11 Combined Stresses in Forests 231
and includes: changes in gene expression, biochemical changes in photosynthesis,
changes in allocation patterns (e.g., reduction in leaf area) and an upregulation of
plant defense compounds (Peñuelas et al. 2013 ; Breda et al. 2006 ). Acclimation
may also induce feedbacks affecting the activity of secondary factors that can am-
plify the physiological stress further (see examples below). Anthropogenic factors
or human influences play a role in influencing forest conditions in almost every
ecosystem through deforestation, pollution, altered fire regimes, and the introduc-
tion of invasive species. These processes alter the presence and abundance of biotic
agents at multiple trophic levels and play a major role in affecting the sensitivity of
the forest to stress through changes in soil conditions, stand density, and structure.
The overview presented in Fig. 11.4 helps to highlight some key features in un-
derstanding the dynamics of stress in forest stands. First, unraveling the sequence of
triggers for different stressors is crucial for understanding how stress comes about.
Second, it is important to consider the impact of any given stress event as part of a
longer-term regime of stressors that continually shape the sensitivity of the forest
stand to subsequent stressors (Dreesen et al. 2014 ). High severity events that reduce
stand density through mortality or canopy collapse can have a stabilizing effect on
Fig. 11.4 A generalized framework for understanding the roles of primary and secondary stress-
ors, conditioning factors, and human influences to define physiological stress. Acclimation repre-
sents a feedback on conditioning factors such as stress history and stand dynamics