Browsing by Subject "Drought"
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Item Biochar And Ecosystem Recovery: Evaluating Artificial Regeneration Outcomes And Understory Plant Community Dynamics In Response To Top-Dress Amendments In Northern Minnesota(2024-05) Ackerman, SophieRecurring and severe droughts present significant obstacles to successful forest regeneration. Biochar soil amendments have emerged as a promising possible solution, providing the dual benefit of mitigating climate change through carbon sequestration while enhancing forest soil health in areas facing regeneration challenges. These amendments improve seedling drought resilience through enhanced cation exchange capacity, water retention, and nutrient availability. However, existing research in forested systems is limited, and primarily focuses on short-term impacts of the amendment on soil nutrients, physical properties, and microbial communities. Researchers have identified a need for long-term, site-specific research regarding how biochar affects forests' growth and stand dynamics. Over a five-year period, we investigated the effects of two different doses of top-dressed biochar soil amendments on tree growth across four different species. Our study did not reveal significant positive effects of biochar on seedling growth or survival over this period. However, it also did not indicate any negative influence on seedling survival or growth. Notably, each species exhibited distinct response patterns, suggesting potential trends deserving further investigation and emphasizing the importance of temporal scale in such studies. Additionally, our analysis of vegetative community composition and structure in response to the biochar amendments revealed distinct species-community responses, indicating complex underlying mechanisms warranting further investigation. In summary, this study contributes to a deeper understanding of biochar’s role in post-disturbance vegetation recovery and sheds light on the variability in seedling response patterns influenced by biochar soil amendment. Emphasizing the significance of species-specific responses, as well as the potential ecosystem-wide cascading effects, our findings highlight the complexity of biochar applications and their interactions with both tree species and understory vegetation over time. This research adds to the growing body of knowledge on biochar applications in forestry, underscoring its potential benefits in reforestation and carbon sequestration efforts. Furthermore, it enhances our understanding of the potential long-term effects of biochar soil amendments on forest health, stand dynamics, and sustainable forest management practices in a changing climate.Item Developing Salt-Tolerant Sod Mixtures for Use as Roadside Turf in Minnesota(Center for Transportation Studies University of Minnesota, 2014-12) Friell, Joshua; Watkins, Eric; Horgan, BrianFailure of roadside grass installations due to high levels of road salt is a common occurrence in Minnesota. Several species that are not currently included in the MnDOT recommendations for these sites have performed well in low-input turfgrass evaluations in Minnesota and warranted evaluation for salt tolerance and suitability for roadside environments. The goal of this project was to develop a recommended mixture or a set of mixtures that provide salt-tolerant sod for roadsides. In the first part of this research, cultivars of cool-season turfgrass were assessed for their ability to establish and survive on roadsides in Minnesota. Concurrently, these grasses were evaluated in a hydroponic system in the greenhouse for salinity tolerance. Together, these studies identified several species and cultivars that were promising for use on Minnesota roadsides. These top-performing grasses were then evaluated in a series of mixtures in three research trials: (1) a roadside evaluation at two locations in Minnesota; (2) a sod strength trial planted at two locations in Minnesota; and (3) an acute drought evaluation utilizing an automated rainout shelter. From these results, we identified species that should be components of a salt-tolerant turfgrass mixture for use on roadsides in Minnesota. Mixtures that included high proportions of fine fescues, especially hard fescue and slender creeping red fescue, performed the best in our trials indicating that these species should be utilized in MnDOT recommendations for turf grown on roadsides.Item An ecological and evolutionary perspective on functional diversity in the genus Salix(2010-05) Savage, Jessica AnneNatural selection, along with biochemical and architectural constraints can limit the trait combinations expressed by plants, creating functional trade-offs across species. These trade-offs often play a critical role in limiting species distributions by preventing them from performing well under all environmental conditions. For this dissertation, I examined the role of functional trade-offs in limiting species distributions at two geographic scales, focusing on species in the genus Salix (the willows). First, I examined whether species exhibited niche differentiation across a local water availability gradient, and