Tree-ring analysis and stem biomass estimation of limba (Terminalia superba Engl. & Diels) in the framework of sustainable forest management

Sustainable forest management primarily depends on the availability of quantitative data, often not available in tropical forests. The lack of long-term data on tree growth, wood production and carbon stocks hampers the implementation of sustainable forest management and possible financing by Kyoto Protocol related tools like REDD+ and Clean Development Mechanisms. Africa is the continent with the lowest percentage of sustainably managed forests and especially African rainforests are currently poorly sampled. Data on wood production and carbon stocks can be collected in several ways but mostly originate from large-scale inventories or repeated measurements of permanent sample plots. These methods are time- and labour-intensive in tropical forests. Moreover, the inventory data require a temporal scale that enables long-term planning. Measurements of carbon stock include the weighing of aboveground biomass of complete trees. Pantropical models are based on these destructive measurements and use variables such as diameter, height and wood density to estimate aboveground biomass in the world’s tropical forests. These models are based on biomass measurements of a large range of tree species and diameters. African rainforests remain underrepresented in these datasets, creating large uncertainties on their carbon stock estimates. Tree-ring observations and detailed information of wood density enable the collection of long-term, accurate data on wood production and carbon stocks in a less destructive, less expensive way. Both approaches are performed on Terminalia superba Engl. & Diels, a long-lived pioneer species. Stem disks and increment cores of planted and natural forest trees were collected in Ivory Coast and in the southernmost part of the Mayombe Forest in the Democratic Republic of Congo. Annual tree rings are observed in both study regions. Growth of T. superba appears to be related to global climate variables like the El Niño-Southern Oscillation in both study regions but a clear link with local precipitation patterns is only found in the Mayombe Forest. Growth curves were also more directly used to document sustainable forest management. On the one hand, growth curves offer the opportunity to calculate growth-based variables for sustained yield like biological rotation ages and associated minimum logging diameters, variables that appeared site-specific. On the other hand, a study on growth changes revealed that T. superba generally reaches the canopy without releases or suppressions, validating that this species does not demand for intensive silvicultural management. Sustainable forest management of T. superba nevertheless appears dependent on forest type and cannot just be scaled up to the species level. Wood density was measured directly at high resolutions (50 μm) on increment cores from the Ivorian sample trees, using state-of-the-art helical X-ray scanning. Combining annual tree-ring data and pith-to-bark wood density profiles enables the extraction of annual wood densities. Wood density increases from pith to bark and allows for detailed estimates of carbon stocks over time using individual ring data. The assumed overestimation of carbon stocks in tropical Africa was confirmed and related to the use of fixed instead of annual wood densities. Furthermore, pantropical models, including those with wood density as a variable, probably even lead to higher overestimates. This work confirms the huge potential of tree-ring analysis and detailed wood density data for long-term planning of sustainable forest management and carbon stocks. Both methods can be used complementary to inventories, combining the diversity of inventory variables (height, recruitment, mortality) with tree rings that extend decades to centuries into time. A larger sample size, including older trees, is recommended to lengthen regional tree-ring chronologies for future dendroclimatological studies. It would also enable a better detection of sampling biases, more reliable estimates of the culmination of growth variables and serve as a solid base for bootstrapping. Apart from the tree level and the inter-annual level, the obtained high-resolution wood density profiles offer also possibilities to study intra-annual density variations and their possible relation with climate and wood anatomy

Tree line dynamics in the tropical African highlands: identifying drivers and dynamics

Questions: What are the potential drivers of tree line change in the tropical African highlands? Are the temperature-sensitive tree lines in these highlands shifting as a result of climate change? Significance: The high-altitude forests provide important ecosystem services for the vulnerable environment of the tropical highlands. Climate change is expected to have pronounced effects on the tree line limit of these forests. Afro-alpine tropical tree lines are therefore potentially valuable as a proxy of climate change and the related response of ecosystems in the tropical highlands. Location: Tropical African highlands. Results: The influence of climatic factors in the African tropical highlands is significantly different compared to other regions. The potentially determining factors for tree line distribution in tropical Africa are temperature, precipitation and cloudiness, carbon balance, fire and anthropo-zoogenic impacts. Despite recent temperature increase, tree lines have not risen to higher altitudes in the tropical African highlands. Instead, high human pressure has caused stabilization and even recession of the tree lines below their natural climatic limit, particularly through livestock herding. But, even neglecting human pressure, there might be a lag in response time between temperature and tree line change. Conclusions: The actual drivers of tree line change in the African tropical highlands are mainly fire and anthropogenic pressure rather than climate change. But long-term drought periods can be a trigger for fire-induced deforestation of the tree line vegetation. Additionally, in volcanic active mountains, volcanic activity is also a potentially limiting factor for the tree line distribution. Tree line dynamics can thus not be used as a proxy of climate change for the African tropical highlands

