The processes that regulate nutrient access by forest trees exhibit substantial variability on both large and small spatial scales. Explicit study of this spatial variability promotes a better understanding of the structure and function of forests. While the importance of space in ecological processes is being increasingly appreciated, there are major gaps in our knowledge about how space influences plant nutrient supply, particularly within a forest stand. This dissertation consists of three chapters that examine the patterns, drivers, and implications of spatial variation in three main processes that make nutrients available to trees: throughfall nutrient deposition, soil nutrient mineralization, and root system development. In Chapter 1, I use data from a field experiment to examine the effect of fertilization on nutrient transfer from the canopy to the soil via throughfall and litterfall in a tropical rainforest. I demonstrate that at small spatial scales, canopy density controls the flux of nutrients in throughfall, while at large scales, soil fertility is an important control, especially for phosphorus. I also show throughfall can be as important as litterfall in the recycling of certain essential nutrients like potassium, and depending on soil fertility, phosphorus. In Chapter 2, I investigate the small scale spatial patterning in soil nitrogen, a nutrient that frequently limits tree growth, in a temperate forest. By quantifying the degree of spatial inequality and autocorrelation in two plots characterized by different dominant tree species, I show that soil extractable nitrogen pools and net nitrogen mineralization fluxes exhibit a high degree of spatial patterning at scales less than 5 meters, with a majority of nitrogen availability contained within hotspots comprising a small proportion of soil area. I also demonstrate that this spatial patterning affects seedling access to soil nitrogen, which has consequences for seedling growth and survival. Chapter 3 examines how tree species and tree size affect the spatial distributions of root systems in two temperate tree species and explores how differences in root spatial coverage could affect tree nutrient access. I find that the spatial distributions of tree root systems can exhibit dramatic differences between species, with a tradeoff between root spatial coverage and total root length. I also discover that the effect of root spatial coverage on soil nutrient access is highly dependent on the spatial patterning of the soil nutrient, such that tree access to patchy nutrients varies greatly based on tree location within the local soil environment, even for medium-size trees. Together, these chapters characterize important patterns and mechanisms of spatial variation in the processes that regulate tree nutrient access.

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Understanding of the hydrological cycle and processes such as interception span as far back as the times of the Renaissance, when Leonardo da Vinci (1452-1519) first described it. However, there remains a gap in the knowledge of both canopy and litter interception in South African forest hydrology. Interception is typically considered to constitute only a small portion of total evaporation and in some models is disregarded or merely lumped with total evaporation, and not considered as a separate process. Interception is a threshold process, as a certain amount of water is required before successive processes such as infiltration and runoff can take place. Therefore an error introduced in modelling interception, especially disregarding it, will automatically introduce errors in the calibration of subsequent models/processes. In this study, field experiments to assess these two poorly understood hydrological processes, viz. canopy and litter interception were established for the three main commercial forestry genera in South Africa, namely, Pinus, Acacia and Eucalyptus as well as an indigenous Podocarpus henkelii stand, thus, accounting for interception of “broad leaf”, “compound leaf” and “needle leaf” trees in order to provide further insight into these processes. The study took place at two locations in the KwaZulu-Natal Midlands over a period of three years. The first site is the Two Streams catchment, located in the Seven Oaks area, about 70km north-east of Pietermaritzburg where the study on the commercial plantation species took place. The second site was the Podocarpus henkelii stand in Karkloof near Howick, 40km north of Pietermaritzburg. From the field data collected (cf. Chapter 2) it was observed that canopy storage capacity, an important parameter governing interception, was not constant and changed with rainfall intensity, with lower intensity events resulting in a higher storage capacity. Building on these findings, a physically based canopy interception model that is based on the well known Gash model was developed, and is referred to herein as the “variable storage Gash model”. While canopy interception is dependent on many factors including the storage capacity, potential evaporation, rainfall intensity and rainfall duration, the litter interception is largely dependent on the storage capacity due to the evaporative drivers under the canopy such as radiation, temperature and