Troy W. Ocheltree

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

Over the last century, many grasslands worldwide have transitioned from a graminoid to a tree/shrub-dominated state in a short period of time, a phenomenon referred to as woody encroachment. Positive feedbacks and bi-stability are thought to be important drivers of woody encroachment, but there is little empirical evidence to suggest that positive feedbacks accelerate the woody encroachment of mesic grasslands. In mesic tallgrass prairie, shrub establishment does not directly facilitate seedling establishment. Yet, shrub establishment may facilitate the clonal spread of existing shrubs into nearby patches, because clonal reproduction might circumvent barriers that typically limit woody seedlings. Our results show that when Cornus drummondii (the predominate encroacher of mesic tallgrass prairie) extends rhizomatous stems into open grasslands, these stems use the same deep soil water sources as mature stems—thereby avoiding competition with grasses and gaining access to a reliable water source. In addition...

Natural selection drives clinal life history patterns in the perennial sunflower species, Helianthus maximiliani

In plants, ecologically important life history traits often display clinal patterns of population divergence. Such patterns can provide strong evidence for spatially varying selection across environmental gradients but also may result from nonselective processes, such as genetic drift, population bottlenecks and spatially restricted gene flow. Comparison of population differentiation in quantitative traits (measured as QST) with neutral molecular markers (measured as FST) provides a useful tool for understanding the relative importance of adaptive and nonadaptive processes in the formation and maintenance of clinal variation. Here, we demonstrate the existence of geographic variation in key life history traits in the diploid perennial sunflower species Helianthus maximiliani across a broad latitudinal transect in North America. Strong population differentiation was found for days to flowering, growth rate and multiple size‐related traits. Differentiation in these traits greatly exceeds neutral predictions, as determined both by partial Mantel tests and by comparisons of global QST values with theoretical FST distributions. These findings indicate that clinal variation in these life history traits likely results from local adaptation driven by spatially heterogeneous environments.

Perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in West Bengal, India

excess of 4600 m gk g −1 . Stable isotope ratios of waters from constructed, perennial ponds indicate the ponds are chiefly recharged during the summer monsoon, and subsequently undergo extensive evaporation during the dry (winter) season. In contrast, groundwaters with high As concentrations plot along the local meteoric water line (LMWL) near where the annual, volume‐weighted mean precipitation values for d 2 Ha ndd 18 O would plot. The stable isotope data demonstrate that groundwaters are directly recharged by local precipitation without significant evaporation, and thus are not recharged by, nor mixed with, the pond waters. Furthermore, reactive transport modeling indicates that dissolved organic matter (DOM) derived from pond waters does not fuel microbial respiration and As mobilization at depth in the underlying aquifer because travel times for pond‐derived DOM exceed groundwater ages by thousands of years. Instead, organic matter within the aquifer sediments must drive dissimilatory iron reduction and As release to groundwaters. Citation: Datta, S., A. W. Neal, T. J. Mohajerin, T. Ocheltree, B. E. Rosenheim, C. D. White, and K. H. Johannesson (2011), Perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in West Bengal, India, Geophys. Res. Lett., 38, L20404, doi:10.1029/2011GL049301.

Thirteen decades of foliar isotopes indicate declining nitrogen availability in central North American grasslands

*Humans are increasing both the deposition of reactive nitrogen (N) and concentrations of atmospheric CO(2) on Earth, but the combined effects on terrestrial ecosystems are not clear. In the absence of historical records, it is difficult to know if N availability is currently increasing or decreasing on regional scales. *To determine the nature and timing of past changes in grassland ecosystem dynamics, we measured the composition of stable carbon (C) and N isotopes in leaf tissue from 545 herbarium specimens of 24 vascular plant species collected in Kansas, USA from 1876 to 2008. We also parameterized a simple model of the terrestrial N cycle coupled with a stable isotope simulator to constrain processes consistent with observed patterns. *A prolonged decline in foliar N concentrations began in 1926, while a prolonged decline in foliar delta(15)N values began in 1940. Changes in the difference between foliar and atmospheric C isotopes reveal slightly increased photosynthetic water use efficiency since 1876. *The declines in foliar N concentrations and foliar delta(15)N suggest declining N availability in these grasslands during the 20th century despite decades of anthropogenic N deposition. Our results are consistent with progressive-nitrogen-limitation-type hypotheses where declines in N availability are driven by increased ecosystem N storage as a result of increased atmospheric CO(2).

Design of the AmeriFlux Portable Eddy Covariance System and Uncertainty Analysis of Carbon Measurements

The AmeriFlux network continues to improve the understanding of carbon, water, and energy fluxes across temporal and spatial scales. The network includes 120 research sites that contribute to the understanding of processes within and among ecosystems. To improve the networks ability and confidence to synthesize data across multiple sites, the AmeriFlux quality assurance and quality control laboratory was established to reduce the within- and among-site uncertainties. This paper outlines the design of the portable eddy covariance system (PECS) and subsequent data processing procedures used for site comparisons. Because the PECS makes precision measurements of atmospheric CO2, the authors also present the results of uncertainty analyses in determining the polynomials for an infrared gas analyzer, estimating the CO2 in secondary standards, and estimating ambient CO2 in field measurements. Under field conditions, drift in the measurement of CO2 increased the uncertainty in flux measurements across 7 days by 5% and was not dependent on the magnitude or direction of the flux. The maximum relative flux measurement error for unstable conditions was 10.03 mol CO2 m 2 s 1 .

