Jennie R. McLaren

Plant identity influences decomposition through more than one mechanism.

Plant litter decomposition is a critical ecosystem process representing a major pathway for carbon flux, but little is known about how it is affected by changes in plant composition and diversity. Single plant functional groups (graminoids, legumes, non-leguminous forbs) were removed from a grassland in northern Canada to examine the impacts of functional group identity on decomposition. Removals were conducted within two different environmental contexts (fertilization and fungicide application) to examine the context-dependency of these identity effects. We examined two different mechanisms by which the loss of plant functional groups may impact decomposition: effects of the living plant community on the decomposition microenvironment, and changes in the species composition of the decomposing litter, as well as the interaction between these mechanisms. We show that the identity of the plant functional group removed affects decomposition through both mechanisms. Removal of both graminoids and forbs slowed decomposition through changes in the decomposition microenvironment. We found non-additive effects of litter mixing, with both the direction and identity of the functional group responsible depending on year; in 2004 graminoids positively influenced decomposition whereas in 2006 forbs negatively influenced decomposition rate. Although these two mechanisms act independently, their effects may be additive if both mechanisms are considered simultaneously. It is essential to understand the variety of mechanisms through which even a single ecosystem property is affected if we are to predict the future consequences of biodiversity loss.

Shrub encroachment in Arctic tundra: Betula nana effects on above‐ and belowground litter decomposition

Rapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.

Asynchrony among local communities stabilises ecosystem function of metacommunities

Abstract Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species‐level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1–315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

The Effects of Fertilization and Herbivory on the Phenology of the Understory Vegetation of the Boreal Forest in Northwestern Canada

Abstract Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring temperatures, and increased soil mineralization, have resulted in shifts in the timing of plant phenological events. We assessed the effects of fertilizer and herbivory on the phenology of 7 plant species from a boreal forest understory using long-term experimental plots in the southwest Yukon. Fertilizer and fencing treatments were initiated in 1990, and discontinued in half of each plot in 2000. There were few effects on phenology of either fertilizer or fencing. In some species, fertilizer affected the final phenological stage reached, but the presence and direction of the effect was species-dependent. Epilobium angustifolium was the only species where the timing of phenological stages responded to fertilization; early phenological stages were advanced with fertilizer. First leaf expansion for Arctostaphylos uva-ursi occurred earlier in fenced plots, although this effect disappeared in plots where the fencing treatment had been discontinued. We conclude that previously observed changes in species abundance with fertilizer treatments are likely not caused by changes in the phenology of these species, and are more likely due to fertilization imposed changes in vegetative growth.

Seasonal patterns of soil nitrogen availability in moist acidic tundra

Our ability to predict effects of changing soil nitrogen (N) in Arctic tundra has been limited by our poor understanding of the intra-annual variability of soil N in this strongly seasonal ecosystem. Studies have shown that microbial biomass declines in spring accompanied by peaks in inorganic nutrients. However, subsequent to this early pulse, there are few high temporal resolution measurements during the growing season. We hypothesized that (1) low N would be maintained throughout the growing season, (2) peaks of total free primary amines (TFPA), ammonium (NH4+), and nitrate (NO3−) would follow a sequential pattern driven by mineralization and nitrification, and (3) a peak in soil N would occur as plants senesce. We conducted weekly measurements of TFPA, NH4+, and NO3− in two tundra sites, from soil thaw in spring to freeze in fall. At each site, NH4+ peaks were followed by smaller peaks in NO3−, supporting the hypothesis that excess NH4+ would be nitrified. Furthermore, peaks in NH4+ were observed both...

Few effects of plant functional group identity on ecosystem properties in an annual desert community

Desertification is leading to large-scale changes in vegetation structure resulting from increased grazing pressure and drought which may, in turn, have further effects on ecosystem functioning. We examine how the changing functional group identity of plants may influence a range of biotic and abiotic ecosystem properties. To explore this question, we use a functional group removal experiment in which single functional groups (graminoids, legumes and non-leguminous forbs) were experimentally removed from an annual plant community in the Negev Desert, Israel. We conducted the experiment in both a high- and a low-resource environment to determine if identity effects are context dependent. We found full biomass compensation by remaining functional groups for the removal of any functional group, often with more, rather than larger, individuals comprising the compensatory growth. We also found few effects overall of functional group identity on ecosystem properties, with some dependence on environmental context. We found that the functional group with the largest proportional biomass often, but not always, had the largest effect on ecosystem properties. We contrast these results with those from previous removal experiments, the majority of which have been conducted in perennial ecosystems, and hypothesize that the transient nature of annual communities leads to fewer plant–soil interactions in the long term, and as a result fewer effects on ecosystem properties.

