Arbuscular mycorrhizal fungal diversity and potential association networks among African tropical forest trees.

Tropical forests represent one of the most diverse and productive ecosystems on Earth. High productivity is sustained by efficient and rapid cycling of nutrients, which is in large part made possible by symbiotic associations between plants and mycorrhizal fungi. In these associations, an individual plant typically associates simultaneously with multiple fungi and the fungi associate with multiple plants, creating complex networks among fungi and plants. However, there are few studies that have investigated mycorrhizal fungal composition and diversity in tropical forest trees, particularly in Africa, or that assessed the structure of the network of associations among fungi and trees. In this study, we collected root and soil samples from Ise Forest Reserve (Southwest Nigeria) and used a metabarcoding approach to identify the dominant arbuscular mycorrhizal (AM) fungal taxa in the soil and associating with ten co-occurring tree species to assess variation in AM communities. Network analysis was used to elucidate the architecture of the network of associations between fungi and tree species. A total of 194 Operational Taxonomic Units (OTUs) belonging to six AM fungal families were identified, with 68% of all OTUs belonging to Glomeraceae. While AM fungal diversity did not differ among tree species, AM fungal community composition did. Network analyses showed that the network of associations was not significantly nested and showed a relatively low level of specialization (H2 = 0.43) and modularity (M = 0.44). We conclude that, although there were some differences in AM fungal community composition, the studied tree species associate with a large number of AM fungi. Similarly, most AM fungi had great host breadth and were detected in most tree species, thereby potentially working as interaction network hubs.

Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change.

Trees continuously regulate leaf physiology to acquire CO2 while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO2 (iCO2 ) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long-term records of tree-ring carbon isotope signatures with leaf physiological measurements of Quercus rubra (Quru) and Liriodendron tulipifera (Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (Anet ), and stomatal conductance to water (gs ) since 1940. We first show 16%-25% increases in tree iWUE since the mid-20th century, primarily driven by iCO2 , but also document the individual and interactive effects of nitrogen (NOx ) and sulfur (SO2 ) air pollution overwhelming climate. We find evidence for Quru leaf gas exchange being less tightly regulated than Litu through an analysis of isotope-derived leaf internal CO2 (Ci ), particularly in wetter, recent years. Modeled estimates of seasonally integrated Anet and gs revealed a 43%-50% stimulation of Anet was responsible for increasing iWUE in both tree species throughout 79%-86% of the chronologies with reductions in gs attributable to the remaining 14%-21%, building upon a growing body of literature documenting stimulated Anet overwhelming reductions in gs as a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.

Protection from overheating of simulated Sceloporus horridus lizards in a biosphere reserve of seasonally dry tropical forest in central Mexico.

In seasonally dry ecosystems, loss of vegetation cover leads to warmer microclimates that can increase lizards' body temperatures to the point of threatening their performance. Preserving vegetation by establishing protected areas may mitigate these effects. We used remote sensing to test these ideas in the Sierra de Huautla Biosphere Reserve (REBIOSH) and surrounding areas. First, we determined whether vegetation cover was higher in the REBIOSH compared to adjacent unprotected areas to the north (NAA) and south (SAA). Then, we used a mechanistic niche model to test whether simulated Sceloporus horridus lizards in the REBIOSH experienced a cooler microclimate, higher thermal safety margin, longer foraging duration, and lower basal metabolic rate compared to adjacent unprotected areas. We compared these variables between 1999, when the reserve was declared, and 2020. We found that vegetation cover increased from 1999 to 2020 in all three areas; it was higher in the REBIOSH than in the more anthropized NAA, and was intermediate in the less anthropized SAA in both years. The microclimate temperature decreased from 1999 to 2020 and was lower in the REBIOSH and SAA than in the NAA. Thermal safety margin increased from 1999 to 2020; it was higher in the REBIOSH than in the NAA and intermediate in the SAA. Foraging duration increased from 1999 to 2020 and was similar among the three polygons. Basal metabolic rate decreased from 1999 to 2020 and was higher in the NAA than in the REBIOSH and SAA. Our results suggest that the REBIOSH provides cooler microclimates that increase the thermal safety margin and lower the metabolic rate of this generalist lizard compared to the NAA, and that the REBIOSH could contribute to increased vegetation cover in its surroundings. Besides, protecting original vegetation cover is an important part of climate change mitigation strategies more generally.

