Belowground biomass response to nutrient enrichment depends on light limitation across globally distributed grasslands

Anthropogenic activities are increasing nutrient inputs to ecosystems worldwide, with consequences for global carbon and nutrient cycles. Recent meta-analyses show that aboveground primary production is often co-limited by multiple nutrients; however, little is known about how root production responds to changes in nutrient availability. At twenty-nine grassland sites on four continents, we quantified shallow root biomass responses to nitrogen (N), phosphorus (P) and potassium plus micronutrient enrichment and compared below- and aboveground responses. We hypothesized that optimal allocation theory would predict context dependence in root biomass responses to nutrient enrichment, given variation among sites in the resources limiting to plant growth (specifically light versus nutrients). Consistent with the predictions of optimal allocation theory, the proportion of total biomass belowground declined with N or P addition, due to increased biomass aboveground (for N and P) and decreased biomass belowground (N, particularly in sites with low canopy light penetration). Absolute root biomass increased with N addition where light was abundant at the soil surface, but declined in sites where the grassland canopy intercepted a large proportion of incoming light. These results demonstrate that belowground responses to changes in resource supply can differ strongly from aboveground responses, which could significantly modify predictions of future rates of nutrient cycling and carbon sequestration. Our results also highlight how optimal allocation theory developed for individual plants may help predict belowground biomass responses to nutrient enrichment at the ecosystem scale across wide climatic and environmental gradients

Dynamics of invasion and native species recovery following fire in coastal sage scrub

Coastal sage scrub (CSS) is an important vegetation type in Southern California, which is under threat from multiple environmental changes such as nitrogen deposition, invasion by exotic species, and accelerating fire regimes. While previous studies have found that CSS can recover pre -fire cover through a successional process, there is concern that high exotic annual grass abundance after fire could prevent native shrub recovery, creating an alternate stable state which requires an active management for restoration of native shrub cover. This study evaluated the performance of focal native and exotic species at early demographic stages (germination, first-year growth and survival) in one recently burned site, and then placed these short-term results in context by examining long-term vegetation dynamics across four CSS sites in San Diego County. Exotic species out-performed native species during all early demographic stages, especially following fire. In the long-term (20 years) cover by native CSS shrubs returned to pre-fire levels at three of the four sites monitored. These results indicate that invasion slows the rate of CSS successional recovery post-fire, and that native CSS vegetation can recover given a long-enough fire return interval. Accelerating fire frequencies, however, may be creating invaded "apparent stable states," because full native shrub recovery does not effectively occur between closely spaced fire

Belowground biomass response to nutrient enrichment depends on light limitation across globally distributed grasslands

Anthropogenic activities are increasing nutrient inputs to ecosystems worldwide, with consequences for global carbon and nutrient cycles. Recent meta-analyses show that aboveground primary production is often co-limited by multiple nutrients; however, little is known about how root production responds to changes in nutrient availability. At twenty-nine grassland sites on four continents, we quantified shallow root biomass responses to nitrogen (N), phosphorus (P) and potassium plus micronutrient enrichment and compared below- and aboveground responses. We hypothesized that optimal allocation theory would predict context dependence in root biomass responses to nutrient enrichment, given variation among sites in the resources limiting to plant growth (specifically light versus nutrients). Consistent with the predictions of optimal allocation theory, the proportion of total biomass belowground declined with N or P addition, due to increased biomass aboveground (for N and P) and decreased biomass belowground (N, particularly in sites with low canopy light penetration). Absolute root biomass increased with N addition where light was abundant at the soil surface, but declined in sites where the grassland canopy intercepted a large proportion of incoming light. These results demonstrate that belowground responses to changes in resource supply can differ strongly from aboveground responses, which could significantly modify predictions of future rates of nutrient cycling and carbon sequestration. Our results also highlight how optimal allocation theory developed for individual plants may help predict belowground biomass responses to nutrient enrichment at the ecosystem scale across wide climatic and environmental gradients.This work was generated using data from the Nutrient Network (http://www.nutnet.org) experiment, funded at the site-scale by individual researchers. Coordination and data management have been supported by funding to E. Borer and E. Seabloom from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) and Long Term Ecological Research (NSF-DEB-1234162 to Cedar Creek LTER) programs, and the Institute on the Environment (DG-0001-13).http://link.springer.com/journal/100212020-03-11hj2019Mammal Research InstituteZoology and Entomolog