Lori A. Biederman

Biochar and its effects on plant productivity and nutrient cycling: a meta-analysis.

Biochar is a carbon‐rich coproduct resulting from pyrolyzing biomass. When applied to the soil it resists decomposition, effectively sequestering the applied carbon and mitigating anthropogenic CO2 emissions. Other promoted benefits of biochar application to soil include increased plant productivity and reduced nutrient leaching. However, the effects of biochar are variable and it remains unclear if recent enthusiasm can be justified. We evaluate ecosystem responses to biochar application with a meta‐analysis of 371 independent studies culled from 114 published manuscripts. We find that despite variability introduced by soil and climate, the addition of biochar to soils resulted, on average, in increased aboveground productivity, crop yield, soil microbial biomass, rhizobia nodulation, plant K tissue concentration, soil phosphorus (P), soil potassium (K), total soil nitrogen (N), and total soil carbon (C) compared with control conditions. Soil pH also tended to increase, becoming less acidic, following the addition of biochar. Variables that showed no significant mean response to biochar included belowground productivity, the ratio of aboveground : belowground biomass, mycorrhizal colonization of roots, plant tissue N, and soil P concentration, and soil inorganic N. Additional analyses found no detectable relationship between the amount of biochar added and aboveground productivity. Our results provide the first quantitative review of the effects of biochar on multiple ecosystem functions and the central tendencies suggest that biochar holds promise in being a win‐win‐win solution to energy, carbon storage, and ecosystem function. However, biochars impacts on a fourth component, the downstream nontarget environments, remain unknown and present a critical research gap.

Spatial Heterogeneity in Soil Microbes Alters Outcomes of Plant Competition

Plant species vary greatly in their responsiveness to nutritional soil mutualists, such as mycorrhizal fungi and rhizobia, and this responsiveness is associated with a trade-off in allocation to root structures for resource uptake. As a result, the outcome of plant competition can change with the density of mutualists, with microbe-responsive plant species having high competitive ability when mutualists are abundant and non-responsive plants having high competitive ability with low densities of mutualists. When responsive plant species also allow mutualists to grow to greater densities, changes in mutualist density can generate a positive feedback, reinforcing an initial advantage to either plant type. We study a model of mutualist-mediated competition to understand outcomes of plant-plant interactions within a patchy environment. We find that a microbe-responsive plant can exclude a non-responsive plant from some initial conditions, but it must do so across the landscape including in the microbe-free areas where it is a poorer competitor. Otherwise, the non-responsive plant will persist in both mutualist-free and mutualist-rich regions. We apply our general findings to two different biological scenarios: invasion of a non-responsive plant into an established microbe-responsive native population, and successional replacement of non-responders by microbe-responsive species. We find that resistance to invasion is greatest when seed dispersal by the native plant is modest and dispersal by the invader is greater. Nonetheless, a native plant that relies on microbial mutualists for competitive dominance may be particularly vulnerable to invasion because any disturbance that temporarily reduces its density or that of the mutualist creates a window for a non-responsive invader to establish dominance. We further find that the positive feedbacks from associations with beneficial soil microbes create resistance to successional turnover. Our theoretical results constitute an important first step toward developing a general understanding of the interplay between mutualism and competition in patchy landscapes, and generate qualitative predictions that may be tested in future empirical studies.

Comment on "Worldwide evidence of a unimodal relationship between productivity and plant species richness"

Fraser et al. (Reports, 17 July 2015, p. 302) report a unimodal relationship between productivity and species richness at regional and global scales, which they contrast with the results of Adler et al. (Reports, 23 September 2011, p. 1750). However, both data sets, when analyzed correctly, show clearly and consistently that productivity is a poor predictor of local species richness.

Herbivores safeguard plant diversity by reducing variability in dominance

Summary 1.Reductions in community evenness can lead to local extinctions as dominant species exclude subordinate species; however, herbivores can prevent competitive exclusion by consuming otherwise dominant plant species, thus increasing evenness. While these predictions logically result from chronic, gradual reductions in evenness, rapid, temporary pulses of dominance may also reduce species richness. Short pulses of dominance can occur as biotic or abiotic conditions temporarily favor one or a few species, manifested as increased temporal variability (the inverse of temporal stability) in community evenness. Here, we tested whether consumers help maintain plant diversity by reducing the temporal variability in community evenness. 2.We tested our hypothesis by reducing herbivore abundance in a detailed study of a developing, tallgrass prairie restoration. To assess the broader implications of the importance of herbivory on community evenness as well as potential mechanisms, we paired this study with a global herbivore-reduction experiment. 3.We found that herbivores maintained plant richness in a tallgrass prairie restoration by limiting temporary pulses in dominance by a single species. Dominance by an annual species in a single year was negatively associated with species richness, suggesting that short pulses of dominance may be sufficient to exclude subordinate species. 4.The generality of this site-level relationship was supported by the global experiment in which inter-annual variability in evenness declined in the presence of vertebrate herbivores over timeframes ranging in length from 2-5 years, preventing declines in species richness. Furthermore, inter-annual variability of community evenness was also negatively associated with pre-treatment species richness. 5.Synthesis: A loss or reduction of herbivores can destabilize plant communities by allowing brief periods of dominance by one or a few species, potentially triggering a feedback cycle of dominance and extinction. Such cycles may not occur immediately following the loss of herbivores, being delayed until conditions allow temporary periods of dominance by a subset of plant species. This article is protected by copyright. All rights reserved.

