11 Combined Stresses in Forests 233
of Liquidambar styraciflua exposed to elevated [CO 2 ] experienced larger declines
in leaf area index during water deficit than stands exposed to ambient conditions
(Warren et al. 2011 ). These patterns presumably arise because gains in plant growth
and tree size realized under elevated [CO 2 ], increases inter-tree competition for wa-
ter during drought. Experiments assessing the interactive effect of drought, elevated
temperature, and [CO 2 ] conclude that elevated temperature and [CO 2 ] can amelio-
rate the effects of potentially stressful water deficits from higher temperature at
moderate drought intensities (Duan et al. 2013 ) but does not influence leaf water
relations and time to mortality when water deficit is extreme (Duan et al. 2014 ).
11.2.2 Primary and Secondary Factor Interactions
Drought affects multiple physiological pathways in trees that can influence both the
attractiveness of the host to particular pest species (e.g., increases in tissue carbo-
hydrate and nitrogen concentration) and constitutive and induced defense mecha-
nisms. Moderate water stress can promote defense through increases in secondary
metabolites in foliage (Ayres and Lombardero 2000 ), while severe water stress can
result in tougher foliage that reduces defoliation damage (Steinbauer 2001 ). How-
ever, acclimation to water stress can also enhance folivory activity in some species.
Rivas-Ubach et al. ( 2014 ) concluded that increased production of compounds as-
sociated with osmoprotection (potassium, sugars, and antioxidants), a response that
increases the tolerance of a low water potential in Quercus ilex, promoted more
severe attack from defoliating insects, highlighting the complexity of drought–pest
interactions (Fig. 11.5).
Moderate water stress can also promote secondary metabolite production in stems
(Jactel et al. 2012 ), resulting in increased resistance to damage from pests such as
stem borers and fungi (Fig. 11.6). Under severe water stress, the capacity of the host
to divert carbohydrates to production of defense compounds decreases (Rouault
et al. 2006 ), thereby reducing resistance to pests. For example, drought reduces the
capacity of Eucalyptus globulus to produce bark exudates as a defense against the
stem borer Phorocantha mastersi (Pook and Forrester 1984 ). Severe water deficit
can result in increased concentration of compounds favoring fungal development,
such as glucose which has been shown to stimulate growth of Armillaria spp and
enable them to grow in the presence of normally inhibitory phenols (Wargo 1996 ).
11.2.3 The Significance of Conditioning Factors
An important conditioning factor affecting the sensitivity of plants is stress his-
tory. Dreesen et al. ( 2014 ) examined the impacts of one-off and repeated periods
of drought and/or heat stress in herbaceous plant assemblages. Drought and heat
treatments reduced leaf survival to a larger extent than either heat or drought alone,
and the occurrence of successive drought and heat treatments with a low recovery
window (2 weeks) increased the leaf sensitivity to the combined stress treatment.