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persist in mature leaves and affect subsequent herbivores, even from different guilds (Neuvonen et al. 1988, HanhimS.ki
1989). The delayed effects (DIR) can remain effective on folivores consuming mature leaves produced the following
summer (Hanhimfiki 1989). Induced resistance in mountain birch foliage is triggered most effectively by damage at the time
when there is peak feeding by the most destructive insect pest of mountain birch, the geometrid, Epirrita autumnata (Tenow
1972, Haukioja et al. 1988). Since defoliation tends to effect all members of the birch chewing guild similarly (Hanhim_iki
1989), the outcomes of defoliation in birch foliage seem to represent a general response, and are not specifically targeted
against Epirrita autumnata or other early season defoliators.
Induced amelioration in mountain birch is triggered either by removal of the apical tips of dormant twigs (Haukioja et
al. 1990) or by damage to expanding buds or tiny leaves (Senn and Haukioja 1994). In practice, the former type of damage
may result from winter browsing by mammals and the latter from insect damage very early in the season. Winter browsing
makes the forthcoming foliage more preferable for a suite of different types of herbivores (Danell and Huss-Danell 1985),
although the effects on insect performance tend to be mild (Neuvonen and Danell 1987, Haukioja et al. 1990). Larvae of
Epirrita autumnata start feeding at, or even before, budbreak, and are known to destroy apical buds from branches (Haukioja
et al. 1990). Early leaf damage might affect foliage quality in the same year while bud removal influences growth in the
following season. In practice this means that depending on the type of target tissue and timing of the damage, larvae of
Epirrita autumnata are potentially capable of inducing either IA, RIR or DIR in birch foliage (Haukioja 1991a).
The case with the pine is different. Most studies conducted on pines have not reported strong qualitative changes in
needle chemistry after defoliation and, accordingly, the effects on insect performance also have been mild or insignificant
(e.g., NiemelS. et al. 1991, Watt et al. 1991, Reich et al. 1993). However, some studies have revealed defoliation-induced
chemical changes in pines. Wagner and Evans (1985) reported that defoliation of Pinus ponderosa seedlings increased the
production of both phenols and proteins in mature and immature foliage. Honkanen and Haukioja (1994) demonstrated
increased nitrogen needle concentration after severe defoliation of Scots pine. These results indicate that the outcomes of
defoliation in pines can vary substantially, and so are the consequences on herbivores.
Pruning or removal of pine buds or twigs causes more lush growth of needles and shoots (e.g., Nuorteva and Kurkela
1993, Honkanen et al. 1994), just like in mountain birch. Pine browsers, particularly moose, are known to cause IA
(L6yttyniemi 1985), but it is not known whether insect herbivores of pine are able to induce such a response.
In summary, both in the case of mountain birch and Scots pine it is possible to induce variable effects - better or
worse foliage quality, or no change at all - by selectively damaging different types of tissues and by varying the timing of the
treatment.
THE RESOURCE STATUS OF BIRCH AND PINE FOR INDUCED RESPONSES:
ADEQUACY OF THE C/N HYPOTHESIS
The simplest explanation for the induced resistance is that chewing or browsing of plant biomass removes nutrients
and such losses lower foliage quality for later herbivores (Tuomi et al. 1984). If the loss of nutrients is critical for induced
resistance, the tree's status can be remedied by fertilization which usually increases the nutrient concentration in foliage. The
crucial question here is whether fertilization decreases the resistance of the tree and cancels any negative effects of defoliation.
In the case of the mountain birch, Haukioja and Neuvonen (1985) reported that nitrogen fertilization did not alleviate
the detrimental effects of defoliation (DIR) on Epirrita larvae. This contradicts the predictions of the carbon-nutrient balance
(C/N) hypothesis, which ascribes the resistance of deciduous trees to carbon-based secondary compounds which generally
decrease when trees obtain extra nutrients. However, Bryant et al. (1993) found that fertilized paper birches, Betula
papyrifera, lost their ability for DIR. They criticized the mountain birch experiment by Haukioja and Neuvonen (1985) for
simultaneous application of the fertilizing and defoliation treatments; the trees might not have had time to benefit from
increased nutrient availability. However, another experiment with the mountain birch-Epirrita system (Ruohom_iki et al. in
press), accounted for the criticism by Bryant et al. (1993) and yet reported the same result of Haukioja and Neuvonen (1985).
Furthermore, the strength of the DIR did not decline with shading, as predicted by the C/N hypothesis (Ruohom_iki et al. in
press). It is possible that mountain birch and paper birch behave differently, or the insects used in the two experiments
showed differential tolerance to the altered carbon-based compounds, or that changes in carbon and nitrogen were not crucial
for the herbivore. In any case, it is obvious that for birches the C/N hypothesis did not provide the general explanation for
induced responses.
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