Thesis (M.S.)--University of Delaware, 2006. / Principal faculty advisor: Delphis F. Levia, Dept. of Geography. Includes bibliographical references.

This study investigates the effects of fire on the chemistry and flux of precipitation diverted to the forest floor as stemflow and throughfall by observing the impact of the June 2013 Thompson Ridge Wildfire in the Jemez River Basin of New Mexico. The loss of canopy cover from wildfire drastically modifies landscapes and alters ecosystems as fire replaces leafy canopies with charred branches and trunks, changes soil composition and erosion processes, and affects hydrologic flow paths and water chemistry. In order to track these changes, throughfall and stemflow collectors were installed beneath burned and unburned canopies in two catchments impacted by the Thompson Ridge Fire. Throughfall, stemflow, and open precipitation samples were analyzed for major cations, anions, dissolved inorganic and organic carbon, trace metals, and rare earth elements to determine how fire affects the chemical composition of the precipitation that interacts with burned canopies. Precipitation samples collected from both burned and unburned sites during the 2014 summer monsoon season show variations across burn severity, specifically in calcium, strontium, phosphate, and dissolved inorganic carbon concentrations, and across collector type with stemflow concentrations generally higher than throughfall and open precipitation concentrations. A stem count model was used to determine tree density for individual plots and catchments from LiDAR images taken before the 2013 fire. The stem count model was used to upscale event and monsoon season solute fluxes from plot to catchment scale. Higher nutrient concentrations combined with higher volumes of precipitation diverted as stemflow in burned forests have a multiplicative effect resulting in greater nutrient fluxes via stemflow creating nutrient hot spots surrounding burned tree trunks. Upscaling these plot scale concentrations and solute fluxes allows this study to represent changes to an entire catchment and quantify effects of wildfire on chemical loads and water chemistry.

The Appalachian Region is a rich and diverse forest ecosystem impacted by present and past mining activities. The Surface Mining Control and Reclamation Act (SMCRA) of 1977 was enacted to resolve many of the environmental problems caused by surfacing mining, such as landslides, erosion, flooding, and poor water quality. As with many solutions, this one came with its own set of environmental problems due to compaction and the introduction of aggressive non-native grasses and shrubs altering hydrologic processes and ecosystem function. The Forestry Reclamation Approach (FRA) is a method for re-establishing forested ecosystems on mined lands. This project evaluated the effect of FRA on throughfall by comparing 10-, 20-, and 100-year old tree plots consisting of coniferous or deciduous trees. Throughfall rates were significantly impacted by tree type and age. Coniferous trees intercepted more rainfall than deciduous ones and the older trees tended to intercept the least. Presence/absence of leaves impacted throughfall depths for deciduous trees. Throughfall was significantly impacted by storm event characteristics. Results may help guide management of forested watersheds regarding strategies to reduce water yields on mined lands.

Western spruce budworm (Choristoneura occidentalis) outbreaks periodically disturb Western US conifer forests by defoliating canopies, which could alter the quantity and chemistry of throughfall delivered to the forest floor. Our objectives were to: i) quantify throughfall water, carbon (C), nitrogen (N), and phosphorus (P) fluxes under budworm-impacted canopies, and ii) examine the influence of herbivore intensity on flux magnitudes. In June 2015, we installed throughfall collectors in two watersheds experiencing high and background levels of herbivory. In each watershed, four plots, each with three throughfall collectors, were established (n=24) collectors), and two bulk rainfall collectors were installed in areas without canopy cover. Throughfall and rainfall were collected from late June to early November 2015. Samples were analyzed for dissolved organic carbon (DOC), ammonium (NH4-N), and soluble reactive phosphorus (SRP). Over the sampling period, throughfall fluxes ranged 8.57 to 47.59 kg/ha for DOC, 0.004-0.011 kg/ha for NH4-N, and 007 - 0.29 kg/ha for SRP. Percent throughfall was slightly, but not significantly, higher in the high (48%) compared to the background watershed (42%). There were no differences in solute concentrations among the watersheds. Net throughfall fluxes, the sum of canopy uptake and leaching and dry/fog deposition, differed significantly for NH4-N by herbivory level and through time for NH4-N and DOC but not SRP. Over time, net NH4-N throughfall fluxes showed a clear transition from net uptake of NH4-N to net leaching of NH4-N in the high herbivory watershed. There was also a clearn NH4-N pulse in the high herbivory watershed after the first, but not subsequent, rainfall events. In this N-limited forest, altered throughfall N may affect soil nutrient cycling and downstream water quality.

