Graduate Student Inquiries and Research
Prospective Students
Students interested in graduate research can be supported by teaching assistantships, grants and research fellowships. I am an advisor for students in three degree programs. If the student is primarily interested in environmental biology and physiology with additional experience in molecular and cellular physiology, then they should apply to the Intercollege Graduate Degree Program in Plant Biology . If their interests are more ecological, and wish to work in the areas of plant physiological ecology, ecosystem ecology or global change biology, then the Intercollege Graduate Program in Ecology is probably their best choice. If the student wants to combine basic environmental plant physiology and ecology with aspects of fruit crop production and management, then the Graduate Program in Horticulture is probably their best choice. Unsure? Contact me: dme9@psu.edu
Current Graduate Student Research
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Luke McCormack: Impacts of Elevated CO2 on fine root and mycorrhizal dynamics I am interested in the affects of global change on terrestrial ecosystem functioning and nutrient cycling. Most of my work has been focused on the changing dynamics of fine roots and their fungal symbionts under elevated CO2. In the future I plan to look more specifically on how colonization rates of mycorrhizas may be impacted by elevated CO2 in temperate forests and how these changes may in turn affect nutrient acquisition and ecosystem NPP. |
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Jane Wubbels: Linking species distributions to tree water relations in a temperate forest in central Pennsylvania Collaborators: Kate McCulloh, Rick Meinzer I work in the area of tree water relations in a mixed temperate forest. I am interested in how tree water use and water availability affect species distribution. My research is part of a larger project, called the Shale Hills Critical Zone Observatory. The critical zone refers to the Earth’s surface and as far into the crust and the atmosphere as organism interactions extend. The primary objective at the Shale Hills CZO is to identify processes that affect the weathering of parent material to form regolith. Within this framework, my goal is to determine how trees affect the weathering process, as well as potential gradients that may arise due to the preferential distribution of species within the watershed by characterizing the role of woody vegetation in the hydrological cycle. I am investigating species-level differences in how trees take up, transport, and transpire water. Based on preliminary surveying data, I found that within a genus, species tended to preferentially occur in certain areas of the watershed. When correlated with soil moisture data, these distribution patterns appeared to follow a soil moisture gradient with species spatially segregating in either the wetter valley area or the drier midslopes and ridges. Based on these patterns, I hypothesized that the species in the drier areas would have relatively more conservative hydraulic architecture than those of the same genus that preferentially occur on wetter soils. I measured xylem vulnerability, hydraulic conductivity, and leaf specific conductivity to characterize each species’ architecture. To test an alternative hypothesis that species are not any more or less conservative in their water use, but rather have access to different water sources, I also collected samples for stable isotope analysis of 18O and 2H. These data will be compared with soil water and ground water isotopic data to determine from where trees draw their water. |
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Quanying Du: Possible mechanisms contributing to nitrogen effects on root longevity Collaborators: Dali Guo, Zhengquan Wang Anthropogenic nitrogen deposition in temperate forests causes ecosystem-level nitrogen and carbon cycle shifts. Nitrogen enrichment along soil nitrogen gradients has commonly been associated with increased root turnover, but not always. More consistently, roots of higher nitrogen content have been associated with faster respiration rates, an indicator of metabolic activity. Despite the inconsistency in previous observations, many studies indicate enhanced metabolic activities and higher susceptibility to herbivores are two possible mechanisms that would lead to shorter root life spans under increased nitrogen supply. 1) Elevated nitrogen levels in previously nitrogen-limited environments could cause roots to be more palatable to herbivores. The increase of palatablity reduces root longevity. Insecticide treatment thus should reduce the risk of root mortality. 2) The generation of reactive oxygen species under oxidative stress may accelerate root aging in nitrogen-rich environments. The plan is to measure root respiration in conjunction with enzyme activity and membrane stability of roots of different age and under different levels of soil nitrogen status. |
