Native plant communities persist by quashing the establishment of new arrivals but disturbance and changes in the resource regime can tip the competitive balance in favor of the newcomers. Although invasive traits are well characterized, their success inside mature communities is rarely tested. In this experiment, we evaluated the ability of one of the most aggressive global invaders, Johnsongrass (Sorghum halepense L.), to establish inside a monoculture of a late-successional perennial bunchgrass stand (switchgrass; Panicum virgatum L.) in central Texas. Specifically, we monitored the growth and persistence of Johnsongrass and switchgrass seeds and rhizome fragments in a factorial experimental design with and without nitrogen fertilizer (NH4NO3) and supplementary irrigation. Johnsongrass emerged two weeks ahead of switchgrass, on average, with seeds and rhizomes of both species emerging at similar times. Emergence and height gain was not affected by irrigation as natural rainfall was high in the first half of the growing season. Nitrogen fertilizer input increased the height of recruits across species and propagules, and most strongly in Johnsongrass rhizome plants. Johnsongrass persistence was strongly correlated with early emergence. The persistence of switchgrass recruits was not affected by emergence time but was decreased by fertilizer, suggesting that fertilizer favored mature switchgrass plants more than switchgrass recruits. In the end, no switchgrass and < 2

Common garden experiments are indoor or outdoor plantings of species or populations collected from multiple distinct geographic locations, grown together under shared conditions. These experiments examine a range of questions for theory and application using a variety of methods for analysis. The eight papers of this special feature comprise a cross section of contemporary approaches, summarized and synthesized here by what they tell us about the relationships between climate‐related trait spectra and fitness optima. Four of the eight papers are based on field experiments in prairie, desert, Mediterranean and boreal biomes. Representative of many common garden experiments, these experiments reveal consistent evidence of traits varying with population climate provenance, but evidence of a tradeoff between growth and tolerance traits or of consistent fitness optimization at home is scant, in contrast to trait theory. Two synthesis papers highlight dominant patterns of trait divergence, including for an exotic invasive species. One theoretical paper warned that unknown kinship relationships between populations can result in the misidentification of adaptive trait divergence. A third synthesis paper formulated novel and ambitious goals for common‐garden studies through including measurement of response variables at multiple levels of biological organization. The featured papers discuss multiple avenues for improving common garden studies. Genomic analysis, together with the quantification of kinship relationships, will continue to reveal the influence of environmental drivers on gene selection. Measuring a more complete set of fitness traits, especially for traits related to regeneration, will permit the development of projection models to explicitly link trait spectra, climate patterns and fitness consequences. More standardized data reporting will additionally improve abilities to synthesize findings across experiments. Testing population performance in competition with other species will produce more robust fitness comparisons between genotypes, especially for slower‐growing genotypes in higher‐resource environments. Adding gardens in and beyond climatic edge locations will furthermore strengthen the understanding of population failure and species exclusion. Finally, there is unrealized potential in adding ecosystem‐level observations to common‐garden studies that will enhance integrative analysis across scales of biological organization and scientific domains. Synthesis . With novel, creative designs, data integration and synthesis, common garden experiments will continue to advance the understanding of trait ensembles interacting with climate across scales of biological organization, provide pivotal data for global change models and guide ecological applications such as restoration of habitats for rare and climate sensitive species.

Purpose Allelopathy is a plant interaction in which a donor species releases chemicals that suppress the development of receptor species. Allelopathy has been suggested as one explanation for catastrophic loss of native biodiversity in some invaded biomes; however, experimental tests of this hypothesis have had inconsistent results. Here, we examine if a previous finding of strong allelopathic effects of the warm-season, invasive C4 grass Bothriochloa ischaemum on North American prairie grasses can be reproduced in a different geographic setting. Methods We examined the effects of sterilized (autoclaved or microfiltered) and unsterilized leachate on germination and the effect of sterilized leachate on seedling growth, including five native species and two exotic warm season grass species. For nine weeks, seedlings were irrigated with water or autoclaved leachate from B. ischaemum or Schizachyrium scoparium , a native species. Results Germination rates were significantly suppressed only in the two invasive species and only when treated with sterilized leachates. Seedling biomass at harvest was largely insensitive to leachate application. Conclusions The present study did not replicate earlier results, though many details of the experimental designs were similar. However, we used sterilized leachates and soils, whereas the previous study did not, which could indicate mediation by indirect microbial effects in the previous study. In addition, historic differences in the introduction of B. ischaemum in Oklahoma and Texas, along with climatic differences, may have affected the evolution of allelopathy post-invasion. Future studies would benefit from comparisons of allelopathic effects across invasive species’ ecotypes, using sterilized and unsterilized extracts.

