Climate Change Alters Seedling Emergence and Establishment in an Old-Field Ecosystem

Background: Ecological succession drives large-scale changes in ecosystem composition over time, but the mechanisms whereby climatic change might alter succession remain unresolved. Here, we asked if the effects of atmospheric and climatic change would alter tree seedling emergence and establishment in an old-field ecosystem, recognizing that small shifts in rates of seedling emergence and establishment of different species may have long-term repercussions on the transition of fields to forests in the future. Methodology/Principal Findings: We introduced seeds from three early successional tree species into constructed old-field plant communities that had been subjected for 4 years to altered temperature, precipitation, and atmospheric CO 2 regimes in an experimental facility. Our experiment revealed that different combinations of atmospheric CO2 concentration, air temperature, and soil moisture altered seedling emergence and establishment. Treatments directly and indirectly affected soil moisture, which was the best predictor of seedling establishment, though treatment effects differed among species. Conclusions: The observed impacts, coupled with variations in the timing of seed arrival, are demonstrated as predictors o

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

Root Damage by Insects Reverses the Effects of Elevated Atmospheric CO2 on Eucalypt Seedlings

Predicted increases in atmospheric carbon dioxide (CO(2)) are widely anticipated to increase biomass accumulation by accelerating rates of photosynthesis in many plant taxa. Little, however, is known about how soil-borne plant antagonists might modify the effects of elevated CO(2) (eCO(2)), with root-feeding insects being particularly understudied. Root damage by insects often reduces rates of photosynthesis by disrupting root function and imposing water deficits. These insects therefore have considerable potential for modifying plant responses to eCO(2). We investigated how root damage by a soil-dwelling insect (Xylotrupes gideon australicus) modified the responses of Eucalyptus globulus to eCO(2). eCO(2) increased plant height when E. globulus were 14 weeks old and continued to do so at an accelerated rate compared to those grown at ambient CO(2) (aCO(2)). Plants exposed to root-damaging insects showed a rapid decline in growth rates thereafter. In eCO(2), shoot and root biomass increased by 46 and 35%, respectively, in insect-free plants but these effects were arrested when soil-dwelling insects were present so that plants were the same size as those grown at aCO(2). Specific leaf mass increased by 29% under eCO(2), but at eCO(2) root damage caused it to decline by 16%, similar to values seen in plants at aCO(2) without root damage. Leaf C:N ratio increased by >30% at eCO(2) as a consequence of declining leaf N concentrations, but this change was also moderated by soil insects. Soil insects also reduced leaf water content by 9% at eCO(2), which potentially arose through impaired water uptake by the roots. We hypothesise that this may have impaired photosynthetic activity to the extent that observed plant responses to eCO(2) no longer occurred. In conclusion, soil-dwelling insects could modify plant responses to eCO(2) predicted by climate change plant growth models