Biotic rescaling reveals importance of species interactions for variation in biodiversity responses to climate change.

Generality in understanding biodiversity responses to climate change has been hampered by substantial variation in the rates and even directions of response to a given change in climate. We propose that such context dependencies can be clarified by rescaling climate gradients in terms of the underlying biological processes, with biotic interactions as a particularly important process. We tested this rescaling approach in a replicated field experiment where entire montane grassland communities were transplanted in the direction of expected temperature and/or precipitation change. In line with earlier work, we found considerable variation across sites in community dynamics in response to climate change. However, these complex context dependencies could be substantially reduced or eliminated by rescaling climate drivers in terms of proxies of plant-plant interactions. Specifically, bryophytes limited colonization by new species into local communities, whereas the cover of those colonists, along with bryophytes, were the primary drivers of local extinctions. These specific interactions are relatively understudied, suggesting important directions for future work in similar systems. More generally, the success of our approach in explaining and simplifying landscape-level variation in climate change responses suggests that developing and testing proxies for relevant underlying processes could be a fruitful direction for building more general models of biodiversity response to climate change.

Plant effects on and response to soil microbes in native and non-native Phragmites australis.

Plant-soil feedbacks (PSFs) mediate plant community dynamics and may plausibly facilitate plant invasions. Microbially mediated PSFs are defined by plant effects on soil microbes and subsequent changes in plant performance (responses), both positive and negative. For microbial interactions to benefit invasive plants disproportionately, native and invasive plants must either (1) have different effects on and responses to soil microbial communities or (2) only respond differently to similar microbial communities. In other words, invasive plants do not need to cultivate different microbial communities than natives if they respond differently to them. However, effects and responses are not often explored separately, making it difficult to determine the underlying causes of performance differences. We performed a reciprocal-transplant PSF experiment with multiple microbial inhibition treatments to determine how native and non-native lineages of Phragmites australis affect and respond to soil bacteria, fungi, and oomycetes. Non-native Phragmites is a large, fast-growing, cosmopolitan invasive plant, whereas the North American native variety is comparatively smaller, slower growing, and typically considered a desirable wetland plant. We identified the effects of each plant lineage on soil microbes using DNA meta-barcoding and linked plant responses to microbial communities. Both Phragmites lineages displayed equally weak, insignificant PSFs. We found evidence of slight differential effects on microbial community composition, but no significant differential plant responses. Soils conditioned by each lineage differed only slightly in bacterial community composition, but not in fungal composition. Additionally, native and non-native Phragmites lineages did not significantly differ in their response to similar soil microbial communities. Neither lineage appreciably differed when plant biomass was compared between those grown in sterile and live soils. Targeted microbial inhibitor treatments revealed both lineages were negatively impacted by soil bacteria, but the negative response was stronger in non-native Phragmites. These observations were opposite of expectations from invasion theory and imply that the success of non-native Phragmites, relative to the native lineage, does not result from its interaction with soil microorganisms. More broadly, quantifying plant effects on, and responses to soil microbes separately provides detailed and nuanced insight into plant-microbial interactions and their role in invasions, which could inform management outcomes for invasive plants.

Macroecological context predicts species' responses to climate warming.

Context-dependencies in species' responses to the same climate change frustrate attempts to generalize and make predictions based on experimental and observational approaches in biodiversity science. Here, we propose predictability may be enhanced by explicitly incorporating macroecological context into analyses of species' responses to climate manipulations. We combined vascular plant species' responses to an 8-year, 12-site turf transplant climate change experiment set in southwestern Norway with climate niche data from the observed 151 species. We used the difference between a species' mean climate across their range and climate conditions at the transplant site ("climate differences") to predict colonization probability, extinction probability, and change in abundance of a species at a site. In analyses across species that ignore species-specific patterns, colonization success increased as species' distribution optima were increasingly warmer than the experimental target site. Extinction probability increased as species' distribution optima were increasingly colder than the target site. These patterns were reflected in change in abundance analyses. We found weak responses to increased precipitation in these oceanic climates. Climate differences were better predictors of species' responses to climate manipulations than range size. Interestingly, similar patterns were found when analyses focused on variation in species-specific responses across sites. These results provide an experimental underpinning to observational studies that report thermophilization of communities and suggest that space-for-time substitutions may be valid for predicting species' responses to climate warming, given other conditions are accounted for (e.g., soil nutrients). Finally, we suggest that this method of putting climate change experiments into macroecological context has the potential to generalize and predict species' responses to climate manipulations globally.

