Successional Shifts Research Articles

Drought Decreases Growth and Increases Mortality of Coexisting Native and Introduced Tree Species in a Temperate Floodplain Forest

Forest dieback and mortality events induced by drought stress are widely reported. However, few studies have jointly examined the role played by drought on growth and mortality in tree species inhabiting floodplain forests. Here, we focused on mortality events occurring since the early 2000s on large areas in a floodplain forest located within the Ticino regional park in Northwest Italy, where affected native (pedunculate oak, Quercus robur L.) and introduced tree species (black locust, Robinia pseudoacacia L.) coexist. We related growth with climate data and drought severity to discern if these species were similarly affected by drought. Then, we: (i) evaluated the presence of pathogens of the genus Phytophthora in recently dead oak trees since this was the most affected species and pathogens are often associated with oak decline cases; and (ii) compared xylem vessel diameter and tree-ring C isotope discrimination (δ13C) to highlight differences in water-use strategies between living and dead trees in both species. The radial growth of living and dead trees started diverging in the 1970s, although only after warm-drought periods occurred during 1990s did this divergence become significant. Growth of trees that died responded more negatively to drought than in the case of living trees. Moreover, trees that died formed smaller xylem vessels in the past than living trees and also showed more negative δ13C values in both tree species, indicating a higher intrinsic water-use efficiency in living than in dead trees. The pathogen Phytophthora cinnamomi Rands was only detected in one recently dead tree, suggesting that it is unlikely that dead oaks were predisposed to drought damage by the pathogen. We conclude that a climate shift from wet to warm-dry summer conditions in the early 1990s triggered forest dieback and induced mortality in both tree species. Temperate floodplain forests are susceptible to drought-induced dieback. The drought-sensitivity of both species could lead to successional shifts driven by a reduction of N inputs through N-fixing by black locust and the replacement of oak by drought-tolerant species.

Evidence of Ash Tree (Fraxinus spp.) Specific Associations with Soil Bacterial Community Structure and Functional Capacity

The spread of the invasive emerald ash borer (EAB) across North America has had enormous impacts on temperate forest ecosystems. The selective removal of ash trees (Fraxinus spp.) has resulted in abnormally large inputs of coarse woody debris and altered forest tree community composition, ultimately affecting a variety of ecosystem processes. The goal of this study was to determine if the presence of ash trees influences soil bacterial communities and/or functions to better understand the impacts of EAB on forest successional dynamics and biogeochemical cycling. Using 16S rRNA amplicon sequencing of soil DNA collected from ash and non-ash plots in central Ohio during the early stages of EAB infestation, we found that bacterial communities in plots with ash differed from those without ash. These differences were largely driven by Acidobacteria, which had a greater relative abundance in non-ash plots. Functional genes required for sulfur cycling, phosphorus cycling, and carbohydrate metabolism (specifically those which breakdown complex sugars to glucose) were estimated to be more abundant in non-ash plots, while nitrogen cycling gene abundance did not differ. This ash-soil microbiome association implies that EAB-induced ash decline may promote belowground successional shifts, altering carbon and nutrient cycling and changing soil properties beyond the effects of litter additions caused by ash mortality.

Principal factors controlling the species richness of European fens differ between habitat specialists and matrix‐derived species

AbstractAimWe present the first continental‐scale study of factors controlling the species richness of groundwater‐fed fens, comparing land snails, vascular plants and bryophytes. We separately analyse two ecologically distinct groups differing in conservation value and colonization/extinction dynamics, that is habitat specialists, and matrix‐derived species. Considering the island‐like nature of fen habitats, we hypothesize larger differences in the species richness–environment relationships between habitat specialists and matrix‐derived species than among the taxonomic entities.LocationSeven European regionsMethodsRichness was counted at 373 well‐preserved fens with undisturbed hydrology using the same protocols. Relationships between the species richness and water pH, waterlogging, climate and geography were explored by GLMs.ResultsLand snail richness responded mainly to water pH, regardless of habitat specialization. Richness of vascular plant and bryophyte specialists was strongly driven by geographical location of the sites, while that of matrix‐derived species was driven by waterlogging and water pH. The richness of matrix‐derived species of all taxa significantly increased with the decreasing waterlogging. Residual richness of specialists of all taxa decreased towards southern Europe.Main conclusionsIn island‐like terrestrial habitats, differences between specialists and matrix‐derived species may outweigh differences among taxa, unless there is one strong physiological determinant of species richness such as pH in land snails. The richness of specialists seems to be strongly related to difficult‐to‐measure regional factors such as historical frequency and connectivity of fen habitats. The richness of matrix‐derived species depends mainly on local conditions, such as pH and waterlogging, determining the degree of habitat contrast against the surrounding matrix. Sufficient waterlogging maintains a high representation of habitat specialists in fen communities, and disturbance of water regime may cause the increase in the number of matrix‐derived species and potentially trigger successional shifts towards non‐fen communities.