The role of protected areas co-management in enhancing resistance and resilience of deciduous forest ecosystem to extreme climatic events in Bangladesh.

Due to ongoing and projected climate change as well as increasing anthropogenic disturbances, the tropical deciduous forest has been experiencing a decline in its biomass and productivity. To mitigate this adverse effect, many tropical countries have adopted forest co-management engaging local communities. However, the effects of co-management on the resistance and resilience of forest ecosystems to extreme climatic events have rarely been tested. The present study investigates the effects of co-management on resistance and resilience to extreme climatic events in two major tropical deciduous forest protected areas of Bangladesh, namely Madhupur National Park (MNP) and Bhawal National Park (BNP), through remotely sensed satellite data. We used the Google Earth Engine platform to access the Landsat images from 1990 to 2020 for a comprehensive assessment of the forest cover condition under two major management regimes (i.e., traditional and co-management). We find that co-management slows down the rate of forest destruction, where the rate of forest destruction was 108 ha year-1 in MNP and 121 ha year-1 in BNP during the year 1990-2008 under traditional forest management system. Under the co-management regime, forest cover increased by 19 ha year-1 and 41 ha year-1 from 2009 to 2020 respectively in MNP and BNP. Our study finds a highly significant correlation between rainfall (p < 0.001) and forest health, although co-management had poor impacts on forest resistance and resilience in case of extreme climatic events, such as drought and heavy rainfall. We find, no significant impacts of co-management on resistance and resilience to drought in MNP, and on resistance and resilience to heavy rainfall in MNP and BNP. In BNP, the impacts of co-management on resistance (p < 0.05) and resilience (p < 0.01) of forest to drought were highly significant. Forest co-management although have the potentials to reduce the deforestation rate by mitigating anthropogenic disturbances, its capacity to tackle the adverse impact of climate change was limited in our study. An adaptive co-management model, therefore, is crucial for mainstreaming the adverse effect of climate change on the tropical deciduous forest to harness the maximum potential of community participation in forest resources management.

Stasis in forest regeneration following deer exclusion and understory gap creation: A 10-year experiment.

Chronically elevated ungulate browse pressure in temperate forests worldwide often generates ecological legacies characterized by low plant diversity and contributes to the formation of dense, nearly monodominant, and highly recalcitrant layers of understory vegetation. Once established, these recalcitrant layers combined with continued browsing may jointly constrain tree establishment and diversity so completely that understory recovery may be unattainable without mitigating browse pressure, the recalcitrant layer, or both. Here, we investigate the independent and synergistic effects of both white-tailed deer (Odocoileus virginianus) browsing and hay-scented fern (Dennstaedtia punctilobula) competition on tree regeneration in a 10-year experiment. Specifically, we examine how tree seedling establishment, growth, and composition are filtered by fern cover versus fern removal (gaps), browser presence versus absence (exclosures), and their combined effects during 10 years at three hardwood forest sites in Pennsylvania, USA. Fern gaps enhanced establishment for multiple tree species, increasing seedling density and diversity, particularly in the first 3 years post-treatment, and enhancing richness (≤1 species) over the course of the experiment. Excluding deer for a decade increased the height growth of other regeneration and altered species composition, but had no effect on diversity, richness, and density. Notably, we observed higher Prunus serotina seedling densities outside exclosures, possibly due to greater secondary dispersal. We argue that browsing legacies in second growth forests established at the turn of the last century created two conditions inimical to diverse forest regeneration: an overstory dominated by two species, P. serotina and Acer rubrum (86% of basal area), and a dense recalcitrant understory layer dominated by a native fern. The first condition limits propagule supply, the second strongly filters seedling establishment, and both create impoverished forest understories composed of few individuals and species. In undisturbed forest understories, the inertia toward impoverishment was sustained across the decade even where browsing was eliminated. Consequently, stand replacing disturbances (whether natural or anthropogenic) that disrupt the understory layer and reinitiate succession may be necessary to propel forests out of their current stasis and down a pathway leading to greater diversity.

Litter quality and decomposition responses to drought in a northeastern US deciduous forest.