Plant Size and Competitive Dynamics along Nutrient Gradients.

Resource competition theory in plants has focused largely on resource acquisition traits that are independent of size, such as traits of individual leaves or roots or proportional allocation to different functions. However, plants also differ in maximum potential size, which could outweigh differences in module-level traits. We used a community ecosystem model called mondrian to investigate whether larger size inevitably increases competitive ability and how size interacts with nitrogen supply. Contrary to the conventional wisdom that bigger is better, we found that invader success and competitive ability are unimodal functions of maximum potential size, such that plants that are too large (or too small) are disproportionately suppressed by competition. Optimal size increases with nitrogen supply, even when plants compete for nitrogen only in a size-symmetric manner, although adding size-asymmetric competition for light does substantially increase the advantage of larger size at high nitrogen. These complex interactions of plant size and nitrogen supply lead to strong nonlinearities such that small differences in nitrogen can result in large differences in plant invasion success and the influence of competition along productivity gradients.

One hundred and six years of change in a Sonoran Desert plant community: Impact of climate anomalies and trends in species sensitivities.

A major restriction in predicting plant community response to future climate change is a lack of long-term data needed to properly assess species and community response to climate and identify a baseline to detect climate anomalies. Here, we use a 106-year dataset on a Sonoran Desert plant community to test the role of extreme temperature and precipitation anomalies on community dynamics at the decadal scale and over time. Additionally, we tested the climate sensitivity of 39 desert plant species and whether sensitivity was associated with growth form, longevity, geographic range, or local dominance. We found that desert plant communities had shifted directionally over the 106 years, but the climate had little influence on this directional change primarily due to nonlinear shifts in precipitation anomalies. Decadal-scale climate had the largest impact on species richness, species relative density, and total plant cover, explaining up to 26%, 45%, and 55% of the variance in each, respectively. Drought and the interaction between the frequency of freeze events and above-average summer precipitation were among the most influential climate factors. Increased drought frequency and wetter periods with frequent freeze events led to larger reductions in total plant cover, species richness, and the relative densities of dominant subshrubs Ambrosia deltoidea and Encelia farinosa. More than 80% of the tested species were sensitive to climate, but sensitivity was not associated with a species' local dominance, longevity, geographic range, or growth form. Some species appear to exhibit demographic buffering, where when they have a higher sensitivity to drought, they also tend to have a higher sensitivity to favorable (i.e., wetter and hotter) conditions. Overall, our results suggest that, while decadal-scale climate variation substantially impacts these desert plant communities, directional change in temperature over the last century has had little impact due to the relative importance of precipitation and drought. With projections of increased drought in this region, we may see reductions in total vegetation cover and species richness due to the loss of species, possibly through a breakdown in their ability to demographically buffer climatic variation, potentially changing community dynamics through a change in facilitative and competitive processes.

The contrasting roles of growth traits and architectural traits in diversity maintenance in clonal plant communities.

