11 Combined Stresses in Forests 235
11.2.4 Contribution from Human Influences
The effects of pollution such as nitrogen deposition can play a significant role in
forest declines in the northern hemisphere. High rates of tree mortality in Japanese
red pine ( Pinus densiflora) forests were found to be correlated with early pheno-
logical development in south-facing stands and exposure to extremely low air tem-
peratures (Shan 2000 ). Acid rain played a crucial role in reducing frost hardiness,
thereby increasing the sensitivity of foliage that had developed early in the growing
season (Shan 2000 ).
Trees in highly disturbed agricultural landscapes may succumb to stress from a
variety of sources. Landsberg and Wylie [25] proposed a conceptual model of the
initiation and development of rural dieback in Eucalyptus spp. Factors controlling
leaf nitrogen, populations of leaf feeding insects or defoliation can directly promote
dieback (Landsberg 1983 ). Changes in these factors arise from a variety of differ-
ent sources including: climate extremes, salinity, excessive nutrients, changes in
conditions for insects and their predators, soil compaction, and increased competi-
tion with agricultural crops. Interestingly, they showed that the nitrogen concentra-
tion of resprouting foliage produced by weakened defoliated trees made them more
attractive to leaf feeding insects (White 1984 ), leading to a cycle of defoliation and
incomplete recovery, progressive dieback, and sometimes mortality. The case of
rural dieback in Australian eucalypts demonstrates the multifaceted nature of some
tree declines and the problems with treating single causal factors when attempting
to manage 170 fragmented and highly disturbed forests and woodlands.
11.3 How Does the Contribution from Different Stressors
Affect the Magnitude and Direction of the Stress
Response?
It is helpful to view the impact of multiple stressors in terms of whether the com-
bination of two or more stressors produces antagonistic, additive, or synergistic
outcomes for plant function such as growth or photosynthesis. Antagonistic stress
combinations produce a response that is less than would be expected from add-
ing the impact of two hypothetical stressors, stress A and stress B (the additive
response). Synergistic stress combinations result in a response that is greater than
the additive response, implying an amplifying effect from the interaction of stress A
and stress B. Manipulative experiments that simulate two common stress combina-
tions, drought and defoliation, show that growth responses can range from antago-
nistic through to synergistic. In fast-growing E. globulus, treatments involving 50 %
defoliation at a low water availability enhanced growth relative to that of plants
in the undefoliated, low water stress treatment (Pinkard et al. 2011 ). One might
expect defoliation to reduce water loss under mild drought, thereby reducing the
decline in water deficit and plant growth. However, the interaction of defoliation
of Quercus robur and Pinus pinaster (85 and 50 % defoliation, respectively) and
water deficit tends to produce additive or synergistic growth outcomes (Gieger and