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anches dead or dying with borer-related bark ridges obvious on the bark surface; 2 = more than half of the crown branches
dead or dying with bark ridges obvious and borer exit holes often present; and 3 = tree dead, usually with borer-related bark
ridges and exit holes present. Bark ridges (callus tissue) occur on branch and trunk sections in response to larval feeding
(Anderson 1944, Barter 1957, Wilson and Haack 1990, Herms 1991). The presence of borer-related bark ridges was scored
in years 12 and 15. Completion of larval development was also scored in years 12 and 15 by noting the presence of borer exil
holes. Tree survival was computed for each of the 218 families in years 2, 6, 8, 10, 12, 15, and 16.
Historical temperature data were obtained from the Michigan Department of Agriculture, Climatological Division
(MDA 1974). Average annual mean temperature was considered the most relevant climatic variable to use when considering
global warming. The average annual mean temperature assigned to each seed source was obtained from a map showing
annual mean temperatures for Michigan averaged over the 30-year period 1940-1969 (Fig. 1, Table 1, MDA 1974). This was
done by locating all seed collection sites on the map and then assigning a particular annual mean temperature value to each
seed source based on its location relative to the map isotherms. Although actual temperature data for each of the 26 collections
sites would have been preferred, this was not possible because many collection sites had no nearby weather stations.
Additional climatic data for each of the 26 collection sites were obtained in similar fashion from other maps (Table 1).
Using the map shown in Figure 1, the annual mean temperature values assigned to the 218 seed sources ranged from
40° to 47°F (4.4*-8.3°C; Table 1). Actual average annual mean temperature values from several official weather stations for
this same area and time period ranged from 39.7* to 47.1"F (4.30-8.4"C; MDA 1971). The average annual mean temperature
of the plantation site was about 49.00F (9.4°C; MDA 1971). Overall, assuming that the birch seed sources were genetically
acclimated to the average annual mean temperature of their original sites, then the birch trees growing at the plantation site
were theoretically experiencing levels of climatic warming equivalent to 1.9° to 9.3*F (1.1 ° to 5.2°C; Table 2). For ease of
discussion, I will use the term climatic differential (CD) to represent the difference (in °C) between the average annual mean
temperatures of the seedlot origin and the plantation site.
Table 2.--Average annual mean temperature data and percent of paper birch trees (both live and dead) that had bronze
birch borer-induced ridges and/or borer exit holes along their lower trunk in years 12 (1987) and 15 (1990) postestablishment
by climatic differential treatment.
Mean temperature
difference between
Corresponding origin and Percent of trees with Percent of trees with
isotherms 2 plantation site 3 borer-induced ridges 4 borer exit holes 4
CD j °F °C *F °C Yr 12 Yr 15 Yr 12 Yr 15
CD t ° 46, 47 7.7, 8.3 2 1.1 30ab 32b 10b 13b
CD2 ° 44, 45 6.7, 7.2 4 2.2 24b 33b 8b 18b
CD3 ° 42, 43 5.6, 6.1 6 3.3 39ab 46b 12b 25b
CD4 ° 40, 41 4.4, 5.0 8 4.4 50a 60a 23a 40a
CD = Climatic differential, which is a term that reflects the difference between the average annual mean temperature
of the seed sources within a particular treatment group and the average annual mean temperature of the plantation
site, which was 49°F (9.4"C). The value 1° in CD1 °, for example, reflects the annual mean temperature difference
between the origin and the plantation site, e.g., 1.I*C (see footnote 3 below).
2The isotherms of average annual mean temperature in *F (and the approximate °C equivalent), as given in Fig. 1,
that correspond to each CD grouping.
3For each CD grouping, the mean temperature difference between the origin and the plantation site was calculated
as follows. For CDI*, the corresponding isotherms were 46* and 47°F. However, these isotherms span the temperature
range from 46* to 47.9°F, with a mean of about 47"E Thus, the temperature difference between the origin and the :
plantation site in this case is 2*F (49°-47 *) or about 1.I*C. These values represent the theoretical level of climatic
warming that the corresponding trees were experiencing while growing at the plantation site in southern Michigan.
4 Percent of trees with borer-induced ridges and/or borer exit holes along the lower trunk; percent values followed by
the same letter within a column are not significantly different
followed by the Dunn's test for multiple comparisons).
238
(p