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Marc Goebel: Factors influencing fine root decomposition - Belowground carbon sequestration and nutrient cycling in temperate forest ecosystems Collaborators: Douglas Archibald, Sarah Hobbie, Jacek Oleksyn, Peter Reich Marc has been partially supported by the National Science Foundation, College of Agricultural Sciences, Horticulture Department and the Ecology Program. Biological processes belowground promote carbon and nutrient cycling and therefore influence the productivity, structure, and diversity of plant communities. In a variety of plant communities, the belowground net primary production is at least that of aboveground, contributing significantly to the recycling of plant tissue-bound carbon and nutrients into the soil through fine root decomposition.A common garden approach of 14 tree species will be used to investigate (1) the impact of species-specific differences in structural and functional characteristics of fine root orders on the process of decomposition among temperate tree species and (2) predict rates of in situ root decomposition based on initial root tissue quality and soil characteristics. Based on age and functional differences fine root tissues vary in quality and anatomical characteristics. Under the assumption that root tissues with different anatomy and tissue quality decompose at different rates, I stratified fine roots (< 1 mm) by root order and placed them into separate compartments of root litterbags over the time period of three years. The goal of this experiment is to examine if rates of carbon and nutrient cycling vary among root orders of fine roots < 1 mm. In addition to the root litterbag study, I observed undisturbed fine roots in situ, severed from the stem, using minirhizotron technology. Vital staining was used to refine estimates of the death of roots based on external appearance, which was then used with minirhizotron images of roots severed from the stem. Rates of decomposition will be estimated from the duration between root death and total disappearance of fine roots. Previous research on root decomposition, using roots with a diameter less than 2 mm) showed that certain factors of root tissues are more influential during decay than others. The analysis of decayed root orders for the content of carbon, nitrogen, calcium, magnesium, phosphorous and aluminum, as well as, esterfied phenolics and the lignin content will allow a detailed evaluation of these intrinsic factors. In addition, I will examine the relationship of soil properties, the extrinsic factors, collected for each plot of the common garden setting, to further improve the analytical power of my research. Ultimately, the comparison of two belowground methods examining rates of decomposition by root order will provide better understanding of belowground decomposition processes among different temperate tree species. |
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Kevin Mueller: Tree species effects on carbon cycling in soil Collaborators: Kate Freeman, Jon Chorover, Jacek Oleksyn, Peter Reich, Sarah Hobbie, Susan Trumbore Kevin has been partially supported by NSF IGERT: Biogeochemical Research Initiative for Education, the Ecology Program and the Horticulture Department. Despite the prominent position of soil organic matter (SOM) in the global carbon (C) cycle and the promise for temperate forests to act as C sinks, the biotic controls over recalcitrant SOM formation in these ecosystems are not well understood. We are investigating plant controls on SOM formation and C sequestration in soils at a unique temperate forest common garden experiment in central Poland. The accumulation of plant inputs above- and belowground for 35 years in 14 species plots provides a powerful tool for exploration of the influence of tree species on soil C dynamics. Specifically, we plan to examine the relationship between SOM and plant characteristics such as potential growth rate, nutrient conservation strategy, and plant litter production and chemistry. Does variation in certain measurable plant traits influence soil C dynamics in a predictable fashion? Bomb radiocarbon analysis will be utilized to estimate recent C input and stability (C sink potential) in soil. In addition, we are using GC-MS and NMR analysis of the biochemical composition of SOM to determine the relative importance of plant inputs (root and leaf material) to soil C storage. The relationships revealed in this study will be useful in modeling the effects of changes in tree species composition on C dynamics in temperate forests. |