Nutrient Addition Shifts Plant Community Composition Towards Earlier Flowering Species in Some Prairie Ecoregions in the U.S. Central Plains

The distribution of flowering across the growing season is governed by each species’ evolutionary history and climatic variability. However, global change factors, such as eutrophication and invasion, can alter plant community composition and thus change the distribution of flowering across the growing season. We examined three ecoregions (tall-, mixed, and short-grass prairie) across the U.S. Central Plains to determine how nutrient (nitrogen (N), phosphorus, and potassium (+micronutrient)) addition alters the temporal patterns of plant flowering traits. We calculated total community flowering potential (FP) by distributing peak-season plant cover values across the growing season, allocating each species’ cover to only those months in which it typically flowers. We also generated separate FP profiles for exotic and native species and functional group. We compared the ability of the added nutrients to shift the distribution of these FP profiles (total and sub-groups) across the growing season. In all ecoregions, N increased the relative cover of both exotic species and C3 graminoids that flower in May through August. The cover of C4 graminoids decreased with added N, but the response varied by ecoregion and month. However, these functional changes only aggregated to shift the entire community’s FP profile in the tall-grass prairie, where the relative cover of plants expected to flower in May and June increased and those that flower in September and October decreased with added N. The relatively low native cover in May and June may leave this ecoregion vulnerable to disturbance-induced invasion by exotic species that occupy this temporal niche. There was no change in the FP profile of the mixed and short-grass prairies with N addition as increased abundance of exotic species and C3 graminoids replaced other species that flower at the same time. In these communities a disturbance other than nutrient addition may be required to disrupt phenological patterns.

Phenological Monitoring Aids Habitat Management of Threatened Plant

ABSTRACT: Western prairie fringed orchid (WPFO, Platanthera praeclara Sheviak & Bowles) is a federally threatened Great Plains forb incapable of regeneration if above-ground parts are damaged within a given growing season. The invasion of smooth brome (Bromus inermis Leyss) reduces diversity and alters resources in northern tall grass prairies, including those occupied by WFPO. Prescribed fire is most effective for brome reduction when it is timed to its phenology. This timing could also interfere with the growth of non-target species, including WPFO. Our objective was to document the emergence and growth of WPFO and to determine the degree of phenological overlap between WFPO and smooth brome. We monitored orchid phenology to detect emergence dates and tracked growth of flowering and non-flowering WFPO. We found that WFPO individuals likely to flower emerge earlier and their growth is more rapid than plants not destined to flower. We augmented orchid monitoring with observations of smooth brome for two years and found that the majority of reproductive orchids emerged before the dates when brome reached its reported stage of greatest susceptibility to fire. We conclude that late spring prescribed fires are likely to damage WPFO individuals most likely to reproduce. Fall burns may provide an alternative strategy for prairie management at WPFO sites.

On the importance of accurate reporting: a response to comments on ‘Biochar and its effects on plant productivity and nutrient cycling: a meta-analysis’

