236 P. Mitchell et al.
Thomas 2002 ; Jacquet et al. 2014 ). These conflicting results suggest that the host
physiology, conditioning factors such as tree age, and the simulated intensity of the
primary and secondary stressors can determine the magnitude and direction of the
response.
The intensity of the primary stressor has the potential to modulate the impact
of additional secondary and conditioning factors. As a primary stressor such as
drought progresses, cell expansion and growth are often the first casualties of wa-
ter deficit, followed by a decline in the rate of photosynthesis (Hsiao et al. 1976 ),
and the eventual breakdown of water and sugar transport (Hölttä et al. 2009 ). The
addition of a secondary stressor at a given drought intensity, can act to preserve or
further disrupt changes to these processes. Of the few studies to test for interactions
between stress type (drought and simulated herbivory) and intensity (moderate and
high), Bansal et al. ( 2013 ) reported a larger impact on growth in Pinus sylvestris in
the combined stress treatment than in the single stress treatments at the moderate
intensity (Bansal et al. 2013 ). However, under high intensity stress, the reduction
in growth was largest in the drought treatments, regardless of levels of herbivory
Fig. 11.6 Relationship between the level of damage ( effect size) associated with pests of woody
organs or foliage and a species-specific index of water stress (see text). For woody organs, condi-
tions of low water stress may promote allocation of carbohydrates produced by photosynthesis
to growth rather than defense; the host may demonstrate less resistance to pests of woody organs
under these conditions. Many studies show that under moderate levels of water stress, resistance
to pests of woody organs increases, but severely water-stressed trees are less likely to be resistant
to these pests. For foliar organs, low levels of water stress induce nutritional changes in the foli-
age that can leave trees less resistant to defoliation pests. Under moderate levels of water stress,
the host resistance to foliar pests may increase, due to the production of defense compounds in
the leaves. Under severe water stress conditions, while production of defense compounds may
be more limited, physical changes such as increases in leaf toughness may promote resistance.
(Adapted from Jactel 2012 ).