Allometric relationships for eight species of 4-5 year old nitrogen-fixing and non-fixing trees.

Allometric equations are often used to estimate plant biomass allocation to different tissue types from easier-to-measure quantities. Biomass allocation, and thus allometric equations, often differs by species and sometimes varies with nutrient availability. We measured biomass components for five nitrogen-fixing tree species (Robinia pseudoacacia, Gliricidia sepium, Casuarina equisetifolia, Acacia koa, Morella faya) and three non-fixing tree species (Betula nigra, Psidium cattleianum, Dodonaea viscosa) grown in field sites in New York and Hawaii for 4-5 years and subjected to four fertilization treatments. We measured total aboveground, foliar, main stem, secondary stem, and twig biomass in all species, and belowground biomass in Robinia pseudoacacia and Betula nigra, along with basal diameter, height, and canopy dimensions. The individuals spanned a wide size range (<1-16 cm basal diameter; 0.24-8.8 m height). For each biomass component, aboveground biomass, belowground biomass, and total biomass, we determined the following four allometric equations: the most parsimonious (lowest AIC) overall, the most parsimonious without a fertilization effect, the most parsimonious without canopy dimensions, and an equation with basal diameter only. For some species, the most parsimonious overall equation included fertilization effects, but fertilization effects were inconsistent across fertilization treatments. We therefore concluded that fertilization does not clearly affect allometric relationships in these species, size classes, and growth conditions. Our best-fit allometric equations without fertilization effects had the following R2 values: 0.91-0.99 for aboveground biomass (the range is across species), 0.95 for belowground biomass, 0.80-0.96 for foliar biomass, 0.94-0.99 for main stem biomass, 0.77-0.98 for secondary stem biomass, and 0.88-0.99 for twig biomass. Our equations can be used to estimate overall biomass and biomass of tissue components for these size classes in these species, and our results indicate that soil fertility does not need to be considered when using allometric relationships for these size classes in these species.

Symbiotic nitrogen fixation does not stimulate soil phosphatase activity under temperate and tropical trees.

Symbiotic nitrogen (N)-fixing plants can enrich ecosystems with N, which can alter the cycling and demand for other nutrients. Researchers have hypothesized that fixed N could be used by plants and soil microbes to produce extracellular phosphatase enzymes, which release P from organic matter. Consistent with this speculation, the presence of N-fixing plants is often associated with high phosphatase activity, either in the soil or on root surfaces, although other studies have not found this association, and the connection between phosphatase and rates of N fixation-the mechanistic part of the argument-is tenuous. Here, we measured soil phosphatase activity under N-fixing trees and non-fixing trees transplanted and grown in tropical and temperate sites in the USA: two sites in Hawaii, and one each in New York and Oregon. This provides a rare example of phosphatase activity measured in a multi-site field experiment with rigorously quantified rates of N fixation. We found no difference in soil phosphatase activity under N-fixing vs. non-fixing trees nor across rates of N fixation, though we note that no sites were P limited and only one was N limited. Our results add to the literature showing no connection between N fixation rates and phosphatase activity.

Trait-based filtering mediates the effects of realistic biodiversity losses on ecosystem functioning.

Biodiversity losses are a major driver of global changes in ecosystem functioning. While most studies of the relationship between biodiversity and ecosystem functioning have examined randomized species losses, trait-based filtering associated with species-specific vulnerability to drivers of diversity loss can strongly influence how ecosystem functioning responds to declining biodiversity. Moreover, the responses of ecosystem functioning to diversity loss may be mediated by environmental variability interacting with the suite of traits remaining in depauperate communities. We do not yet understand how communities resulting from realistic diversity losses (filtered by response traits) influence ecosystem functioning (via effect traits of the remaining community), especially under variable environmental conditions. Here, we directly test how realistic and randomized plant diversity losses influence productivity and invasion resistance across multiple years in a California grassland. Compared with communities based on randomized diversity losses, communities resulting from realistic (drought-driven) species losses had higher invasion resistance under climatic conditions that matched the trait-based filtering they experienced. However, productivity declined more with realistic than with randomized species losses across all years, regardless of climatic conditions. Functional response traits aligned with effect traits for productivity but not for invasion resistance. Our findings illustrate that the effects of biodiversity losses depend not only on the identities of lost species but also on how the traits of remaining species interact with varying environmental conditions. Understanding the consequences of biodiversity change requires studies that evaluate trait-mediated effects of species losses and incorporate the increasingly variable climatic conditions that future communities are expected to experience.

