Transpiration dynamics support resource partitioning in African savanna trees and grasses

Citation: Holdo, R. M., & Nippert, J. B. (2015). Transpiration dynamics support resource partitioning in African savanna trees and grasses. Ecology, 96(6), 1466-1472. doi:10.1890/14-1986.1It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap-flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water-use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water-use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree-grass dynamics under altered precipitation regimes

Savanna Tree Seedlings are Physiologically Tolerant to Nighttime Freeze Events

Citation: O'Keefe, K., Nippert, J. B., & Swemmer, A. M. (2016). Savanna Tree Seedlings are Physiologically Tolerant to Nighttime Freeze Events. Frontiers in Plant Science, 7, 13. doi:10.3389/fpls.2016.00046Freeze events can be important disturbances in savanna ecosystems, yet the interactive effect of freezing with other environmental drivers on plant functioning is unknown. Here, we investigated physiological responses of South African tree seedlings to interactions of water availability and freezing temperatures. We grew widely distributed South African tree species (Colophospermum mopane, Combretum apiculaturn, Acacia nigrescens, and Cassia abbreviate) under well watered and water-limited conditions and exposed individuals to nighttime freeze events. Of the four species studied here, C. mopane was the most tolerant of lower water availability. However, all species were similarly tolerant to nighttime freezing and recovered within one week following the last freezing event. We also show that water limitation somewhat increased freezing tolerance in one of the species (C. mopane). Therefore, water limitation, but not freezing temperatures, may restrict the distribution of these species, although the interactions of these stressors may have species specific impacts on plant physiology. Ultimately, we show that unique physiologies can exist among dominant species within communities and that combined stresses may play a currently unidentified role in driving the function of certain species within southern Africa

Lowveld Savanna Bush Cutting Alters Tree-Grass Interactions

Savannas are characterized by the coexistence of trees and grasses, and their interactions are modified by water availability and herbivore activity. Many savannas are experiencing bush encroachment, resulting in reduced herbaceous productivity and shifts in savanna structure. This study aims to understand the effects of tree density and tree cutting on herbaceous productivity, water use, and herbivore abundance in a mopane-dominated lowveld savanna. We present data from a 4-year mopane-cutting experiment in the Mthimkhulu Game Reserve bordering Kruger National Park (South Africa). We established three 60x60 m plots for experimental manipulation where mopane stems and re-sprouting shoots were cut 2-3 times per year (2015-2019). We established transects within the plots to measure grass productivity and herbivore activity (counts of tracks and dung). Additionally, we measured root non-structural carbohydrates within cut and uncut mopane to assess the impact of cutting on energy storage. We used stable isotopes of xylem and soil water at multiple depths to infer changes in functional water use of coexisting mopane and grass species. Cutting had limited effects on mopane survival during this 5-year period, but re-sprouting stems had reduced height and starch concentrations than uncut trees. Cutting mopane resulted in shallower-soil water use in 2017 and tended to increase variability in root water uptake across multiple soil depths. The cut treatment tended to have higher grass cover and productivity than the control treatment by the 3rd growing season. Visitation of grazers increased in the cut plots relative to uncut plots by 2017, suggesting increased grass cover promotes grazer visitation. These results emphasize the importance of top-down drivers on savanna tree cover and the impacts of bush encroachment on grass biomass and herbivore presence. We suggest repeated cutting or browsing pressure is needed to suppress woody cover and increase grass production in lowveld savannas

Lack of eutrophication in a tallgrass prairie ecosystem over 27 years

Many North American grasslands are receiving atmospheric nitrogen (N) deposition at rates above what are considered critical eutrophication thresholds. Yet, potential changes in grassland function due to anthropogenic N deposition are poorly resolved, especially considering that other dynamic factors such as land use and precipitation can also affect N availability. To better understand whether elevated N deposition has altered ecosystem structure or function in North American grasslands, we analyzed a 27-year record of ecophysiological, community, and ecosystem metrics for an annually burned Kansas tallgrass prairie. Over this time, despite increasing rates of N deposition that are within the range of critical loads for grasslands, there was no evidence of eutrophication. Plant N concentrations did not increase, soil moisture did not decline, forb diversity did not decline, and the relative abundance of dominant grasses did not shift toward more eutrophic species. Neither aboveground primary productivity nor N availability to plants increased. The fates of deposited N in grasslands are still uncertain, and could include management losses through burning and grazing. However, evidence from this grassland indicates that eutrophication of North American grassland ecosystems is not an inevitable consequence of current levels of N deposition