wind speed being moderated by the above canopy. From these finding, a litter interception model based on idealised drying curves from litter samples collected at the study sites was also developed (cf. Chapter 3). From the field data, it was found that the canopy interception for Eucalyptus grandis, Acacia mearnsii and Pinus patula was 14.9, 27.7 and 21.4% of mean annual precipitation (MAP) respectively. The simulated canopy interception using the “variable storage Gash model” was 16.9%, 26.6% and 23.3% for E. grandis, A. mearnsii and P. patula respectively. The litter interception measured for E. grandis, A. mearnsii and P. patula was found to be 8.5, 6.6 and 12.1% of MAP respectively, while the simulated litter interception using the idealised drying curve model corresponded well with the measured results and were 10.1%, 5.4% and 13.4% for E. grandis, A. mearnsii and P. patula respectively. The idealised drying curve model is site and species specific and is therefore not transferable to other locations. Conversely, the “variable storage Gash model” is transferable as it is not site and species specific, and relies on readily measureable and available information. Building on field studies, this was then used to simulate the canopy interception for Eucalyptus, Acacia mearnsii and Pinus in South Africa (including Lesotho and Swaziland) for all quinary catchments in which commercial forestry could be grown, i.e. a mean annual precipitation of greater than 600 mm.year-1 (cf. Chapter 4). It was found that, depending on the location and genus, canopy interception loss can be as high as 100 to 300 mm per year or approximately 10% to 40% of MAP. This relates to a mean interception loss of between 1.0 and 3.0 mm per rainday, highlighting the spatial variability of canopy interception. To further investigate the spatial variability of canopy interception, at various spatial scales, remote sensing technology was applied to estimate leaf area index (LAI) for use in modelling/estimating canopy storage capacity and canopy interception (cf. Chapter 6). The NDVI, SAVI and Vogelmann 1 vegetation indices were used in the estimation of the LAI. It was found the Vogelmann 1 index produced the best results. As models to estimate canopy interception typically require LAI and storage capacity, it was calculated that the ability to estimate these parameters over large areas is valuable for water resources managers and planners. An often neglected consideration of canopy and litter interception is its role in determining the water use efficiency (WUE) of a forest stand (cf. Chapter 5). This component of the study was undertaken in an indigenous Podocarpus henkelii stand as well as a commercial Pinus patula stand in Karkloof in the KwaZulu-Natal Midlands. The sap flow (transpiration) was measured in both the P. henkelii and P. patula stands using the using the Heat Pulse Velocity (HPV) technique in order to determine the productive green water use. The canopy and litter interception was measured in the P. henkelii site, but was modelled in the P. patula site using the “variable storage Gash” and idealised drying curve models, in order to estimate the non-productive green water use. It was found that the canopy and litter interception for P. henkelii was 29.8% and 6.2% respectively, while the modelled canopy and litter interception for P. patula was 22.1% and 10.7% respectively. If only the productive green water use (transpiration) is considered, then the water use efficiency of P. henkelii and P. patula was found to be 7.14 g.mm-1 and 25.21 g.mm-1 respectively. However, from a water management perspective it is important to consider the total green water use efficiency (transpiration + interception), which reveals a significantly lower water use efficiency of 3.8 g.mm-1 and 18.8 g.mm-1 for P. henkelii and P. patula respectively. To extend the study to a globally relavent issue, the possible impact of climate change on canopy interception was investigated, as forests growth is critically linked to climate (cf. Chapter 7). To achieve this, the CABALA model was used to model LAI and transpiration of Eucalyptus grandis and Pinus patula under 9 different climate change scenarios, including changes in temperature, rainfall and atmospheric CO2. The simulated LAI values from the CABALA model for all 9 climate scenarios were then used to simulate canopy interception using the “variable storage Gash model”. Results show that LAI may increase by as much as 24% and transpiration may decrease by as much as 13%, depending on the scenario, location and tree species. However, it was found that canopy interception does not change greatly, leading to the conclusion that under climate change conditions, canopy interception may not become a more dominant component of the hydrological cycle than it currently is as the changes under climate change are likely to be less than the natural variability from year to year. However, canopy interception remains an important consideration for water resources management and planning both currently and in the future. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

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Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 204-236). Also available on the World Wide Web.