N supply mediates the radiative balance of N2 O emissions and CO2 sequestration driven by N-fixing vs. non-fixing trees.

Forests are a significant CO2 sink. However, CO2 sequestration in forests is radiatively offset by emissions of nitrous oxide (N2 O), a potent greenhouse gas, from forest soils. Reforestation, an important strategy for mitigating climate change, has focused on maximizing CO2 sequestration in plant biomass without integrating N2 O emissions from soils. Although nitrogen (N)-fixing trees are often recommended for reforestation because of their rapid growth on N-poor soil, they can stimulate significant N2 O emissions from soils. Here, we first used a field experiment to show that a N-fixing tree (Robinia pseudoacacia) initially mitigated climate change more than a non-fixing tree (Betula nigra). We then used our field data to parameterize a theoretical model to investigate these effects over time. Under lower N supply, N-fixers continued to mitigate climate change more than non-fixers by overcoming N limitation of plant growth. However, under higher N supply, N-fixers ultimately mitigated climate change less than non-fixers by enriching soil N and stimulating N2 O emissions from soils. These results have implications for reforestation, suggesting that N-fixing trees are more effective at mitigating climate change at lower N supply, whereas non-fixing trees are more effective at mitigating climate change at higher N supply.

Naïve plant communities and individuals may initially suffer in the face of reintroduced megafauna: An experimental exploration of rewilding from an African savanna rangeland.

Excluding large native mammals is an inverse test of rewilding. A 25-year exclosure experiment in an African savanna rangeland offers insight into the potentials and pitfalls of the rewilding endeavor as they relate to the native plant community. A broad theme that has emerged from this research is that entire plant communities, as well as individual plants, adjust to the absence of herbivores in ways that can ill-prepare them for the return of these herbivores. Three lines of evidence suggest that these "naïve" individuals, populations, and communities are likely to initially suffer from herbivore rewilding. First, plots protected from wild herbivores for the past 25 years have developed rich diversity of woody plants that are absent from unfenced plots, and presumably would disappear upon rewilding. Second, individuals of the dominant tree in this system, Acacia drepanolobium, greatly reduce their defences in the absence of browsers, and the sudden arrival of these herbivores (in this case, through a temporary fence break), resulted in far greater elephant damage than for their conspecifics in adjacent plots that had been continually exposed to herbivory. Third, the removal of herbivores favoured the most palatable grass species, and a large number of rarer species, which presumably would be at risk from herbivore re-introduction. In summary, the native communities that we observe in defaunated landscapes may be very different from their pre-defaunation states, and we are likely to see some large changes to these plant communities upon rewilding with large herbivores, including potential reductions in plant diversity. Lastly, our experimental manipulation of cattle represents an additional test of the role of livestock in rewilding. Cattle are in many ways ecologically dissimilar to wildlife (in particular their greater densities), but in other ways they may serve as ecological surrogates for wildlife, which could buffer ecosystems from some of the ecological costs of rewilding. More fundamentally, African savannah ecosystems represent a challenge to traditional Western definitions of "wilderness" as ecosystems free of human impacts. We support the suggestion that as we "rewild" our biodiversity landscapes, we redefine "wildness" in the 21st Century to be inclusive of (low impact, and sometimes traditional) human practices that are compatible with the sustainability of native (and re-introduced) biodiversity.

Logarithmic scales in ecological data presentation may cause misinterpretation.

Scientific communication relies on clear presentation of data. Logarithmic scales are used frequently for data presentation in many scientific disciplines, including ecology, but the degree to which they are correctly interpreted by readers is unclear. Analysing the extent of log scales in the literature, we show that 22% of papers published in the journal Ecology in 2015 included at least one log-scaled axis, of which 21% were log-log displays. We conducted a survey that asked members of the Ecological Society of America (988 responses, and 623 completed surveys) to interpret graphs that were randomly displayed with linear-linear or log-log axes. Many more respondents interpreted graphs correctly when the graphs had linear-linear axes than when they had log-log axes: 93% versus 56% for our all-around metric, although some of the individual item comparisons were even more skewed (for example, 86% versus 9% and 88% versus 12%). These results suggest that misconceptions about log-scaled data are rampant. We recommend that ecology curricula include explicit instruction on how to interpret log-scaled axes and equations, and we also recommend that authors take the potential for misconceptions into account when deciding how to visualize data.