Even though drought impacts on tree physiology have been identified, whether drought affects leaf litter chemistry that, in turn, influences litter decay rates is still poorly understood. We compared litter quality and decomposition for two cohorts of leaves from five co-occurring seasonally deciduous tree species: Acer saccharum, Tilia americana, Quercus rubra, Quercus alba, and Ostrya virginiana. One cohort experienced a growing-season drought, and the other cohort came from the same trees in the ensuing, post-drought growing season. Leaf litter production was greater for drought litter than post-drought litter for all five species. Specific leaf area and nitrogen concentrations were 20% greater for the drought cohort than the post-drought cohort. Concentrations of non-structural carbohydrates were about 14% greater for the drought cohort, except for greater values for post-drought A. saccharum litter. Pectin in the middle lamella of leaf litter was 31% lower for the drought cohort compared to post-drought cohort. We found few differences in litter decay rates between drought and post-drought cohorts, although Q. rubra litter had more decomposition for the post-drought cohort than the drought cohort, whereas A. saccharum litter had more decomposition for the drought cohort than the post-drought cohort. Leaf litter decay rates for the drought cohort were related to litter nitrogen and lignin concentrations, whereas decay rates for the post-drought cohort were related to litter carbohydrate concentrations. Our findings suggest that the role of drought events on seasonally deciduous forest ecosystems must recognize species-specific, idiosyncratic responses in leaf litter quality and decomposition.

Tree height and leaf drought tolerance traits shape growth responses across droughts in a temperate broadleaf forest.

As climate change drives increased drought in many forested regions, mechanistic understanding of the factors conferring drought tolerance in trees is increasingly important. The dendrochronological record provides a window through which we can understand how tree size and traits shape growth responses to droughts. We analyzed tree-ring records for 12 species in a broadleaf deciduous forest in Virginia (USA) to test hypotheses for how tree height, microenvironment characteristics, and species' traits shaped drought responses across the three strongest regional droughts over a 60-yr period. Drought tolerance (resistance, recovery, and resilience) decreased with tree height, which was strongly correlated with exposure to higher solar radiation and evaporative demand. The potentially greater rooting volume of larger trees did not confer a resistance advantage, but marginally increased recovery and resilience, in sites with low topographic wetness index. Drought tolerance was greater among species whose leaves lost turgor (wilted) at more negative water potentials and experienced less shrinkage upon desiccation. The tree-ring record reveals that tree height and leaf drought tolerance traits influenced growth responses during and after significant droughts in the meteorological record. As climate change-induced droughts intensify, tall trees with drought-sensitive leaves will be most vulnerable to immediate and longer-term growth reductions.

Tracking the phenology of photosynthesis using carotenoid-sensitive and near-infrared reflectance vegetation indices in a temperate evergreen and mixed deciduous forest.

Photosynthetic phenology is an important indicator of annual gross primary productivity (GPP). Assessing photosynthetic phenology remotely is difficult for evergreen conifers as they remain green year-round. Carotenoid-based vegetation indices such as the photochemical reflectance index (PRI) and chlorophyll/carotenoid index (CCI) are promising tools to remotely track the invisible phenology of photosynthesis by assessing carotenoid pigment dynamics. PRI, CCI and the near-infrared reflectance of vegetation (NIRV ) index may act as proxies of photosynthetic efficiency (ɛ), an important parameter in light-use efficiency models, or direct proxies of photosynthesis. To understand the physiological mechanisms reflected by PRI and CCI and the ability of vegetation indices to act as proxies of photosynthetic activity for estimating GPP, we measured leaf pigment composition, PRI, CCI, NIRV and photosynthetic activity at the leaf and canopy scales over 2 years in an evergreen and mixed deciduous forest. PRI and CCI captured the large seasonal carotenoid/chlorophyll ratio changes and good relationships were observed between PRI-ɛ and CCI-photosynthesis and NIRV -photosynthesis. PRI-, CCI- and NIRV -based models effectively tracked observed seasonal GPP. We propose that carotenoid-based and near-infrared reflectance vegetation indices may provide useful proxies of photosynthetic activity and can improve remote sensing-based models of GPP in evergreen and deciduous forests.

Structure and parameter uncertainty in centennial projections of forest community structure and carbon cycling.