Abrupt transition of mesic grassland to shrubland: evidence for thresholds, alternative attractors, and regime shifts

Ecosystems with alternative attractors are susceptible to abrupt regime shifts that are often difficult to predict and reverse. In this study, we quantify multiple system dynamics to determine whether the transition of mesic grassland to shrubland, a widespread phenomenon, represents a linear reversible process, a nonlinear but reversible threshold process, or a transition between alternative attractors that is nonlinear and prone to hysteresis. Using a 28-yr data set with annual resolution and extensive spatial replication, we found that shrub cover is correlated with distinct thresholds of fire and C4 grass cover, resulting in temporal bimodality of shrub cover and abrupt shifts of shrub cover despite gradual changes in grass cover. These abrupt increases in shrub cover are the most rapid ever reported in grasslands, and illustrate internal thresholds that separate grasslands and shrublands. Nonlinear transitions from low to high shrub cover were also closely associated with positive feedback mechanisms that alter fire and competition (r2 = 0.65), suggesting that grasslands and shrublands could show hysteresis, and by definition exist as alternative attractors. Thus, the response of this ecosystem to anthropogenic activity should tend to be rapid, nonlinear, and perhaps difficult to reverse. Regime shifts in this mesic grassland were predictable: we found that grassland and shrubland attractors were differentiated by critical thresholds of ∼50–70% grass cover, 5–10% shrub cover, and a fire return interval of ∼3 yr. These thresholds may provide adaptive potential for managing nonlinear behavior in socio-ecological systems in a changing environment.

USING NOCTURNAL COLD AIR DRAINAGE FLOW TO MONITOR ECOSYSTEM PROCESSES IN COMPLEX TERRAIN

This paper presents initial investigations of a new approach to monitor ecosystem processes in complex terrain on large scales. Metabolic processes in mountainous ecosystems are poorly represented in current ecosystem monitoring campaigns because the methods used for monitoring metabolism at the ecosystem scale (e.g., eddy covariance) require flat study sites. Our goal was to investigate the potential for using nocturnal down-valley winds (cold air drainage) for monitoring ecosystem processes in mountainous terrain from two perspectives: measurements of the isotopic composition of ecosystem-respired CO2 (delta13C(ER)) and estimates of fluxes of CO2 transported in the drainage flow. To test if this approach is plausible, we monitored the wind patterns, CO2 concentrations, and the carbon isotopic composition of the air as it exited the base of a young (approximately 40 yr-old) and an old (>450 yr-old) steeply sided Douglas-fir watershed. Nocturnal cold air drainage within these watersheds was strong, deep, and occurred on more than 80% of summer nights. The depth of cold air drainage rapidly increased to tower height or greater when the net radiation at the top of the tower approached zero. The carbon isotope composition of CO2 in the drainage system holds promise as an indicator of variation in basin-scale physiological processes. Although there was little vertical variation in CO2 concentration at any point in time, we found that the range of CO2 concentration over a single evening was sufficient to estimate delta 13C(ER) from Keeling plot analyses. The seasonal variation in delta 13C(ER) followed expected trends: during the summer dry season delta 13C(ER) became less negative (more enriched in 13C), but once rain returned in the fall, delta 13C(ER) decreased. However, we found no correlation between recent weather (e.g., vapor pressure deficit) and delta 13C(ER) either concurrently or with up to a one-week lag. Preliminary estimates suggest that the nocturnal CO2 flux advecting past the 28-m tower is a rather small fraction (<20%) of the watershed-scale respiration. This study demonstrates that monitoring the isotopic composition and CO2 concentration of cold air drainage at the base of a watershed provides a new tool for quantifying ecosystem metabolism in mountainous ecosystems on the basin scale.

Changes in stomatal conductance along grass blades reflect changes in leaf structure

Identifying the consequences of grass blade morphology (long, narrow leaves) on the heterogeneity of gas exchange is fundamental to an understanding of the physiology of this growth form. We examined acropetal changes in anatomy, hydraulic conductivity and rates of gas exchange in five grass species (including C(3) and C(4) functional types). Both stomatal conductance and photosynthesis increased along all grass blades despite constant light availability. Hydraulic efficiency within the xylem remained constant along the leaf, but structural changes outside the xylem changed in concert with stomatal conductance. Stomatal density and stomatal pore index remained constant along grass blades but interveinal distance decreased acropetally resulting in a decreased path length for water movement from vascular bundle to stomate. The increase in stomatal conductance was correlated with the decreased path length through the leaf mesophyll. A strong correlation between the distance from vascular bundles to stomatal pores and stomatal conductance has been identified across species; our results suggest this relationship also exists within individual leaves.

Community traitscape of foliar nitrogen isotopes reveals N availability patterns in a tallgrass prairie

Background and aimsNutrients are important determinants of community assembly, yet the roles they play in structuring plant communities are still poorly understood. One inferential approach to understanding how environmental factors structure plant communities is examining the distribution of key functional traits among species of a community—a community traitscape.MethodsTo better understand how nitrogen (N) and water availability structure grasslands, we measured N concentrations and isotope ratios for 366 herbaceous species in a mesic N-limited temperate grassland, Konza Prairie. We also compared foliar N concentrations and N isotopes between Konza species and a global dataset.ResultsSpecies that had either high foliar N concentrations or high δ15NL were not necessarily more or less abundant on the landscape nor more or less likely to be found in uplands, grazed areas, or burned areas. Apparently there are unique hot spots of high N availability at Konza and the typical non-Fabaceae Konza species occupies sites with greater N availability than found globally.ConclusionsAlthough nascent, the Konza traitscapes suggest that plant diversity in nutrient-limited communities might be strongly dependent on high-nutrient availability sites that enable high-fertility species to persist in a matrix of low nutrient availability.

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.