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Extreme climatic events such as regional droughts are likely to produce rapid, profound and long-lasting effects on ecosystems and landscapes if large numbers of individuals of dominant or key structural species are killed (IPCC 1996). In tropical forests, drought-related tree mortality can produce important compositional shifts by removing drought-sensitive species (Condit 1998). In temperate broadleaved forests, drought-induced gaps may become the dominant disturbance and therefore be the major mechanism of tree-replacement (Clinton et al. 1993). In semi-arid woodlands, drought may cause massive dieoffs that contribute to ecotonal shifts along moisture gradients (Betancourt et al. 1993; Allen & Breshears 1998; Swetnam & Betancourt 1998) and accelerate shrub invasion (Grover & Musick 1990).
If global-scale models are to incorporate interannual climatic variability, mechanistic insights will be needed to link climatically anomalous events with physiological or demographic processes (Walker 1996) such as drought-induced tree mortality. Although factors such as environmental stress, biomass allocation patterns, ontogenic changes in susceptibility, growth variability, previous growth trends and genetic variability in drought resistance, that predispose adult trees to remain alive or die during severe droughts (Cobb et al. 1994), are critical for understanding and predicting community-level responses to altered climatic regimes, they are poorly understood.
Tree mortality events related to increased ENSO-related variability (e.g. the warm events of 1983 and 1997, Condit et al. 1995; Williamson et al. 2000, respectively) have been monitored in permanent plots in tropical regions. This has allowed quantification of mortality rates and assessment of the resilience of ecosystems to episodic droughts, whereas, in temperate regions, examination of annual rings allows retrospective analysis of the consequences and causes of drought-induced mortality (Cherubini et al. 2002). Dating of death and establishment, in combination with live tree ring width data, offers information on mortality and establishment rates as well as on growth responses of trees surviving droughts that occurred more than 100 years ago (e.g. Kitzberger et al. 1995; Villalba & Veblen 1997a; Villalba & Veblen 1998). On the other hand, for more recent mortality events, tree rings offer a unique opportunity to compare recent growth conditions and long-term growth patterns between trees that died and those that have survived the drought (Pedersen 1998; Ogle et al. 2000; Wyckoff & Clark 2002). Such comparisons provide information on how biomass allocation patterns, drought acclimation, tree decline (expressed as climatic sensitivity) and long-term growth patterns may predispose individual trees to mortality.
Drought may produce instantaneous large-scale water shortage (such that supply falls below the normal range of variability), and thus have an adverse effect on growth. However, plasticity may produce phenotypes that can offset the potential growth-limiting effects of drought, via a range of physiological and morphological process. If instantaneous effects predominate, trees in more water-limited situations will be more prone to die. Predictions at different spatial scales (from coarser to finer) are that trees located at the dry end of a rainfall gradient, on sun-facing slopes, on dry ridges or on shallow soils will be more prone to die, as will exposed individuals. Trees that experience higher interannual growth variability and higher climatic sensitivity are expected to have increased mortality risk. On the other hand, plasticity may lead to drought-induced death occurring more in trees that normally grow in an environment with a relatively constant water supply. These individuals, which develop shallow root systems and have high above/below-ground biomass ratios, may suffer more from extreme water shortage and high temperatures than trees periodically exposed to such shortages. Trees located closer to streams, in deeper, well-developed soils or growing at higher densities would then be expected to suffer higher mortality.
Over a longer time frame, and if background mortality is more diffuse, these processes may interact with overall decline in populations. The decline-disease theory (Manion 1981) suggests that a concatenation of stress factors acting over long periods during a trees’ lifetime may be responsible for its death. Integration of the multiple stresses faced by a tree leads to a growth decline and this is proposed to be the main factor involved in the onset of tree weakening and increased susceptibility to subsequent stress (Monserud 1976; Pedersen 1998). Manion's model predicts that factors such as previous droughts or insect attacks may lead to symptoms of decline, including partial crown dieback, insect and hemiparasite infestations, sharp growth declines and slow sustained radial growth, and the affected trees will become susceptible to death from factors such as a new drought. The cumulative effects of repeated drought on tree mortality are important in the context of communities changing in response to changes in mode and amplitude of climatic variability. For example, red oak mortality in the Appalachians during droughts in the 1970s was, in part, related to a prior severe drought in 1925 (Stringer et al. 1989).
