1. Introduction
Climatic changes are expected to drastically modify growing conditions for forest trees in the coming decades [
1], primarily driven by changes in the frequency, intensity and duration of extreme events [
2]. Reports of forest dieback associated with drought and heat events are becoming increasingly common and have been reported from a wide variety of forest types [
3]. Further examples of drought-induced forest dieback have recently emerged from southwestern United States [
4,
5], the Mediterranean Basin [
6] and from southwestern Australia [
7,
8]. Water-limited forest environments (e.g., Mediterranean-climate ecosystems, MCEs) are thought to be particularly prone to dieback due to projected reductions in rainfall and increasing temperatures [
2,
9]. In fact, assessing the impacts of drought-induced mortality has been recently recognized as a research priority for Mediterranean ecosystems [
10].
Extreme drought and heat waves have the potential to cause catastrophic ecosystem type changes, transforming forest composition, structure and ecosystem functioning on a sub-decadal timescale [
11] (although see discussion on existence of stabilizing processes minimizing and counteracting the effects of these events [
12]). Differential species-specific susceptibility to drought can favor more tolerant species, thus shifting forest composition [
13,
14]. For example, a rapid (<5 year) landscape-scale shift of a woody ecotone between semiarid ponderosa pine forest and piñon-juniper woodland in response to a severe drought in northern New Mexico was reported by Allen and Breshears [
15] and has persisted for more than 40 years. Such ecosystem responses will be determined by the intrinsic traits of species to withstand drought (resistance traits), respond to drought (resilience traits), or the ability to adapt or migrate [
16]. Compositional shifts in ecosystems are expected in the coming decades with climatic changes [
17].
In contrast to many temperate species, those from MCEs are recognized as being highly resilient to disturbances through a wide range of morphological, ecological and phenological adaptations [
18,
19]. After disturbance, resprouting is a common trait in many trees and shrubs in MCEs and allows for the persistence of many perennial plant species [
20]. Australian Myrtaceae (especially
Eucalyptus and its allies) are well known as successful epicormic and basal resprouters [
21], as well as
Quercus and
Adenostoma in California, North America [
22,
23] and
Acacia and
Olea in South Africa [
24]. However, little is known about the response of these species to drought and the role of drought in driving shifts in composition.
There are several studies that have investigated the susceptibility and response of Mediterranean species following drought events, for example, in mixed conifer forests in the western United States [
25], in woody vegetation in central and Southern Spain [
26], and in pine forests in France [
6]. Determining the relative susceptibility of co-occurring tree species to drought, and their recovery, is particularly important for determining the potential for compositional changes and ecosystem shifts with continued climate change. Here, we aim to determine the susceptibility and response pattern of the two co-occurring dominant overstory tree species
Eucalyptus marginata Donn ex Sm. and
Corymbia calophylla R. Br. K.D. Hill and L.A.S. Johnson, to extreme drought in a Mediterranean-type forest in southwestern Australia. Specifically, we examine their relative probability of experiencing: (a) partial crown dieback, (b) complete crown dieback; (c) resprouting; and (d) tree mortality, assessed 16 months following the drought.
3. Results and Discussion
Eucalyptus marginata had a higher probability of experiencing both partial (
F = 4.11,
p = 0.0431) and complete crown dieback (
F = 28.33,
p < 0.0001) compared to
C. calophylla (
Figure 3). Of those trees experiencing dieback, both species had an equal probability of being able to resprout following dieback (
F = 0.35,
p = 0.5554), despite
E. marginata having significantly taller sprout heights (3.3m
vs. 2.3m,
F = 4.30,
p = 0.0389) compared to
C. calophylla by 16 months following drought. Overall,
E. marginata had a higher probability of tree mortality after 16 months (
F = 7.53,
p = 0.0063) as more of this species experienced dieback at the outset.
This study has highlighted contrasting strategies and thresholds to a sudden and severe drought by co-occurring species in a Mediterranean-climate type forest. We have shown that after 16 months
E. marginata had a higher probability of being affected by the drought, and a lower probability of surviving the drought compared with
C. calophylla. These findings after 16 months are different from recent work [
8] that found at six months there was no difference between these species in terms of the percentage of trees that remained alive. Thus, it seems that longer term shifts in composition may develop at these sites. Such shifts in composition have been noted elsewhere following drought events, such as in semiarid ponderosa pine forest and pinyon-juniper woodland in northern New Mexico [
15] and in pinyon-juniper woodland in northern Arizona [
34]. Nonetheless, although such drought events have the potential to induce changes in forest composition directly by altering species-specific tree survival patterns, as seen here, permanent vegetation change will occur only if recruitment patterns are also affected [
14]. This will be the focus of future research.
Figure 3.
The probability of Corymbia calophylla and Eucalyptus marginata trees experiencing: (a) partial crown dieback; (b) complete crown dieback; (c) resprouting (of trees that experienced dieback); and (d) overall tree mortality, following drought in the Northern Jarrah Forest, Western Australia. An asterisks indicates significantly higher probabilities from generalized mixed models at alpha = 0.05.
Figure 3.
The probability of Corymbia calophylla and Eucalyptus marginata trees experiencing: (a) partial crown dieback; (b) complete crown dieback; (c) resprouting (of trees that experienced dieback); and (d) overall tree mortality, following drought in the Northern Jarrah Forest, Western Australia. An asterisks indicates significantly higher probabilities from generalized mixed models at alpha = 0.05.
There are clear differences in drought resistance among co-occurring species, which can originate from different physiological responses to drought [
34,
35] and species-site interactions [
36]. The hydraulic framework proposed by McDowell
et al. [
11] suggests that trees are more likely to die via hydraulic failure under sudden and intense droughts, however, strict control of stomata closure under those conditions is predicted to be beneficial. Although both species in this study are able to tolerate drought by closing stomata early in the drought cycle,
C. calophylla seems to have a greater capacity to do this than
E. marginata [
37]. This might explain the lower mortality in
C. calophylla during the sudden and severe drought event in this study. A more detailed investigation of the differential physiological characteristics of the co-occurring species at the study sites is needed.
Eucalyptus marginata is known to maintain relatively high transpiration rates during summer [
38], owing to its deep root system accessing stored water [
39]. These high transpiration rates may make this species more vulnerable on shallow soils that have limited connection to groundwater during extended periods of drought [
40]. Given that the drought-affected sites in this study are associated with sites at high elevations, on steep slopes, and close to rock outcrops [
28], this event illustrates the site-driven vulnerability of
E. marginata during drought. Site-driven responses have also been observed in a mixed hardwood-loblolly pine forest in Georgia, in terms of a soil moisture and transpiration interaction. The study showed that in the upslope section, which had shallower soils, transpiration became limited by soil moisture [
41]. This is because more water is stored in deeper soils, and removing the same amount of water from a section of deep soil and from a shallow soil results in a faster depletion of moisture in the shallow soil [
41].
Given the patterns outlined above, one could suggest that C. calophylla has strong resistance traits to drought (e.g., osmotic adjustment), and, since there was no difference in the level of resprouting between the species, both species have equally strong resilience traits following drought. It is currently unknown whether these differential responses will lead to longer term changes in co-dominance within drought-affected patches. If C. calophylla is able to maintain functioning crowns, produce flowers, attract pollinators to such a contracted resource and set viable seed more quickly than E. marginata, it has the potential to drive longer term compositional changes. Further research will offer critical information on longer-term health trajectories, canopy seed bank dynamics, natural recruitment, the drought-fire interactions that are likely to occur in this system, and the implications of shifting forest composition on faunal species that are reliant on these canopy species.