Post-wildfire shrub dynamics and ecological site controls in a sagebrush steppe: Successional shift or enhanced visibility?

ABSTRACTShrub invasion after disturbance affects agriculture and conservation in arid rangelands. After a 2010 wildfire in southwestern Wyoming, USA ranchers perceived rabbitbrush (Chrysothamnus viscidiflorus) had increased in burned sagebrush (Artemisia tridentata) dominated areas. Using paired burned-unburned transects stratified across four ecological sites, we determined that ecological site, not fire, explained C. viscidiflorus density. Fire reduced A. tridentata but only increased the visibility, not the density, of the shorter C. viscidiflorus portion of the shrub community. We must differentiate between actual community shifts versus perceived shifts by placing plant succession and disturbance within the context of Ecological Site Descriptions (ESDs).

Biodiversity–ecosystem function relationships change through primary succession

Ecologists traditionally use environmental parameters to predict successional shifts in compositional characteristics of local species assemblages (environmental control). Another important focus in ecology is to understand functional roles of species assemblages in determining local environmental properties (diversity control). Then, the question emerges: which is the cause, and which is the consequence? To clarify the causal relationships between species assemblages and environmental properties, we focused on seral changes in species/functional diversity of vascular plants in tundra ecosystems of the High Arctic. We found that, although species richness was influenced by soil properties in the earlier stages of primary succession, the causalities were reversed in the later stages. We also found functional differentiation among coexisting species in the later stage, suggesting that the ‘complementarity effect’ of diversity on ecosystem functions likely increased with ecosystem development through time. By contrast, particular species had little disproportional influence on soil properties, suggesting that the ‘selection effect’ as an alternative mechanism was less important. This result was likely attributed to the importance of facilitation in the marginal High Arctic environment. Plant–microsite associations are shaped by feedback mechanisms and therefore, neither plant nor microsite is a single absolute predictor of the other. Although our observational study has limitations, we demonstrates a possibility that the relative magnitude of the influence of one on the other can change in the process of succession, emphasizing that the causalities underlying biodiversity–ecosystem function relationships change through succession.

Modelling moose-forest interactions under different predation scenarios at Isle Royale National Park, USA.

Loss of top predators may contribute to high ungulate population densities and chronic over-browsing of forest ecosystems. However, spatial and temporal variability in the strength of interactions between predators and ungulates occurs over scales that are much shorter than the scales over which forest communities change, making it difficult to characterize trophic cascades in forest ecosystems. We applied the LANDIS-II forest succession model and a recently developed ungulate browsing extension to model how the moose population could interact with the forest ecosystem of Isle Royale National Park, USA, under three different wolf predation scenarios. We contrasted a 100-yr future without wolves (no predation) with two predation scenarios (weak, long-term average predation rates and strong,higher than average rates). Increasing predation rates led to lower peak moose population densities, lower biomass removal rates, and higher estimates of forage availability and landscape carrying capacity, especially during the first 40 yr of simulations. Thereafter, moose population density was similar for all predation scenarios, but available forage biomass and the carrying capacity of the landscape continued to diverge among predation scenarios. Changes in total aboveground live biomass and species composition were most pronounced in the no predation and weak predation scenarios. Consistent with smaller-scale studies, high browsing rates led to reductions in the biomass of heavily browsed Populus tremuloides, Betula papyrifera, and Abies balsamea, and increases in the biomass of unbrowsed Picea glauca and Piceamariana, especially after the simulation year 2050, when existing boreal hardwood stands at Isle Royale are projected to senesce. As a consequence, lower predation rates corresponded with a landscape that progressively shifted toward dominance by Piceaglauca and Piceamariana, and lacking available forage biomass. Consistencies with previously documented small-scale successional shifts, and population estimates and trends that approximate those from this and other boreal forests that support moose provide some confidence that these dynamics represent a trophic cascade and therefore provide an important baseline against which to evaluate long-term and large-scale effects of alternative predator management strategies on ungulate populations and forest succession.

