<p>  </p> <p><em>Santalum</em> spp., known globally as sandalwood, are highly sought after for their aromatic oil-rich heartwood and have been exploited throughout their range. Six of the 19 <em>Santalum</em> species are endemic to the main Hawaiian Islands, where they are known locally as ʻiliahi. Excessive harvesting led to the extirpation of Hawaiian sandalwood from 90% of its historic range by 1840. There is limited peer-reviewed literature concerning the propagation of Hawaiian sandalwood, and methods developed for other non-Hawaiian species cannot be directly adopted due to differences in sandalwood species physiology, available host species, and growing environment. This, combined with increased interest in growing Hawaiian sandalwood, prompts the need for the development of propagation protocols based on empirical research. The primary knowledge gaps in propagation include best practices for producing high-quality seedlings in the nursery (e.g., fertilizers and hosts) and silvicultural practices for maximizing outplanting survival and growth. The Hawaii Island endemic <em>Santalum paniculatum</em> has the largest remnant population and distribution and high commercial value, making it an ideal species to focus our study on. We conducted two experiments to evaluate the response of <em>S. paniculatum</em> seedlings to propagation methods employed with Australian and Indian sandalwood, although with species of hosts native to Hawaii. The first experiment was a nursery growth trial that evaluated the quality of <em>S. paniculatum </em>seedlings in response to nutrient availability (controlled-release fertilization, control), chelated iron fertilizer (applied, control), and species of pot host (<em>Acacia koa</em>, <em>Dodonaea viscosa</em>, control). The quality of seedlings was determined by measurements of height, root collar diameter, dry mass, root shoot ratio, chlorophyll index, and nutrient status (N & Fe concentration). Nutrient availability had the greatest impact on seedling quality and increased height, root collar diameter, dry mass, chlorophyll index, and nutrient status. Chelated iron fertilizer effectively improved seedling quality (height, collar, dry mass, chlorophyll content, and Fe concentration) in a nutrient-limiting environment, although a nutrient-rich environment diminished its effect with sufficient iron levels. The host species treatment had the least influence on seedling quality and only influenced haustoria formation by causing more haustoria in <em>A. koa</em>-paired compared to <em>D. viscosa</em>-paired and control seedlings. Although the pot host had the lowest effect on seedling quality during nursery propagation, it provided benefits in the field planting phase of the project.</p> <p>The second experiment of the project assessed the survival and performance of field-planted <em>S. paniculatum</em> in response to (1) nursery fertilization, (2) an <em>A. koa</em> pot host, and (3) an <em>A. koa </em>field host. Nursery fertilization had the greatest effect on performance and enhanced survival, height, height growth, collar, collar growth. The survival rate of unfertilized seedlings was 43.3% (± 5.9) compared to 86.9% (± 4.2) for fertilized seedlings. The pot host improved height, height growth, collar, collar growth of seedlings, but it did not influence survival. The intermediate field host significantly improved survival from 52.7% (± 7.8) to 78.0% (± 5.6) and only affected the height measurements and not the collar. There was a significant interaction between the field host and nursery fertilizer treatment associated with the fertilized seedlings planted with field hosts having lower water potential than the fertilized seedling planted without a field host. Our results demonstrated that supplemental nursery nutrition, pot hosts, and intermediate <em>A. koa </em>field hosts benefited <em>S. paniculatum</em> regeneration establishment in different ways. Furthermore, the effect of the hosting treatments may become more pronounced in the field over time as more haustoria connections are formed. This research project provides essential baseline information that helps to enhance the current methodology and inform future decision-making concerning the propagation of <em>S. paniculatum </em>and other Hawaiian <em>Santalum</em> species. </p>

<p dir="ltr">Deciduous leaf habits have evolved multiple times across many lineages in response to stresses like drought, cold, or darkness. This short, seasonal leaf lifespan allows trees to invest in photosynthesis during prime conditions and retreat to dormancy to survive less favorable conditions. The consequence of short leaf lifespan is that trees must perform an entire year's carbon capture into 6-8 months. This leads to leaves that are cheaper to produce than longer lived evergreen counterparts. As soon as challenging conditions have passed the leaves of deciduous trees expand rapidly; and this expansion has huge impacts on local ecosystems. Other plants like spring ephemerals have evolved to complete the majority of their life cycle before the upper canopy closes off. During the summer, deciduous leaves gather huge amounts of carbon for the trees to survive their dormancy. Finally, as the trees prepare to enter dormancy, nutrients are withdrawn from leaves as the chlorophyll is metabolized, causing them to transition from bright green to shades of red and yellow. In addition to other plants, people find the annual process of renewal on bud burst and tragic decline during senescence fascinating and culturally important. The aim of my thesis is to expand our understanding of winter deciduous leaves through every major stage of development, as well as to investigate how this process may shift due to climate change.</p>