investigated the extent that plant function and functional similarity influence species distributions and patterns of co-occurrence. For this analysis, I examined species distributions in relation to physiological and functional data collected in the field and in a greenhouse common garden. I also estimated a phylogeny of the species to examine patterns of species phylogenetic community structure and trait evolution. Second, I examined whether there was evidence for a trade-off between freezing tolerance and growth that could explain species range limits. This analysis involved comparing species freezing tolerances and growth rates under different environmental conditions and determining the extent that these traits related to species distributions. Overall, I found evidence that functional trade-offs are important in determining species distributions both within local plant communities and across broader geographic distributions. On a local level, species exhibit niche differentiation across a water availability gradient, and this differentiation is driven by a functional trade-off between drought tolerance and relative growth rate. Traits related to species drought tolerance and recruitment strategies also demonstrate correlated evolution with species water availability niches. At a broader geographic scale, species demonstrate a trade-off between freezing tolerance and growth. This trade-off is primarily the result of species dependence on photoperiod cues for growth regulation. The strong correlation between species growth rates, their freezing tolerances, and their modeled geographic ranges suggests that this trade-off could influence their geographic distributions. This research demonstrates the importance of functional trade-offs in determining the distributions of species in an ecological and economically important genus. It also demonstrates the value of integrative research that draws on physiological, evolutionary and ecological methodology.Item Examining the drivers of current and future changes in Central U.S. warm-season rainfall(2014-09) Harding, Keith John IliffWarm-season precipitation in the Central U.S. is highly variable, as severe droughts and flooding often occur in consecutive years or simultaneously. Some of the most highly productive agricultural lands are present within the region despite susceptibility to warm-season rainfall extremes. Climate change is expected to increase precipitation extremes globally, but how warm-season Central U.S. precipitation will be affected is unclear. In this study, I examine the drivers of current and future warm-season precipitation in the region as well as how the basic characteristics of summer rainfall may be affected by climate change through the use of gridded observations, reanalysis datasets, and dynamical downscaling of global climate models (GCMs). It is demonstrated that the negative phase of the Pacific-North American (PNA) teleconnection pattern enhances heavy precipitation events over the Upper Midwest by modulating the strength of the Great Plains Low Level Jet (GPLLJ), possibly enabling greater medium range prediction of Midwest heavy rain events. Similarly, I aim to reduce uncertainty in long-term projections of how precipitation may be affected by climate change by examining shortfalls in GCM-simulated warm-season precipitation and demonstrating improvement with dynamical downscaling. Using the Weather Research and Forecasting (WRF) model, two GCMs are dynamically downscaled in one historical and three future timeslices with varying anthropogenic forcing. Future warm-season precipitation in these simulations is more intense, less frequent, and occurs with more days between rain events, similar to trends in observations that show large increases in extreme rainfall events and rainfall intensity. The intensification of extreme rainfall events in future simulations is the strongest during the April-July, associated with a strengthening of the GPLLJ during those months. Heavier rainfall rates during extreme precipitation events are related to a stronger cold pool and mesohigh, which force stronger moisture convergence above the cold pool in the presence of additional low-level moisture and a drier mid-troposphere. Overall, the identification of plausible physical mechanisms that might contribute to the enhancement of heavy rainfall events in the region enables greater confidence in future projections of extreme rainfall events.Item Modeling the impact of iIrrigation on precipitation over the Great Plains.