Sphagnum establishment in alkaline fens: Importance of weather and water chemistry.

Sphagnum expansion to alkaline fens has accelerated during the last decades in Europe, leading to changes in diversity, habitat distributions and carbon storage. The causes are still not clearly understood and involve an interplay between climate change, hydrology, nutrient supply and Sphagnum physiology. We conducted a 4-year field experiment in eight fens in Central European highlands and assessed survival and establishment of individual apical shoot fragments of S. flexuosum, S. warnstorfii and S. squarrosum transplanted along the microtopographical gradient. In a laboratory experiment, we tested combined effects of desiccation and high calcium bicarbonate concentration on Sphagnum survival. We found that in unflooded positions, living shoots of Sphagnum and brown mosses lowered [Ca2+] and pH in their capillary water, in contrast to dead fragments; yet without differences between species. Survival and expansion of Sphagnum fragments, which did not die of acute calcium toxicity during first weeks/months, was negatively affected by dry weather and alkaline water chemistry, reflecting Sphagnum intolerance to desiccation and to combined high [Ca2+] and pH. Shoot fragments expanded to patches only when precipitation was high. Interestingly, non-toxic concentration of calcium bicarbonate reduced desiccation damage in Sphagnum, probably through protection of membranes or other cell components. This mechanism would facilitate Sphagnum survival in elevated, frequently desiccated microhabitats of calcareous fens such as brown-moss hummocks. However, since water-retaining capacity of few Sphagnum shoots is insufficient to change water chemistry in its surroundings, surface acidification may occur only once the environment (e.g. sufficient humidity) enabled expansion to larger mats. Then, the retained rainwater together with hardly decomposable Sphagnum litter would separate mire surface from groundwater, speeding up successional shift towards poor fens. Sphagnum expansion to alkaline fens is therefore more likely in humid regions.

Variation in plant community composition and vegetation carbon pools a decade following a severe fire season in interior Alaska

AbstractQuestionsHow does fire severity, measured as depth of burn of ground layer fuels, control the regeneration of understorey species across black spruce‐dominated stands varying in pre‐fire organic layer depths? Are successional shifts from evergreen to deciduous understorey vegetation more likely to occur with greater depth of burn? Does a shift in understorey vegetation community towards more deciduous species influence carbon accumulation in vegetation biomass?LocationNorthern boreal forest, interior Alaska.MethodsWe sampled 32 stands in interior Alaska that burned in 2003 and 2004 in which depth of burn had been recorded soon after fire. In 2014 we characterized tree density, understorey vegetation composition, above‐ and below‐ground vegetation carbon pools, and a suite of environmental variables. We used ANOVA and multivariate redundancy analysis models to analyse the dominant controls on vegetation composition and carbon pools.ResultsFire severity was a strong control on post‐fire tree and non‐vascular species composition. Ten years post‐fire, sites that experienced deeper burning of organic layers had a higher abundance of deciduous tree species, fire‐adapted mosses, forbs and graminoids, and lower abundances of evergreen shrubs and Sphagnum mosses. Environmental variables (elevation, soil bulk density and mineral soil pH, moisture and temperature) served as important controls on tree and vascular understorey species composition. We found no evidence that carbon pools associated with recovering vegetation biomass were influenced by landscape position, fire severity or environmental variables.ConclusionsBoth fire frequency and depth of burn are projected to increase with a warmer climate in interior Alaska. These shifts in fire regime are expected to favour the regeneration of deciduous species over conifers, which have the potential to regulate successional pathways and large‐scale trends in albedo, which both ultimately could influence ecosystem–climate feedbacks and dampen future fire cycles. Our results show that understorey species composition is controlled by fire severity, suggesting that more severe burning causes a decline in the self‐replacement of conifer communities across a range of hydrologic settings. Overall, our results suggest a loss of resilience in black spruce forests in the face of a changing fire regime. However, we found no evidence that fire severity influences vegetation carbon pools 10 yrs post‐fire, suggesting that there is less landscape variation in biomass carbon pools than in vegetation composition and succession during early post‐fire succession.