Secondary forest regrowth shapes community succession and biogeochemistry for decades, including in the Upper Great Lakes region. Vegetation models encapsulate our understanding of forest function, and whether models can reproduce multi-decadal succession patterns is an indication of our ability to predict forest responses to future change. We test the ability of a vegetation model to simulate C cycling and community composition during 100 years of forest regrowth following stand-replacing disturbance, asking (a) Which processes and parameters are most important to accurately model Upper Midwest forest succession? (b) What is the relative importance of model structure versus parameter values to these predictions? We ran ensembles of the Ecosystem Demography model v2.2 with different representations of processes important to competition for light. We compared the magnitude of structural and parameter uncertainty and assessed which sub-model-parameter combinations best reproduced observed C fluxes and community composition. On average, our simulations underestimated observed net primary productivity (NPP) and leaf area index (LAI) after 100 years and predicted complete dominance by a single plant functional type (PFT). Out of 4,000 simulations, only nine fell within the observed range of both NPP and LAI, but these predicted unrealistically complete dominance by either early hardwood or pine PFTs. A different set of seven simulations were ecologically plausible but under-predicted observed NPP and LAI. Parameter uncertainty was large; NPP and LAI ranged from ~0% to >200% of their mean value, and any PFT could become dominant. The two parameters that contributed most to uncertainty in predicted NPP were plant-soil water conductance and growth respiration, both unobservable empirical coefficients. We conclude that (a) parameter uncertainty is more important than structural uncertainty, at least for ED-2.2 in Upper Midwest forests and (b) simulating both productivity and plant community composition accurately without physically unrealistic parameters remains challenging for demographic vegetation models.

Extensive land cover change across Arctic-Boreal Northwestern North America from disturbance and climate forcing.

A multitude of disturbance agents, such as wildfires, land use, and climate-driven expansion of woody shrubs, is transforming the distribution of plant functional types across Arctic-Boreal ecosystems, which has significant implications for interactions and feedbacks between terrestrial ecosystems and climate in the northern high-latitude. However, because the spatial resolution of existing land cover datasets is too coarse, large-scale land cover changes in the Arctic-Boreal region (ABR) have been poorly characterized. Here, we use 31 years (1984-2014) of moderate spatial resolution (30 m) satellite imagery over a region spanning 4.7 × 106  km2 in Alaska and northwestern Canada to characterize regional-scale ABR land cover changes. We find that 13.6 ± 1.3% of the domain has changed, primarily via two major modes of transformation: (a) simultaneous disturbance-driven decreases in Evergreen Forest area (-14.7 ± 3.0% relative to 1984) and increases in Deciduous Forest area (+14.8 ± 5.2%) in the Boreal biome; and (b) climate-driven expansion of Herbaceous and Shrub vegetation (+7.4 ± 2.0%) in the Arctic biome. By using time series of 30 m imagery, we characterize dynamics in forest and shrub cover occurring at relatively short spatial scales (hundreds of meters) due to fires, harvest, and climate-induced growth that are not observable in coarse spatial resolution (e.g., 500 m or greater pixel size) imagery. Wildfires caused most of Evergreen Forest Loss and Evergreen Forest Gain and substantial areas of Deciduous Forest Gain. Extensive shifts in the distribution of plant functional types at multiple spatial scales are consistent with observations of increased atmospheric CO2 seasonality and ecosystem productivity at northern high-latitudes and signal continental-scale shifts in the structure and function of northern high-latitude ecosystems in response to climate change.

Interaction of abiotic factor on soil CO2 efflux in three forest communities in tropical deciduous forest from India.

Environmental factors along with soil physico-chemical properties play a significant role on the diurnal trend of soil CO2 efflux. Soil CO2 efflux in Indian tropical forests is poorly studied. We studied the soil CO2 efflux in a representative tropical deciduous forest at Katerniaghat Wildlife Sanctuary (KWLS), Uttar Pradesh. The three forest communities namely dry mixed (DMF), Sal mixed (SMF), and Teak plantation (TPF) were selected for measuring soil CO2 efflux in the summer season during April to May 2017 using automated LI-COR 8100 soil CO2 flux system. Soil physico-chemical parameters were also studied in the three abovementioned forest communities. We also measured the different microclimatic variables at forest understorey in all three communities during the summer season. Total day time soil CO2 efflux of 826.70, 1089.24, and 828.94 (µmolCO2 m-2d-1) was observed in TPF, SMF, and DMF respectively. Soil CO2 efflux observed significant differences (P < 0.01) among the three forest communities studied for the summer season in tropical deciduous forest of Terai Himalaya. Average soil CO2 efflux rate (µmol CO2 m-2 s-1) of 4.06 ± 0.36, 5.03 ± 0.45, and 4.37 ± 0.79 was observed in TPF, SMF, and DMF, respectively, which is positively correlated with total organic carbon (TOC) and water holding capacity (WHC) among soil physico-chemical variables. Among microclimatic variables, soil temperature (ST, °C) and air temperature (AT, °C) observed strong positive correlation with day time soil CO2 efflux in all three communities. Significant increase in soil CO2 flux was observed with increasing air and soil temperature (AT and ST) in DMF and SMF. Maximum TOC of 19.23 g Kg-1 was observed in SMF among all communities in the summer season. The result showed that soil CO2 efflux is closely associated with TOC, WHC, AT, and ST for Indian deciduous forest ecosystems.