Widely distributed species are often locally adapted to climate gradients across their ranges. But little is known about the patterns of intraspecific adaptation in desert shrubs. We examined the questions of local adaptation in multiple populations of two common shrub species of the winter‐wet Mojave Desert in North America in a multiple common garden experiment. Plants were raised in the greenhouse and transplanted at the age of 1 year. Ambrosia dumosa is a drought‐deciduous low shrub and Larrea tridentata is an exceptionally long‐lived evergreen. Over 4 years, we monitored growth, survivorship, leaf and reproductive cover and once measured leaf N content, δ 13 C and SLA. We hypothesized that populations of both species would be differentiated along a growth–survivorship trade‐off according to homesite aridity. Both species exhibited previously undocumented population differences along gradients of winter precipitation and temperature. In general, populations from more winter‐mesic regions had faster growth in more mesic gardens and lower survivorship in the most arid garden. Homesites with more variable summer precipitation had greater growth for A. dumosa populations, but lower growth for L. tridentata . Among L. tridentata populations, leaf cover correlated positively with growth and negatively with survival time. For A. dumosa populations, growth and survival could not be attributed to specific traits across gardens. However, larger transplants had generally lower growth rates and higher survival rates across gardens, except in the driest garden, where the population averages of intrinsic water use efficiency (iWUE) and stem growth rate were positively correlated. Synthesis . Two dominant species of the Mojave Desert adapted locally to variation in winter and summer precipitation and temperature. They did so in different ways, suggesting that L. tridentata mitigated the risk of hydraulic failure, while A. dumosa optimized carbon assimilation for growth.

Background The paper by Korboulewsky and co-authors in this issue ofPlant and Soiladdress some of the central questions of critical zone ecohydrology: how do plants interact with rocks that exclude roots but hold plant-available water? Scope I compare plant water uptake from stony soils and fractured bedrock in the critical zone, suggesting that the two cases may represent endpoints of a continuum along which the proportion of available space for root growth changes. Conclusions Rhizosphere models could be improved and generalized by structuring the layers of the critical zone into volume fractions that can be rooted and fractions from which roots are excluded. I hypothesize that plant-available water capacity of the rooted fraction governs productivity, while plant-available water in the unrooted fraction governs drought resilience.

This chapter provides an overview of the variety of methods available that scientists have developed to explore the natural and material world. Each method is accompanied by discussion of a rich set of procedures that extend beyond just experiments that many believe to be the way that science works. This limited view of scientific research is frequently found in classroom discussions, textbooks, lab manuals, professional development plans, online resources, classroom posters, state science standards, and even implied by the requirements found in many science competitions. To counter this pervasive misconception, this chapter offers a more complete illustration of the ways that investigations may occur in science by providing a detailed overview of all scientific methodology in a useful tool we call the “Modes of Scientific Inquiry” (MSI) flowchart. This flowchart provides examples of qualitative and quantitative methods, offers distinctions between descriptive, correlational, comparative and experimental designs, observational attributes, useful analytical approaches, and graphical representations of results from such investigations. Science teachers, other formal and informal science educators and students will find this overview useful in discussions about how science works and, more importantly, in conducting authentic scientific research.

(1) To develop a 3D root distribution model for piñon-juniper woodland using only tree species, sizes and locations as input. (2) To interpret a two-year time series of soil moisture relative to root distributions. The study was conducted in a piñon (Pinus edulis (Englem.)) -juniper (Juniperus monosperma (Englem.) Sarg.) woodland in New Mexico. We extracted roots from 720 soil blocks (30 cm × 10 cm × 10 cm) cut from the walls of three 10-m long and 1.5-m deep trenches. Roots were sorted by species and diameter class. Distribution models were developed for the dry weight of roots ≤5 mm in diameter. Soil water content and water potentials were measured in soil profiles under tree cluster and canopy gaps for 2 years, including a protracted dry-down period. Piñon had twice the root dry mass of juniper, similar to the ratio of canopy projection areas. Root densities were ca. 50% lower in soils under canopy gaps compared to tree clusters and the species ratio did not significantly differ between clusters and gaps. Piñon root density declined faster with soil depth and distance from the stem compared to juniper. A hard caliche layer at 60–80 cm soil depth had no apparent effect on the already low root density at that depth. Overall, the models explained 66% (piñon) and 54% (juniper) of the spatial variation in root density at the scale of sampled soil blocks. During an 8-month dry period, soil moisture declined faster in regions of higher root density: in shallow soil and under tree clusters. This left a reserve of plant-available soil water in the deep soil under canopy gaps. Horizontal variation in root density in these open woodlands is predictable and an important component of the dynamic interactions between plant and soil. Under wet and dry conditions, soil water content is substantially different under tree clusters and canopy gaps.