I hypothesized that Douglas-fir trees (Pseudotsuga menziesii) standing apart from other trees ('open-grown') will intercept more rainfall than Douglas-fir trees standing near other trees ('closed-canopy'). Open-grown trees differ structurally and are more common in urban settings, yet have been infrequently studied. Existing literature, based primarily on closed-canopy trees, suggests Douglas-fir trees in Pacific Northwest forests intercept approximately 25% of rainfall annually. Because open-grown trees have more vertical leaf area than individual trees in closed-canopy forests, I expected to find higher interception by open-grown trees. I collected throughfall under four open-grown Douglas-firs using six static collectors ('buckets') per tree, and two closed-canopy Douglas-firs using six buckets per tree. I compared their throughfall to the incident rainfall in two adjacent open-field buckets. Gross interception was measured in 53 collections during rainy weather from 16Nov07 to 31Mar08. Over the same period, rainfall per hour, wind speed, gust speed, wind direction, temperature and relative humidity were collected at a weather station located within 1 km of the site. For comparison, average hourly rainfall at Portland International Airport from 1950 to 2005, for the same months of the collection period, showed a comparable number of medium- to high-intensity storms, but more low-intensity storms. I found that incident rainfall for the adjacent open-field buckets totaled 65.6cm and 71.6cm over the study period. Interception values for closed-canopy trees averaged 26%, corresponding to the literature, with results of 22 and 30%. Interception values for open-grown trees averaged 31%, with results ranging from 15 to 45%. Three of the 24 buckets returned overall negative interception rates over five months. Given the lower storm intensity of 2007-08, interception rates may be somewhat high, compared to the historical average. The negative interception rates at three buckets were likely due to their locations under high drip points, as has been observed in other studies. Considering the wide range of canopy architecture among open-grown trees, the high variability in interception was not surprising. My hypothesis was supported by the data, but requires more testing to better generalize these results. Future studies that link open grown tree canopy morphological characteristics to interception are warranted.

Discrimination of different tree crowns based on their 3D shapes is essential for a wide range of forestry applications, and, due to its complexity, is a significant challenge. This study presents a modified 3D shape descriptor for the perception of different tree crown shapes in discrete-return LiDAR point clouds. The proposed methodology comprises of five main components, including definition of a local coordinate system, learning salient points, generation of simulated LiDAR point clouds with geometrical shapes, shape signature generation (from simulated LiDAR points as reference shape signature and actual LiDAR point clouds as evaluated shape signature), and finally, similarity assessment of shape signatures in order to extract the shape of a real tree. The first component represents a proposed strategy to define a local coordinate system relating to each tree to normalize 3D point clouds. In the second component, a learning approach is used to categorize all 3D point clouds into two ranks to identify interesting or salient points on each tree. The third component discusses generation of simulated LiDAR point clouds for two geometrical shapes, including a hemisphere and a half-ellipsoid. Then, the operator extracts 3D LiDAR point clouds of actual trees, either deciduous or evergreen. In the fourth component, a longitude-latitude transformation is applied to simulated and actual LiDAR point clouds to generate 3D shape signatures of tree crowns. A critical step is transformation of LiDAR points from their exact positions to their longitude and latitude positions using the longitude-latitude transformation, which is different from the geographic longitude and latitude coordinates, and labeled by their pre-assigned ranks. Then, natural neighbor interpolation converts the point maps to raster datasets. The generated shape signatures from simulated and actual LiDAR points are called reference and evaluated shape signatures, respectively. Lastly, the fifth component determines the similarity between evaluated and reference shape signatures to extract the shape of each examined tree. The entire process is automated by ArcGIS toolboxes through Python programming for further evaluation using more tree crowns in different study areas. Results from LiDAR points captured for 43 trees in the City of Surrey, British Columbia (Canada) suggest that the modified shape descriptor is a promising method for separating different shapes of tree crowns using LiDAR point cloud data. Experimental results also indicate that the modified longitude-latitude shape descriptor fulfills all desired properties of a suitable shape descriptor proposed in computer science along with leaf-off, leaf-on invariance, which makes this process autonomous from the acquisition date of LiDAR data. In summary, the modified longitude-latitude shape descriptor is a promising method for discriminating different shapes of tree crowns using LiDAR point cloud data.

