No description available.

Diss. (sammanfattning) Umeå : Sveriges lantbruksuniversitet, 2009. / Härtill 4 uppsatser.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1967. / Includes bibliographical references (p. 57-58).

Trembling aspen (Populus tremuloides Michx.) were observed growing along roads far north from the area where it dominates, in sites dominated by black spruce (Picea mariana (Mill.) B.S.P.) forests. This study examined the distribution of aspen at an early development stage and the conditions in which they are growing in a black spruce/feathermoss forest type in northwestern Abitibi, Quebec six years following harvesting. In this region, aspen are appearing in logged and burned areas that had been previously dominated by black spruce. The relationship of aspen growth with Ca availability and mineral soil access is the main focus of the study. Soil and foliar samples from aspen seedlings were collected from roadside, slash and cutover locations during the summer of 2003. Trees were also measured for height and basal diameter. Microsites where aspen was growing and where it was absent were compared to determine whether aspen was associated with specific microsites soil properties. The results suggest that there are differences in the growing conditions for aspen between different locations but that the trees are growing successfully in all of the three location types. In the cutovers, aspen seedlings were consistently found in association with patches of Polytrichum moss. All the sets of data indicate that Ca availability and access to mineral soil are not the main factors influencing the distribution of aspen but that soil pH, or a factor relating to pH, may be important.

Trembling aspen (Populus tremuloides (Michx.)) has become established along a mining road in the lower northern Quebec region (49°39' to 49°45'N and 79°01' to 79°02' W) in areas that have not supported aspen in the past. The primary means of regeneration of aspen is through vegetative root suckers, but it is doubtful that it could progress through this region at such a fast rate solely by asexual reproduction. Aspen are known to produce large amounts of seeds that could account for aspen's rapid progression, but sexual reproduction of aspen is rare; very little is known about Populus tremuloides seed germination and seed survival in natural conditions. The aim of this study was to determine seedbed conditions that facilitate the emergence and survival of Populus tremuloides seedlings, and to identify the key factors involved in these processes. Observations of established saplings and a sowing experiment were conducted on both post harvest and post fire sites. Our data on established saplings showed that there were seed-origin trembling aspen located in both post-harvest and burned areas indicating that Populus tremuloides can and has established from seed in areas where aspen was previously absent. Both our sowing experiment and our greenhouse experiment showed that the emergence of seedlings was relatively low, but that Polytrichum strictum moss and exposed mineral soil depressions were the best seedbeds for seed germination. Extreme temperatures and availability of moisture appeared to be controlling factors on seed germination, but ultimately the physical and biological characteristics of each substrate influenced the substrate's response to these factors in a different way. We conclude that sexual reproduction of trembling aspen may and has occurred in this region, and is promoted by the presence of exposed mineral soil and Polytrichum strictum moss. / Keywords: Bryophytes, Populus tremuloides, Polytrichum, Sphagnum, seedling, seedbed, germination, seed.

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.

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.

Diss. (sammanfattning) Umeå : Sveriges lantbruksuniversitet, 2005. / Härtill 4 uppsatser.

Thesis (M.S.)--University of Wyoming, 2007. / Title from PDF title page (viewed on Nov. 26, 2008). Includes bibliographical references (p. 24-33).

Quaking aspen (Populus tremuloides) is the most widely distributed tree in North America (Lindroth and St Clair 2013), and a keystone species in our western montane forests (Worrall et al. 2015). Aspen has become a model organism for studies of genetics and physiology in woody plants (Bradshaw et al. 2000, Taylor 2002). Aspen is also economicallyimportant (Worrall et al. 2015) – wood is harvested for various uses, its scenic beauty helps sustain the tourism economy in many areas, and it has recently been studied as a possible source of biofuel (Sannigrahi et al. 2010). Aspen is also a species of conservation concern, due to recent large-scale deterioration and decline of many aspen forests in the last two decades (Worrall et al. 2013). Several causal factors have been identified: fire suppression (Calder et al. 2011, Smith et al. 2011), increased ungulate herbivory (Kay and Bartos 2000), disease (Marchetti et al. 2011), and climate change (Worrall et al. 2013). My thesis focuses on two different biotic stressors of aspen: a fungal pathogen and ungulate herbivory. Understanding the relationship between aspen and their biotic stressors adds to our knowledge of aspen ecology and helps manage the increasing risk of decline in our aspen forests. Chapter 1 is a study of the relationship between aspen and a necrotrophic fungal pathogen (Drepanopeziza sp.) during a major disease outbreak in 2015. I quantified the relationship between Drepanopeziza infection severity and aspen leaf functional traits, including morphological, chemical and phenological traits. I found that severe Drepanopeziza infection was associated with low concentrations of a key class of herbivore defense compounds (phenolic glycosides), and strongly associated with early budbreak and leaf-out in aspen stands. The association between infection and early budbreak was likely caused by unusually rainy conditions in May of 2015, which may have exposed leaf tissue to wet conditions that favor thedispersal of Drepanopeziza spores. Chapter 2 is an experiment designed to determine whether the mode and timing of herbivory can influence aspen's defensive response. I specifically asked whether removing leaves, twigs and meristems together and removing leaves alone had unique effects on aspen sucker growth, survival, and phytochemistry. Additionally, I applied these simulated herbivory treatments to suckers on different dates to see whether early- or late-summer herbivory had greater effects on suckers. I found strong mode and timing effects on growth and survival, but not foliar chemistry.