Determinants of woody encroachment and cover in African savannas

Savanna ecosystems are an integral part of the African landscape and sustain the livelihoods of millions of people. Woody encroachment in savannas is a widespread phenomenon but its causes are widely debated. We review the extensive literature on woody encroachment to help improve understanding of the possible causes and to highlight where and how future scientific efforts to fully understand these causes should be focused. Rainfall is the most important determinant of maximum woody cover across Africa, but fire and herbivory interact to reduce woody cover below the maximum at many locations. We postulate that woody encroachment is most likely driven by CO2 enrichment and propose a two-system conceptual framework, whereby mechanisms of woody encroachment differ depending on whether the savanna is a wet or dry system. In dry savannas, the increased water-use efficiency in plants relaxes precipitation-driven constraints and increases woody growth. In wet savannas, the increase of carbon allocation to tree roots results in faster recovery rates after disturbance and a greater likelihood of reaching sexual maturity. Our proposed framework can be tested using a mixture of experimental and earth observational techniques. At a local level, changes in precipitation, burning regimes or herbivory could be driving woody encroachment, but are unlikely to be the explanation of this continent-wide phenomenon

Moving carbon between spheres, the potential oxalate-carbonate pathway of Brosimum alicastrum Sw.; Moraceae.

Aims The Oxalate-Carbonate Pathway (OCP) is a biogeochemical process that transfers atmospheric CO2 into the geologic reservoir as CaCO3; however, until now all investigations on this process have focused on species with limited food benefits. This study evaluates a potential OCP associated with Brosimum alicastrum, a Neotropical species with agroforestry potential (ca. 70–200 kg-nuts yr−1), in the calcareous soils of Haiti and Mexico. Methods / results Enzymatic analysis demonstrated significant concentrations of calcium oxalate (5.97 % D.W.) were associated with B. alicastrum tissue in all sample sites. The presence of oxalotrophism was also confirmed with microbiological analyses in both countries. High concentrations of total calcium (>7 g kg−1) and lithogenic carbonate obscured the localised alkalinisation and identification of secondary carbonate associated with the OCP at most sample sites, except Ma Rouge, Haiti. Soils adjacent to subjects in Ma Rouge demonstrated an increase in pH (0.63) and CaCO3 concentration (5.9 %) that, when coupled with root-like secondary carbonate deposits in Mexico, implies that the OCP does also occur in calcareous soils. Conclusions Therefore this study confirms that the OCP also occurs in calcareous soils, adjacent to B. alicastrum, and could play a fundamental and un-accounted role in the global calcium-carbon coupled cycle

Influence of season and method of topkill on resprouting characteristics and biomass of Quercus nigra saplings from a southeastern U.S. pine-grassland ecosystem

© 2014, Springer Science+Business Media Dordrecht. The resprouting ability of woody plants in frequently burned ecosystems may be influenced by the season and method of topkill. We conducted an experiment to test for the effects of season and method of topkill on aboveground biomass, belowground biomass, and mortality of hardwoods found in a southeastern U.S. pine-grassland. We predicted that topkill occurring during the growing season and topkill by fire would have greater negative impacts on resprouting and root growth and result in greater mortality. We conducted a shadehouse experiment in north Florida in which we applied topkill treatments (burn, clip, and no-topkill) in three seasons (dormant, early growing, and mid growing) to Quercus nigra (water oak) saplings. Plants were destructively sampled 12 months post-treatment to measure aboveground and belowground biomass. Saplings topkilled in the early and mid growing seasons had reduced growth and greater mortality one-year post-treatment compared to plants topkilled in the dormant season. While there was no difference in one-year post-treatment biomass or mortality of saplings between the two methods of topkill, clipped plants had more stems and shorter average stem height than plants topkilled by fire. Root growth continued despite topkilling for all seasons and was greatest for no-topkill plants. These results suggest that while topkill reduces biomass, hardwoods have evolved to maintain belowground biomass reserves, enabling genets to resprout following subsequent topkilling and to persist through frequent disturbances