Changes in dispersal and light capturing traits explain post‐abandonment community change in semi‐natural grasslands

AbstractQuestionDoes the role of dispersal and light limitation for community assembly change in temperate semi‐natural dry grasslands (SNDGs) post‐abandonment?LocationTemperate calcareous semi‐natural dry grasslands, western Estonia (58.642 N, 23.516 E).MethodsWe compared taxonomic and functional community structure of semi‐natural dry grasslands in Estonia at two time periods: first in 1975 when traditional grazing prevailed, and again in 2013 when management had ceased for at least 30 yr. In order to assess functional community structure, we used community‐weighted mean trait values and mean pair‐wise distance as metrics of functional diversity for traits related to plant dispersal and adaptations to reduced light availability. Successional shifts in functional community structure were examined using regression and multivariate analysis.ResultsChanges in community assembly of SNDGs post‐abandonment were largely driven by more closed vegetation structure and subsequently reduced light availability, as well as altered availability of dispersal vectors. Importance of long‐distance dispersal via grazing animals decreased while importance of bird dispersal increased under abandonment. At the same time, the importance of competition for light increased under abandonment. Results indicate that typical SNDG species are especially susceptible to abandonment because they are light‐demanding and depend on dispersal by wind or epizoochory (i.e. on the fur of animals), and their decline likely leads to reduced small‐scale species richness.ConclusionsVegetation changes in abandoned SNDGs show that regular management, by creating characteristic light and dispersal conditions, is highly important for maintaining both taxonomic and functional diversity. In particular, typical grassland species might be prone to local extinction as taller vegetation develops post‐abandonment. The importance of dispersal processes for community assembly observed in grazed SNDGs thus highlights that successful grassland conservation should include maintenance of dispersal conditions. Grassland conservation should aim to preserve the whole ecosystem and account for the multiple ecosystem processes (e.g. light availability, dispersal), which simultaneously structure plant community composition.

Functional convergence and phylogenetic divergence during secondary succession of subtropical wet forests in Puerto Rico

AbstractQuestionUnderstanding how the relative importance of different community assembly processes changes during secondary succession of diverse systems remains elusive. Functional and phylogenetic approaches that place species along continuous axes of niche differentiation and evolutionary relatedness, however, are deepening our understanding of the mechanisms that drive successional dynamics. We ask whether successional shifts in the functional and phylogenetic composition of post‐agricultural tropical forests provide evidence for niche partitioning or competitive dominance hierarchies as drivers of successional change.LocationSubtropical wet forests, Puerto Rico.MethodsWe combined data on four functional traits [leaf dry mass per area (LMA), wood density (WD), maximum height (Hmax), seed dry mass] and a well‐resolved molecular phylogeny to characterize taxonomic, functional and phylogenetic composition of sapling and adult tree communities along a regionally replicated chronosequence. We used a null model approach to assess how functional and phylogenetic diversity change with forest age.ResultsCorresponding increases of community‐weighted mean LMA, Hmax and seed mass with forest age reflected a shift in dominance of species with acquisitive resource‐use strategies and small seeds towards species with more conservative resource use and larger seeds. A negative relationship between forest age and local diversity of Hmax and seed mass suggested increased importance of competitive hierarchies for light capture and shade‐tolerant regeneration in older forests. In contrast, the colonization of palms in older forest plots led to a positive relationship between forest age and local phylogenetic diversity, suggesting functional convergence of distantly related lineages on traits that confer competitive dominance under low resource conditions.ConclusionsWe linked both functional and phylogenetic dimensions of community diversity with successional trajectories of post‐agricultural tropical forests. Contrasting patterns of these dimensions of diversity shed light on the underlying community assembly processes. We argue that integrating traits and phylogeny with specific hypotheses about physiological and historical mechanisms is essential for advancing our understanding of the drivers of community change during succession.