Photosynthetic tolerance to non-resource stress influences competition importance and intensity in an invaded estuary.

In an attempt to clarify the role of environmental and biotic interactions on plant growth, there has been a long-running ecological debate over whether the intensity and importance of competition stabilizes, increases or decreases across environmental gradients. We conducted an experiment in a Chinese estuary to investigate the effects of a non-resource stress gradient, soil salinity (from 1.4‰ to 19.0‰ salinity), on the competitive interactions between native Phragmites australis and invasive Spartina alterniflora. We linked these effects to measurements of photosynthetic activities to further elucidate the underlying physiological mechanism behind the competitive interactions and the driver of invasion. The experiments revealed that while biomass of both species decreased in the presence of the other, competition did not alter photosynthetic activity of either species over time. P. australis exhibited high photosynthetic activity, including low chlorophyllase activity, high chlorophyll content, high stomatal conductance and high net photosynthetic rate, at low salinity. Under these conditions, P. australis experienced low competitive intensity, leading to high biomass production and competitive exclusion of S. alterniflora. The opposite was observed for S. alterniflora: while competitive intensity experienced by P. australis increased with increasing salinity, and photosynthetic activity, biomass, competitive dominance and the importance of competition for P. australis growth decreased, those of S. alterniflora were stable. These findings demonstrate that S. alterniflora invasion driven by competitive exclusion are likely to occur and expand in high salinity zones. The change in the nature of competition along a non-resource stress gradient differs between competitors likely due to differences in photosynthetic tolerance to salinity. The driver of growth of the less-tolerant species changes from competition to non-resource stress factors with increasing stress levels, whereas competition is constantly important for growth of the more-tolerant species. Incorporating metrics of both competition intensity and importance, as well as linking these competitive outcomes with physiological mechanisms, is crucial to understanding, predicting, and mediating the effects of invasive species in the future.

Flowering phenology shifts in response to biodiversity loss.

Observational studies and experimental evidence agree that rising global temperatures have altered plant phenology-the timing of life events, such as flowering, germination, and leaf-out. Other large-scale global environmental changes, such as nitrogen deposition and altered precipitation regimes, have also been linked to changes in flowering times. Despite our increased understanding of how abiotic factors influence plant phenology, we know very little about how biotic interactions can affect flowering times, a significant knowledge gap given ongoing human-caused alteration of biodiversity and plant community structure at the global scale. We experimentally manipulated plant diversity in a California serpentine grassland and found that many plant species flowered earlier in response to reductions in diversity, with peak flowering date advancing an average of 0.6 days per species lost. These changes in phenology were mediated by the effects of plant diversity on soil surface temperature, available soil N, and soil moisture. Peak flowering dates were also more dispersed among species in high-diversity plots than expected based on monocultures. Our findings illustrate that shifts in plant species composition and diversity can alter the timing and distribution of flowering events, and that these changes to phenology are similar in magnitude to effects induced by climate change. Declining diversity could thus contribute to or exacerbate phenological changes attributed to rising global temperatures.

The symbionts made me do it: legumes are not hardwired for high nitrogen concentrations but incorporate more nitrogen when inoculated.

High tissue nitrogen (N) concentrations in N-fixing legumes may be driven by an evolutionary commitment to a high N strategy, by higher N availability from fixation, or by some other cause. To disentangle these hypotheses, we asked two questions: are legumes hardwired to have high N concentrations? Aside from delivering fixed N, how does inoculation affect legume N concentrations? In order to understand drivers of plant stoichiometry, we subjected four herbaceous legume species to nine levels of N fertilization in a glasshouse. Half of the individuals were inoculated with crushed nodules, whereas the other half remained uninoculated and could not fix N. Across four legume species, we found that tissue stoichiometry and nutrient content were more plastic than has been described for any other plant species. In addition, inoculated plants had higher tissue N concentrations than N fixation activity alone can explain. Rather than being hardwired for high N or phosphorus (P) demand, the legumes we examined were highly flexible in their nutrient allocation. Understanding the drivers of legume N concentrations is essential to understanding the role of N fixers in community- and ecosystem-level processes.

