Factors influencing the health of quaking aspen (Populus tremuloides Michx.)
Date
2015
Authors
Dudley, Megan Marie, author
Tisserat, Ned A., advisor
Negron, Jose, advisor
Jacobi, William R., committee member
Shepperd, Wayne D., committee member
Newman, Steven E., committee member
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Abstract
In Chapter 1 of this dissertation, we analyzed a series of increment cores collected from 260 adult dominant or co-dominant quaking aspen (Populus tremuloides Michx.) trees from national forests across Colorado and southern Wyoming in 2009 and 2010. Half of the cores were collected from trees in stands with a high amount of crown dieback, and half from lightly damaged stands. We define the level of stand damage based on stand survey data, in which lightly damaged stands had average crown dieback of 16%, and heavily damaged stands averaged 41%. Upon analysis, two-thirds of the cores collected did not exhibit radial growth correlated with region-wide patterns (e.g. climate) and were classified as having a low cohesive response (LCR). The site variable most predictive of whether a stand exhibited high cohesive response (HCR) or low cohesive response was site elevation, followed by aspect, slope, and canopy closure. Sites with HCR stands were more likely to have aspen bark beetle damage, white rot, and Cryptosphaeria canker. We did not detect relationships between tree growth and summer precipitation from 1900-2008, but there was a relationship between growth and annual precipitation. A growth model included maximum May and July temperatures, as well as the current and previous year’s annual precipitation. Historically, Cytospora canker of quaking aspen was thought to be caused primarily by Cytospora chrysosperma. However a new and widely-distributed Cytospora species on quaking aspen has recently been described (tentatively named Cytospora notastroma). In Chapter 2 of this dissertation, we show the relative pathogenicity of both species. Small-diameter aspen trees were inoculated with one or two isolates each of C. chrysosperma and C. notastroma in a greenhouse, outdoor setting, and in environmental growth chambers. Results indicate that both species are pathogenic to drought-stressed trees and that C. chrysosperma was more aggressive than C. notastroma at both warm and cool temperatures. Neither species caused significant canker growth on trees that were not drought stressed. In Chapter 3, we investigated the abundance and frequency of C. notastroma, relative to C. chrysosperma. We wished to estimate the relative abundance of known Cytospora species on quaking aspen throughout portions of the Rocky Mountain region, and to construct species-level phylogenies based upon isolates obtained from infected aspen. We report that both C. chrysosperma and C. notastroma are quite common on quaking aspen, although we recovered C. chrysosperma slightly more often (48% of sequenced cultures) than C. notastroma (42 % of sequenced cultures). We also recovered a third, previously-described species, C. nivea in 9% of sequenced cultures. We also found that Cytospora species often co-occur on the same host tree (25% of trees sampled), and that evidence of recombination or possible hybridization between the species exists. The aspen bark beetle, Trypophloeus populi, is known as a stress-related damage agent on quaking aspen. In a previous study, we often found T. populi attacking host trees also infected with Cytospora canker. In Chapter 4 of this dissertation, we wished to determine whether T. populi is a potential vector of Cytospora canker, and whether Cytospora inoculum could be recovered from adult beetles or gallery tissues. We did not recover any Cytospora isolates from 161 adult T. populi beetles cultured, and only two Cytospora isolates from 42 beetle galleries and seven adult aspen. We suspect that these isolates, cultured from two trees, were a result of a previous infection, as both host trees had extensive cankers as well as Cytospora fruiting bodies.
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Subject
climate
Populus tremuloides
Cytospora chrysosperma
Cytospora notastroma
growth
Trypophloeus populi