Stable carbon and oxygen isotopes in tree rings show physiological responses of Pericopsis elata to precipitation in the Congo Basin

In equatorial regions, where tree rings are less distinct or even absent, the response of forests to high-frequency climate variability is poorly understood. We measured stable carbon and oxygen isotopes in anatomically distinct, annual growth rings of four Pericopsis elata trees from a plantation in the Congo Basin, to assess their sensitivity to recorded changes in precipitation over the last 50 y. Our results suggest that oxygen isotopes have high common signal strength (EPS = 0.74), and respond to multi-annual precipitation variability at the regional scale, with low δ18O values (28–29‰) during wetter conditions (1960–1970). Conversely, δ13C are mostly related to growth variation, which in a light-demanding species are driven by competition for light. Differences in δ13C values between fast- and slow-growing trees (c. 2‰), result in low common signal strength (EPS = 0.37) and are driven by micro-site conditions rather than by climate. This study highlights the potential for understanding the causes of growth variation in P. elata as well as past hydroclimatic changes, in a climatically complex region characterized by a bimodal distribution in precipitation

High-resolution proxies for wood density variations in Terminalia superba

Background and Aims Density is a crucial variable in forest and wood science and is evaluated by a multitude of methods. Direct gravimetric methods are mostly destructive and time-consuming. Therefore, faster and semi- to non-destructive indirect methods have been developed. Methods Profiles of wood density variations with a resolution of approx. 50 µm were derived from one-dimensional resistance drillings, two-dimensional neutron scans, and three-dimensional neutron and X-ray scans. All methods were applied on Terminalia superba Engl. & Diels, an African pioneer species which sometimes exhibits a brown heart (limba noir). Key Results The use of X-ray tomography combined with a reference material permitted direct estimates of wood density. These X-ray-derived densities overestimated gravimetrically determined densities non-significantly and showed high correlation (linear regression, R2 = 0·995). When comparing X-ray densities with the attenuation coefficients of neutron scans and the amplitude of drilling resistance, a significant linear relation was found with the neutron attenuation coefficient (R2 = 0·986) yet a weak relation with drilling resistance (R2 = 0·243). When density patterns are compared, all three methods are capable of revealing the same trends. Differences are mainly due to the orientation of tree rings and the different characteristics of the indirect methods. Conclusions High-resolution X-ray computed tomography is a promising technique for research on wood cores and will be explored further on other temperate and tropical species. Further study on limba noir is necessary to reveal the causes of density variations and to determine how resistance drillings can be further refine

Triaging borderline/mild dyskaryotic Pap cytology with p16/Ki-67 dual-stained cytology testing: Cross-sectional and longitudinal outcome study

Background: Women with borderline/mildly dyskaryotic (BMD) cytology smears are currently followed up with repeat testing at 6 and 18 months. The objective of this study is to analyse the cross-sectional and longitudinal performance of p16/Ki-67 dual-stained cytology for the detection of cervical intraepithelial neoplasia (CIN) grade 3 or worse (CIN3+) and CIN2+ in women with BMD, and to compare the results with baseline human papillomavirus (HPV) testing. Methods: Conventional Pap cytology specimens of 256 women with BMD were dual stained for p16/Ki-67 retrospectively, and compared with baseline HPV results and long-term follow-up results. Results: p16/Ki-67 dual-stained cytology showed a sensitivity of 100%, a specificity of 64.4% and a negative predictive value (NPV) of 100.% for CIN3+. Human papillomavirus testing demonstrated similar sensitivity (96.3%), and NPV (99.1%), but a significantly lower specificity (57.6%; P=0.024) for CIN3+. Sensitivity, specificity and NPV for CIN2+ of dual-stained cytology were 89.7%, 73.1% and 95.1%, respectively, which was similar when compared with HPV testing. Dual-stained cytology showed a significant lower referral rate than HPV testing (43.6% vs 49.1%; P=0.043). During long-term follow-up, no CIN3+ lesions developed in HPV-positive, dual-stained negative women. Conclusions: Comparable sensitivity and NPV of dual-stained cytology for CIN3+, combined with a significantly higher specificity, makes p16/Ki-67 dual-stained cytology a viable alternative to HPV testing for triaging BMD