240 P. Mitchell et al.
responses, and process-based/spatially explicit models could be achieved using ap-
proaches such as hierarchical Bayesian models (Cable et al. 2009 ; Metcalf et al.
2009 ; Ogle and Barber 2008 ).
11.6 Conclusions
The extent and magnitude of impacts from multiple stressors in forests are likely to
become larger in response to future shifts in climate and land-use intensification.
Building on existing perspectives of stress dynamics, we have presented a concep-
tual framework that allows us to generalize about the nature of stress interactions
with regard to a range of impacts, such as protracted forest declines and episodic
forest collapse. This perspective highlights some key issues in understanding the
mechanisms underlying stress in forest ecosystems: (1) The specific sequence of
triggers for different stressors is crucial for defining their physiological response
and impact; (2) any single event needs to be viewed as part of a longer-term re-
gime of stressors that continually shapes the sensitivity of the forest stand to sub-
sequent stressors; (3) 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. Increasing intensity of any
given stress can lead to threshold-type responses that exacerbate or diminish effects
from other stressors; and (4) recovery patterns are facilitated by changes in water
and carbon balance and can inform overall levels of stress that are not necessarily
apparent during the period of distress. While there is much progress to be made in
translating our mechanistic understanding of multiple stressors into models for pre-
dicting broad-scale impacts, this chapter raises some pertinent issues for researchers
dealing with combined stress in complex ecosystems.
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