Plant communities often exhibit high diversity, even though pairwise experiments usually result in competitive hierarchies that should result in competitive exclusion. Such experiments, however, do not typically allow expression of spatial traits, despite theoretical studies showing the potential importance of spatial mechanisms of diversity maintenance. Here we ask whether, in a clonal plant model system, spatial trait variation is more likely than growth trait variation to maintain diversity. We used a field-calibrated, spatially explicit model to simulate communities comprising sets of four simulated species differing in only one of a suite of architectural or growth traits at a time, examining their dynamics and long-term diversity. To compare trait manipulation effects across traits measured in different units, we scaled traits to have identical effects on initial productivity. We found that in communities of species differing only in an architectural trait, all species usually persist, whereas communities of species differing only in a growth trait experienced rapid competitive exclusion. To examine the roles of equalizing and stabilizing mechanisms in maintaining diversity, we conducted reciprocal invasion experiments for species pairs differing only in single traits. The results suggest that stabilizing mechanisms cannot account for the observed long-term co-occurrence. Strong positive correlations between diversity and similarity both in monoculture carrying capacity and reciprocal invasion ability suggesting equalizing mechanisms may instead be responsible.

The role of plant functional groups mediating climate impacts on carbon and biodiversity of alpine grasslands.

Plant removal experiments allow assessment of the role of biotic interactions among species or functional groups in community assembly and ecosystem functioning. When replicated along climate gradients, they can assess changes in interactions among species or functional groups with climate. Across twelve sites in the Vestland Climate Grid (VCG) spanning 4 °C in growing season temperature and 2000 mm in mean annual precipitation across boreal and alpine regions of Western Norway, we conducted a fully factorial plant functional group removal experiment (graminoids, forbs, bryophytes). Over six years, we recorded biomass removed, soil microclimate, plant community composition and structure, seedling recruitment, ecosystem carbon fluxes, and reflectance in 384 experimental and control plots. The dataset consists of 5,412 biomass records, 360 species-level biomass records, 1,084,970 soil temperature records, 4,771 soil moisture records, 17,181 plant records covering 206 taxa, 16,656 seedling records, 3,696 ecosystem carbon flux measurements, and 1,244 reflectance measurements. The data can be combined with longer-term climate data and plant population, community, ecosystem, and functional trait data collected within the VCG.

Time lags and the balance of positive and negative interactions in driving grassland community dynamics.

Both facilitative and competitive interactions occur simultaneously among plants, and the net balance between them can vary over time. Despite this, recent model-fitting studies have found that negative interactions predominate. This suggests that more complex models may be necessary to uncover facilitation. Here we fitted models including seasonality, interannual variation, and time lags to survey data to test for patterns in positive and negative interactions among plants in a Michigan dry sand prairie. We hypothesized that interactions would be generally facilitative in this dry environment. Results indicate that most immediate (direct) interactions among dominant species are actually competitive, although interactions were more facilitative over the drier summer season. Interestingly, lagged density dependence was strong for all species in both seasons; it was positive for conspecific interactions and both positive and negative for heterospecific interactions. Observed lagged density dependence is likely due to effects from litter and/or past storage in rhizomes. Conspecific immediate and lagged interactions tended to be stronger than heterospecific interactions, suggesting that population dynamics in this community are driven mostly by conspecifics. Overall, the presence of strong lagged density dependence in this system suggests that it may be more widespread in plants than previously thought.

Differences in rhizosphere microbial communities between native and non-native Phragmites australis may depend on stand density.

Microorganisms surrounding plant roots may benefit invasive species through enhanced mutualism or decreased antagonism, when compared to surrounding native species. We surveyed the rhizosphere soil microbiome of a prominent invasive plant, Phragmites australis, and its co-occurring native subspecies for evidence of microbial drivers of invasiveness. If the rhizosphere microbial community is important in driving plant invasions, we hypothesized that non-native Phragmites would cultivate a different microbiome from native Phragmites, containing fewer pathogens, more mutualists, or both. We surveyed populations of native and non-native Phragmites across Michigan and Ohio USA, and we described rhizosphere microbial communities using culture-independent next-generation sequencing. We found little evidence that native and non-native Phragmites cultivate distinct bacterial, fungal, or oomycete rhizosphere communities. Microbial community differences in our Michigan survey were not associated with plant lineage but were mainly driven by environmental factors, such as soil saturation and nutrient concentrations. Intensive sampling along transects consisting of dense monocultures of each lineage and mixed zones revealed bacterial community differences between lineages in dense monoculture, but not in mixture. We found no evidence of functional differences in the microbial communities surrounding each lineage. We extrapolate that the invasiveness of non-native Phragmites, when compared to its native congener, does not result from the differential cultivation of beneficial or antagonistic rhizosphere microorganisms.