Hot drought is a climate phenomenon that has lately received much attention for its potential to disrupt forest function worldwide. A sharp increase in tree mortality associated with this climate pattern are often cast as a disturbance, in which high temperature is responsible for causing exceptional rates of mortality. The alternative interpretation is simply that drought kills trees in a density-regulating manner and within the bounds of normal forest function. To evaluate the evidence for disturbance versus regulating dynamics, we conducted censuses across 30 plots in the Edwards Plateau region of central Texas, USA, four years after the hot drought of 2011. The purpose was to explain variation in population responses to drought, including crown mortality, resprouting rate and sapling survivorship in terms of physical site factors, community characteristics and local climate data. Through model selection analysis, we identified the most parsimonious binomial regression models for the three most common species. In Ashe juniper populations, overall crown mortality was 20% and all predictive factors indicated the influence of population-regulating dynamics. In live oak (Quercus virginiana & Q. fusiformis), which had a crown mortality rate of 23%, the influence of regulating factors was less prominent, but there was also no evidence that crown death was linked to heat exposure. However, resprouting in both species appeared to be inhibited by heat exposure, as was crown mortality in the understory species Texas persimmon (Diospyros texana). Along with overall high levels of sapling survivorship, these patterns suggest that Ashe juniper woodlands, despite having been relatively hard hit by the 2011 drought, are not particularly threatened by hot drought events, although some subordinate species may be. Density-regulating mechanisms of forest drought response are often underplayed, but they are far easier to represent in vegetation models than disturbance dynamics.

Plant water use is an important component in the function of Earth's critical zone and this can be examined by decomposing isotope composition of xylem water into contributions from precipitation stored in shallow soil layers and deeper groundwater. The usual procedure for estimating the proportional use of groundwater by plants is to sample the isotope composition of soil and groundwater and determine the most probable mixing coefficients from all potential sources. Here we propose and test a novel method for achieving the same goal without sampling soil water. The method is based on analyzing variability in the stem water isotope ratios of several members of a community and the known isotope ratio of groundwater to 'triangulate' the unknown isotope ratio of stored rainwater. Using a simple water balance model, parameterized to produce the best fit between actual and estimated stem water isotope ratios, we simulated seasonal variation in the volume and isotope ratio of rainwater storage, along with species-specific groundwater use ratios. The method was applied to eight woody plant species growing on two rocky outcrops in the South China karst. Estimated average proportional groundwater use over two seasons varied between 14% and 62% and was site-dependent. For the majority of species, groundwater use increased as estimated stored rainwater volume declined. The two species with highest groundwater use were taller, deciduous or semi-deciduous trees with lower wood densities. While the new method was inspired by the inability to sample water stored in the rocky outcrops, it may have broader use in any environment where the spatial variability of soil water isotope composition is a barrier to estimating average groundwater use ratios. The broader adoption of this or equivalent methods would greatly improve the study of the Earth's critical zone.

Karst regions, which account for about 15% of the terrestrial surface area, are characte-rized by specific hydrogeological structure different from most non-karst regions. Thus, many research methods that are used in non-karst regions cannot be directly used in karst regions. This issue is especially relevant to research on plant water sources. In this paper, origins and possible solutions to the common problems associated with research on water sources used by karst plant species were reviewed. Four questions were addressed: 1) why is it important to determine plant water source in karst regions? 2) Why are stable isotopes used? 3) What are the challenges associated with using stable isotopes in karst regions? 4) What are the probable solutions for these challenges? This review emphasized the advantages of using stable isotope techniques to identify sources of water used by karst plant species and the challenges associated with satisfying the prerequisites of this method. It is suggested that sources of water used by plant species in karst regions need not to be divided into specific depths and the method of identifying sources of water used by plant species based on their hydrologic properties was much applicable.

Invasive plant species often dominate native species in competition, augmenting other potential advantages such as release from natural enemies. Resource pre-emption may be a particularly important mechanism for establishing dominance over competitors of the same functional type. We hypothesized that competitive success of an exotic grass against native grasses is mediated by establishing an early size advantage. We tested this prediction among four perennial C4 warm-season grasses: the exotic weed Johnsongrass (Sorghum halepense), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius) and switchgrass (Panicum virgatum). We predicted that a) the competitive effect of Johnsongrass on target species would be proportional to their initial biomass difference, b) competitive effect and response would be negatively correlated and c) soil fertility would have little effect on competitive relationships. In a greenhouse, plants of the four species were grown from seed either alone or with one Johnsongrass neighbor at two fertilizer levels and periodically harvested. The first two hypotheses were supported: The seedling biomass of single plants at first harvest (50 days after seeding) ranked the same way as the competitive effect of Johnsongrass on target species: Johnsongrass < big bluestem < little bluestem/switchgrass, while Johnsongrass responded more strongly to competition from Johnsongrass than from native species. At final harvest, native plants growing with Johnsongrass attained between 2-5% of their single-plant non-root biomass, while Johnsongrass growing with native species attained 89% of single-plant non-root biomass. Fertilization enhanced Johnsongrass' competitive effects on native species, but added little to the already severe competitive suppression. Accelerated early growth of Johnsongrass seedlings relative to native seedlings appeared to enable subsequent resource pre-emption. Size-asymmetric competition and resource-pre-emption may be a critical mechanism by which exotic invasive species displace functionally similar native species and alter the functional dynamics of native communities.