Mixed‐severity natural disturbance regime dominates in an old‐growth Norway spruce forest of northwest Russia

AbstractQuesionsWhat were the long‐term disturbance rates (including variability) and agents in pristine Norway spruce‐dominated (Picea abies (L.) Karst.) forests? Have soil moisture conditions influenced disturbance rates across this boreal spruce‐dominated forest? Were the temporal recruitment patterns of canopy dominants associated with past disturbance periods?LocationInterfluvial region of Northern Dvina and Pinega rivers, Arkhangelsk, northwest Russia.MethodsWe linked dendrochronological data with tree spatial data (n trees = 1659) to reconstruct the temporal and spatial patterns of canopy gaps in a 1.8‐ha area from 1831–2008, and to develop a growth‐release chronology from 1775–2008.ResultsNo evidence of stand‐replacing disturbances was found within selected forest stands over the studied period. Forest dynamics were driven by small‐ to moderate‐scale canopy disturbances, which maintained a multi‐cohort age structure. Disturbance peaks were observed in the 1820s, 1920s, 1970s and 2000s, with decadal rates reaching 32% of the stand area disturbed.ConclusionsThe overall mean decadal rate was 8.3% canopy area disturbed, which suggests a canopy turnover time of 122 yr, with a 95% confidence envelop of 91–186 yr. Bark beetle outbreaks (possibly exacerbated by droughts) and wind‐storms emerged as the principal disturbance agents. Recruitment of both Norway spruce and downy birch was associated with periods of increased canopy disturbance. Moisture conditions (moist vs mesic stands) were not significantly related to long‐term disturbance rates. The studied spruce‐dominated boreal forests of this region apparently exhibited long‐term forest continuity under this mixed‐severity disturbance regime. These disturbances caused considerable structural alterations to forest canopies, but apparently did not result in a pronounced successional shifts in tree species composition, rather occasional minor enrichments of birch in these heavily spruce‐dominated stands.

Biomass removal by clipping and raking vs burning for the restoration of abandoned Stipa‐dominated European steppe‐like grassland

AbstractQuestionDo restoration measures involving yearly biomass removal over 9 yr have a beneficial effect on the structure and composition of long‐term abandoned European steppe‐like grassland? What is the effect of a single accidental fire event on this grassland community?LocationContinental dry steppe‐like grassland, Suatu and Puini, Transylvania, Romania.MethodsIn two long‐term abandoned grassland sites, we experimentally carried out annual biomass removal by clipping and raking, and by raking alone over a period of 9 yr. We analysed the effect of the two planned treatments in both sites and that of an accidental fire in one of the sites on the structure and composition of vegetation. We applied a predominantly functional trait‐based analysis, but the dominant species Stipa pulcherrima and Carex humilis were treated separately.ResultsAnnual biomass removal applied for 9 yr at one site favoured sub‐dominant grasses and short‐lived forbs over long‐lived forbs and the two dominant species, and generated a successional shift to more open grassland. The clipping and raking treatment promoted plot‐scale species richness when the grassland species pool was large enough. The number and abundance of Red‐Listed species were not influenced by our treatments. The single accidental fire event at one of the sites negatively affected the two dominant species and promoted one of the sub‐dominant grasses. In addition, burning in combination with abandonment and presence of stoloniferous grass species benefiting from fire decreased species richness. Variation in weather conditions, in particular the series of dry years during our experiment, caused significant changes in vegetation cover and the quantity of litter, and decreased the abundance of S. pulcherrima in both sites and that of long‐lived forbs in one site.ConclusionsAccording to our results, biomass removal had a beneficial effect on vegetation structure and composition in long‐term abandoned steppe‐like grassland. Until further experimental data on the effect of burning on temperate dry grassland are available, traditional management practices should be favoured over alternative methods.