Litter drives ecosystem and plant community changes in cattail invasion.

Invaded systems are commonly associated with a change in ecosystem processes and a decline in native species diversity; however, many different causal pathways linking invasion, ecosystem change, and native species decline could produce this pattern. The initial driver of environmental change may be anthropogenic, or it may be the invader itself; and the mechanism behind native species decline may be the human-induced environmental change, competition from the invader, or invader-induced environmental change (non-trophic effects). We examined applicability of each of these alternate pathways in Great Lakes coastal marshes invaded by hybrid cattail (Typha x glauca). In a survey including transects in three marshes, we found that T. x glauca was associated with locally high soil nutrients, low light, and large amounts of litter, and that native diversity was highest in areas of shallow litter depth. We tested whether live T. x glauca plants or their litter induced changes in the environment and in diversity with a live plant and litter transplant experiment. After one year, Typha litter increased soil NH4+ and N mineralization twofold, lowered light levels, and decreased the abundance and diversity of native plants, while live Typha plants had no effect on the environment or on native plants. This suggests that T. x glauca, through its litter production, can cause the changes in ecosystem processes that we commonly attribute to anthropogenic nutrient loading and that T. x glauca does not displace native species through competition for resources, but rather affects them non-trophically through its litter. Moreover, because T. x glauca plants were taller when grown with their own litter, we suggest that this invader may produce positive feedbacks and change the environment in ways that benefit itself and may promote its own invasion.

Quantitative partitioning of regional and local processes shaping regional diversity patterns.

Much recent research explaining plant community diversity has focused on comparing the relative impacts of regional and local processes. We employed a novel analysis to quantify the effects of multiple regional and local processes on species richness, and to make quantitative comparisons of those effects across two sites that differ in plot-level species richness, productivity and environmental conditions. While abiotic stress and competition limited richness within the communities at both sites, only differences in the overall pool of species at the site, likely resulting from long-distance dispersal and climate fluctuations, explained the differences in plot-level richness between sites. Patterns in local richness may be driven by a temporal storage effect, with greater richness in the site with greater climatic variability. By identifying both the factors that impact diversity within communities and those that vary systematically across communities, our integrated approach provides a better understanding of regional diversity patterns.

Sources of diversity in a grassland metacommunity: quantifying the contribution of dispersal to species richness.

Metacommunity theory suggests a potentially important role for dispersal in diversity maintenance at local, as well as regional, scales. In addition, propagule addition experiments have shown that dispersal often limits local diversity. However, actual dispersal rates into local communities and the contribution of immigrants to observed local diversity are poorly known. We present a new approach that partitions the diversity of a target community into dispersal-maintained and dispersal-independent components. Specifically, we quantify distances through space and time to the nearest potential seed source for naturally occurring recruits in target communities by using hierarchical data on species pools (local, site, region, and seed bank). Using this "recruit tag" approach, we found that dispersal contributed 29%-57% of the seedling diversity in perennial grasslands with different successional histories. However, both dispersal and seedling mortality remained remarkably constant, in absolute terms, over succession. The considerable loss of diversity over secondary succession (66%), therefore, could be understood only by considering how these processes interact with the decreasing disturbance rate (i.e., frequency of gaps) in later-successional sites. We conclude that a metacommunity perspective is relevant and necessary to understand the diversity and community assembly of this study system.

