Wilma J. Blaser

Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration.

During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N-fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its effects upon pools of soil N, P, and carbon (C), and availabilities of N and P. We also evaluated whether these effects induced feedbacks upon the growth of understory vegetation and encroaching shrubs. Dichrostachys cinerea shrubs increased total N and P pools, as well as resin-adsorbed N and soil extractable P in the top 10-cm soil. Shrubs and understory grasses differed in their foliar N and P concentrations along gradients of increasing encroachment, suggesting that they obtained these nutrients in different ways. Thus, grasses probably obtained them mainly from the surface upper soil layers, whereas the shrubs may acquire N through symbiotic fixation and probably obtain some of their P from deeper soil layers. The storage of soil C increased significantly under D. cinerea and was apparently not limited by shortages of either N or P. We conclude that the shrub D. cinerea does not create a negative feedback loop by inducing P-limiting conditions, probably because it can obtain P from deeper soil layers. Furthermore, C sequestration is not limited by a shortage of N, so that mesic savanna encroached by this species could represent a C sink for several decades. We studied the effects of woody encroachment on soil N, P, and C pools, and availabilities of N and P to Dichrostachys cinerea shrubs and to the understory vegetation. Both N and P pools in the soil increased along gradients of shrub age and cover, suggesting that N fixation by D. cinerea did not reduce the P supply. This in turn suggests that continued growth and carbon sequestration in this mesic savanna ecosystems are unlikely to be constrained by nutrient limitation and could represent a C sink for several decades.

Facilitative or competitive effects of woody plants on understorey vegetation depend on N‐fixation, canopy shape and rainfall

A recent meta-analysis suggested that differences in rainfall are a cause of variation in tree-grass interactions in savannas, with trees facilitating growth of understorey grasses in low-rainfall ...

Population regulation by enemies of the grass Brachypodium sylvaticum: demography in native and invaded ranges

The enemy-release hypothesis (ERH) states that species become more successful in their introduced range than in their native range because they leave behind natural enemies in their native range and are thus released from enemy pressures in their introduced range. The ERH is popularly cited to explain the invasive properties of many species and is the underpinning of biological control. We tested the prediction that plant populations are more strongly regulated by natural enemies (herbivores and pathogens) in their native range than in their introduced range with enemy-removal experiments using pesticides. These experiments were replicated at multiple sites in both the native and invaded ranges of the grass Brachypodium sylvaticum. In support of the ERH, enemies consistently regulated populations in the native range. There were more tillers and more seeds produced in treated vs. untreated plots in the native range, and few seedlings survived in the native range. Contrary to the ERH, total measured leaf damage was similar in both ranges, though the enemies that caused it differed. There was more damage by generalist mollusks and pathogens in the native range, and more damage by generalist insect herbivores in the invaded range. Demographic analysis showed that population growth rates were lower in the native range than in the invaded range, and that sexually produced seedlings constituted a smaller fraction of the total in the native range. Our removal experiment showed that enemies regulate plant populations in their native range and suggest that generalist enemies, not just specialists, are important for population regulation.

Mixed fitness effects of grass endophytes modulate impact of enemy release and rapid evolution in an invasive grass

AbstractEndophytic fungi in grasses are often considered to be mutualistic because they can increase host resistance to herbivory and drought. However, not all endophytes are beneficial to their hosts, but may instead be specialist enemies. Brachypodium sylvaticum is an invasive grass in the USA. In its European native range, it is nearly always infected by the host-specific endophyte Epichloë sylvatica. While this fungus decreases herbivory, it also decreases the growth rate and size of infected plants, making them less competitive. After showing that B. sylvaticum has lost its endophyte in the invaded range, we use greenhouse assays to deconfound the effects of endophyte infection and range origin to test assumption of the evolution of increased competitive abilities (EICA) hypothesis. Brachypodium in its invaded range appears to have lost tolerance mechanisms present in the native range, allowing Epichloë to greatly increase seedling mortality and reduce growth rates. Additionally, there is some evidence for increased competitive abilities in the form of increased seedling growth rates in the invasive range. Together, these results provide strong support of the EICA hypothesis.n

Climate-smart sustainable agriculture in low-to-intermediate shade agroforests

Meeting demands for agricultural production while maintaining ecosystem services, mitigating and adapting to climate change and conserving biodiversity will be a defining challenge of this century. Crop production in agroforests is being widely implemented with the expectation that it can simultaneously meet each of these goals. But trade-offs are inherent to agroforestry and so unless implemented with levels of canopy cover that optimize these trade-offs, this effort in climate-smart, sustainable intensification may simply compromise both production and ecosystem services. By combining simultaneous measurements of production, soil fertility, disease, climate variables, carbon storage and species diversity along a shade-tree cover gradient, here we show that low-to-intermediate shade cocoa agroforests in West Africa do not compromise production, while creating benefits for climate adaptation, climate mitigation and biodiversity. As shade-tree cover increases above approximately 30%, agroforests become increasingly less likely to generate win–win scenarios. Our results demonstrate that agroforests cannot simultaneously maximize production, climate and sustainability goals but might optimise the trade-off between these goals at low-to-intermediate levels of cover.Agroforests can combine crop production with ecosystem service provisioning, but how they should be implemented to balance their costs and benefits remains unknown. This study finds that low-to-intermediate shade cocoa agroforests in West Africa benefit biodiversity and climate without compromising production.