11 Combined Stresses in Forests 225
that chronic distress dominates any acquired resistance. In this generalized model of
stress, improved stress resistance in response to the initial stress involves energetic
costs and changes and the expression of different genes to trigger a suite of acclima-
tion processes (e.g., heat shock proteins, osmoregulatory compounds) that enhance
resistance to subsequent stress (Steinberg et al. 2008 ). If the stress is maintained,
exhaustion eventuates, causing chronic damage and a collapse of cellular functions
(e.g., membrane integrity, photosynthetic apparatus). While Selye’s three-phase
stress model was originally formulated to describe human physiology, it provides
a simple model of how stress-defense systems might develop in individual plants.
Because tree species are long-lived, they may be exposed to multiple cycles
of stress and/or various types of stress that act in concert to bring about changes
in plant functioning and survival. In response to a myriad of stress combinations,
trees have evolved many strategies to resist, tolerate, and recover during periods of
stress. Climate change and other human influences and disturbance have the poten-
tial to introduce novel combinations of stressors that make predicting impact from
multiple stressors exceedingly difficult. For example, changes in temperature and
atmospheric [CO 2 ] will modify the range of “normal conditions” at which species
will operate, which could have implications for recovery rates and effectiveness of
acclimation processes during acute or chronic stress events.
To date, the study of forest stress within the fields of forest pathology, entomol-
ogy, ecology, and tree physiology has taken different perspectives regarding the sig-
nificance of multiple stressors. Forest pathology and entomology have sometimes
assumed that epidemics of insects or fungi and the associated stress were dominated
by single causal factors (Mueller-Dombois 1986 ). This perspective has often failed
to explain the causes and consequences of major pest outbreaks in forests, because
it tended to ignore other contributing factors such as stand-level dynamics and cli-
matic variation (Mueller-Dombois 1987 ; Akashi and Mueller-Dombois 1995 ). Plant
physiologists tend to explore stress by minimizing inherent complexities of stress
events through careful experimental manipulation that focuses on specific respons-
es to stressors such as drought/water deficit or salinity. These studies provide an
important mechanistic basis for how plants respond and cope with stress, but are
rarely of sufficient scale and design to properly consider the impact of multiple
stressors and changes in their intensity, duration, or frequency. Ecologists attempt to
explore the impacts of one or multiple stressors in the field through observation of
natural and human-induced gradients in environmental conditions. However, these
studies are often retrospective and must disentangle layers of complexity from ob-
served impacts and scant mechanistic information. A more holistic picture of forest
responses to stress involves an appreciation of the mechanistic and physiological in-
sights within the context of complex trophic interactions, spatial and temporal vari-
ation in the landscape, and their role in triggering a hierarchy of responses within a
population or ecosystem. To start gaining a deeper understanding of environmental
stress and its multifaceted nature, it is important to consider these challenges using
conceptual frameworks through which the system can be viewed.
Understanding changes in forest health in the face of rapid climate change pres-
ents further challenges surrounding how we utilize the wealth of climate projections