Response of Solidago canadensis clones to competition.

Transplants of ten Solidago canadensis clones were grown under high and low competition in the field to determine whether clones differed in survival, growth, and reproduction under natural conditions. Transplants had higher probability of survival and flowering and were larger in all measures of size when competition was experimentally reduced. Clones differed in almost all these measures of success, but only when variance among transplants within clones was reduced by excluding transplants that experienced heavy herbivore damage. Differences among clones were more apparent under low competition than under high competition, despite higher coefficients of variation within clones under low competition. Adjusting transplant size for initial size (parent ramet rhizome mass) did not change these results, although clones did differ in parent rhizome mass. All of these results suggest that there is little potential for selection to discriminate among these clones. Despite the strong differences in transplant performance between the competition treatments and among clones, the clones did not differ in competitive ability-almost none of the clone x competition interactions were significant. In addition, the measures of success of each clone were usually positively correlated between the high and low competition treatments, suggesting there were no tradeoffs between performance under high and low competition for these clones.

The effects of size of opening in vegetation and litter cover on seedling establishment of goldenrods (Solidago spp.).

We investigated the effects of size of opening in the vegetation and litter cover on seedling establishment of two species of goldenrods (Solidago spp.) in an abandoned field in southwestern Michigan, U.S.A. Seeds of S. canadensis and S. juncea were sown into clipped plots, ranging from 0 cm (control, unclipped) to 100 cm in diameter, with and without litter. Seedling emergence, survival and growth were followed for one year. Soil moisture was not significantly different among the opening sizes, but, within a size, tended to be lower when litter was removed. Light intensity at the soil surface was positively related to opening size early in the growing season, but later in the growing season reached a maximum in intermediate-sized openings and then leveled off.Litter strongly inhibited seedling emergence in both species. Emergence of S. canadensis seedlings was lower in 0 and 10 cm openings than in the larger openings, while emergence of S. juncea seedlings was lower in the largest openings (100 cm) than in all the smaller openings. In contrast, seedling growth and probability of survival increased with diameter of opening for both species. Some seedlings of S. juncea did survive in complete vegetation cover (controls, 0 cm openings) while seedlings of S. canadensis survived only in openings of at least 30 cm diameter. Thus, S. juncea had a smaller minimum opening size for seedling establishment than S. canadensis, although both species grew and survived best in the largest openings made in the experiment.

Early development of bacterial community diversity in emergently placed urinary catheters.

BACKGROUND: Approximately 25% of hospitalized patients have a urinary catheter, and catheter associated urinary tract infection is the most common nosocomial infection in the US, causing >1 million cases/year. However, the natural history of the biofilms that rapidly form on urinary catheters and lead to infection is not well described. FINDINGS: We characterized the dynamics of catheter colonization among catheters collected from 3 women and 5 men in a trauma burn unit with different indwelling times using TRFLP and culture. All patients received antibiotic therapy. RESULTS: Colony-forming units increased along the extraluminal catheter surface from the catheter balloon to the urethra, but no trend was apparent for the intraluminal surface. This suggests extraluminal bacteria come from periurethral communities while intraluminal bacteria are introduced via the catheter or already inhabit the urine/bladder. Richness of operational taxonomic units (OTUs) increased over time on the intraluminal surface, but was constant extraluminally. CONCLUSIONS: OTU community composition was explained best by time rather than axial location or surface. Our results suggest that catheter colonization can be very dynamic, and possibly have a predictable succession.

Species-specific SSR alleles for studies of hybrid cattails (Typha latifolia x T. angustifolia; Typhaceae) in North America.