In their comment, Jeffery et al., make several criticisms of our meta-analysis (Biederman & Harpole 2012). Specifically, they argued that we (i) failed to provide a ‘thorough assessment of all the different socioeconomic, biophysical, and ecological components from a systems analysis perspective’; (ii) that we did not assess all of the potential effects of biochar; and (iii) our conclusions were ‘beyond what can be robustly concluded based on the data presented in the article’. We address these criticisms below. Furthermore, Jeffery et al. correctly state that we failed to address their 2011 meta-analysis in our study. Unfortunately, the timing of our literature searches missed this contribution. As we now have the opportunity to compare these two analyses, we find that the primary difference in our findings appears to be the result of several problematic decisions made by Jeffery et al. in their analysis that caused them to underestimate biochar’s mean effect with exaggerated confidence. A comparison of these methods and their consequences is also provided below. Biochar has been argued to have potential to alleviate several global problems by providing multiple and simultaneous ‘wins’: as a by-product of bioenergy production, by increasing carbon sequestration, and through the improvement of agroecosystem function (Laird, 2008). The case for carbon storage is compelling given the estimated half-life of biochar and other forms of black carbon in the soil is on the order of millennia (Lehmann et al., 2006; Novak et al., 2010). However, there are many sociological and market uncertainties that need to be resolved before net bioenergy and sequestration benefits are possible (Hill et al., 2006; Stoms et al., 2012). The specific objective of our (Biederman & Harpole 2012) meta-analysis, however, was to evaluate the third potential ‘win’ of biochar: multiple ‘ecosystem functions including plant productivity, nutrient uptake, soil properties, and on ecosystem services, such as crop yield.’ This focus on ecosystem responses necessarily did not include wider topics, such as the socioeconomic effects of biomass production or a systems analysis perspective. Second, Jeffery et al. are correct that we did not assess every potential effect of biochar on ‘soil-dwelling and aquatic biota (including induced toxicity), gas fluxes and emissions, sustainable biomass provisioning for biochar production, eutrophication of aquatic environments, potential for contamination or bioaccumulation of contaminants, including overall food and health safety.’ Again, some of these important topics, such as food safety, were well beyond our stated objective. In other cases, such as gas fluxes and emissions, the dynamic and complex nature of these responses requires careful and singular treatment. Finally, there was often insufficient published data, on soil biota for example, available to conduct a rigorous meta-analysis. We, in fact, discussed this issue and the critical need for such studies; for example, ‘It is imperative that we understand how biochar interacts with all aspects of the environment prior to its widespread application.’ A meta-analysis is necessarily constrained by the availability of published studies: it can highlight the state of our current knowledge by identifying the central tendencies in the results of multiple independent studies, and where the data do not exist, highlight the state of what we do not know and the critical studies that need to be done. Therefore, contrary to the claim of Jeffery et al. that ‘the range of ecosystem functions analyzed is highly limited and so not sufficiently representative’, our comprehensive analysis of available published data represents the current state or our knowledge of the effects of biochar addition on multiple ecosystem functions. That there are at present insufficient or even no studies that provide additional desired data is a different matter beyond the scope of a meta-analysis. We agree with Jeffery et al. that there is a need for more studies on a broader range of response variables, and we argued this fact in our article. For example, we calculate the average study length to highlight the lack of long-term data. Correspondence: W. Stanley Harpole, tel. +515 294 7253, fax 515 294 1337, e-mail: harpole@iastate.edu

Spatial heterogeneity in species composition constrains plant community responses to herbivory and fertilisation

Environmental change can result in substantial shifts in community composition. The associated immigration and extinction events are likely constrained by the spatial distribution of species. Still, studies on environmental change typically quantify biotic responses at single spatial (time series within a single plot) or temporal (spatial beta diversity at single time points) scales, ignoring their potential interdependence. Here, we use data from a global network of grassland experiments to determine how turnover responses to two major forms of environmental change - fertilisation and herbivore loss - are affected by species pool size and spatial compositional heterogeneity. Fertilisation led to higher rates of local extinction, whereas turnover in herbivore exclusion plots was driven by species replacement. Overall, sites with more spatially heterogeneous composition showed significantly higher rates of annual turnover, independent of species pool size and treatment. Taking into account spatial biodiversity aspects will therefore improve our understanding of consequences of global and anthropogenic change on community dynamics.

Herbivory and eutrophication mediate grassland plant nutrient responses across a global climatic gradient

Plant stoichiometry, the relative concentration of elements, is a key regulator of ecosystem functioning and is also being altered by human activities. In this paper we sought to understand the global drivers of plant stoichiometry and compare the relative contribution of climatic vs. anthropogenic effects. We addressed this goal by measuring plant elemental (C, N, P and K) responses to eutrophication and vertebrate herbivore exclusion at eighteen sites on six continents. Across sites, climate and atmospheric N deposition emerged as strong predictors of plot-level tissue nutrients, mediated by biomass and plant chemistry. Within sites, fertilization increased total plant nutrient pools, but results were contingent on soil fertility and the proportion of grass biomass relative to other functional types. Total plant nutrient pools diverged strongly in response to herbivore exclusion when fertilized; responses were largest in ungrazed plots at low rainfall, whereas herbivore grazing dampened the plant community nutrient responses to fertilization. Our study highlights (1) the importance of climate in determining plant nutrient concentrations mediated through effects on plant biomass, (2) that eutrophication affects grassland nutrient pools via both soil and atmospheric pathways and (3) that interactions among soils, herbivores and eutrophication drive plant nutrient responses at small scales, especially at water-limited sites.