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It is often speculated that non-native invasive species undergo rapid changes in their phenotypic properties (i.e., traits) that provide adaptive advantage in their new environment. However, few studies have directly compared traits of invasive non-native species with their native counterparts to reveal whether such phenotypic changes occur, and which stages of initial introduction and subsequent invasion contribute to these shifts. We studied trait variation of an invasive tree, Pinus contorta, which is native to northwestern North America and invasive in the Patagonia region of South America (i.e., Argentina and Chile). Commercial plantations of P. contorta were introduced extensively in Patagonia from the 1970s onward, from an unknown seed origin within the Pacific Northwest, USA, where three sub-species are found, including subsp. contorta, latifolia, and murrayana. We employed a home-versus-away study approach, where we compared mean growth, defense, and reproduction trait values, and mean within-stand trait variation (Coefficient of Variation, CV) of Patagonia plantations, with the three native sub-species. We further compared mean traits, and trait CVs between invasive P. contorta and the Patagonia plantations from which they escaped. Patagonia plantations shared the most similar mean trait values with subsp. latifolia and murrayana, suggesting possible source populations. However, both mean trait values and trait CVs of Patagonia plantations differed from all three native sub-species, indicating potential founder effects, population bottlenecks, and/or plastic responses to their new environment that occurred during or after introduction. We also found evidence for selective change during invasion; however, these differences did not suggest growth traits were prioritized over defense traits, which was inconsistent with hypotheses that invaders exhibit an evolutionary trade-off between defense traits and growth traits. Our study highlights that processes occurring both at first introduction and establishment, as well as the subsequent invasion phase can influence the phenotype of successful invaders. [ABSTRACT FROM AUTHOR]

In nutrient poor and dry forest sites common to northern Scandinavia, Scots pine (Pinus sylvestris L.) is the most common species both in managed and natural forests. However, Scots pine is sensitive to competition during establishment. Harvesting of all trees within a given area, i.e., clear-cutting, liberates regenerating seedlings from competition with mature trees. However, recently, clear-cut-free or continuous cover forestry has been the subject of substantial debate. When choosing a management method, it is important to recognize how competitive interactions direct the success of Scots pine regeneration. We studied Scots pine regeneration at three environments: beneath the canopy of mature trees, at the canopy edge in full sunlight, and distant from the canopy with no influence of mature trees. We imposed three treatments in each of these environments: root isolation (i.e., trenching), nitrogen (N) fertilization, and control plots. Root isolation enhanced seedling performance under the canopy of mature trees. Nitrogen fertilization enhanced seedling performance to a greater extent in the clear-cut than at the forest edge. However, N fertilization had no effect under the canopy. In the N-fertilized plots, we measured higher N content in the soil under the canopy than in the open environments, indicating that not all excess N was obtained by the mature trees. N-uptake might have been limited by competition for water in the N-fertilized plots. Our results suggest that belowground competition limits the success of regeneration of Scots pine. However, N fertilization presents a tool to compensate for underground competition along canopy edges. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation represents the largest natural flux of new nitrogen (N) into terrestrial ecosystems, providing a critical N source to support net primary productivity of both natural and agricultural systems. When they are common, symbiotic associations between plants and bacteria can add more than 100 kg N ha−1 y−1 to ecosystems. Yet, these associations are uncommon in many terrestrial ecosystems. In most cases, N inputs derive from more cryptic sources, including mutualistic and/or free-living microorganisms in soil, plant litter, decomposing roots and wood, lichens, insects, and mosses, among others. Unfortunately, large gaps remain in the understanding of cryptic N fixation. We conducted a literature review to explore rates, patterns, and controls of cryptic N fixation in both unmanaged and agricultural ecosystems. Our analysis indicates that, as is common with N fixation, rates are highly variable across most cryptic niches, with N inputs in any particular cryptic niche ranging from near zero to more than 20 kg ha−1 y−1. Such large variation underscores the need for more comprehensive measurements of N fixation by organisms not in symbiotic relationships with vascular plants in terrestrial ecosystems, as well as identifying the factors that govern cryptic N fixation rates. We highlight several challenges, opportunities, and priorities in this important research area, and we propose a conceptual model that posits an interacting hierarchy of biophysical and biogeochemical controls over N fixation that should generate valuable new hypotheses and research. [ABSTRACT FROM AUTHOR]