<p dir="ltr">For several decades, the ecological dominance of white oak (<i>Quercus alba</i>) has been declining throughout the species’ native range in eastern North America with failure to recruit new individuals into the overstory. White oak’s decline is concerning as the species is of great cultural, ecological, and economic value. Planting artificial regeneration is one approach to bolstering flagging natural white oak regeneration insufficient in vigor or quantity to supplant mature canopy white oak. Shelterwood harvests and artificial regeneration alone or in combination are frequently suggested to be an effective means of securing sufficient white oak regeneration in central hardwood understories. Because there is a much more comprehensive body of work examining northern red oak (<i>Quercus rubra</i>) than white oak artificial regeneration, managerial prescriptions for artificial regeneration of white oak are commonly generalized from northern red oak prescriptions. If the two species are silvically different, however, they should be managed differently to achieve maximum effectiveness of regenerative prescriptions.</p><p><br></p><p dir="ltr">I conducted both a silvicultural field trial and a more controlled shade and competition study to examine artificial white oak regeneration responses to light and competition gradients. In the silvicultural field trial, I tested the impacts of varied lengths of competition control, geographical seed source, and canopy cover on growth and survival of artificial white oak regeneration within an expanding shelterwood system. After three growing seasons, my results indicated that seedlings grow and survive at the greatest rates in areas of up to approximately 50% canopy closure, or conditions found in harvest gaps.</p><p dir="ltr">In a shade and competition study, I compared artificial northern red oak and white oak growth, morphology, and physiology responses to three light levels (10% or low, 30% or medium, and full sun or high) under the presence or absence of an invasive competitor (Amur honeysuckle (<i>Lonicera maackii</i>)). After two years, my results indicated that medium light levels resulted in the greatest height and diameter growth as well as the greatest nonstructural carbohydrate amounts in both root and shoot organs of both species. Interestingly, my physiology results indicated that northern red oak seedlings displayed lower light compensation points and greater quantum yields than white oak seedlings. These traits potentially indicate greater shade tolerance of northern red oak than white oak. Further, white oak foliar nitrogen in shaded treatments, quantum yield, and light compensation points were impacted more severely by competition than equivalent northern red oak measures, indicating that white oak seedlings may not be as well equipped to handle invasive competition pressures. These results indicate that these two upland oak species are fundamentally different, and these differences should be considered when writing management prescriptions.</p>

<div>Anthropogenically driven global change is disrupting ecosystems and habitats of many plant species, straining the ability of native species to survive and reproduce. The overarching goal of this research was to holistically work towards restoration of a threatened tree species by connecting research from different disciplines. In order to do so, the threatened butternut tree (<i>Juglans cinerea</i>) and its hybrids were used as a case study. Hybridization can incorporate stress tolerance in plants and could be a potential restoration tool. Evidence in some wild butternut populations indicates that naturalized hybrids of butternut with Japanese walnut (<i>Juglans ailantifolia</i>) may be more tolerant to butternut canker disease (BCD) than butternut, but this has not been formally tested. Thus, chapter 2 examined potential BCD tolerance within and between unadmixed and hybrid butternut inoculated with two BCD fungal isolates. Differences in canker growth were observed by fungal isolate, which could help to explain some differences in BCD severity found among butternut populations. Smaller and fewer cankers and greater genetic gains were detected in hybrid families, demonstrating that hybrids warrant further evaluation as a possible breeding tool for developing BCD-resistant butternut trees.</div><div>However, even with increased disease tolerance, hybrids must possess similar ecophysiological tolerances to their native progenitor to be an effective replacement. Butternut is extremely cold hardy, but Japanese walnuts are native to a warmer ecosystem, indicating potential disparities in extreme temperature tolerances between the two species and their hybrids. Thus, samples from mature trees were subjected to cold and heat treatments to compare relative extreme temperature tolerances within butternut and between butternut, Japanese walnut, and their hybrids. Within butternut, trees from colder areas exhibited less cold damage than those from warmer areas. Differences in heat damage among provenances occurred but did not follow a clear trend. Butternut exhibited greatest cold tolerance, Japanese walnut exhibited greatest heat tolerance, and hybrids were intermediate. Thus, the utility of hybrids for restoration could be limited at the extremes of the species’ distributions.</div><div>A second, but different type of freeze test was conducted for chapter 4 using seedlings to gain a more nuanced understanding of cold tolerance within butternut and between butternut and its hybrids. No survival or damage differences were detected in butternut provenances, although seedlings from the coldest provenances experienced more delayed budbreak at the two warmest treatments than those from warmer provenances. Interspecific differences were not observed in dieback but were detected in survival and budbreak. The hybrids had greater survival than butternut from warmer provenances at the lowest temperature treatment (-38 °C), but given that temperatures that low are extremely unlikely to occur in those provenances, it is not anticipated to give the hybrids an advantage if planted in those areas. However, the hybrids’ earlier budbreak could limit the success of restoration with these hybrids in the coldest extents of butternut’s range. </div><div>If hybrids, as well as genetically modified (GM) trees, are successfully developed for effective disease tolerance and to serve as an ecologically suitable replacement, success of restoration using hybrids will ultimately depend on those directly responsible for replanting efforts. A survey was administered to land managers in 46 organizations in Indiana to gauge perceptions of hybrid and GM trees, as well as current use of hybrid trees. Land managers had stronger concern for ecological, rather than economic, issues. Agreement was highest for using hybrid and GM trees for “conservation and restoration of at-risk species”, “timber production”, and “non-timber products (fruit, syrup, etc.)”. However, perceptions varied by characteristics, such as concern type, age, and the type of land they managed. Ecological concern and the type of land being managed most strongly predicted current hybrid use. Overall, results indicate the majority of land managers in Indiana would likely be agreeable to recommendations towards using hybrids. However, most nonetheless had strong ecological concerns about their suitability as a native replacement. It is important to note, though, that consistent with the results of previous studies, great variation was seen within the performance and characteristics of the butternut hybrids in chapters 2-4. Thus, it may be possible with careful selection and breeding to harness this variation to develop disease tolerant and ecologically similar hybrids acceptable to land managers.</div>