Interactive Effects of Drought and Fire on Co-Existing Woody and Herbaceous Communities in a Temperate Mesic Grassland

Increased drought and woody encroachment are likely to have substantial and interactive effects on grassland carbon and water cycling in the future. However, we currently lack necessary information to accurately predict grassland responses to drought-by-fire interactions in areas experiencing woody encroachment. A more thorough understanding of these interactive effects on grass-shrub physiology would improve the effectiveness of demographic vegetation models and refine predictions of future changes in grassland ecosystem function. To this end, we constructed passive rainout shelters over mature Cornus drummondii shrubs and co-existing grasses in two fire treatments (1-year and 4-year burn frequency) at the Konza Prairie Biological Station (north-eastern Kansas, USA) that reduced precipitation by 50%. Plant responses to drought and fire were monitored at the leaf-level (gas exchange, predawn and midday water potential, turgor loss point) and the whole-plant level (aboveground biomass). Here, we report results from the 2020 growing season, after three years of treatment. Photosynthetic rates of C. drummondii and Andropogon gerardii, a dominant C4 grass, were lower in drought treatments at the end of the growing season. A. gerardii also exhibited higher photosynthetic rates in the 4-year burn watershed, but C. drummondii rates were not impacted by burn frequency. Predawn and midday leaf water potential for both species, as well as turgor loss point for C. drummondii, were lower in the 4-year burn treatment, indicating increased water stress. This trend was more pronounced in drought shelters for C. drummondii. These results indicate that three years of 50% precipitation reduction has resulted in modest impacts on water stress and gas exchange in both species. Long-term studies of co-existing grasses and shrubs are useful for informing management of woody encroachment during drought and help to identify whether multiple external pressures (drought and fire) are needed to reverse grassland-to-shrubland transitions in temperate mesic grasslands

Electronic excitations stabilized by a degenerate electron gas in semiconductors

Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium-doped GaN up to a temperature of 100 K, mimicking sharp spectral features at high free electron concentrations (3.4E19–8.9E19 cm−3). Our interpretation of the data suggests that a degenerate, three-dimensional electron gas stabilizes a novel class of quasiparticles, which we name collexons. These many-particle complexes are formed by exchange of electrons with the Fermi gas. The potential observation of collexons and their stabilization with rising doping concentration is enabled by high crystal quality due to the almost ideal substitution of host atoms with dopants.DFG, 43659573, SFB 787: Semiconductor Nanophotonics: Materials, Models, Device

Assessing the Roles of Fire Frequency and Precipitation in Determining Woody Plant Expansion in Central U.S. Grasslands

Woody plant expansion into grasslands and savannas is occurring and accelerating worldwide and often impacts ecosystem processes. Understanding and predicting the environmental and ecological impacts of encroachment has led to a variety of methodologies for assessing its onset, transition, and stability, generally relying on dynamical systems approaches. Here we continue this general line of investigation to facilitate the understanding of the roles of precipitation frequency and intensity and fire frequency on the conversion of grasslands to woody‐dominated systems focusing on the central United States. A low‐dimensional model with stochastic precipitation and fire disturbance is introduced to examine the complex interactions between precipitation and fire as mechanisms that may suppress or facilitate increases in woody cover. By using Lyapunov exponents, we are able to ascertain the relative control exerted on woody encroachment through these mechanisms. Our results indicate that precipitation frequency is a more important control on woody encroachment than the intensity of individual precipitation events. Fire, however, exerts a much more dominant impact on the limitation of encroachment over the range of precipitation variability considered here. These results indicate that fire management may be an effective strategy to slow the onset of woody species into grasslands. While climate change might predict a reduced potential for woody encroachment in the near future, these results indicate a reduction in woody fraction may be unlikely when considering anthropogenic fire suppression

Evidence of physiological decoupling from grassland ecosystem drivers by an encroaching woody shrub

Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community. Mechanisms of encroachment are typically assessed using temporal data or experimental manipulations, with few large spatial assessments of shrub physiology. In a mesic grassland in North America, we measured inter- and intra-annual variability in leaf δ¹³C in Cornus drummondii across a grassland landscape with varying fire frequency, presence of large grazers and topographic variability. This assessment of changes in individual shrub physiology is the largest spatial and temporal assessment recorded to date. Despite a doubling of annual rainfall (in 2008 versus 2011), leaf δ¹³C was statistically similar among and within years from 2008-11 (range of −28 to −27‰). A topography*grazing interaction was present, with higher leaf δ¹³C in locations that typically have more bare soil and higher sensible heat in the growing season (upland topographic positions and grazed grasslands). Leaf δ¹³C from slopes varied among grazing contrasts, with upland and slope leaf δ¹³C more similar in ungrazed locations, while slopes and lowlands were more similar in grazed locations. In 2011, canopy greenness (normalized difference vegetation index – NDVI) was assessed at the centroid of individual shrubs using high-resolution hyperspectral imagery. Canopy greenness was highest mid-summer, likely reflecting temporal periods when C assimilation rates were highest. Similar to patterns seen in leaf δ¹³C, NDVI was highest in locations that typically experience lowest sensible heat (lowlands and ungrazed). The ability of Cornus drummondii to decouple leaf physiological responses from climate variability and fire frequency is a likely contributor to the increase in cover and frequency of this shrub species in mesic grassland and may be generalizable to other grasslands undergoing woody encroachment

Intraspecific Trait Variability in Andropogon gerardii, a Dominant Grass Species in the US Great Plains

The climatic conditions in the North American Great Plains are highly variable, characteristic of an inter-continental climate. Antecedent climate history has impacted the flora of Great Plains grasslands, resulting in high species richness as well as dominance by only a few grass species, such as Andropogon gerardii. While the productivity of A. gerardii is well described, the individual physiological, and morphological characteristics that confer species dominance over wide spatial gradients are not clearly understood. We performed a literature search to assess intra-specific trait variability of A. gerardii from as many locations as possible. Ultimately, only 13 locations in the Great Plains have reported common plant functional traits (PFTs) for this species. To best represent site-specific climate conditions, plant functional trait data (8 PFTs) were collected from literature reporting ambient growing conditions, and excluded experimental manipulations. For most PFTs, we found insufficient data to fully quantify the range of variation across the geographical extent of A. gerardii dominance. This is surprising given that we focused on the most abundant grass in one of the most well-studied regions globally. Furthermore, trait data collected from our literature search showed a high degree of variability, but no strong relationships were observed between mean trait values and climate predictors. Our review of the literature on A. gerardii suggests a role for trait variability as a mechanism enabling the dominance of this species across large regions such as the Great Plains of North America

Testing the Children: Do Non-Genetic Health-Care Providers Differ in Their Decision to Advise Genetic Presymptomatic Testing on Minors? A Cross-Sectional Study in Five Countries in the European Union

BACKGROUND: Within Europe many guidelines exist regarding the genetic testing of minors. Predictive and presymptomatic genetic testing of minors is recommended for disorders for which medical intervention/preventive measures exist, and for which early detection improves future medical health. AIM: This study, which is part of the larger 5th EU-framework "genetic education" (GenEd) study, aimed to evaluate the self-reported responses of nongenetic health-care providers in five different EU countries (Germany, France, Sweden, the United Kingdom, and the Netherlands) when confronted with a parent requesting presymptomatic testing on a minor child for a treatable disease. METHODS: A cross-sectional study design using postal, structured scenario-based questionnaires that were sent to 8129 general practitioners (GPs) and pediatricians, between July 2004 and October 2004, addressing self-reported management of a genetic case for which early medical intervention during childhood is beneficial, involving a minor. RESULTS: Most practitioners agreed on testing the oldest child, aged 12 years (81.5% for GPs and 87.2% for pediatricians), and not testing the youngest child, aged 6 months (72.6% for GPs and 61.3% for pediatricians). After multivariate adjustment there were statistical differences between countries in recommending a genetic test for the child at the age of 8 years. Pediatricians in France (50%) and Germany (58%) would recommend a test, whereas in the United Kingdom (22%), Sweden (30%), and the Netherlands (32%) they would not. CONCLUSION: Even though presymptomatic genetic testing in minors is recommended for disorders for which medical intervention exists, EU physicians are uncertain at what age starting to do so in young children