Background and aims: Plant-soil feedbacks (PSFs) play an important role in mediating plant species coexistence, community dynamics and ecosystem functioning. Soil biota (e.g. mutualists, pathogens), nutrient availability and secondary chemicals can drive the strength and direction of PSFs, but the variations and context-dependence of their effects remain unclear.We used a phylogenetically controlled meta-analysis of 57 PSF studies across 166 plant species to explore whether and how these drivers affect individual PSFs (the performance of a species on conspecific versus heterospecific soils) and pairwise PSFs (indicating whether feedbacks promote stable or unstable species coexistence) under various intrinsic, environmental and experimental contexts.Mutualists led to stronger positive individual and pairwise PSFs across various intrinsic and external contexts. However, PSFs became more negative when whole biota was present, with stronger negative effects on native species compared to exotic species and the most negative effects on plants experiencing interspecific competition. Manipulations of pathogens, nutrient availability and secondary chemicals had overall minimal influence on both types of PSFs, but the effect of nutrient availability on pairwise PSFs increased with increasing phylogenetic distance between species.Our study suggests that soil biota is an important driver of PSFs and that plant origin and competitive context should be considered when predicting the role of soil biota in driving PSFs. Finally, we propose several directions for the next generation of PSF experiments towards a better understanding of the relative importance and interactions of different PSF drivers.Methods: Plant-soil feedbacks (PSFs) play an important role in mediating plant species coexistence, community dynamics and ecosystem functioning. Soil biota (e.g. mutualists, pathogens), nutrient availability and secondary chemicals can drive the strength and direction of PSFs, but the variations and context-dependence of their effects remain unclear.We used a phylogenetically controlled meta-analysis of 57 PSF studies across 166 plant species to explore whether and how these drivers affect individual PSFs (the performance of a species on conspecific versus heterospecific soils) and pairwise PSFs (indicating whether feedbacks promote stable or unstable species coexistence) under various intrinsic, environmental and experimental contexts.Mutualists led to stronger positive individual and pairwise PSFs across various intrinsic and external contexts. However, PSFs became more negative when whole biota was present, with stronger negative effects on native species compared to exotic species and the most negative effects on plants experiencing interspecific competition. Manipulations of pathogens, nutrient availability and secondary chemicals had overall minimal influence on both types of PSFs, but the effect of nutrient availability on pairwise PSFs increased with increasing phylogenetic distance between species.Our study suggests that soil biota is an important driver of PSFs and that plant origin and competitive context should be considered when predicting the role of soil biota in driving PSFs. Finally, we propose several directions for the next generation of PSF experiments towards a better understanding of the relative importance and interactions of different PSF drivers.Results: Plant-soil feedbacks (PSFs) play an important role in mediating plant species coexistence, community dynamics and ecosystem functioning. Soil biota (e.g. mutualists, pathogens), nutrient availability and secondary chemicals can drive the strength and direction of PSFs, but the variations and context-dependence of their effects remain unclear.We used a phylogenetically controlled meta-analysis of 57 PSF studies across 166 plant species to explore whether and how these drivers affect individual PSFs (the performance of a species on conspecific versus heterospecific soils) and pairwise PSFs (indicating whether feedbacks promote stable or unstable species coexistence) under various intrinsic, environmental and experimental contexts.Mutualists led to stronger positive individual and pairwise PSFs across various intrinsic and external contexts. However, PSFs became more negative when whole biota was present, with stronger negative effects on native species compared to exotic species and the most negative effects on plants experiencing interspecific competition. Manipulations of pathogens, nutrient availability and secondary chemicals had overall minimal influence on both types of PSFs, but the effect of nutrient availability on pairwise PSFs increased with increasing phylogenetic distance between species.Our study suggests that soil biota is an important driver of PSFs and that plant origin and competitive context should be considered when predicting the role of soil biota in driving PSFs. Finally, we propose several directions for the next generation of PSF experiments towards a better understanding of the relative importance and interactions of different PSF drivers.Conclusion: Plant-soil feedbacks (PSFs) play an important role in mediating plant species coexistence, community dynamics and ecosystem functioning. Soil biota (e.g. mutualists, pathogens), nutrient availability and secondary chemicals can drive the strength and direction of PSFs, but the variations and context-dependence of their effects remain unclear.We used a phylogenetically controlled meta-analysis of 57 PSF studies across 166 plant species to explore whether and how these drivers affect individual PSFs (the performance of a species on conspecific versus heterospecific soils) and pairwise PSFs (indicating whether feedbacks promote stable or unstable species coexistence) under various intrinsic, environmental and experimental contexts.Mutualists led to stronger positive individual and pairwise PSFs across various intrinsic and external contexts. However, PSFs became more negative when whole biota was present, with stronger negative effects on native species compared to exotic species and the most negative effects on plants experiencing interspecific competition. Manipulations of pathogens, nutrient availability and secondary chemicals had overall minimal influence on both types of PSFs, but the effect of nutrient availability on pairwise PSFs increased with increasing phylogenetic distance between species.Our study suggests that soil biota is an important driver of PSFs and that plant origin and competitive context should be considered when predicting the role of soil biota in driving PSFs. Finally, we propose several directions for the next generation of PSF experiments towards a better understanding of the relative importance and interactions of different PSF drivers. [ABSTRACT FROM AUTHOR]

