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One weakness of using data from genetic tests plantings to study the impact of climatic warming, however, is that
other variables besides temperature change when moving seed from its site of origin to the test location. Photoperiod, for
example, is one important variable for which many tree species, including paper birch, are genetically adapted (Vaartaja
1959, Wright 1976, Kozlowski et al. 1991, Farmer 1993). In the northern hemisphere, typically, northern sources tend to
flush earlier and terminate shoot growth earlier when moved southward compared with more local sources (Wright 1976,
Kozlowski et al. 1991). In the present study, birch seed was collected between Lat. 43.3°N and 47.4°N, and then planted at
42.0°N. On the longest day of the year there is about 15.2 hours of light at 42.0°N, 15.5 hours at 43.3°N, and 16.0 hours at
47.4°N. Similarly, on the shortest day of the year, they experience about 9.1 hours of light at 42.0°N, 9.0 hours at 43.3°N, and
8.4 hours at 47.4°N. As a means of minimizing any influence of photoperiod, linear regression analyses were also conducted
on two subsets of the original 26 populations (collection sites) in which the latitudinal spread among the seed sources was
further reduced. In the first case, only the 13populations from the Lower Peninsula of Michigan were tested (populations 14-
26, Fig. 2, Table 1), representing collections within a 2° span of latitude and a 5°F (2.8°C) span in annual mean temperature.
In the second case, 12 birch populations were tested (populations 4-7, 10, 12-18; see Fig. 2, Table 1), representing collections
within a 1.5° span in latitude and a 4°F (2.2°C) span in annual mean temperature.
Analyses were conducted using SAS and SigmaStat. Height and borer data were analyzed on an individual tree basis,
whereas tree survival was calculated on a family basis (n = 218), using only those birch families that had at least one tree
surviving through the second growing season (1977). Each tree was assigned to one of four "treatments" based on the
average annual mean temperature of its origin. The four groupings were isotherms 40 ° and 41°F, 42 ° and 43°F, 44 ° and 45°F,
and 46 oand 47°F (Table 2). The climatic differential term assigned to each grouping was CD1 ° (46-47°F), CD2°(44°-45°F),
CD3 ° (42°-43°F), and CD4 ° (40°-41°F; Table 2). These terms, CDI°-CD4 °, reflect the average climatic differential in °C fbr
each treatment group, e.g., for CD1 ° the midpoint climatic differential value is actually 1.I°C and represents the average
degree of climatic warming that the trees in isotherms 460F and 47°F were theoretically experiencing (Table 2). Analysis of
variance (ANOVA) was used to test for differences among treatments; ap level of 0.05 was used for significance. Tree
height and survival data were analyzed using the GLM procedure of SAS, followed by the Student-Newman-Keuls means
separation test. The arcsin square root transformation was performed on the survival data prior to analysis. Borer data were
analyzed using the Kruskal-Wallis nonparametric test of SigmaStat, which performs an ANOVA on ranks; Dunn's test was
used to make multiple comparisons among treatment means.
RESULTS
Paper birch growth, survival, and susceptibility to bronze birch borer attack were significantly affected by the
climatic differential between the origin of the seed source and the plantation site (Fig. 3). Overall, height growth decreased
as the climatic differential increased (Fig. 3A). Differences in height growth were already evident by the end of the second
growing season in 1977 (p < 0.0001; Fig. 3A). By the end of the sixth growing season (1985), CDI ° trees were the tallest,
with tree height becoming successively shorter for the CD2 °, CD3 °, and CD4 ° trees (p < 0.0001 ; Fig. 3A).
As the trees aged, tree survival tended to decrease as the climatic differential increased (Fig. 3B). From year 2 (1977)
to year 8 (1983) post-establishment, little tree mortality occurred, but thereafter tree mortality accelerated, especially among
the trees experiencing the greatest climatic differential (CD4°; Fig. 3B). Treatment survival values first became significantly
different in year 12 post-establishment (1987; p < 0.006), when survival for the CD4 ° trees was lower than that for trees from
the other three groups. A similar pattern of tree survival continued into years 15 (1990; p < 0.001) and 16 (1991; p < 0.0008)
post-establishment.
As the trees became older, tree susceptibility to the bronze birch borer increased with increasing climatic differential
(Fig. 3C). Of the 146 paper birch trees that died between 1985 and 1991, 143 (98%) of them had signs of borer attack along
their lower trunk. A similar pattern emerged during each of the 3 years when borer damage was assessed (1985, 1987, 1990;
p < 0.001 for each), i.e., CD4 ° trees experienced the highest levels of borer attack, while CDI ° trees experienced the lowest
levels (Fig. 3C). As would be expected from the above discussion, the occurrence of borer-related bark ridges and exit holes
was more common on CD4 ° trees than on CD 1° trees (Table 2).
Regarding the two subsets of birch populations in which the latitudinal spread was reduced, paper birch growth,
survival, and susceptibility to bronze birch borer were all significantly affected by the climatic differential between the origin
of the seed source and the plantation site. In the first case, linear regression analysis of the 13 populations from Michigan's
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