Increasing demand for water supplies in semi-arid regions, such as San Antonio, has sparked an interest in potential recharge management through brush control. Two shallow caves under woody plant cover in northern Bexar County, Texas were chosen as study sites where a detailed water budget would be developed. The Headquarters Cave site measures natural rainfall and cave recharge while the Bunny Hole site is instrumented to measure throughfall, stemflow, surface runoff, and cave recharge. Large scale rainfall simulation was used at Bunny Hole to apply water directly above the cave footprint allowing us to determine how recharge differs between natural and simulated rainfall events. Under natural conditions, Headquarters Cave recharged 15.05% of the annual rainfall while Bunny Hole received 4.28%. Natural canopy throughfall measured 59.96% of the water budget; stemflow accounted for 0.48% and canopy interception was 39.56%; no surface runoff was measured. Rainfall simulations conducted at Bunny Hole resulted in an average of 74.5% throughfall, 5.3% stemflow, 20.2% canopy interception, 2.8% surface runoff, and 6.9% cave recharge; simulation intensities were typically higher than natural event intensities. General water budgets across the Edwards Plateau have concluded that evapotranspiration represents 65% of total annual rainfall while percolation and storage accounts for 30% and the remaining 5% is runoff. These studies have been focused on broad water budget parameters while this study looks at more detailed components. No other study to date has been able to combine throughfall, stemflow, surface runoff, and vertical recharge monitoring to quantify the water budget in the Edwards Plateau; these parameters are instrumental in determining a detailed water budget in juniper rangelands. Results from this study illustrate the significance of all aspects of the water budget and are the first to yield a firm measurement of actual upland recharge.

Due largely to fire exclusion and land use changes, upland oak ecosystems in the central and eastern U.S. are shifting dominance from fire-tolerant oaks (Quercus spp.) to shade-tolerant, fire-sensitive species (mesophytes). This shift has been hypothesized to occur via a positive feedback loop termed mesophication, where mesophytes create shaded understory that limits oak growth and wetter fuels and soils, decreasing forest flammability. To determine how canopy water partitioning varies between oaks and mesophytes, I measured stemflow, throughfall, and surface soil moisture monthly over a 14-month period for overstory and midstory trees of oaks (Q. alba, Q. falcata) and hypothesized mesophytes (Carya tomentosa, Acer rubrum, Ulmus alata) in northern Mississippi. Overstory oaks partitioned 5.1% of rainwater into stemflow, while mesophytic species partitioned 7.2%, leading to 3.5% wetter soils under mesophytes. The hydrology of mesophyte canopies may reduce forest flammability and promote conditions favorable for mesophyte regeneration, ultimately compromising long-term oak regeneration.

