Counting the costs of multiple stressors: is the whole greater than the sum of the parts?

Describing a recent drought-related forest dieback event in southwestern Australia in 2011 (Matusick 2012), a colleague remarked upon the distinctive sounds of wood boring beetles feeding on weakened eucalypt trees during one of the most severe droughts on record (K. Ruthrof, personal communication). For this insect population, normally scarce and benign, drought stress had most likely triggered a surge in their abundance , thereby amplifying declines in forest health during an already stressful event. Observations of an apparent coincidence of stressors such as drought and pests are of course common across many ecosystems (Mattson and Haack 1987, Ayres and Lombardero 2000). The subsequent impacts on forest function and structure from stressors such as drought and herbivory represent complex interactions between abiotic and biotic factors (Raffa et al. 2008). During its lifetime, a tree faces a diverse range of conditions arising from the combination of climatic, soil and pest dynamics and disturbances that may induce periods of stress, culminating in reduced growth, vigour, canopy dieback or mortality. Based on the sequence, timing and intensity of these stress-ors, their associated impacts may be persistent or acute and manifest over different temporal and spatial scales. For example , recent observations of episodic mortality from acute drought stress across the USA, Europe and Australia highlight the role of single extreme events (Martínez-Vilalta and Piñol 2002, Breshears et al. 2005, Matusick 2012), while longer term forest declines result from persistent and subtle stress interactions that make identifying specific causes more difficult (Manion 1981, Close and Davidson 2004). It is increasingly suggested that forests in the future will experience increased levels of stress as a result of the direct impacts of shifting climatic patterns, increasing atmospheric [CO 2 ], land use change and associated feedbacks on processes such as pest demographics, invasive species and host susceptibility (Intergovernmental Panel on Climate Change 2007). Any assessment of future forest vulnerability must consider the differential contributions from single and multiple stress factors and how their timing, intensity and frequency may interact to affect individuals, stands and ecosystems. Climate is arguably the most influential factor affecting vulnerability in managed and unmanaged forests due to its role as a primary or underlying stress, that precedes or enables the arrival and expansion of associated biotic stressors (Figure 1a) (Kurz et al. 2008). Drought itself is not a proximal stress but the cause of multiple stresses that impact on trees (Figure 1a) and …