CHANGES IN THE UNDERSTORY PLANT COMMUNITY AND ECOSYSTEM PROPERTIES ALONG A SHRUB DENSITY GRADIENT

Climate warming is projected to alter the vegetation community composition of arctic and alpine ecosystems including an increase in the relative abundance and cover of deciduous shrubs. This change in plant functional group dominance will likely alter tundra ecosystem structure and function. We conducted an observational study to quantify how the understory vegetation community and ecosystem properties varied along a shrub density and altitudinal gradient in a tundra alpine ecosystem in south-west Yukon. Although there was weak association between shrub density and species richness of understory community, there were large differences in functional group abundance between the different shrub densities; forb cover increased at lower elevations with higher shrub density at the expense of cryptogam and dwarf shrub cover. Litter mass, light interception, and soil carbon:nitrogen ratios all increased with shrub density. Sites with shrubs had higher summer soil temperatures, lower summer soil moisture, and lowe...

Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities

R code, derived metrics, and limited metadata associated with Wilcox et al. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters. When using this data or code, please cite the original publication: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre, G. Houseman, F. Isbell, D. Johnson, J. Alatalo, A. Baldwin, E. Bork, E. Boughton, W. Bowman, A. Britton, J. Cahill, S. Collins, G-Z. Du, A. Eskelinen, L. Gough, A. Jentsch, C. Kern, K. Klanderud, A. Knapp, J. Kreyling, Y. Luo, J. McLaren, P. Megonigal, V. Onipchenko, J. Prevey, J. Price, C. Robinson, O. Sala, M. Smith, N. Soudzilovskaia, L. Souza, D. Tilman, S. White, Z. Xu, L. Yahdjian, Q. Yu, P. Zhang, Y, Zhang. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters vol(iss):xx-xx. Additionally, please cite the Figshare file set: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre. (2017). Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities. Figshare. https://dx.doi.org/10.6084/m9.figshare.5384167. R Code The analysis proceeds in several steps, which can be viewed most easily by examining the ~/Wilcox_etal_DerivedData_and_Code/analysis_scripts/main_text_scripts/patches_source_all_scripts.R file. Questions about the code or analysis should be directed to Kevin Wilcox (wilcoxkr@gmail.com) or Andrew Tredennick (atredenn@gmail.com). Derived Data We provide the full set of metrics (e.g., alpha, beta, and gamma stability and diversity) for each of our study sites. The main analysis and all figures in the paper can be reproduced using these metrics. Metrics were calculated from time series of abundance data from 62 grassland sites around the globe, although primarily from North America and Europe. The data is part of the CoRRE Data Base (http://corredata.weebly.com/), and those interested in using proprietary data not included in this fileset are encouraged to contact the CoRRE data base maintainers (http://corredata.weebly.com/contact.html).

Effects of Increased Soil Nutrients on Seed Rain: A Role for Seed Dispersal in the Greening of the Arctic?

Abstract Warming temperatures in the Arctic are resulting in greater plant growth, particularly deciduous shrubs, in a phenomenon termed the “Greening of the Arctic.” Local expansion of deciduous shrubs is most likely resulting from vegetative growth, while the contribution of recruitment from seed is unknown. Here we compare seeds dispersed from plant communities created by experimental nutrient addition for 5 or 22 years with those from a community of sedges, deciduous shrubs, and evergreen shrubs at ambient soil nutrients. Nutrient addition decreased species richness and diversity and shifted the plant community toward dominance by dwarf birch, Betula nana, and a forb, Rubus chamaemorus. Generally, the composition of the seed rain resembled the adjacent plant community for deciduous shrubs, but not for other growth forms. Total seed abundance and proportion and total abundance of deciduous shrub seeds were greater adjacent to plots fertilized for 22 years. The deciduous shrub seed response was driven by a dramatic increase in seeds dispersed by Betula, resulting in lower taxa diversity, but not richness, of seed rain. These results suggest that increased shrub abundance will affect local seed dispersal, providing additional propagules for germination and increasing opportunities for reproduction by seed to be an important factor in the Greening of the Arctic.