Forests of the southeastern U.S. play an important role in meeting the increasing demand for forest products, and represent an important carbon (C) sink that can be managed as a potential tool for mitigating atmospheric CO2 concentrations and global climate change. However, realizing this potential depends on full accounting of the ecosystem carbon (C) budget. The separate evaluation of root-derived, autotrophic (RA) and microbially-derived heterotrophic (RH) soil respiration in response to management and climate change is important, as environmental and ecological factors often differentially affect these components, and RH can be weighed against net primary productivity (NPP) to estimate the C sink or source status of forest ecosystems. The objective of this research was to improve the quantitative and mechanistic understanding of soil respiratory fluxes in managed loblolly pine (Pinus taeda L.) plantations of the southeastern U.S. To achieve this overall objective, three studies were implemented to: 1) estimate the proportion and seasonality of RH:RS in four stand age classes, and identify relationships between RH:RS and stand characteristics 2) evaluate the effects of forest nutrient management and throughfall reduction on factors that influence RH and decomposition dynamics, including litter quality, microbial biomass, and enzyme activity and 3) evaluate the sensitivity of sources of RH (mineral soil-derived heterotrophic respiration; RHM, and leaf litter-derived heterotrophic respiration; RHL) to varying soil and litter water content over the course of a dry down event, and assess whether fertilization influences RH. Stand age and measurement season each had a significant effect on RH:RS (P < 0.001), but there were no interactive effects (P = 0.202). Mean RH:RS during the 12-month study declined with stand age, and were 0.82, 0.73, 0.59, and 0.50 for 3-year-old, 9-year-old, 18- year-old, and 25-year-old stands, respectively. Across all age classes, the winter season had the highest mean RH:RS of 0.85 while summer had the lowest of 0.55. Additionally, there were highly significant (P < 0.001) and strong (r > 0.5) correlations between RH:RS and peak LAI, stem volume, and understory biomass. Fertilization improved litter quality by significantly decreasing lignin:N and lignin:P ratios, caused a shift in extracellular enzyme activity from mineral soil N- and P-acquiring enzyme activity to litter C-acquiring enzyme activity, and increased microbial biomass pools. Throughfall reduction decreased litter quality by increasing lignin:N and lignin:P, but also increased C-acquiring enzyme activity. RHL was more sensitive to water content than RHM, and increased linearly with increasing litter water content (R2 = 0.89). The contribution of RHL to RH was greatest immediately following the wetting event, and decreased rapidly to near-zero between three – 10 days. RHM also had a strong relationship with soil water content (R2 = 0.62), but took between 200 – 233 days to attain near-zero RHM rates. Fertilization had no effect on RHM (P = 0.657), but significantly suppressed RHL rates after the wetting event (P < 0.009). This research provides estimates of RH:RS in managed loblolly pine systems that can be used to improve regional ecosystem C modeling efforts, and demonstrates the need to consider the impact of stand age and seasonal patterns to identify the point at which plantations switch from functioning as C sources to C sinks. Additionally, it demonstrates that the controls over RH are dynamic and influenced in the short-term by fertilization and changed precipitation regimes, with the greatest impact on properties affecting litter RH compared to mineral soil. Future research should work to improve the mechanistic understanding of the seasonal and spatial variability of RH and related controlling biotic and abiotic parameters to remedy the variability in existing RS and ecosystem C models. Understanding how management and climate change may impact factors that control RH will ultimately improve our understanding of what drives changes in forest C fluxes. / Ph. D.

Rainfall interception is a primary control over the moisture input to a forested ecosystem through the partitioning of precipitation into throughfall, stemflow, and an evaporated component (i.e. the interception loss). Rainfall interception is a spatially and temporally varying process at multiple scales, but heterogeneity in interception processes are poorly understood and poorly described in the literature. We need to know how net precipitation varies in ecosystems because natural systems are driven by non-linear ecohydrological processes where mean values cannot capture localized effects or the cumulative consequences associated with an extremely heterogeneous input. In this thesis, we present two studies that investigate the heterogeneity of interception loss and throughfall in a forested catchment in the western Cascades range of Oregon. In one study, we examined the spatio-temporal patterns among point measurements of throughfall depth and isotopic composition to determine the cause of isotopic differences between throughfall and rainfall. Our results indicated that the residual moisture retained on the canopy from previous events plays a major role in determining the isotopic composition of the next event's throughfall. Differences between the isotopic composition of throughfall samples could indicate further partitioning of throughfall into various flow-paths from the canopy. The second project examined the question of how vegetation variability and terrain complexity drive interception loss heterogeneity at the whole-catchment scale. We applied a simple interception model to a watershed gridded at a 50 m resolution to investigate the relative importance of topographic and vegetative controls over the spatial variability of interception loss. We found that storm characteristics are crucial regarding the impact of spatial heterogeneities in vegetation and evaporation rates. In the Pacific Northwest climate, interception loss is not highly variable for the majority of the year because the annual precipitation is dominated by large storms with low interception losses. However, the net precipitation input to a watershed becomes extremely heterogeneous in the summer due to high interception loss variability. Summer interception loss could be an important control over the spatial variability of the availability of moisture, coinciding with when vegetation is most water-limited. / Graduation date: 2013