PREMISE: Studies of hybridizing species are facilitated by the availability of species-specific molecular markers for identifying early- and later-generation hybrids. Cattails are a dominant feature of wetland communities, and a better understanding of the prevalence of hybrids is needed to assess the ecological and evolutionary effects of hybridization. Hybridization between Typha angustifolia and T. latifolia produce long-lived clones, known as Typha ×glauca, which are considered to be invasive. Although morphological variation in cattails makes it difficult to recognize early- and later-generation hybrids, several dominant, species-specific RAPD markers are available. Our goal was to find codominant, species-specific markers with greater polymorphism than RAPDs, to identify later-generation hybrids more efficiently. • METHODS: We screened nine SSR (simple sequence repeat) loci that were described from populations in Ukraine, and we surveyed 31 cattail populations from the upper Midwest and eastern USA. • KEY RESULTS: Seven SSR loci distinguished the parent taxa and were consistent with known species-specific RAPD markers, allowing easier detection of backcrossing. We used linear discriminant analysis to show that F(1) hybrid phenotypes were intermediate between the parent taxa, while those of backcrossed plants overlapped with the hybrids and their parents. Log(leaf length/leaf width), spike gap length, spike length, and stem diameter explained much of the variation among groups. • CONCLUSIONS: We provide the first documentation of backcrossed plants in hybridizing cattail populations in Michigan. The diagnostic SSR loci we identified should be extremely useful for examining the evolutionary and ecology interactions of hybridizing cattails in North America.

Microhabitat associations and seedling bank dynamics in a neotropical forest.

We conducted a rigorous test of tropical tree seedling microhabitat differentiation by examining microhabitat associations, survival and growth of established seedlings of ten tropical tree species representing a four-factor gradient in seed size. Eight microhabitat variables describing soil and light conditions were measured directly adjacent to each of 588 seedlings within twelve 10x100 m belt transects at Paracou, French Guiana, and at 264 reference points along the transects. From these measurements, we defined three principal components describing soil richness, soil softness and canopy openness. Six of ten species (in 9 of 30 total cases) were distributed non-randomly with respect to microhabitat along at least one principal component. However, few species demonstrated clear microhabitat specialization. All shifts in distribution relative to reference points were in the same direction (richer, softer soil). Furthermore, of 135 pairwise comparisons among the species, only 7 were significantly different. More than three-fourths of all seedlings (75.3%) survived over the 2-year monitoring period, but survival rates varied widely among species. In no case was the probability of survival influenced by any microhabitat parameter. Relative height growth rates for the seedlings over 2 years varied from -0.031 cm cm(-1) year(-1) (Dicorynia guianensis, Caesalpiniaceae) to 0.088 cm cm(-1) year(-1) (Virola michelii, Myristicaceae). In only 4 of 30 cases was height growth significantly associated with one of the three principal components. Because the conditions in this study were designed to maximize the chance of finding microhabitat differentiation among a group of species differing greatly in life history traits, the lack of microhabitat specialization it uncovered suggests that microhabitat partitioning among tropical tree species at the established seedling stage is unlikely to contribute greatly to coexistence among these species.

The effect of mean and variance in resource supply on survival of annuals from Mediterranean and desert environments.

Resource availability is often characterized by mean annual amounts, while ignoring the spatial variation within habitats and the temporal variation within a year. Yet, temporal and spatial variation may be especially important for identifying the source of stress in low productivity environments such as deserts where resources are often pulsed and resource renewal events are separated by long periods of low resource availability. Therefore, the degree of stress will be determined in part by the length of time between recharge events. Here, we investigated the effect of timing and total amount of water application on two congeneric pairs, each with a population from a low (desert) and a high (Mediterranean) productivity habitat. As expected, highest survival and greatest growth were found at low or intermediate recharge intervals, and the magnitude of response to increases in total seasonal amounts was greater for Mediterranean species than desert species. The species that had greater survival switched in the hierarchy under high total water depending on interval length. These results demonstrate that temporal variation in resource availability can be as important as annual total amounts for plant performance and that response to temporal dynamics can vary between species. This has implications for community-level processes, as competitive hierarchies may switch based on resource dynamics rather than only total availability.