Plant community response to novel silvicultural treatments in Great Lakes northern hardwoods

The objective of this dissertation is to assess plant community response across a range of silvicultural disturbances and test ecological hypotheses to better inform ecologists and forest managers. To provide context for the utility of revising silvicultural systems, I review natural disturbance regimes and historical practices that have shaped contemporary Great Lakes northern hardwood forests (Chapter 2). Further, I identify important ways to expand the silvicultural toolbox and better emulate natural disturbance regimes. Building on this theoretical underpinning, I investigate the initial regeneration and plant community response to two novel silvicultural experiments: the Northern Hardwood Experiment for Enhancing Diversity (NHSEED) near Alberta, Michigan, and a strip clearcut experiment near Mountain Iron, Michigan. Three themes emerged from the findings in this dissertation. First, seedlings and saplings receive few benefits from reduced canopy cover if they cannot overcome additional limitations. For example, yellow birch (Betula alleghaniensis Britt.) seedling density was better predicted by conspecific overstory basal area and litter depth variation than silvicultural treatments (Chapter 3), and sugar maple recruitment into the sapling size class in clearcut strips may be limited by deer browse (Chapter 5). Second, silvicultural disturbances tend to favor low-mass fruit, long-lived fruit, or vegetative reproduction, except for sugar maple which relies on robust advance regeneration to benefit from overstory disturbances (Chapters 3, 4 and 5). Third, the relationship between disturbance severity and diversity is not conclusive. Initial responses to silvicultural disturbances did not follow the intermediate disturbance hypothesis, which proposes that diversity is maximized at intermediate levels of disturbance intensity or frequency (Chapter 4). Moreover, taxonomic and phylogenetic diversity do not always respond similarly to disturbances (Chapter 4), suggesting that both indices should be incorporated into informed management decisions. Integrating these findings into management planning may allow better predictions to silvicultural disturbances now and in the future

The biological controls of soil carbon accumulation following wildfire and harvest in boreal forests: A review

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha(-1) year(-1), respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models

Mobile Air Quality Studies (MAQS) - an international project

Due to an increasing awareness of the potential hazardousness of air pollutants, new laws, rules and guidelines have recently been implemented globally. In this respect, numerous studies have addressed traffic-related exposure to particulate matter using stationary technology so far. By contrast, only few studies used the advanced technology of mobile exposure analysis. The Mobile Air Quality Study (MAQS) addresses the issue of air pollutant exposure by combining advanced high-granularity spatial-temporal analysis with vehicle-mounted, person-mounted and roadside sensors. The MAQS-platform will be used by international collaborators in order 1) to assess air pollutant exposure in relation to road structure, 2) to assess air pollutant exposure in relation to traffic density, 3) to assess air pollutant exposure in relation to weather conditions, 4) to compare exposure within vehicles between front and back seat (children) positions, and 5) to evaluate "traffic zone"- exposure in relation to non-"traffic zone"-exposure. Primarily, the MAQS-platform will focus on particulate matter. With the establishment of advanced mobile analysis tools, it is planed to extend the analysis to other pollutants including including NO2, SO2, nanoparticles, and ozone

Transthyretin Is Dysregulated in Preeclampsia, and Its Native Form Prevents the Onset of Disease in a Preclinical Mouse Model

Preeclampsia is a major pregnancy complication with potential short- and long-term consequences for both mother and fetus. Understanding its pathogenesis and causative biomarkers is likely to yield insights for prediction and treatment. Herein, we provide evidence that transthyretin, a transporter of thyroxine and retinol, is aggregated in preeclampsia and is present at reduced levels in sera of preeclamptic women, as detected by proteomic screen. We demonstrate that transthyretin aggregates form deposits in preeclampsia placental tissue and cause apoptosis. By using in vitro approaches and a humanized mouse model, we provide evidence for a causal link between dysregulated transthyretin and preeclampsia. Native transthyretin inhibits all preeclampsia-like features in the humanized mouse model, including new-onset proteinuria, increased blood pressure, glomerular endotheliosis, and production of anti-angiogenic factors. Our findings suggest that a focus on transthyretin structure and function is a novel strategy to understand and combat preeclampsia

Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N-2-fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N-2-fixation with the functioning of vascular plants. Neglecting nonvascular N-2-fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N-2-fixing plants are more common. Adequately accounting for nonvascular N-2-fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N-2-fixation responses to global change factors (elevated CO2, N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N-2-fixation using nine Earth system models. The majority of empirical studies found positive effects of CO2, warming, precipitation, or light on nonvascular N-2-fixation, but N deposition strongly downregulated N-2-fixation in most empirical studies. Furthermore, we found that the responses of N-2-fixation to elevated CO2 were generally consistent between models and very limited empirical data. In contrast, empirical-model comparisons suggest that all models we assessed, and particularly those that scale N-2-fixation with net primary productivity or evapotranspiration, may be overestimating N-2-fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N-2-fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N-2-fixation and that field studies include more targeted measurements to improve model structures and parameterization