Motivations and research objectives: During the passage of rain water through a forest canopy two main processes take place. First, water is redistributed; and second, its chemical properties change substantially. The rain water redistribution and the brief contact with plant surfaces results in a large variability of both throughfall and its chemical composition. Since throughfall and its chemistry influence a range of physical, chemical and biological processes at or below the forest floor the understanding of throughfall variability and the prediction of throughfall patterns potentially improves the understanding of near-surface processes in forest ecosystems. This thesis comprises three main research objectives. The first objective is to determine the variability of throughfall and its chemistry, and to investigate some of the controlling factors. Second, I explored throughfall spatial patterns. Finally, I attempted to assess the temporal persistence of throughfall and its chemical composition. Research sites and methods: The thesis is based on investigations in a tropical montane rain forest in Ecuador, and lowland rain forest ecosystems in Brazil and Panama. The first two studies investigate both throughfall and throughfall chemistry following a deterministic approach. The third study investigates throughfall patterns with geostatistical methods, and hence, relies on a stochastic approach. Results and Conclusions: Throughfall is highly variable. The variability of throughfall in tropical forests seems to exceed that of many temperate forests. These differences, however, do not solely reflect ecosystem-inherent characteristics, more likely they also mirror management practices. Apart from biotic factors that influence throughfall variability, rainfall magnitude is an important control. Throughfall solute concentrations and solute deposition are even more variable than throughfall. In contrast to throughfall volumes, the variability of solute deposition shows no clear differences between tropical and temperate forests, hence, biodiversity is not a strong predictor of solute deposition heterogeneity. Many other factors control solute deposition patterns, for instance, solute concentration in rainfall and antecedent dry period. The temporal variability of the latter factors partly accounts for the low temporal persistence of solute deposition. In contrast, measurements of throughfall volume are quite stable over time. Results from the Panamanian research site indicate that wet and dry areas outlast consecutive wet seasons. At this research site, throughfall exhibited only weak or pure nugget autocorrelation structures over the studies lag distances. A close look at the geostatistical tools at hand provided evidence that throughfall datasets, in particular those of large events, require robust variogram estimation if one wants to avoid outlier removal. This finding is important because all geostatistical throughfall studies that have been published so far analyzed their data using the classical, non-robust variogram estimator. / Motivation und Zielsetzung: Wenn Regen durch ein Kronendach fällt lassen sich zwei Prozesse beobachten: das Regenwasser wird umverteilt und die chemische Qualität des Wassers verändert sich erheblich. Die Prozesse im Kronenraum resultieren in einer hohen Variabilität des Bestandsniederschlags und dessen chemischer Zusammensetzung. Bestandsniederschlag beeinflusst eine Reihe von physikalischen, chemischen und biologischen Prozessen am Waldboden. Daher können Untersuchungen zur Variabilität und zu Mustern im Bestandsniederschlag helfen, bodennahe Prozesse besser zu verstehen. Diese Dissertation behandelt hauptsächlich drei Aspekte. Erstens, die Arbeit beschäftigt sich mit der Erfassung der Variabilität im Bestandsniederschlag und dessen chemischer Zusammensetzung, zudem werden Einflussfaktoren dieser Variabilität