11 Combined Stresses in Forests 227
early 1990s, and by the early 2000s was estimated to threaten 2.5 million ha of rem-
nant eucalypt forests in northern New South Wales and south-eastern Queensland
(Wardell-Johnson et al. 2005 ). Bell miner associated dieback is attributed to the
exclusion of natural enemies of leaf-feeding psyllids by high densities of bell miner
( Manorina melanophrys) populations. Dieback is triggered by increases in defolia-
tion from psyllids that may increase the susceptibility of trees to additional biotic
attack. Dieback in several eucalypt species was thought to be driven by multiple
feedbacks between forest structure and site conditions, physiological responses
that alter foliar chemistry, the abundance of sap-sucking psyllids, and bell miner
populations (Stone 2005 ). At the other end of the forest stress continuum lie those
event-driven changes in forest health caused by acute stress. These stress events
tend to operate at much shorter temporal scales (months to years) and tend to be
driven primarily by climatic factors (Jurskis 2005 ). As discussed in Box 1, episodic
stress events such as droughts are frequently characterized by multiple climatic and
biotic stressors; however, they tend to involve a less complicated set of feedbacks.
It is also worth pointing out that the initial trigger for both types of responses may
be quite similar, i.e., long-term drought, yet differences in the intensity, frequency,
and duration of the primary driver influence the rate at which forests are impacted.
Box 1 A surge in the awareness and study of drought impacts on forests in the last
decade is providing a glimpse of how multiple stressors might combine to affect tree
health and survival as a consequence of global environmental change (Allen et al.
2010 ; van Mantgem et al. 2009 ). Drought-induced tree die-off events are examples
of extreme stress events in forest ecosystems reflecting conditions beyond the toler-
ances of the affected tree species. A survey of published studies documenting epi-
sodic tree die-off events highlights the universal role that drought plays across a
diverse range of forest types, including semi-arid shrub lands through to tropical
rainforests (from Allen et al. 2010 , Mitchell et al. 2014 and unpublished data). A
clear pattern emerging from these studies is that these extreme drought events gen-
erally coincide with elevated temperatures and heat-wave events (Allen et al. 2010 ;
Mitchell et al. 2014 ). This is a well-documented climatological phenomenon that
occurs at regional and continental scales (Vautard et al. 2007 ; Lyon 2011 ). In addi-
tion to heat stress, a large proportion of drought die-off events are associated with
biotic agents (Fig. 11.3). In this survey, 25 of the 67 die-off events had some evi-
dence of biotic agents with defoliating and wood-boring insects being most common
Fig. 11.2 Responses of forest ecosystems to stress can be described as a continuum based on the
duration over which the forest is impacted and the role of different factors in mediating the stress