2 = 0.83) with multiple regression using δ¹³C values in whole-wood samples (i.e., earlywood and latewood) and the site index of stands (where site index is the average height of the dominant 20% trees in the stand at base age 20). The δ¹³C values in tree rings across the planted range of P. radiata in South Africa can therefore be used to identify broad categories of water availability for purposes of increasingly sitespecific silvicultural management.