Food web heterogeneity and succession in created saltmarshes

Summary Ecological restoration must achieve functional as well as structural recovery. Functional metrics for re‐establishment of trophic interactions can be used to complement traditional monitoring of structural attributes. In addition, topographic effects on food web structure provide added information within a restoration context; often, created sites may require spatial heterogeneity to effectively match structure and function of natural habitats. We addressed both of these issues in our study of successional development of benthic food web structure, with focus on bottom‐up‐driven changes in macroinvertebrate consumer assemblages in the saltmarshes of the Venice Lagoon, Italy. We combined quantified estimates of the changing community composition with stable isotope data (13C:12C and 15N:14N) to compare the general trophic structure between created (2–14 years) marshes and reference sites and along topographic elevation gradients within saltmarshes. Macrofaunal invertebrate consumers exhibited local, habitat‐specific trophic patterns. Stable isotope‐based trophic structure changed with increasing marsh age, in particular with regard to mid‐elevation (Salicornia) habitats. In young marshes, the mid‐elevation consumer signatures resembled those of unvegetated ponds. The mid‐elevation of older and natural marshes had a more distinct Salicornia zone food web, occasionally resembling that of the highest (Sarcocornia‐dominated) elevation. In summary, this indicates that primary producers and availability of vascular plant detritus structure consumer trophic interactions and the flow of carbon. Functionally different consumers, subsurface‐feeding detritivores (Oligochaeta) and surface grazers (Hydrobia sp.), showed distinct but converging trajectories of isotopic change over time, indicating that successional development may be asymmetric between ‘brown’ (detrital) guilds and ‘green’ (grazing) guilds in the food web. Synthesis and applications. Created marsh food webs converged into a natural state over about a decade, with successional shifts seen in both consumer community composition and stable isotope space. Strong spatial effects were noted, highlighting the utility of stable isotopes to evaluate functional equivalence in spatially heterogeneous systems. Understanding the recovery of functional properties such as food web support, and their inherent spatial variability, is key to planning and managing successful habitat restoration.

Successional and seasonal variations in soil and litter microbial community structure and function during tropical postagricultural forest regeneration: a multiyear study.

Soil microorganisms regulate fundamental biochemical processes in plant litter decomposition and soil organic matter (SOM) transformations. Understanding how microbial communities respond to changes in vegetation is critical for improving predictions of how land-cover change affects belowground carbon storage and nutrient availability. We measured intra- and interannual variability in soil and forest litter microbial community composition and activity via phospholipid fatty acid analysis (PLFA) and extracellular enzyme activity across a well-replicated, long-term chronosequence of secondary forests growing on abandoned pastures in the wet subtropical forest life zone of Puerto Rico. Microbial community PLFA structure differed between young secondary forests and older secondary and primary forests, following successional shifts in tree species composition. These successional patterns held across seasons, but the microbial groups driving these patterns differed over time. Microbial community composition from the forest litter differed greatly from those in the soil, but did not show the same successional trends. Extracellular enzyme activity did not differ with forest succession, but varied by season with greater rates of potential activity in the dry seasons. We found few robust significant relationships among microbial community parameters and soil pH, moisture, carbon, and nitrogen concentrations. Observed inter- and intrannual variability in microbial community structure and activity reveal the importance of a multiple, temporal sampling strategy when investigating microbial community dynamics with land-use change. Successional control over microbial composition with forest recovery suggests strong links between above and belowground communities.

Soil-foraging animals alter the composition and co-occurrence of microbial communities in a desert shrubland.

Animals that modify their physical environment by foraging in the soil can have dramatic effects on ecosystem functions and processes. We compared bacterial and fungal communities in the foraging pits created by bilbies and burrowing bettongs with undisturbed surface soils dominated by biocrusts. Bacterial communities were characterized by Actinobacteria and Alphaproteobacteria, and fungal communities by Lecanoromycetes and Archaeosporomycetes. The composition of bacterial or fungal communities was not observed to vary between loamy or sandy soils. There were no differences in richness of either bacterial or fungal operational taxonomic units (OTUs) in the soil of young or old foraging pits, or undisturbed soils. Although the bacterial assemblage did not vary among the three microsites, the composition of fungi in undisturbed soils was significantly different from that in old or young foraging pits. Network analysis indicated that a greater number of correlations between bacterial OTUs occurred in undisturbed soils and old pits, whereas a greater number of correlations between fungal OTUs occurred in undisturbed soils. Our study suggests that digging by soil-disturbing animals is likely to create successional shifts in soil microbial and fungal communities, leading to functional shifts associated with the decomposition of organic matter and the fixation of nitrogen. Given the primacy of organic matter decomposition in arid and semi-arid environments, the loss of native soil-foraging animals is likely to impair the ability of these systems to maintain key ecosystem processes such as the mineralization of nitrogen and the breakdown of organic matter, and to recover from disturbance.

