226 P. Mitchell et al.
to predict potential stress dynamics and responses in biological systems such as for-
ests (Bonan 2014 ). For example, projections of reduced water availability and con-
comitant increases in temperature might be predicted with reasonable certainty for a
particular region or landscape. Yet, predicting impacts on a forest ecosystem is diffi-
cult, given that co-occurring tree species respond very differently to drought, owing
to differences in factors such as rooting patterns, water management strategies, and
ontogeny (Koepke 2010 ; Fensham and Fairfax 2007 ; Engelbrecht and Kursar 2003 ;
Mitchell et al. 2008 ). In the case of drought and many other potential environmental
drivers, the resultant physiological stress and the associated impact is not purely
defined by the exposure (i.e., climatic drivers) to stress, but also by how exposure
interacts with the sensitivity of the organism or system to produce an impact on the
system. Sensitivity encompasses many factors, including genetic/phenotypic traits,
soil conditions, and stress history for a particular site. Thus, it is important to con-
sider physiological stress for an individual as an interaction between components of
exposure and sensitivity in determining what factors are important for understand-
ing vulnerability of forests to potential stress-inducing factors (Mitchell et al. 2013 ).
In this chapter, we examine how different abiotic and biotic factors combine
to induce stress in trees, and its impacts on forest health more broadly. Some rel-
evant conceptual frameworks are introduced that help to disentangle interrelations
between the drivers of stress and interpret the range of impacts often described
and observed in forests under stress. Examples of combined stresses are used to
emphasize that physiological stress commonly arises through the joint contribution
of primary, secondary, anthropogenic, and conditioning factors. The relevance of
intensity, frequency, and duration of the individual and combined stress is discussed
in conjunction with how they moderate physiological distress and recovery. A large
focus of this chapter concerns stressors associated with global climate change, with
a particular emphasis on associated increases in drought. However, insights gleaned
from these examples are pertinent to many other types of stresses in natural and
managed forest ecosystems.
11.2 Conceptualizing Multiple Stressors and Their
Consequences for Forests
The causes and consequences of changes in forest health and condition can be
viewed as a continuum of responses that are related to the temporal scale at which
they impact on forest health (Fig. 11.2). At one end of this continuum lie forest
declines or diebacks, which can be characterized as a protracted malfunction of tree
health and a progressive decline in stand vigor and productivity over time (Mueller-
Dombois 1988 ). Forest declines tend to occur over decades or even generations
(Fig. 11.2). Forest declines tend to be driven by a combination of biotic and abi-
otic stressors, often involving multiple trophic-level interactions and a strong role
from human influences (Jurskis 2005 ; Manion 1981 ). An example of forest decline
involving complex trophic interactions is the phenomenon known as Bell miner
associated dieback in Australia. This form of forest decline became common in the