The forest steppe-ecotones of northern Patagonia are particularly sensitive to interannual climatic variability, which affects tree demography both directly (Villalba & Veblen 1997a, 1998) and, indirectly, via fire regimes (Kitzberger & Veblen 1997; Kitzberger et al. 1997; Veblen et al. 1999). This has important consequences for community dominance, landscape composition, configuration and shrub/grass encroachment (Veblen & Lorenz 1987; Kitzberger & Veblen 1999). Episodes of massive mortality of Austrocedrus chilensis (D. Don) Flor. et Boult over the past c. 90 years coincide with exceptionally dry springs and summers during the 1910s, in 1943–44 and in the 1950s (Villalba & Veblen 1997a, 1997b, 1998; Villalba et al. 1998). Similarly, on the eastern side of the Andes, strong recurring earthquakes affect mixed Nothofagus dombeyi (Mirb.) Blume–A. chilensis forest and the occurrence of strong seismic events during periods of drought resulted in localized episodic tree mortality on unstable substrates (Kitzberger et al. 1995).
Abundant dead-standing trees and partial crown dieback are conspicuous features of northern Patagonian Nothofagus-dominated forests (Veblen et al. 1996). Although the aetiology of this dieback is not fully understood, decline has been attributed to post-fire cohort senescence in combination with negative effects triggered by seismic events or previous droughts (Veblen et al. 1996). Understanding the predisposing causes of drought-induced regional-scale Nothofagus mortality in northern Patagonia would allow generation of new conceptual models of forest dynamics that incorporate dieback as a source of opportunity for tree replacement in the context of variable climate and previous disturbances.
During 1998–99, strong La Niña conditions in the western tropical Pacific and an anomalously high polarity of the Antarctic Oscillation (i.e. a strengthened polar vortex; Thompson & Wallace 2000) led to severe drought in northern Patagonia (annual rainfall was the lowest since recordings began in 1905). This climatic event resulted in massive mortality of the 25–40 m tall evergreen tree Nothofagus dombeyi (coihue) near its eastern distributional limit towards the Patagonian steppe. In Nahuel Huapi National Park, more than 11 000 ha of forest (c. 10% of the N. dombeyi forests) had > 25% mortality and c. 680 ha had > 75% of trees killed (Bran et al. 2001).
We examined the factors involved in determining Nothofagus mortality following this drought. At the stand scale, we proposed that if mechanisms related to the instantaneous effects of drought are dominant, episodic mortality would be greatest at the xeric ends of water availability gradients (eastern xeric populations, those with low basal area, on north-facing slopes, rocky ravines or thin soils and non-riparian forests). However, if drought tolerance overcompensates for these effects, we would expect to find higher mortality at intermediate situations (central submesic forests, high basal area, east/west facing slopes, riparian/coastal forests, valley bottoms). If decline predisposes a tree to death, we additionally expected to find higher mortality in forests with evidence of decline (e.g. high density of dead-standing and partially killed trees, heavy infestation with boring insects or mistletoe).
At the individual scale, we expected that more exposed, climatically sensitive trees and trees with large year-to-year growth variation would be more prone to die if instantaneous effects predominated, but trees within closed stands with relatively high leaf area, little climatic sensitivity and low interannual variability in growth would be more affected if drought acclimation was important. On the other hand, if previous decline determines tree death, we expected to find lower mean annual growth and symptoms of decline (insect galleries/woodpecker cavities, partial crown dieback) in trees that died compared with trees that remained alive.