(2011-08) Harding, Keith John IliffSince World War II, the rapid expansion of irrigation throughout the Great Plains has threatened the sustainability of the Ogallala Aquifer. Irrigation has been shown to modify the surface energy and water budgets over the Great Plains by altering the partitioning of latent and sensible heating. An increase in latent heating from irrigation contributes to a cooler and more humid surface, which has competing impacts on convection. In this study, the Weather Research and Forecasting model was modified to simulate the effects of irrigation at sub-grid scales. Nine April-October simulations were completed for different hydrologic conditions over the Great Plains. Data from these simulations was assimilated into a back-trajectory analysis to identify where evapotranspired moisture from irrigated fields predominantly falls out as precipitation. May through September precipitation increased on average over the Great Plains by 4.97 mm (0.91%), with the largest increases during wet years (6.14 mm; 0.98%) and the smallest increases during drought years (2.85 mm; 0.63%). Large precipitation increases occurred over irrigated areas during normal and wet years, with decreases during drought years. On average, only 15.8% of evapotranspired moisture from irrigated fields fell out as precipitation over the Great Plains, resulting in 5.11 mm of May-September irrigation-induced precipitation. The heaviest irrigation-induced precipitation occurred over north-central Nebraska, coincident with simulated and observed precipitation increases. While irrigation resulted in localized and region-wide increases in precipitation, large evapotranspiration increases suggest that irrigation contributes to a net loss of water in the Great Plains.Item Predictive and Explanatory Modeling of Ecosystem Response to Soil Water Stress(2023-05) Sloan, BrandonPlant leaves close their stomatal apertures in response insufficient soil moisture, and, thus, control ecosystem water, carbon, and energy cycles. Many terrestrial biosphere models (TBMs) underlying general circulation models represent this ecosystem response to soil water stress with an empirical correction function (β) of soil moisture---a convenient approach that can produce large prediction uncertainties. To reduce this uncertainty, TBMs have increasingly incorporated physically-based Plant Hydraulic Models (PHMs) at the cost of more poorly-constrained parameters. Therefore, understanding why and when PHM and β predictions diverge would usefully inform model selection within TBMs. Here, we reconcile PHMs and β by illustrating that the soil-plant hydraulic transport in PHMs (parametrized by conductance) couples the effects of soil and atmospheric dryness on stomatal closure, and β emerges as an infinitely-conductive PHM, breaking the coupling. As a result, PHM and β transpiration predictions diverge most for soil-plant systems with low hydraulic conductance (transport-limited) that experience large variability in atmospheric dryness with moderate soil water stress. We confirm these results in a TBM case study at an eddy covariance site, and propose a `dynamic' β that compromises between β and PHMs. Our efforts to extend our analysis across biomes revealed a practical problem: which eddy covariance sites observed soil water stress? Many studies use explanatory models to infer stress signals from eddy covariance data, but the studies vary widely in their goals, selected sites, and inference assumptions. Furthermore, the sensitivity of the inferred stress signals to the numerous assumptions (i.e. robustness) are rarely quantified, giving pause to meta-analyses to identify stressed sites. Here, we develop a framework that quantifies the robustness of inferred soil water stress signals to the most prevalent data and modeling assumptions in literature, and apply this framework to 150 eddy covariance sites. Only 7 sites have a robust soil water stress signal due primarily to poor explanatory model performance. Furthermore, the results challenge common approaches of generalizing site-specific stress signals across biomes. We improve upon the robust stress signal detection by identifying several assumptions to improve explanatory model performance across most sites. Specifically, the response variable for parameter estimation and phenology considerations allow the robust stress framework to identify 30 sites with robust soil water stress signals. Lastly, we provide a user-friendly visual tool that rank-orders sites by the robustness of their soil water stress signals. Our research provides fundamental insights into ecosystem soil water stress prediction, while providing practical guidance on how to infer stress signals from data.Item Source, Fall 2013(University of Minnesota Extension, 2013) University of Minnesota ExtensionItem Understanding the role of plant hydraulic traits in ecological processes(2021-11) Vargas Gutierrez, GermanAmong all ecosystems tropical forests play a disproportionate role in Earth’s carbon cycle as they account for one third of total primary productivity. Global environmental change is causing novel precipitation patterns, shifts in the timing of rainfall events and an increase of the atmospheric water demand in tropical regions. Such changes pose a threat to the ecosystem services provided by tropical forest ecosystems as there is great uncertainty of how tropical forests will respond to warmer and drier climatic conditions. Plant responses to drought occur at different biological and temporal scales, ranging from responses at the tissue level to responses that define species and populations. Plant hydraulic traits have proven to be key in the characterization of how plants respond to drought, as they allow to group species in those that avoid drought and those that resist drought. Therefore, quantifying