Growing season moisture drives interannual variation in woody productivity of a temperate deciduous forest.

The climate sensitivity of forest ecosystem woody productivity (ANPPstem ) influences carbon cycle responses to climate change. For the first time, we combined long-term annual growth and forest census data of a diverse temperate broadleaf deciduous forest, seeking to resolve whether ANPPstem is primarily moisture- or energy-limited and whether climate sensitivity has changed in recent decades characterised by more mesic conditions and elevated CO2 . We analysed tree-ring chronologies across 109 yr of monthly climatic variation (1901-2009) for 14 species representing 97% of ANPPstem in a 25.6 ha plot in northern Virginia, USA. Radial growth of most species and ecosystem-level ANPPstem responded positively to cool, moist growing season conditions, but the same conditions in the previous May-July were associated with reduced growth. In recent decades (1980-2009), responses were more variable and, on average, weaker. Our results indicated that woody productivity is primarily limited by current growing season moisture, as opposed to temperature or sunlight, but additional complexity in climate sensitivity may reflect the use of stored carbohydrate reserves. Overall, while such forests currently display limited moisture sensitivity, their woody productivity is likely to decline under projected hotter and potentially drier growing season conditions.

Spatial structure develops early in forest herb populations, controlled by dispersal and life cycle.

Fine-scale spatial structure is an essential feature of plant populations, controlling pollination, herbivory, pathogen spread, and resource partitioning. Origins of spatial distribution are often obscure in long-established forests, but successional stands offer insight through their physical and compositional simplicity. We tested the hypothesis that spatial structure in forest herb populations arises through a nucleation process in which colonizing species transition from random to clustered distributions through clonal expansion, seed dispersal, and conformity to environmental gradients. Spatial structure was examined in a chronosequence of 40 s growth stands in southeast Ohio, USA. Herbaceous vegetation was recorded in nested plots to describe the evolution of pattern across multiple scales. Spatial distribution was described as the variance:mean ratio of stem number plot-1, and compared between age classes and functional groups. Environmental influence was assessed as the marginal R2 value of environmental models predicting stem number. Herb species responded individualistically to stand age and environmental gradients, although all were to some degree clustered across age classes. Dispersal-limited, non-clonal, and annual species were most strongly clustered, suggesting the importance of seed dispersal range and population growth rate in determining spatial structure. Spatial distribution was weakly related to environmental variables. Clustered distributions established early in succession and remained stable for at least 80 years. Pattern formation can be interpreted in terms of nucleation, as we hypothesized, but clusters form earlier than expected. The spatial structure of herb populations in deciduous forests appears to be governed by patterns established during colonization; environmental filtering appears to play a minor role.

Experimental effects of white-tailed deer and an invasive shrub on forest ant communities.

Ungulate browse and invasive plants exert pressure on plant communities and alter the physical and chemical properties of soils, but little is known about their effects on litter-dwelling arthropods. In particular, ants (Formicidae) are ubiquitous in temperate forests and are sensitive to changes in habitat structure and resources. As ants play many functional roles, changes to ant communities may lead to changes in ecosystem processes. We conducted a long-term experiment that controlled white-tailed deer (Odocoileus virginianus) access and presence of an invasive understory shrub in deciduous forests located in southwestern Ohio, USA from 2011 to 2017. Several leaf-litter ant community responses and litter biomass were measured in five paired deer access and exclosure plots, each with a split-plot removal of Amur honeysuckle (Lonicera maackii). Ant abundance and species richness increased with time in deer exclosures, but not in deer access plots. Honeysuckle removal reduced abundance and richness of ants. There were additive effects of deer and honeysuckle on ant richness, and interactive effects of deer and honeysuckle on ant abundance. Deer exclusion reduced variation in ant composition relative to access plots. There was little evidence that treatments directly influenced species diversity of ants. However, all ant measures were positively related to litter biomass, which was greater in deer exclosures relative to access plots. Our results indicate strong indirect effects of herbivores and honeysuckle on litter-dwelling ants, mediated through changes in litter biomass and likely vegetation structure, which may alter ant-mediated ecosystem processes.

