232 P. Mitchell et al.
forests and reduce the impact of future drought through acclimation and reductions
in competition for soil water (Lloret et al. 2012 ). Patterns in stress response and
recovery reflect the current state and conditioning of the system thereby influencing
the severity of future stresses (Loehle and LeBlanc 1996 ; Niinemets 2010 ). Third,
the contribution from primary, secondary, conditioning, and anthropogenic factors
will vary according to their magnitude (intensity, frequency, and duration) and how
they overlap in time and space. Some relevant examples are presented below that
highlight how plants respond to different combinations of the primary, secondary,
anthropogenic, and conditioning factors presented in Fig. 11.4.
11.2.1 Combinations of Primary Factors
At their extreme, low air temperatures can cause freezing injury including cell burst,
damage to foliar and stem tissues, and death (Clements and Ludlow 1977 ). At sub-
lethal levels, low temperatures capable of causing frosts inhibit rates of photosyn-
thesis through limiting the rates of the biochemical reactions of photosynthesis.
There can also be a light-dependent decrease in photosynthetic efficiency termed
cold-induced photoinhibition, which can amplify the impacts of frost (Davidson
2004 ). Successive sublethal frost events reduced photosynthesis of Eucalyptus
globulus and E. nitens saplings growing in southern Australia between 9 and 17 %.
High early morning light conditions following a frost event contributed to pho-
tosynthetic reduction via photoinhibition, but only before midmorning (Davidson
2004 ). Many tree species can acclimate to frosts which reduces photoinhibitoin
effects (Long et al. 1983 ), although the effectiveness of acclimation varies between
species. For example, photosynthesis of cold-acclimated E. nitens recovered within
a day following a frost event, whereas in E. globulus it took 3 days to recover to
pre-frost levels (Davidson 2004 ).
The combined stressors of waterlogging and salinity are common to many re-
gions where disturbance has led to an increase in dryland salinity from increased
water tables or increases in soil sodicity and reductions in infiltration (Barrett-Len-
nard and Shabala 2013 ). Waterlogging restricts plant growth by inducing hypoxia
in the roots resulting in diminished carbon metabolism and nutrient supply (Trought
and Drew 1980 ). Responses to salinity involve osmotically mediated changes in
water status and toxic effects associated with salt accumulation in tissues (Munns
and Termaat 1986 ). Under waterlogging and saline conditions, hypoxia exacer-
bates these toxic effects and affects plant K+ nutrition (Barrett-Lennard and Shabala
2013 ). The co-occurrence of waterlogging and salinity can induce similar or larger
reductions in gas exchange in eucalypt species depending on species tolerances to
either of these stressors (van der Moezel et al. 1989 ).
The increase in atmospheric [CO 2 ] is thought to increase water-use efficiency
during drought due to decreases in stomatal conductance, a common response ob-
served in tree species exposed to elevated [CO 2 ] (Ainsworth and Rogers 2007 ).
However, this leaf-level response may be negated where increases in leaf growth
and vegetation cover under favorable conditions enhance stress impacts during ad-
verse conditions. This has been demonstrated at the stand-level, where those stands