Testing the trait-based community framework: Do functional traits predict competitive outcomes?

Plant traits can be used to understand a range of ecological processes, including competition with invasive species. The extent to which native and invasive species are competing via limiting similarity or trait hierarchies has important implications for the management of invaded communities. We screened 47 native species that co-occur with Festuca perennis, a dominant invader in California serpentine grassland, for traits pertaining to resource use and acquisition. We then grew F. perennis with 10 species spanning a range of functional similarity in pairwise competition trials. Functionally similar species did not have a strong adverse effect on F. perennis performance as would be expected by limiting similarity theory. Phylogenetic relatedness, which may integrate a number of functional traits, was also a poor predictor of competitive outcome. Instead, species with high specific root length, low root-to-shoot biomass ratio, and low leaf nitrogen concentration were more effective at suppressing the growth of F. perennis. Our results suggest that fitness differences (i.e., trait hierarchies) may be more important than niche differences (i.e., limiting similarity) in structuring competitive outcomes in this system and may be a promising approach for the restoration of invaded systems.

Diversity of nitrogen fixation strategies in Mediterranean legumes.

Symbiotic N2 fixation (SNF) brings nitrogen into ecosystems, fuelling much of the world's agriculture(1) and sustaining carbon storage(2,3). However, it can also cause nitrogen saturation, exacerbating eutrophication and greenhouse warming(4-7). The balance of these effects depends on the degree to which N2-fixing plants adjust how much N2 they fix based on their needs (their SNF 'strategies')(5,6). Genetic, biochemical and physiological details of SNF are well known for certain economically important species(8,9), but the diversity of N2-fixing plants(10) and bacteria(11) is enormous, and little is known about most N2-fixing symbioses in natural ecosystems(12). Here, we show that co-occurring, closely related herbs exhibit diverse SNF strategies. In response to a nitrogen supply gradient, four species fixed less N2 than they needed (over-regulation), two fixed what they needed (facultative) and two did not downregulate SNF (obligate). No species downregulated but fixed more N2 than it needed (under-regulation or incomplete downregulation), but some species under-regulated or incompletely downregulated structural allocation to SNF. In fact, most species maintained nodules (the root structures that house symbionts) when they did not fix N2, suggesting decoupling of SNF activity and structure. Simulations showed that over-regulation of SNF activity is more adaptive than under-regulation or incomplete downregulation, and that different strategies have wildly different effects on ecosystem-level nitrogen cycling.

Realistic diversity loss and variation in soil depth independently affect community-level plant nitrogen use.

Numerous experiments have demonstrated that diverse plant communities use nitrogen (N) more completely and efficiently, with implications for how species conservation efforts might influence N cycling and retention in terrestrial ecosystems. However, most such experiments have randomly manipulated species richness and minimized environmental heterogeneity, two design aspects that may reduce applicability to real ecosystems. Here we present results from an experiment directly comparing how realistic and randomized plant species losses affect plant N use across a gradient of soil depth in a native-dominated serpentine grassland in California. We found that the strength of the species richness effect on plant N use did not increase with soil depth in either the realistic or randomized species loss scenarios, indicating that the increased vertical heterogeneity conferred by deeper soils did not lead to greater complementarity among species in this ecosystem. Realistic species losses significantly reduced plant N uptake and altered N-use efficiency, while randomized species losses had no effect on plant N use. Increasing soil depth positively affected plant N uptake in both loss order scenarios but had a weaker effect on plant N use than did realistic species losses. Our results illustrate that realistic species losses can have functional consequences that differ distinctly from randomized losses, and that species diversity effects can be independent of and outweigh those of environmental heterogeneity on ecosystem functioning. Our findings also support the value of conservation efforts aimed at maintaining biodiversity to help buffer ecosystems against increasing anthropogenic N loading.