Globally, more than half of the world's population is living in urban areas and it is well accepted that human activities (e.g. climate warming, pollution, landscape homogenization) pose a multitude of threats to ecosystems. Largely, human-related impacts on biodiversity will hold consequences for larger ecological processes and research looking into human impacts on sensitive epiphytic lichen and moss communities is an emerging area of research. While seemingly small, lichen and moss communities exist on nearly every terrestrial ecosystem on Earth and contribute to whole-system processes (e.g. hydrology, mineral cycling, food web energetics) worldwide. To further examine human impacts on epiphytic communities, I conducted three studies examining urbanization and climate warming effects on epiphytic lichen and moss biodiversity and ecology. In the first study I revisited a historic urban lichen community study to assess how urban lichen communities have responded to regional air quality changes occurring over the last nearly two decades. I further investigated, for the first time, the biodiversity of urban tree canopy-dwelling lichen communities in a native coniferous tree species, Pseudotsuga menziesii. I found that urban parks and forested areas harbor a species rich community of lichens epiphytes. Further, I found evidence for the distinct homogenization of urban epiphytic lichen communities, suggesting that expanding beyond simplistic measures of biodiversity to consider community composition and functional biodiversity may be necessary when assessing the ecology and potential ecosystem services of epiphyte communites within urbanizing landscapes. Next, I present the first tall tree canopy study across a regional gradient of urbanization near Portland, Oregon, USA. I found that tall tree canopy epiphyte communities change dramatically along gradients of increasing urbanization, most notably by the transitioning of species functional groups from sensitive, oligotrophic species to a dominance of urban-tolerant, eutrophic species. The implications these dramatic shifts in species composition have on essential PNW ecosystem processes, like N-fixation and canopy microclimate regulation, is still not well understood and is difficult to formally evaluate. However, I find strong evidence that native conifer trees in urban areas may provide a diversity of essential ecosystem services, including providing stratified habitat for epiphyte communities and their associated micro arthropod communities and the scavenging of atmospherically deposited nutrients. Future work is needed to understand how losses in canopy N fixation and species with large biomass (both lichens and bryophytes) will affect nutrient and hydrologic cycling in the PNW region, which continue to undergo rapid growth and urbanization. The final chapter investigates the impacts of passive warming by Open Top Chambers (OTCs) in moss-dominated ecosystems located on the Western Antarctic Peninsula, an area of increasing climate warming. I compared species-specific temperature effects, moss canopy morphology, sexual reproductive effort and invertebrate communities between OTC and control moss communities for two moss species, Polytrichastrum alpinum and Sanionia uncinata, that make up over 65% of the terrestrial vegetative cover in the area. I found distinct reproductive shifts in P. alpinum under passive warming compared to controls. Moss communities under warming also had substantially larger total invertebrate communities than those in control moss communities, and invertebrate communities were significantly affected by moss species and moss reproductive effort. Further, substantial species-specific thermal differences among contiguous patches of these dominant moss species were revealed. These results suggest that continued warming will differentially impact the reproductive output of Antarctic moss species and is likely to dramatically alter terrestrial ecosystems dynamics from the bottom up. This combined work provides a diverse contribution to the field of epiphyte ecology and biology by providing new insights on how human impacts will affect epiphyte lichen and moss communities across diverse ecosystems, in light of a rapidly changing planet.