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response of a tree species to the combined stressors of drought and herbivory, and
demonstrates why predicting the impacts from stress combinations under chang-
ing intensities is notoriously complex and difficult. The impact of combined stress
on some element of plant function, e.g., growth, tends to be significant at low-to-
moderate intensities of the stress, whereas as drought stress intensifies, the primary
stressor becomes the dominant impact on plant function. This example helps to
emphasize why consideration of the appropriate physiological thresholds related
to water and carbon balance will help to differentiate between the effects of single
and multiple stressors. Thus, it is critical that researchers can measure and report
stress intensity using parameters such as soil and/or plant water potential, percent-
age change in leaf area or leaf temperature.
11.4 Recovery from Multiple Stressors
An essential component of characterizing the complete response to stress is defin-
ing recovery. Recovery can be defined as the ability of an individual to resume
prestress function, such as the return to some mean growth rate, canopy structure,
or level of productivity. Recovery from stress is often not considered in the study of
stress tolerance in herbaceous species, but for long-lived woody species, assessing
recovery from stress can provide a powerful insight into the resilience of a species
or forest ecosystem. Time to recovery can be a useful metric to understand the se-
verity of the stress event, and tends to increase with increasing impairment of plant
functioning, i.e., hydraulic dysfunction (Brodribb and Cochard 2009 ). Tracking the
trajectory of growth or carbon gain beyond their initial decline or distress period,
helps to reveal the degree to which the stress was transient, delayed, or sustained. It
can allow us to elucidate the key physiological recovery mechanisms involved, such
as remobilization of carbohydrates, hydraulic repair, recovery of leaf biochemistry,
or the action of heat shock proteins (Peñuelas et al. 2013 ). Mechanisms of recovery
are often associated with metabolic costs that delay a return to prestress levels.
Brodribb et al. ( 2010 ) found that the recovery of gas exchange via the restoration of
hydraulic conductance tracks the growth of new xylem tissues, suggesting that the
recovery imposes significant carbon costs after drought. Recovery strategies such
as resprouting, enables a canopy suffering severe damage from dieback or herbivo-
ry to be rebuilt, and for the re-establishment of prestress rates of growth and carbon
gain. While many species such as eucalypts, draw on a large store of nonstructural
carbohydrates for rapid recovery (Pinkard et al. 2011 ), canopy recovery in taxa
such as Pinus spp. is often limited due to a smaller pool of stored carbon (Galiano
et al. 2011 ; Mitchell et al. 2014 ), particularly following multiple stress events. Con-
trasting recovery strategies are reflected in differences in life history, ontogeny and
resource-use, and will have important consequences for water and carbon balance
during and after stress. For example, the ability to resist and recover from repeated
drought events was influenced by plant size in the resprouting Quercus ilex, pre-
sumably due to differences in rooting depth and the size of the carbohydrate store