R code

R code for data analysis of N stocks and accumulation rates across many boreal forest sites.</p

Data

Nitrogen stocks and explanatory variables among boreal wildfire chronosequences.</p

Warming influences carbon and nitrogen assimilation between a widespread Ericaceous shrub and root-associated fungi

High-latitude ecosystems are warming faster than other biomes and are often dominated by a ground layer of Ericaceous shrubs, which can respond positively to warming. The carbon-for-nitrogen (C-for-N) exchange between Ericaceous shrubs and root-associated fungi may underlie shrub responses to warming, but has been understudied. In a glasshouse setting, we examined the effects of warming on the C-for-N exchange between the Ericaceous shrub Empetrum nigrum ssp. hermaphroditum and its root-associated fungi. We applied different 13C and 15N isotope labels, including a simple organic N form (glycine) and a complex organic N form (moss litter) and quantified their assimilation into soil, plant biomass, and root fungal biomass pools. We found that warming lowered the amount of 13C partitioned to root-associated fungi per unit of glycine 15N assimilated by E. nigrum, but only in the short term. By contrast, warming increased the amount of 13C partitioned to root-associated fungi per unit of moss 15N assimilated by E. nigrum. Our study suggests that climate warming affects the short-term exchange of C and N between a widespread Ericaceous shrub and root-associated fungi. Furthermore, while most isotope tracing studies use labile N sources, we demonstrate that a ubiquitous recalcitrant N source may produce contrasting results

Predictors of taxonomic and functional composition of black spruce seedling ectomycorrhizal fungal communities along peatland drainage gradients

Many trees depend on symbiotic ectomycorrhizal fungi for nutrients in exchange for photosynthetically derived carbohydrates. Trees growing in peatlands, which cover 3% of the earth’s terrestrial surface area yet hold approximately one-third of organic soil carbon stocks, may benefit from ectomycorrhizal fungi that can efficiently forage for nutrients and degrade organic matter using oxidative enzymes such as class II peroxidases. However, such traits may place a higher carbon cost on both the fungi and host tree. To investigate these trade-offs that might structure peatland ectomycorrhizal fungal communities, we sampled black spruce (Picea mariana (Mill.)) seedlings along 100-year-old peatland drainage gradients in Minnesota, USA, that had resulted in higher soil nitrogen and canopy density. Structural equation models revealed that the relative abundance of the dominant ectomycorrhizal fungal genus, Cortinarius, which is known for relatively high fungal biomass coupled with elevated class II peroxidase potential, was negatively linked to site fertility but more positively affected by recent host stem radial growth, suggesting carbon limitation. In contrast, Cenococcum, known for comparatively lower fungal biomass and less class II peroxidase potential, was negatively linked to host stem radial growth and unrelated to site fertility. Like Cortinarius, the estimated relative abundance of class II peroxidase genes in the ectomycorrhizal community was more related to host stem radial growth than site fertility. Our findings indicate a trade-off between symbiont foraging traits and associated carbon costs that consequently structure seedling ectomycorrhizal fungal communities in peatlands

Seedling and sapling recruitment following novel silvicultural treatments in Great Lakes northern hardwoods

Conventional forestry practices in Great Lakes northern hardwoods tend to promote the regeneration of a few economically important tree species, and may consequently contribute to a decline in mid-tolerant species abundance and tree species diversity. Using a greater variety of silvicultural systems to emulate a range of historical disturbances may help to restore historical mid-tolerant species abundance and tree diversity. To test the link between seedling recruitment and sapling recruitment and disturbance-based management systems, we implemented a novel silvicultural experiment using a variety of harvest and site preparation treatments ranging from single-tree selection to clear-cut and scarification, which emulate regionally-specific natural disturbance regimes. We predicted that mid-severity harvest treatments would promote mid-tolerant sapling recruitment, while scarification and the creation of pit-mound topography would favor recruitment of small-seeded species such as yellow birch (Betula alleghaniensis Britt.). We found that seedling and sapling composition was related to silvicultural treatment and driven by an opposing gradient of canopy openness and leaf litter depth along with temporal soil water content variation. Yellow birch seedling density, however, had little relation to silvicultural treatment but was instead predicted by positive relationships with leaf litter depth variation and residual conspecific basal area. Our findings suggest that silvicultural systems aimed at promoting species diversity in northern hardwoods should consider, along with conventional techniques, modifying litter depth and canopy openness while retaining seed trees