Diss. (sammanfattning). Umeå : Sveriges lantbruksuniv. / Härtill 4 uppsatser.

Eighteen stands of bur oak (Quercus macrocarpa Michx.) and trembling aspen (Populus tremuloides Michx.) were sampled and analyzed using dendrochronological methods to study the potential effects on tree growth of emissions from a 132 MW coal-fired generating station. Sixteen stands were sampled within a 16-km radius of the station, and two control stands were sampled outside of the range of influence, at distances . 40 km. All stands showed similar radial growth patterns from 1960-2001, regardless of distance from or direction relative to the generating station, and a number of stands, including the controls, had below average growth after 1970. Both species were significantly affected by climatic factors, showing decreased radial growth with increasing June temperature. The species differed in their growth responses to spring precipitation and temperature in the previous October. One bur oak site displayed marked radial growth decline beginning in the mid-1970s, strongly pronounced following 1977. This decline does not appear to be related to emissions from the station, but is suspected to be a result of poor site conditions (shallow soil developed over calcareous till), confounded by a change in drainage (a road was built adjacent to the stand in 1977, perpendicular to the direction of drainage). The below average growth seen in 1970-2001 across most stands is likely attributable to stand dynamics and age effects.

This dissertation explores the differential effects of an ungulate community (cattle, mule deer, and American elk) on aspen (Populus tremuloides) regeneration pre- and post-fire disturbance. It's first chapter examines the differential effects of cattle, mule deer, and American elk on aspen regeneration across variable topography, climate and forest type; aspen dominant to conifer dominant subalpine forests. We found that each ungulate species, if sufficiently numerous, can cause aspen regeneration failure. Also, high elevation, southern aspect and high winter snow pack increase resilience to ungulate herbivory. Chapter two compares the efficacy of four methods of estimating ungulate impact on aspen. We found that direct methods of estimation (meristem removal, and defoliation) are better indicators of ungulate impact than indirect methods (fecal and camera counts). Therefore, we suggest to management that removal of apical meristems be an indicator for relative ungulate use. In chapters 3 and 4 we use differential ungulate exclosure fencing similar and camera traps to model the year to year and monthly use of aspen by ungulates. We model the per unit animal impact on aspen using photo counts. Our results show that ungulates on a per animal basis utilize aspen similarly. However, when adjusted for body size, native ungulates select aspen more often than cattle. Also, we found that while aspen is more palatable in June it is relative use, as measured by removal of apical meristems is significantly higher throughout July and August. We conclude that aspen is relatively more palatable than grass and forb species late season, and becomes and important forage species late season when forbs and grass species nutrient content drops. In chapter five, we quantify the physiological effects of ungulates on aspen regeneration after fire in order to elucidate the physiological mechanisms underlying plant:animal interactions. We show that ungulate herbivory induces high concentrations of defense chemistry (Phenolics), and reduces palatability through lower foliar nonstructural carbohydrates, and that this high investment in defense and lower growth potential related to low palatability causes severe reductions in aspen vertical growth leading to aspen regeneration failure. Together, we examine how biotic and abiotic factors at the landscape level, as well as defense and physiological function at the leaf chemistry level mediate how ungulates influence aspen regeneration and recruitment

Quaking aspen (Populus tremuloides) has the largest natural distribution of any tree native to North America, ranging from Alaska through the breadth of Canada and south to mid-Mexico. The Laurentide ice sheet occupied most of the current range of P. tremuloides until the late Pleistocene epoch, so this species has undergone a significant, geologically recent range expansion. Surprisingly, range-wide patterns of genetic variation in P. tremuloides have never been described. Using a sample set representing the full longitudinal and latitudinal extent of the species distribution, I have conducted a phylogeographic analysis for P. tremuloides. Preliminary results comparing both nuclear and chloroplast DNA sequences revealed surprisingly low levels of divergence across the range. Because of this remarkably shallow genetic divergence among aspen populations, I used a set of rapidly-evolving molecular markers (microsatellites) to describe patterns of gene flow and diversity and to correlate those patterns with landscape features and histories. I analyzed eight microsatellite loci in 794 individuals from 30 sampling sites. From this multilocus data set, I identified pronounced genetic structuring across the range. Strikingly, sampling sites representing the southwestern portion of the range, the western United States and Mexico, form a distinct cluster. Sites within this southwestern cluster display dramatically reduced within-site genetic diversity but elevated regional genetic diversity, which suggests that populations in the southwestern portion of the range make up a stable edge persisting through multiple climate oscillations. Based on the uniqueness of the southwestern cluster and the climatic differences between the southwest and northern portions of the range, I propose that the southwestern cluster may represent a distinct ecotype. I also identified hotspots of diversity that correspond with potential refugia during the last glacial maximum but additional work is needed to refine these patterns. Further, my findings provide a solid foundation for a range of future studies on adaptive genetic and trait variation in this species.

