View or print this publication - Northern Research Station - USDA ...

DEFENSE THEORIES AND BIRCH RESISTANCE
MATTI ROUSI t, JORMA TAHVANAINEN 2, WILLIAM J. MATTSON 3
and HANNI SIKANEW
_The Finnish Forest Research Institute, Punkaharju Research Station, SF-58450 Punkaharju, Finland
2University of Joensuu, Depamnent of Biology, P.O. Box 111, SF 80101 Joensuu, Finland
3USDA Forest Service, North Central Experimental Station, 1407 Harrison Rd, East Lansing, M148823 USA
INTRODUCTION
Different tree species in northern temperate forests clearly vary in their resistance to browsing mammals (Gill 1993
a,b). In addition to variation at the species level, there are also clear indications of genetic differences in herbivore resistance
within species. Such variation in resistance has been explained by the duration and intensity of past exposure to herbivores
and pathogens (Leppik 1970, Bryant et al. 1989). It is believed that the centers of genetic diversity for plant species should
be the best places in which to find resistance to their common herbivores and diseases (Vavilov 1920). All of these kinds of
variation have important economic and ecological implications for plantation forestry and forest industries. For example,
hybridization could be used to elevate the resistance of otherwise high quality but susceptible tree species or individuals (see
Rousi 1990).
It has been postulated that there is a trade-off between growth and plant defense (Rhoades 1979, Herms and Mattson
1992). That being the case, practical forestry might not be interested in slow growing genotypes, although they might be
more safe to use. Trees are generally attacked by a myriad of herbivores and if there is a trade-off between resistance and
growth to each of those, then generally resistant genotypes should be very slow growing. Ecological trade-offs in the form
of negative correlations in resistance to different types of herbivores are also possible. Conventional wisdom suggests that
trees which have to compete intensely for nutrients and light, can probably be resistant only to restricted number of herbivores.
In addition to genetic components, environment may also modify plant resistance. If carbon is the limiting factor due
to shading, or to large amounts of available nutrients and water, then defensive secondary metabolite level may decline so
long as the plant still has strong sinks capable of usurping the available photosynthates and nutrients (Mattson 1980, Bryant
el al. 1983).
To test the effect of environment and birch genotype on hare resistance, we carried out several cafeteria experiments
using winter dormant shoots of small birch seedlings and plantlets grown in Punkaharju, East Finland.
RESULTS AND DISCUSSION
Birch Species and Genotypes Vary In Their Hare Resistance
Birch species vary strongly in their hare resistance. According to many studies, Japanese white birches, B.
platyphylla and B. ermanii, are both very resistant. Some other species, such as B. alleghaniensis (Fig. 1) and B. papyrifera,
are among the most susceptible species, whereas B. pendula is somewhat intermediate in resistance. Interestingly, experiments
made in Alaska, Finland, and Japan all give essentially the same results (Chiba and Nagata 1968, Bryant et al. 1989,
Rousi et al. 1989, 1996a). This most probably indicates the wide adaptability of birch: there are no drastic changes in
resistance even if birches are grown as exotics. In addition, it shows that the Japanese mountain hare, Lepus ainu, the Finnish
mountain hare, L. timidius, and the snowshoe hare, L. americanus, make their food selection on the same bases.
Mattson, W.J., Niemel',i, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gem Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
39