The worm burden of tracer kids and lambs browsing heterogeneous vegetation is influenced by strata harvested and not total dry matter intake or plant life form.

This study assessed the effect of total dry matter intake (DMI), plant life form and strata harvested on the gastrointestinal nematode (GIN) worm burdens of tracer kids and lambs browsing heterogeneous vegetation during the rainy season (August-November). The rainy season was divided into 6 2-week periods (P1-P6), and environmental conditions (rainy days, rainfall, temperature and humidity) were recorded daily. Five pairs each of tracer kids and tracer lambs raised free of GIN infections were used. Every 15 days, different pairs of kids and lambs were introduced to a 2.2-ha plot and co-grazed with a flock of 30 sheep and 70 goats for a period of 3 weeks. Feeding behaviour of each pair of tracers was measured in weeks 2 and 3. The continuous bite monitoring method was used to estimate total DMI, DMI of plant life forms and DMI from plants of different strata. After each 3-week period, the tracer pair was maintained indoors for 28 days and necropsied on day 29 to recover the worm burden. The feeding behaviour of the tracers was compared between periods (P2-P6) and between kids and lambs. The differences in the worm burdens of the tracers between periods were not associated with total DMI or DMI from plant life forms. Worm burdens were highest during P5 and P6 in tracer kids and lambs (P < 0.05), suggesting a build-up of infective larvae in the tropical deciduous forest (TDF). The lower worm burdens of tracer kids compared with lambs (P < 0.05) seemed to be associated with less low- and more medium-stratum ingestion.

Sequence of flower and leaf emergence in deciduous trees is linked to ecological traits, phylogenetics, and climate.

While much research has focused on the timing of individual plant phenological events, the sequence of phenological events has received considerably less attention. Here we identify drivers and patterns of flower and leaf emergence sequence (FLS) in deciduous tree species of the Great Lakes region of North America. Five hypotheses related to cold tolerance, water dynamics, seed mass, pollination syndrome, and xylem anatomy type were compared for their ability to explain FLS. Phylogenetic and geographic patterns of FLS were also assessed. We identified additional traits associated with FLS using Random Forest models. Of the hypotheses assessed, those related to species' water dynamics and seed mass had the greatest support. The spatial pattern of FLS was found to be strongly related to minimum monthly temperature and the phylogenetic pattern was clustered among species. Based on results from Random Forest models, species' fruiting characteristics were found to be the most important variables in explaining FLS. Our results show that FLS is related to a suite of plant traits and environmental tolerances. We emphasize the need to expand phenological research to include both the timing and sequence of plant's entire phenology, in particular in relation to plant physiology and global change.

Nutrient foraging strategies are associated with productivity and population growth in forest shrubs.

Background and Aims: Temperate deciduous forest understoreys are experiencing widespread changes in community composition, concurrent with increases in rates of nitrogen supply. These shifts in plant abundance may be driven by interspecific differences in nutrient foraging (i.e. conservative vs. acquisitive strategies) and, thus, adaptation to contemporary nutrient loading conditions. This study sought to determine if interspecific differences in nutrient foraging could help explain patterns of shrub success and decline in eastern North American forests. Methods: Using plants grown in a common garden, fine root traits associated with nutrient foraging were measured for six shrub species. Traits included the mean and skewness of the root diameter distribution, specific root length (SRL), C:N ratio, root tissue density, arbuscular mycorrhizal colonization and foraging precision. Above- and below-ground productivity were also determined for the same plants, and population growth rates were estimated using data from a long-term study of community dynamics. Root traits were compared among species and associations among root traits, measures of productivity and rates of population growth were evaluated. Key Results: Species fell into groups having thick or thin root forms, which correspond to conservative vs. acquisitive nutrient foraging strategies. Interspecific variation in root morphology and tissue construction correlated with measures of productivity and rates of cover expansion. Of the four species with acquisitive traits, three were introduced species that have become invasive in recent decades, and the fourth was a weedy native. In contrast, the two species with conservative traits were historically dominant shrubs that have declined in abundance in eastern North American forests. Conclusions: In forest understoreys of eastern North America, elevated nutrient availability may impose a filter on species success in addition to above-ground processes such as herbivory and overstorey canopy conditions. Shrubs that have root traits associated with rapid uptake of soil nutrients may be more likely to increase in abundance, while species without such traits may be less likely to keep pace with more productive species.