Woody vegetation in grasslands and savannas has increased worldwide over the past 100-200 years. This phenomenon of "woody plant encroachment" (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO2 levels may have favored C-3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna ecosystems and its endemic biodiversity. Traditional management goals aimed at restoring forage and livestock production after WPE have broadened to support a more diverse portfolio of ecosystem services. Accordingly, we focus on how WPE and management actions aimed at reducing woody plant cover influence carbon sequestration, water yield, and biodiversity, and discuss the trade-offs involved when balancing competing management objectives.

AimsInvasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.MethodsWe quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C-4 grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C-4 exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important FindingsAfter 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root: shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root: shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.

Brush removal is widely practiced as a tool for increasing groundwater recharge, but its efficacy depends greatly on the way in which the removed species interact with the hydrological system relative to the vegetation replacing it. We examined the effects of Ashe juniper removal in the recharge zone of the Edwards Aquifer, Texas, USA, a karst aquifer. The study was conducted in an Ashe juniper (Juniperus ashei)-live oak (Quercus fusiformis) woodland on a hill slope composed of rocky, shallow soils over fractured limestone bedrock. Ashe juniper is a native species that has been encroaching grasslands and savannas over the past century. In September 2008, a plot was cleared of 90% of its juniper trees. Tree transpiration, predawn water potentials and vegetation cover across the cleared plot and an adjacent reference site were measured from May 2009 to December 2011. Stand-level tree transpiration from May 2009 to March 2010 was diminished by a severe summer drought in 2009, from which trees were slow to recover. Subsequently, tree transpiration was 5-10x higher in the woodland compared to the clearing. For all of 2011, also a drought year, tree transpiration in the woodland exceeded precipitation inputs, indicating a high capacity for water storage at the study site. However, site differences for oak trees were generally larger than for juniper trees. While juniper removal accounted for a 431 mm year(-1) difference in tree transpiration between sites, vegetation cover in the clearing increased from 42% to 90% over two years, suggesting that understory growth was increasingly compensating for the loss of juniper transpiration. We conclude that the removal of a relatively shallow-rooted tree, when replaced with herbaceous vegetation and low shrubs, has little effect on deep recharge. By contrast, successive years of precipitation extremes may be more effective increasing recharge by lowering the water transport capacity of trees in the aftermath of severe drought. Copyright (C) 2016 John Wiley & Sons, Ltd.

QuestionCan the timing of cutting or burning be used to selectively damage an invasive C-4 grass species while limiting collateral damage to dominant native C-4 grass species in a grassland community?LocationA medium to short grass prairie in central Texas, US.MethodsWe conducted six cut and burn trials between June 2011 and January 2012 during an exceptional drought year. Target species were the native, perennial, C-4 grass Schizachyrium scoparium (little bluestem) and the introduced, perennial, C-4 grass Bothriochloa ischaemum (yellow bluestem). Burn trials were conducted inside a steel barrel using standardized fuel loads. To assess recovery, we counted the first cohort of new tillers that emerged after treatment and tillers that grew in the subsequent spring. We also recorded phenological status, meteorological variables and burn temperatures to determine if tiller recovery was predictable.ResultsBothriochloa ischaemum responded more negatively to burning than to cutting, but these treatments were no different for S.scoparium. Both species produced the smallest number of tillers after the summer burns, and the burn datexspecies interaction was insignificant. On average, peak fire temperature inside tussocks was 100 degrees C higher in B.ischaemum than in S.scoparium. Judging from the reduction in tiller densities in undisturbed plots from spring 2011 to spring 2012, B.ischaemum was also more sensitive to drought. Multivariate models were significant and related burn recovery to peak fire temperature, among other factors, and recovery from cutting to ambient temperature on the day of cutting. In B.ischaemum, plants with higher investment in early reproductive growth suffered more burn damage.ConclusionsWe identified no optimal burn time that maximized discrimination between the two species. Nonetheless, the invasive species was more sensitive to both drought and fire, possibly due to higher heat exposure of growing points. Thus, burning during the summer of a drought year may constitute the sought-after treatment that selectively favours native species. Ultimately, successful management will involve fuel and temperature management combined with assessment of the relative phenologies of the target species prior to burning. This study suggests that a hot fire at a time when the invasive species has a larger investment in early reproductive growth will provide the best control.