234 P. Mitchell et al.
Interestingly, treatments that had longer recovery times (3.5 or 6 weeks) did not af-
fect leaf and whole-plant survival (Dreesen et al. 2014 ). Studies in woody species
show that the impact of repeated drought on photosynthetic capacity is dependent
on the intensity and frequency of the drought regime (Liu et al. 2010 ). Incomplete
recovery in some species was only observed after the third severe drought cycle and
was attributed to stomatal limitations on photosynthesis (Liu et al. 2010 ). In another
study, the increasingly incomplete recovery of photosynthesis was associated with a
reduction in the maximum quantum efficiency relative to control plants, pointing to
significant metabolic impairment of the photosynthetic apparatus (Liu et al. 2010 ;
Gallé and Feller 2007 ).
Soil conditions can exert a strong control on the development of stress from pri-
mary climatic drivers by controlling how plants match water uptake with demand.
While a mismatch in water uptake and demand is characteristic in plants experienc-
ing drought stress, plant water uptake can be impeded in frozen soils. In boreal for-
est ecosystems, a delay in soil warming in frozen soils compared with air warming
at the end of winter can result in plant water deficit even when soils are wet (Repo
et al. 2005 , 2008 ). The disjunct between root dormancy (or slowed metabolism)
and increased shoot growth can induce xylem cavitation (the process of air filling
and blocking xylem conduits), and lead to reductions in tree growth, photosynthetic
efficiency, and plant water potential (Larsen 1993 ). Thus, rapid climate change in
boreal forests that alters patterns in frost and/or soil and air temperatures has the po-
tential to introduce stress combinations via delays in thawing events and/or earlier
starts to spring-time growth.
Fig. 11.5 Summary of
how acclimation to drought
stimulates increases in foli-
vory activity. An increase in
drought stress triggers (a) a
shift in the foliar metabolome
and a concomitant increase
in the concentrations of
antioxidants, nitrogen, and
sugars (b). These changes
in foliar chemistry promote
higher folivory activity on
drought-stressed trees (c).
Increases in folivory can also
stimulate an upregulation in
the concentration of plant
defensive compounds such as
terpenes and phenolics (d).
(Modified from Rivas-Ubach
et al. 2014 )