11 Combined Stresses in Forests 237
(Bansal et al. 2013 ). Recently, Jactel et al. ( 2012 ) conducted a meta-analysis of
drought studies that involved damage by insects and fungi. The study defined the
intensity of plant water deficit using a species-specific index of water stress using
plant water status and the stem hydraulic resistance to cavitation. They concluded
that the impact on tree growth or survival (derived using an effect size method)
increases with the intensity of water stress for foliar pests and pathogens, whereas
the effect from pests on woody organs was dependent on whether the insect or fun-
gal species was a primary (develop on healthy trees) or secondary (develop during
physiological stress) agent Jactel et al. ( 2012 ). Figure 11.6 summarizes some of the
key findings of the meta-analysis of Jactel et al., and shows how risk of damage
is strongly determined by the relationship between intensity of water stress and
changes in the resistance of different tree organs (foliar or woody).
Intensity-dependent responses may also arise because a highly sensitive process
such as growth ceases at a threshold water deficit (Mitchell et al. 2014 ) so that any
further reductions in water deficit combined with injury from biotic attack have
minimal effect on growth-related responses. Figure 11.7 illustrates a hypothetical
Fig. 11.7 A hypothetical scenario illustrating how the importance of primary, secondary, and mul-
tiple stressors changes across different stress intensities. The two stressors, herbivory ( solid black
line) and drought ( solid blue line), and their interaction for any given intensity are represented
by the drought × herbivory line ( gray, dot-dash). The trajectory of the additive impact of the
two stressors, i.e., drought + herbivory is given as a reference ( dashed line). At high intensities,
drought may reduce plant functioning to zero (death), whereas herbivory has less impact on func-
tion. The positioning of the drought x herbivory line below the additive impact line indicates a
synergistic effect (multiple stress impact is greater than the sum), and above the additive impact
line indicates an antagonistic effect (multiple stress impact is less than the sum). In this scenario,
the stress impacts of drought x herbivory on tree vigor are determined by “multiple stress effects”
at low to moderate intensities up until some threshold value ( vertical dashed line). Beyond this
threshold, the drought x herbivory line converges on the drought stress line, indicating tree func-
tion to be solely determined by the “primary stress effect” (drought in this case; after Mitchell et al.
2013 ). Copyright Oxford University Press.