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

late season defoliation decreases the growth potential of next year's pine needles more than early season defoliation
(Honkanen et al. 1994). This presumably relates to the long developmental time of pine buds (Agren and Axelsson 1985)
and that defoliation removes the local storage in pine needles. We assume that late season defoliation of birch mainly hinders
provisioning of the general resource pool (Fig. 1).
In conclusion, both the type of the damaged tissue andtiming of the damage contribute to the post-defoliation
responses of mountain birch and Scots pine. The important points are whether the damaged tissue was a physiological sink
or a source, and which sinks the damaged foliage was provisioning at the time of the damage. Thus IA, DIR, RIR, as well as
local changes in growth activity, can all be understood as outcomes of the same basic explanation.
DISCUSSION
The system of sink-source gradients is part of the mechanism regulating the allocation of resources within a plant.
Therefore, it offers a basis for explanations about spatial and temporal patterns of resources, and therefore also for chemical
compounds which are potentially important for herbivores and about changes in these compounds after herbivory. This is not
to say that we regard resource availability unimportant; the availability of carbon and mineral nutrients obviously represents
modifying and constraining factors for tree-herbivore interactions. Among other things, resources modify sink strengths of
meristems and plant apical dominance (Cline 1991).
The G/D and, especially, the C/N hypotheses, emphasize plant quality and different ways that shortages of resources
may cause chemical changes which are assumed to be important for herbivores. The S/S hypothesis, instead, emphasizes the
ability of meristems to manufacture new biomass via spatially and temporally variable allocation of resources to specific
meristems. Limitations in resource allocation may alter foliage quality either via the primary or the secondary leaf chemistry,
or, most probably, both. The SIS hypothesis does not make a priori predictions of the importance of these chemical changes
for herbivores.
We assume that the basic explanation for DIR, and perhaps for localized RIR, is the weakened state of meristems.
This leads to a lower competitive ability in the plant sink-source system. Altered resistance is an automatic outcome of these
changes. There are four aspects of induced tree responses which support this. First, defoliation/clipping-induced changes in
plant growth activity are basically local phenomena. This also refers to the largely unspecific changes in primary and
secondary chemistry which produce effects classified as RIR, DIR and IA in trees like birches. This clearly contrasts with
some damage-induced systemic responses which effectively spread within and among plants (Walker-Simmons and Ryan
1977, Farmer and Ryan 1990, Takabayashi et al. 1991). They concern well known and obviously strictly defensive traits like
production and transfer of the proteinase inhibitor inducing factor (PIIF) e.g. in many herbaceous plants (McFarland and
Ryaa 1974), or the resin duct system of conifers (e.g., Blanche et al. 1992). Second, the responses of birch foliage quality to
defoliations or clippings is of a general nature and affects all species of chewing insect herbivores tested so far in a similar
way (Hanhimfiki 1989). Third, the existence of IA demonstrates that reasons other than defense (or recovery) must be sought
when interpreting plant responses to herbivory. Such seemingly nonadaptive responses are real and presumably reflect
constraints in plant design. Fourth, birch and pine are surprisingly insensitive to the amount of lost biomass: limited as well
as intensive defoliations cause fairly similar degrees of responses (Haukioja and Neuvonen 1987, Honkanen and Haukioja
1994). This is consistent with sink strength regulated photosynthesis which is common in plants (e.g., Wardlaw 1990). All
these aspects indicate that defoliation-induced changes in carbon-based potentially defensive chemistry, as well as in the
primary chemistry, have explanations other than defense, or the relative availability of different types of resources. Note that
acceptance of this explanation does not deny the importance of plant chemistry on herbivores. However, the question of
whether induced tree resistance, measured as a decrease in insect performance after previous foliar damage, is a true defense
or not, is outside the scope of this paper.
The S/S hypothesis has some obvious practical implications. For instance, branch or ramet specific responses of trees
to defoliations (Haukioja et al. 1983, Tuomi et al. 1988b, L_ngstr6m et al. 1990, Hanhim_iki and Senn 1992, Honkanen et al.
1994, Honkanen and Haukioja 1994) become easily comprehensible. This fact has made it possible to use individual ramets
of the same multi-stemmed tree as units on which different treatments have been applied. The question of whether such
branch-specific defoliations are as effective in inducing induced resistance as more extensive tree-wide defoliations has an
ahnost paradoxical answer: at least in pine, branches show stronger, not weaker, responses after branch-wide than after treewide
defoliations (Honkanen and Haukioja 1994). This result agrees with the S/S hypothesis and, to our understanding,
6