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Thomas Adams: Root growth plasticity and longevity of temperate trees species that vary widely in specific root length. Tom is partially supported by the National Science Foundation A widely held view in the field of root ecology is that small diameter roots with low tissue density, resulting in a corresponding high specific root length (SRL), tend to have shorter lifespans compared with low SRL, coarse roots (Eissenstat and Yanai 1997). These small diameter roots are thought to have relatively low construction costs and therefore can be readily produced to take advantage of favorable soil microsites. Additionally, these roots can be easily shed under stressful conditions without great expense to the parent plant. As such, these ephemeral roots are generally formed by fast-growing, early successional species. There is strong evidence that high SRL roots have shorter lifespans than low SRL roots within a single plant’s root system (Anderson et al. 2003, Wells and Eissenstat 2001); however, there is only limited evidence that this pattern exists among root systems of different species that vary in SRL. Establishing that a relationship does indeed exist between SRL, ability to proliferate and lifespan would provide valuable insights into belowground resource acquisition and interspecific competition, which in turn will have implications for plant community structure, successional patterns, and invasibility. I therefore plan to investigate whether co-occurring trees species with low SRL have longer root longevity than species with high SRL and whether these low-SRL species have lower root production and less plasticity in favorable soil conditions than species of higher SRL. To do this, I plan to conduct a long-term root demography study, using clear acrylic minirhizotron tubes, to track the roots of six tree species of widely divergent SRL. Trees were planted in 1996 in a common garden at the Pennsylvania State University’s Russell E. Larson Agricultural Research Center. I will use half of the minirhizotron tubes to obtain information on root longevity and production and these tubes will also serve as controls for the other half of the minirhizotron tubes which will be used in a fertilizer-enrichment study to investigate root plasticity and patch proliferation. |
Recent Past Graduate Students |
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Huining Xu: Influence of crop load on belowground carbon allocation in Concord grape. M.S. in Plant Biology, 2008 Collaborators: Alan Lakso, Rick Dunst, Kelly Link Huining has been supported by an IBIOS fellowship, the Viticulture Consortium and NY Wine & Grape Foundation  Large crops may substantially compete with root growth and activity in grapes and other fruit tress, possibly leading to a decline in plant vigor. I predict that high crop loads will reduce fine-root production, longevity, respiration and concentration of carbohydrates and nitrogen. My study site is a mature Concord grape vineyard in Fredonia, NY. In 2004, 2005, and 2006, at 30 days after bloom, crops were adjusted to four levels by cluster thinning, with targets of 100, 75, 50 and 25% of full crop in a completely randomized block design with 3 treatment vines and one buffer vine on each side of the treatment vines. Fine root production and lifespans have been observed using minirhizotrons beginning in 2004. I am also determining the effect of different crop loads on root respiration using a gas exchange system with an IRGA and carbohydrate and nitrogen concentration by colorimetric and elemental analysis, respectively (2006). I will estimate the total amount and spatial distribution of the vine root system by roots collected from soil cores in 2006 before veraison. These data will be used to calculate the total carbohydrate requirement and carbon allocation of the whole root system. Publications:
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Luis Valenzuela: Root and leaf responses to different levels of reproduction in Vaccinium corymbosum. PhD in Horticulture, 2008 Collaborators: Kathy Demchak, Elsa Sanchez Luis has been supported by Consejo Nacional de Ciencia y Tecnologia (CONCYT) fellowship and the National Science Foundation.  Optimizing carbon (C) allocation is key to maximizing fruit production without diminishing long-term plant vigor. Fruit production constitutes a large C sink, which competes with the production, maintenance, and storage pools of shoots and roots. The level that a crop affects root and shoot processes depends on the photosynthetic capacity of the plant. Thus, a whole-plant C budgeting approach is necessary to fully understand the effects of crop load on plant vigor. Fine root carbon costs are the least understood, but can have high construction and maintenance costs. Roots that turnover more quickly have higher production costs. Similarly, leaf costs are governed by the number of leaves produced, how frequently they need to be replaced to maintain a given canopy size, and the costs (dark respiration) required for their maintenance. In addition, leaf C assimilation may increase somewhat with increases in crop. Therefore, I will determine how blueberry root and leaf production, turnover and maintenance as well as whole-canopy C assimilation, are affected by different levels of reproduction. Blueberry plants may be particularly unique in this regard. Blueberries and other members of the Ericales are very distinct from other plant orders because of the unique root architecture. Blueberry roots are extremely fine, extensively branched, lack root hairs and are colonized by ericoid mycorrhizal fungi. This may cause the carbon costs of blueberry roots to differ markedly from other plants. Publications:
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| Taryn Bauerle, PhD in Horticulture, 2007 Grapevine water relations in Oakville, CA: root demographics and physiology in response to plant water deficit
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| Jennifer M. Withington, PhD in Ecology, 2005 Fine root production and lifespan in eleven temperate tree species growing in a common garden in Poland.  Leaf lifespan and structure have been linked to plant competition and nutrient cycling. Analyses of leaf structure and leaf lifespan on global data sets provide strong evidence for long leaf lifespan coupled with low specific leaf area and low mass-based N concentrations. Because fine roots share many characteristics with leaves (e.g. resource acquisition, ephemeral nature), we hypothesized that fine root and leaf lifespan should be correlated, and fine roots lifespan should couple with root traits.Our common garden in central Poland consisted of replicated, monospecific plots of five hardwood and six conifer species. We used minirhizotrons to observe root production and lifespan over four years. Minirhizotrons are used to nondestructively observe roots. We tested the important assumption that tube material does not influence root behavior using butyrate and acrylic tubes in plots of three hardwoods and three conifers. Root survivorship near acrylic tubes was shorter for the conifers and longer for the hardwoods, indicating that multi-species lifespan data can be influenced by tube material. Compared to butyrate, acrylic tube standing crop data were more similar to standing crop estimates from soil cores, suggesting acrylic tubes provide the more accurate data in our study systems. Our species had one peak of root production in summer, while the site experienced no summer drought. Maximum peak production shifted similarly for all species from year to year indicating a strong influence of external factors. Though the eleven tree species exhibited a wide range in leaf and fine root lifespans, leaf lifespan was not correlated with fine root lifespan. Root nitrogen:carbon ratio was inversely correlated with root lifespan. Other root traits, such as diameter and specific root length, were not correlated with root lifespan. Our studies show that tissue structure and patterns of longevity aboveground can contrast markedly with patterns belowground. To better understand larger-scale nutrient cycling in ecosystems as well as patterns associated with belowground growth strategies, we need to understand similarities of roots across species. We can do this by observing root production and lifespan patterns in multi-year, multi-species studies. Publications:
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| Jenny Edwards, M.S. in Ecology, 2005 Reasons for differential leaf calcium concentrations in forest trees  Variation in leaf-litter Ca concentration among fourteen tree species growing in a common garden in central Poland was linked to variation in soil pH, exchangeable Ca, soil base saturation, forest floor turnover rates, and earthworm abundance. Given the potential importance of tissue Ca to biogeochemical processes, in this study we investigated two potential controls on leaf Ca concentrations. We investigated whether species with high Ca concentrations in green leaves and leaf litter access soil Ca to a greater extent than low-Ca species by growing more roots in high-Ca soil horizons. We also determined whether differences in Ca concentration of leaves and litter were due to differences in nutritive demand for Ca for growth by measuring seedlings of six of the fourteen species grown under controlled conditions of varying Ca supply. Root distribution in the field was determined in all 14 tree species by profile wall mapping of excavated pits. There was no correlation between root count density (number of roots m-2) and exchangeable soil Ca. Across species we found a positive correlation of leaf litter Ca and density of roots 45-100 cm deep in the soil, suggesting that a deeper root distribution confers an advantage in Ca acquisition. Variation among species in leaf Ca concentration of seedlings in the greenhouse was positively correlated with leaf Ca concentrations of mature trees in the field, indicating that the same rank variation in leaf Ca among species existed under controlled Ca supply. Species also differed in growth response to Ca supply. Tilia, the species with the highest leaf Ca in the field, had only 10% as great biomass, height and Ca concentration at low as at high Ca supply. The other species exhibited no significant differences in biomass or height at different levels of Ca supply. Our data support the hypothesis that variation among species in leaf and litter Ca concentration of mature trees in the field is a trait associated with innate physiological differences. Additionally, our data support the hypothesis that species with high- as opposed to low- Ca in leaves and litter have access to greater pools of Ca in deep soils.Publications:
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