In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives.

A major goal in ecology is to understand mechanisms that increase invasion success of exotic species. A recent hypothesis implicates altered species interactions resulting from ungulate herbivore overabundance as a key cause of exotic plant domination. To test this hypothesis, we maintained an experimental demography deer exclusion study for 6 y in a forest where the native ungulate Odocoileus virginianus (white-tailed deer) is overabundant and Alliaria petiolata (garlic mustard) is aggressively invading. Because population growth is multiplicative across time, we introduce metrics that correctly integrate experimental effects across treatment years, the cumulative population growth rate, λc, and its geometric mean, λper-year, the time-averaged annual population growth rate. We determined λc and λper-year of the invader and of a common native, Trillium erectum. Our results conclusively demonstrate that deer are required for the success of Alliaria; its projected population trajectory shifted from explosive growth in the presence of deer (λper-year = 1.33) to decline toward extinction where deer are excluded (λper-year = 0.88). In contrast, Trillium's λper-year was suppressed in the presence of deer relative to deer exclusion (λper-year = 1.04 vs. 1.20, respectively). Retrospective sensitivity analyses revealed that the largest negative effect of deer exclusion on Alliaria came from rosette transitions, whereas the largest positive effect on Trillium came from reproductive transitions. Deer exclusion lowered Alliaria density while increasing Trillium density. Our results provide definitive experimental support that interactions with overabundant ungulates enhance demographic success of invaders and depress natives' success, with broad implications for biodiversity and ecosystem function worldwide.

Phosphorus cycling in deciduous forest soil differs between stands dominated by ecto- and arbuscular mycorrhizal trees.

Although much is known about how trees and their associated microbes influence nitrogen cycling in temperate forest soils, less is known about biotic controls over phosphorus (P) cycling. Given that mycorrhizal fungi are instrumental for P acquisition and that the two dominant associations - arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi - possess different strategies for acquiring P, we hypothesized that P cycling would differ in stands dominated by trees associated with AM vs ECM fungi. We quantified soil solution P, microbial biomass P, and sequentially extracted inorganic and organic P pools from May to November in plots dominated by trees forming either AM or ECM associations in south-central Indiana, USA. Overall, fungal communities in AM and ECM plots were functionally different and soils exhibited fundamental differences in P cycling. Organic forms of P were more available in ECM plots than in AM plots. Yet inorganic P decreased and organic P accumulated over the growing season in both ECM and AM plots, resulting in increasingly P-limited microbial biomass. Collectively, our results suggest that P cycling in hardwood forests is strongly influenced by biotic processes in soil and that these are driven by plant-associated fungal communities.

Multiscale modeling of spring phenology across Deciduous Forests in the Eastern United States.

Phenological events, such as bud burst, are strongly linked to ecosystem processes in temperate deciduous forests. However, the exact nature and magnitude of how seasonal and interannual variation in air temperatures influence phenology is poorly understood, and model-based phenology representations fail to capture local- to regional-scale variability arising from differences in species composition. In this paper, we use a combination of surface meteorological data, species composition maps, remote sensing, and ground-based observations to estimate models that better represent how community-level species composition affects the phenological response of deciduous broadleaf forests to climate forcing at spatial scales that are typically used in ecosystem models. Using time series of canopy greenness from repeat digital photography, citizen science data from the USA National Phenology Network, and satellite remote sensing-based observations of phenology, we estimated and tested models that predict the timing of spring leaf emergence across five different deciduous broadleaf forest types in the eastern United States. Specifically, we evaluated two different approaches: (i) using species-specific models in combination with species composition information to 'upscale' model predictions and (ii) using repeat digital photography of forest canopies that observe and integrate the phenological behavior of multiple representative species at each camera site to calibrate a single model for all deciduous broadleaf forests. Our results demonstrate variability in cumulative forcing requirements and photoperiod cues across species and forest types, and show how community composition influences phenological dynamics over large areas. At the same time, the response of different species to spatial and interannual variation in weather is, under the current climate regime, sufficiently similar that the generic deciduous forest model based on repeat digital photography performed comparably to the upscaled species-specific models. More generally, results from this analysis demonstrate how in situ observation networks and remote sensing data can be used to synergistically calibrate and assess regional parameterizations of phenology in models.