the sources of variation in plant hydraulic traits presents a promising way to move forward our understanding of how tropical forests will respond to climate-change. The main objective of this research was to study the aspects that drive variation in plant hydraulic traits in tropical plant species, and how nutrient availability affects tropical forest responses to drought. In Chapter 1, I performed a study in which I investigated if it is possible to generalize tree species drought tolerance and growing strategies using their leaf habit (i.e., deciduous, evergreen and semi-deciduous). In this study I found that in the tropical dry forest deciduous species tend to share similarities in their physiological properties, contrary to evergreen species that tend to be physiologically different across sites. Generally deciduous species will show a hydraulically risky growing strategy, and evergreen species a hydraulically safe growing strategy. In Chapter 2, I carried out a large-scale ecosystem manipulation experiment to study how both water and nutrient availability control primary productivity in a tropical dry forest. I found that nutrient availability had a stronger but weak effect on forest productivity that a decrease in soil moisture. Forest productivity was more sensitive to inter-annual variation in climatic conditions, as we observed large decreases in productivity in warmer and drier years when compared to the effect of the experimental treatments. In Chapter 3, I performed a data synthesis to study how environment and evolutionary history shape plant hydraulic trait variation among evergreen and deciduous species across the world tropics. Through this pantropical data synthesis, I was able to point out biases in the assessment of tropical plants drought tolerance and that environment play a stronger role in shaping plant hydraulic traits than species’ evolutionary history. I also demonstrated that deciduous species tend to have similar vulnerability to drought across environmental conditions, while evergreen species hydraulic traits vary as a function of water availability. Collectively, these three studies help us to understand the drivers of drought tolerance in tropical plants and how environmental variation shapes responses to drought in the tropical forests.Item Using functional traits to understand community assembly, responses to drought, and restoration in tropical dry forests(2017-12) Werden, Leland KendallTropical forests have been extensively degraded and deforested. Recent global restoration initiatives, such as the Bonn challenge, have emerged in an attempt to reverse these trends. To ensure these initiatives are effective, continued effort must be made to integrate ecological theory with restoration practice. It is imperative that some of this effort is focused on tropical dry forests (TDFs), as they are critically endangered and their restoration is understudied. Conservation efforts in NW Costa Rica have been effective in passively regenerating extensive areas of TDF, but the presence of degraded Vertisols in this region present a unique challenge and requires an active restoration approach. Furthermore, functional traits have been used to predict the outcomes of applied restoration of tropical wet forests, but their utility had not been evaluated in TDF. The goal of this research was therefore to determine how tree species in passively and actively restored TDFs use different functional strategies, to cope with stressful environmental conditions such as extreme drought and growing in degraded soils. In Chapters 1 and 2 I focused on determining how the functional strategies of tree species drive patterns in passively restored TDFs. In Chapter 1, I studied how abiotic and biotic gradients predict the landscape scale occurrence of TDF tree species, and I found that functional traits clarify community assembly mechanisms along these gradients in passively regenerating TDFs. In Chapter 2, I focused on the hydraulic responses of woody species to extreme drought and I found that trees and lianas have overlapping water-use strategies, but different in their leaf economic traits. In Chapters 3 and 4 I focused on using a similar functional trait-driven approach to actively restore TDF on degraded Vertisols. In Chapter 3, I implemented a 32 species trial to select native TDF species for Vertisol restoration. My results suggest that functional traits most predictive of survivorship and growth in TDF restoration correspond to how species capture carbon and tolerate drought. Finally, for Chapter 4, I used a 6 hectare Vertisol restoration project to conduct the first empirical test of how species with contrasting functional strategies perform at different TDF successional stages. The results from this study suggest that resource acquisition strategies of TDF tree species can be used to predict species’ responses to changes in microclimatic conditions over succession. Collectively these four studies contribute significantly to our understanding of how functional strategies of TDF tree species dictate their responses to drought and gradients in abiotic conditions in both passively and actively restored TDF.