untersucht. Des Weiteren beschäftigt sich die Arbeit mit räumlichen Mustern des Bestandsniederschlagswassers, und drittens wird die zeitliche Stabilität des Bestandsniederschlags und dessen chemischer Zusammensetzung betrachtet. Untersuchungsgebiete und Methoden: Diese Dissertation basiert auf Untersuchungen in einem tropischen Bergregenwald in Ecuador, sowie Studien in tropischen Tieflandregenwäldern in Brasilien und Panama. Die ersten zwei Studien untersuchen Bestandsniederschlag und dessen chemische Zusammensetzung mit Hilfe deterministischer Methoden. Die Arbeit in Panama nutzt geostatistische Methoden zur Beschreibung von Bestandsniederschlagsmustern und verfolgt somit einen stochastischen Ansatz. Ergebnisse und Schlussfolgerungen: Die Variabilität des Bestandsniederschlages ist hoch; das heißt, die Menge des auf den Waldboden tropfenden Wassers kann sich je nach Standort stark unterscheiden. Diese räumliche Variabilität des Bestandsniederschlags ist in tropischen Wäldern höher als in vielen gemäßigten Waldökosystemen, was nicht allein auf verschiedenen Eigenschaften der Ökosysteme zurückzuführen ist. Vielmehr erklären sich die Unterschiede auch aus verschiedenen Waldnutzungen. Abgesehen von biologischen Faktoren beeinflusst die Regenmenge die Variabilität des Bestandsniederschlags erheblich. Die chemische Zusammensetzung des Bestandsniederschlags weist eine noch höhere Variabilität als der Bestandsniederschlag selbst auf. Unterschiede zwischen tropischen und gemäßigten Wäldern lassen sich hier allerdings nicht erkennen, weshalb die hohe Diversität tropischer Ökosysteme die Heterogenität der chemischen Zusammensetzung des Bestandsniederschlags nicht ausreichend erklärt. Eine Vielzahl anderer Faktoren kontrolliert deshalb die Variabilität der Bestandsniederschlagschemie, beispielsweise die Konzentration gelöster Stoffe im Regenwasser oder die Dauer von Trockenperioden. Deren hohe temporale Variabilität ist verantwortlich für die geringe zeitliche Stabilität von Depositionsmessungen. Im Gegensatz dazu ist die temporale Persistenz von Messungen der Bestandsniederschlagsmenge hoch. Insbesondere die Ergebnisse aus Panama zeigen, dass feuchte und trockene Messpunkte über einen Zeitraum von zwei Regenzeiten fortbestehen. Die räumlichen Bestandsniederschlagsmuster im letztgenannten Untersuchungsgebiet sind schwach bzw. weisen die Struktur eines reinen Nugget-Models auf. Die geostatistische Analyse zeigt, dass vor allem die Daten großer Regenereignisse eine robuste Modellierung des Variogramms erfordern, wenn die willkürliche Entfernung von Fernpunkten in den Daten vermieden werden soll. Dieses Resultat ist insbesondere deshalb von Bedeutung, da alle bisherigen Bestandsniederschlagsstudien den klassischen, nicht-robusten Schätzer benutzen, obwohl das Auftreten von Extremwerten in Bestandsniederschlagsdaten für viele Ökosysteme zu erwarten ist.

Oaks (Quercus spp.) are a dominant genus in forests across the United States that have been declining due to fire suppression and forest mesophication. The reduction of these species may alter forest hydrologic and biogeochemical cycling. Canopy-derived nutrients and interspecific temporal distribution of leaves were quantified under oak and hickory (Carya spp.) species in Mississippi during 2014-2016. Throughfall quantity and chemistry were measured during every storm event under oak and hickory species. Interspecific leaf litter was collected weekly to quantify the timing of leaf fall and leaf litter nutrient content. Throughfall volume and solute fluxes were impacted by seasonality. Mg2+ and DOC were greater in throughfall than precipitation. Leaf loss was slower in oak species during leaf fall. Slower decay in oak litter may correlate with higher C/N ratios compared to hickory species. Results of this study indicate oak species are an important contributor to forest hydrology and nutrient cycling.