Quaking aspen (Populus tremuloides) is an important species ecologically and culturally in the western U.S., where it is one of the only broadleaf trees in the mostly conifer-dominated forests. Aspen management in the West has focused on regenerating existing stands vegetatively through root suckering, however this approach is restricted to locations where aspen currently exists and limits the genetic diversity of aspen populations. Planting nursery-grown aspen seedlings offers a potential method for overcoming these limitations, but it has received little attention in the U.S. Intermountain West (IW) to date. In order for this approach to be more broadly implemented, nursery protocols designed to grow high-quality aspen seedlings are needed, along with an understanding of what the major challenges to seedling establishment will be. With the assistance of my committee, my research tested a seedling-based approach to aspen restoration in an IW context in two phases. I first used nursery protocols developed for boreal aspen to grow seedlings collected from IW sources in order to assess whether modification of the protocols would be necessary. I then planted the seedlings I grew at three sites in southwestern Utah and monitored their growth and survival over two years. Results from the nursery phase suggest that protocols will need to be modified in order to produce a more consistent response from IW aspen seedlings. In the field, only 10% of the seedlings survived, though the majority of survival occurred in just two locations where soil moisture remained highest during the driest part of the early summer. These results provide useful information to direct future research and suggest that with a better understanding of appropriate site selection, seedling-based aspen restoration could still become a viable management tool in the IW.

Herbivory is one major force accelerating aspen decline in North America, but it is unclear why herbivores prefer certain aspen stands over others, or over other plant species in the understory. In this dissertation, I determined the influence of nutrients and plant secondary compounds (PSC), physiological state, chemical composition, and prior experience on aspen preference by sheep in controlled pen experiments. In addition, I explored the relationship between herbivory, regeneration, recruitment, and other landscape elements for specific aspen stands within Wolf Creek Ranch in northern Utah using biomass and chemical composition of the understory and chemical defenses of juvenile aspen trees (i.e., the foodscape). Aspen intake was enhanced when lamb diets contained a high crude protein to energy ratio or when the basal diet contained a low density of energy. Intake was depressed as concentrations of PG (phenolic glycosides) increased in aspen leaves or when lambs were fed a high energy to protein ratio. The effects of nutrients on aspen intake were greater when phenolic glycosides in aspen were present at low concentrations. However, when given a choice between aspen leaves of high or low PG content, lamb preference depended more on aspen nutrient and mineral availability, or on prior diet, than on defense chemistry. On the landscape, I found that stands at low elevations with low abundance of nutrients in the understory are more likely to experience less regeneration and recruitment than those growing within nutrient-rich sites. Aspen browsing was negatively correlated with PG content in aspen stands, and elk presence (measured via fecal pellets) was negatively correlated with abundance of understory protein. In conclusion, aspen herbivory appears to be controlled by the interplay between types and amounts of nutrients offered by the landscape and the chemical composition of aspen stands. A clear assessment of these variables on the landscape, i.e., the foodscape, will aid in the development of novel management strategies aimed at providing nutrients (e.g., through supplements, introduced forages) at strategic locations in order to reduce aspen herbivory within at-risk aspen stands.

No description available.

Populus tremuloides Michx. (trembling aspen) is a tree species native to much of North America, characterized by an excurrent crown with horizontal to ascending branches and a dominant terminal leader. An unusual clone of trembling aspen was discovered in the 1940s near Hafford, Saskatchewan. This clone demonstrates abnormal crown morphology, in which vigorous shoots bend down, ultimately leading to an overall twisted or crooked appearance. The objectives of the present study were to investigate the mechanism of shoot bending by (1) characterizing the process and timing of bending, (2) evaluating structural aspects of developing wild-type and crooked aspen shoots, and (3) comparing anatomical features of bending shoots with wild-type shoots. L-system reconstruction models of 3-D digitized shoot development revealed dramatic bending midway through the growing season. Morphological analyses revealed that crooked aspen shoots had greater taper compared to the wild-type, typically known to create shoots resist deflection and bending. However, preliminary strength analyses indicated that crooked aspen shoots were less rigid, with smaller values of Young’s modulus compared to wild-type shoots. Anatomical investigations revealed differences in several structural tissues between developing wild-type and crooked aspen shoots, and differences within crooked aspen shoots. Primary phloem fibres on the upper side of bending shoots maintained relatively large lumens while those on the lower side were fully lignified, similar to those of mature vertically oriented wild-type leader shoots. These differences may result in differential extension growth early in development, and/or uneven mechanical support later on, ultimately resulting in bending due to self-weight. Gelatinous fibres (G-fibres), characteristic of tension wood (TW), were found throughout older wild-type and vertically oriented crooked aspen shoots; however, G-fibres were only found on the lower side of crooked aspen shoots. These lateral differences could have contributed to shoot bending by actively bending shoots downwards, or lack of TW on the upper side may not have prevented biomechanical bending from self weight. Nevertheless, shoot bending stops at the end of the growing season, suggesting that the mechanisms involved in creating bent shoots are only functional during the first growing season. / February 2006

No description available.

No description available.