Wildfire disturbance is important for tree regeneration in boreal ecosystems. A considerable amount of literature has been published on how wildfires affect boreal forest regeneration. However, we lack understanding about how soil-mediated effects of fire disturbance on seedlings occur via soil abiotic properties versus soil biota. We collected soil from stands with three different severities of burning (high, low and unburned) and conducted two greenhouse experiments to explore how seedlings of tree species (Betula pendula, Pinus sylvestris and Picea abies) performed in live soils and in sterilized soil inoculated by live soil from each of the three burning severities. Seedlings grown in live soil grew best in unburned soil. When sterilized soils were reinoculated with live soil, seedlings of P. abies and P. sylvestris grew better in soil from low burn severity stands than soil from either high severity or unburned stands, demonstrating that fire disturbance may favor post-fire regeneration of conifers in part due to the presence of soil biota that persists when fire severity is low or recovers quickly post-fire. Betula pendula did not respond to soil biota and was instead driven by changes in abiotic soil properties following fire. Our study provides strong evidence that high fire severity creates soil conditions that are adverse for seedling regeneration, but that low burn severity promotes soil biota that stimulates growth and potential regeneration of conifers. It also shows that species-specific responses to abiotic and biotic soil characteristics are altered by variation in fire severity. This has important implications for tree regeneration because it points to the role of plant–soil–microbial feedbacks in promoting successful establishment, and potentially successional trajectories and species dominance in boreal forests in the future as fire regimes become increasingly severe through climate change. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition can change the carbon (C) sink of northern coniferous forests by changing the balance between net primary production and soil respiration. We used a field experiment in an N poor Pinus sylvestris forest where five levels of N (0, 3, 6, 12, and 50 kg N ha−1 yr−1, n = 6) had been added annually for 12–13 years to investigate how litter C inputs and soil respiration, divided into its autotrophic and heterotrophic sources, respond to different rates of N input, and its subsequent effect on soil C storage. The highest N addition rate (50 kg N ha−1 yr−1) stimulated soil C accumulation in the organic layer by 22.3 kg C kg−1 N added, increased litter inputs by 46%, and decreased soil respiration per mass unit of soil C by 31.2%, mainly by decreasing autotrophic respiration. Lower N addition rates (≤ 12 kg N ha−1 yr−1) had no effect on litter inputs or soil respiration. These results support previous studies reporting on increased litter inputs coupled to impeded soil C mineralization, contributing to enhancing the soil C sink when N is supplied at high rates, but add observations for lower N addition rates more realistic for N deposition. In doing so, we show that litter production in N poor northern coniferous forests can be relatively unresponsive to low N deposition levels, that stimulation of microbial activity at low N additions is unlikely to reduce the soil C sink, and that high levels of N deposition enhance the soil C sink by increasing litter inputs and decreasing soil respiration. [ABSTRACT FROM AUTHOR]

Anthropogenic fires and land-use change, including the conversion from native to exotic species canopies, are two major types of disturbances that strongly affect the functioning of forest ecosystems around the world. These disturbances alter the resource availability for plants, which may lead to changes in species richness. Here we examined the relative effects of canopy cover type, light availability and soil nutrient (N and P) availability on species richness, including invasive species, at different post-fire plant systems. Additionally, we tested the resource heterogeneity hypothesis (RHH) for plant diversity, which proposes that diversity is higher in habitats with spatially heterogeneous resources. We evaluated four different canopy cover types, including mature and second-growth Nothofagus pumilio forests, treeless prairie, Pinus sylvestris afforestations, all of which were converted from mature N. pumilio forests. Using generalized mixed-effects model correlations, we determined (1) the relative influence of canopy cover type, light and soil nutrient availability on understory species richness and (2) the relationship between species richness and resource heterogeneity. We found that canopy cover type was the factor that best explained species richness, much more than fine-scale light and soil nutrient availability. Additionally, we found that the more homogeneous the light environment the higher the number of exotic species (mainly found in the prairie where the highest light intensity occurred), which is contrary to what the RHH states. In conclusion, canopy cover type, a stand-scale driver, and not fine-scale resource (light, N and P) availability, was most important for explaining native and exotic (including invasive) species understory richness in a landscape affected by anthropogenic fires and posterior land-use change. [ABSTRACT FROM AUTHOR]

We investigated among and within species variation in several litter chemical properties, including protein complexation capacity (PCC), for six plant species across a boreal forest chronosequence in northern Sweden across which stand fertility declines sharply with stand age. We hypothesized (1) that evergreen species which dominate in late-successional stands would exhibit higher PCCs than deciduous species that dominate in young stands, (2) that individual species would increase their PCCs in response to nutrient limitation as succession proceeds, and (3) that differences in PCC among litter types would determine their interactive effects with proteins on soil N and C mineralization. The data demonstrated a high PCC, but a low PCC per unit of soluble phenol, for two deciduous species that dominate in early-successional high fertility stands, providing mixed support for our first hypothesis. No species demonstrated a significant correlation between their PCC and stand age, which did not support our second hypothesis. Finally, a soil incubation assay revealed that litter extracts for three of the six species had negative interactive effects with added proteins on N mineralization rates, and that all six species demonstrated positive interactive effects with protein on C mineralization. This pattern did not provide strong support for our third hypothesis, and suggests that N immobilization was likely a more important factor regulating N mineralization than stabilization of proteins into tannin complexes. These data suggest that multiple interactive mechanisms between litter extracts and proteins likely occur simultaneously to influence the availability of N in soils. [ABSTRACT FROM AUTHOR]

Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ 15 N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ 15 N declined by 0.6-1.6‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.