Direct and Indirect Drivers of Moss Community Structure, Function, and Associated Microfauna Across a Successional Gradient

Relative to vascular plants, little is known about what factors control bryophyte communities or how they respond to successional and environmental changes. Bryophytes are abundant in boreal forests, thus changes in moss community composition and functional traits (for example, moisture and nutrient content; rates of photosynthesis and respiration) may have important consequences for ecosystem processes and microfaunal communities. Through synthesis of previous work and new analyses integrating new and published data from a long-term successional gradient in the boreal forest of northern Sweden, we provide a comprehensive view of the biotic factors (for example, vascular plant productivity, species composition, and diversity) and abiotic factors (for example, soil fertility and light transmission) that impact the moss community. Our results show that different aspects of the moss community (that is, composition, functional traits, moss-driven processes, and associated invertebrate fauna) respond to different sets of environmental variables, and that these are not always the same variables as those that influence the vascular plant community. Measures of moss community composition and functional traits were primarily influenced by vascular plant community composition and productivity. This suggests that successional shifts in abiotic variables, such as soil nutrient levels, indirectly affect the moss community via their influence on vascular plant community characteristics, whereas direct abiotic effects are less important. Among the moss-driven processes, moss litter decomposition and moss productivity were mainly influenced by biotic variables (notably the community characteristics of both vascular plants and mosses), whereas moss functional traits (primarily specific leaf area and tissue nutrient concentrations) also were important in explaining moss di-nitrogen-fixation rates. In contrast, both abiotic and biotic variables were important drivers of moss microfaunal community structure. Taken together, our results show which abiotic and biotic factors impact mosses and their associated organisms, and thus highlight that multiple interacting factors need to be considered to understand how moss communities, associated food webs, and the ecosystem processes they influence will respond to environmental change.

Ecosystem‐level consequences of symbiont partnerships in an N‐fixing shrub from interior Alaskan floodplains

In long‐lived N‐fixing plants, environmental conditions affecting plant growth and N demand vary at multiple temporal and spatial scales, and symbiont assemblages on a given host and patterns of allocation to nodule activities have been shown to vary according to environmental factors, suggesting that hosts may alter partner choice and manipulate symbiont assemblages based on shifting plant needs. This study assessed economic trade‐offs among N‐fixing symbionts of thin‐leaf alder (Alnus tenuifolia) by examining whether alder–Frankia associations change in response to the up‐regulation (P fertilization) and down‐regulation (N‐fertilization) of N‐fixation activities, and whether these changes are associated with differences among Frankia partners in their relative C cost and/or N benefit to A. tenuifolia.Relative to control plots, alder in +P plots had significantly higher nodule biomass and N‐fixation rates; these parameters were significantly lower in +N plots, translating to stand‐level N inputs that were more than an order of magnitude greater in +P than +N plots. Nodule respiration and N‐fixation rates were positively correlated, and analyses revealed that alder employs mechanisms to increase the efficiency of C use when N fixation is up‐regulated. Of the eight Frankia OTUs (operational taxonomic units) identified, two were dominant, with significant differences in Frankia OTU composition across samples being explained by fertilization treatment. Dominant OTUs had similar up‐ and down‐regulatory responses to treatments but differed in C costs of N fixation, while the most abundant sub‐dominant failed to up‐regulate N‐fixation rates in +P plots. Differences among Frankia OTUs in traits relevant to plant performance may form the basis for host choice and explain successional shifts in alder–Frankia assemblages. We suggest that manipulation of Frankia assemblages is an adaptation for a species such as A. tenuifolia that maintains a high dependence on atmospheric N across radically different growth environments. Because of the strong effects of thin‐leaf alder on soil N contents, it appears that host changes in C partitioning to nodule activities, including the physiological consequences of host specificity for Frankia assemblages, results in alder both driving and responding to environmental heterogeneity at small to large spatial scales.

Ectomycorrhizal fungal succession coincides with shifts in organic nitrogen availability and canopy closure in post-wildfire jack pine forests

Successional changes in belowground ectomycorrhizal fungal (EMF) communities have been observed with increasing forest stand age; however, mechanisms behind this change remain unclear. It has been hypothesized that declines of inorganic nitrogen (N) and increases of organic N influence changes in EMF taxa over forest development. In a post-wildfire chronosequence of six jack pine (Pinus banksiana) stands ranging in age from 5 to 56 years, we investigated EMF community composition and compared shifts in taxa with detailed soluble inorganic and organic N data. Taxa were identified by internal transcribed spacer rDNA sequencing, and changes in community composition evaluated with non-metric multi-dimensional scaling (NMDS). Dissimilarities in the community data were tested for correlations with N variables. We observed a successional shift along NMDS axis 1 from such taxa as Suillus brevipes and Thelephora terrestris in sites age 5 and 11 to species of Cortinarius and Russula, among others, in the four older sites. This change was positively correlated with soluble organic N (SON) (r(2) = 0.902, P = 0.033) and free amino-acid N (r(2) = 0.945, P = 0.021), but not inorganic N. Overall, our results show a successional shift of EMF communities occurring between stand initiation and canopy closure without a change in species of the dominant plant-host, and associated with SON and free amino-acid N in soil. It is uncertain whether EMF taxa are responding to these organic N forms directly, affecting their availability, or are ultimately responding to changes in other site variables, such as belowground productivity.

