Arboriculture & Urban Forestry 33(1): January 2007 23 weakening effects of predisposing stresses (Manion 1981). The recovery effort depletes the tree’s energy reserves and compromises its defense systems, creating vulnerability to agents known as the contributing factors that ultimately may cause more damage than the inciting factors (Sinclair and Hudler 1988). Contributing factors are conspicuous, biotic, and often the final cause of death. However, they are fre- quently falsely blamed as being solely responsible for tree death instead of indicators of more prolonged stresses (Manion 1981). Wood-boring beetles, canker fungi, and root- decay fungi are all examples of contributing factors. The combined effects of sequential predisposing, inciting, and contributing factors can be described as a conceptual down- ward “spiral” of interactions leading to tree death (Manion 1981). The two-lined chestnut borer—and, to a lesser extent, Armillaria root rot fungi—are well established as the most important contributing factors involved with oak decline in North America (Dunbar and Stephens 1975; Wargo 1977; Haack and Benjamin 1982). Because these pathogens are only harmful if oaks are weakened before infection, the over- all objective of this study is to determine predisposing and inciting factors responsible for bur oak decline in Winnipeg. This was accomplished by comparing characteristics of healthy and declined bur oaks, including present-day tree and site conditions, and dendrochronologic evidence in relation to urban development and climate data. MATERIALS AND METHODS Area of Study Winnipeg, Manitoba, Canada, is a city with an area of 465 km2 (186 mi2) and a population of roughly 620,000 residents (Statistics Canada 2004) found at the junction of the Red and Assiniboine rivers on the Canadian prairies. Urban develop- ment began in the 1860s near the present-day center of the city and expanded rapidly in the 1870s (Dafoe 1998). Many of the city’s neighborhoods were built in the 1940s as a response to a postwar housing shortage (Dafoe 1998). In 1945, Winnipeg contained approximately 50,000 dwellings, and by 1960, that number had more than doubled (City of Winnipeg 2004). Further suburban development continued at a rapid pace until ≈1980 when rapid construction moved be- yond the city limits to surrounding municipalities. As part of the Red River valley, Winnipeg’s soil tends to be black heavy clay with high fertility and poor drainage. The natural vegetation before European settlement was a mix of forest near the waterways shifting to grassland further away as moisture became less available. The arrival of European immigrants meant that almost all trees were harvested for firewood and construction materials in the mid-1800s leaving a barren landscape (Ross 1856; Dafoe 1998; St. George and Nielsen 2002). As a result, many of Winnipeg’s present-day trees germinated during the regenerative time period of the mid- to late 1800s when there was little competition for light or water from established, dominant specimens. Tree Selection Bur oak trees included in the study were randomly selected from an eligible pool within the city of Winnipeg in the summer of 2002 and visually classified according to their crown dieback levels as either healthy (<5% dieback) or de- clined (>25% crown dieback). Sample selection was re- stricted to those trees that (1) were located at least 100m(330 ft) from roads where toxic deicing salts are applied in the winter, (2) were not near any obvious recent major distur- bances, (3) did not have wounds affecting more than 25% of the trunk circumference, and (4) had a trunk diameter at breast height (dbh; measured at 1.4 m [4.5 ft] from the ground) of at least 15 cm (6 in). Selected bur oaks were located mainly in parks, private yards, boulevards, cemeter- ies, golf courses, and along riverbanks. In total, 120 trees (68 healthy, 52 declined) were selected. Data Collection To characterize the selected trees and their growing environ- ments, measurements were made of the trees’ dbh, distance to nearest visible human-caused disturbance (such as pavement, buildings, or a sewer system), and level of surrounding tree competition within a radius of the tree’s height. Because fo- liar nutrient levels can be used to detect potential soil prob- lems (Dyer and Mader 1986; Kozlowski et al. 1991), leaf samples were collected from all trees (50 leaves from the edge of the crown) and analyzed for nutrient contents. The severity of trunk wounding, presence or absence of a buttress (tapering at the base of the stem that may indicate that the soil grade has not been raised), and presence and abundance of urban space (concrete, gravel, buildings) within a radius of each tree’s height were visually rated. A subsample of 22 trees was selected for soil sampling and analysis of soil vari- ables, including texture, bulk density, and chemical proper- ties. Increment cores were collected at breast height from all 120 trees using a 5.5mm(0.22 in) Hagloff tree corer (Hagloff Inc., Madison, MS). Cores were prepared according to stan- dard methods (Stokes and Smiley 1996). Annual ring widths were measured using a Velmex, Inc. measuring system and crossdated using the COFECHA computer program. Age es- timates derived from cores taken at breast height underesti- mate the true age of the tree by the number of years required to reach breast height. Under ideal nursery conditions, oaks require at least 5 to 6 years to reach breast height (Rick Durand, pers. comm.) and often longer in a forest un- derstory environment where oak seedlings may grow very slowly and have recurring shoot dieback (Abrams 1996). Climate data used in this study were obtained from Environ- ©2007 International Society of Arboriculture
January 2007
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