Canopy accession strategies and climate-growth relationships in Acer rubrum

A pervasive pattern of forest composition change is occurring throughout the Central Hardwood Forest of the eastern US. Acer rubrum has invaded the understory of Quercus stands across a variety of site types. The proliferation of A. rubrum, and that of other shade-tolerant mesophytes, inhibits the regeneration of Quercus. Without alterations in disturbance or climate regimes, composition in invaded stands is expected to shift towards A. rubrum dominance. Canopy accession strategies and climate-growth relationships of A. rubrum are critical factors in this successional shift. We quantified patterns of suppression and release during canopy accession, examined the relationships between climate variables and radial growth, and compared our findings for A. rubrum in an old-growth forest in Tennessee to other studies throughout the region to elucidate broad-scale patterns. The most common mode of A. rubrum canopy recruitment began with a tree originating in a gap followed by accession into the canopy without a period of suppression (61%). The remaining trees experienced a period of suppression before recruiting to the main canopy. A prominent establishment pulse occurred from the 1940s to the 1960s and 93% of trees that recruited to the canopy during this period established in gaps and were never suppressed. The mean age at canopy accession for individuals that experienced suppression was 24yr. The transition from suppression to release phases in radial growth trends was abrupt. The mean interseries correlation and the average mean sensitivity of the A. rubrum tree-ring chronology were comparatively high for the region. Acer rubrum individuals were most productive during cool, wet springs preceded by wet autumns and warm winters. Our results indicate that A. rubrum will remain competitive in the coming decades without a change in current disturbance regimes and the Quercus component will be difficult for managers to maintain in similar stands of the Central Hardwood Forest.

Facilitation drives mortality patterns along succession gradients of aspen‐conifer forests

While it is well established that facilitation and competition are important structuring forces in plant communities, a clear understanding of the interactions between them and how they change through the life stages of plants and affect long‐term plant community development is lacking. We have observed that conifer seedlings are rarely found growing in meadows but readily establish under adjacent aspen stands, particularly at the base of aspen trees, creating the potential for antagonistic interactions in later life stages. To examine these relationships and their potential consequences on forest community development, we characterized patterns of establishment, regeneration, and overstory mortality of aspen (Populus tremuloides) and subalpine fir (Abies lasiocarpa) along a stand composition gradient (aspen dominant → mixed → conifer dominant) that develops in seral aspen forests. We found strong stand effects on the establishment of both aspen and subalpine fir regeneration. Aspen regenerated into meadows from the forest boundary, reached peak densities underneath aspen stands, and decreased significantly in mixed and conifer dominated stands. In contrast, subalpine fir seedlings failed to regenerate in meadows, but established readily underneath aspen, mixed and conifer stands. Within stands, establishing subalpine fir seedlings were strongly aggregated around mature aspen trees, which increased the proximity of the two species 2–10 fold depending on subalpine fir age class and stand type. Both stand type and increased proximity of overstory aspen and fir trees drove mortality patterns of the two species in opposite directions. Overstory aspen mortality increased sharply along the stand composition gradient: aspen (7%), mixed (17%), conifer (49%), while subalpine fir wasn't significantly influenced by stand type. Proximity of overstory aspen and subalpine fir, was associated with increased (2×) aspen mortality under all stand conditions but increased subalpine fir survival resulting in high aspen:fir mortality ratios that likely accelerate successional shifts toward conifer dominance. Our data suggest that environmental conditions that promote facilitator mortality may compromise the ability of facilitation‐dependent forests to regenerate following disturbance. Maintenance of natural disturbance regimes appears to be critical in striking an ecological balance between facilitation and competition that promotes sustainability in succession‐driven plant communities.