Are the patterns of regeneration in the endangeredEucalyptus gunniissp.divaricatashifting in response to climate?

Abstract

Increasing drought frequency is a major driver of changes in forest structure and has been implicated in the decline of the endangered tree species, Eucalyptus gunnii ssp. divaricata (McAulay & Brett) in the Central Plateau region of Tasmania, Australia. In this study, we examined patterns of regeneration, aspects of the water relations of E. gunnii ssp. divaricata and its replacement Eucalyptus pauciflora and, whether shifts in stand dominance have occurred where the subspecies co‐occurs with E. pauciflora could be related to recent changes in climate. Successful E. gunnii ssp. divaricata seedling regeneration was restricted to micro‐sites with relatively deep soils within slight depressions. In contrast, poor E. gunnii ssp. divaricata regeneration and declining adult cohorts of this species all occurred on steeper, concave micro‐sites with shallow soils. This apparent shift in suitable regeneration micro‐site, from sites with shallow to deeper soils, may be linked to an observed 25% reduction in summer rainfall over the last 50 years. On slopes surrounding waterlogged depressions where E. gunnii ssp. divaricata co‐occurs with E. pauciflora, E. pauciflora was in higher abundance than E. gunnii ssp. divaricata in small adult and sapling size‐classes, compared with the adult cohorts (>30 cm d.b.h.), a trend consistent with a shift in stand dominance. Despite existing paradigms related to differential drought tolerance between these two species as a driver of this shift in stand dominance, there were no differences in predawn (Ψpd) water potentials between species. Furthermore, pressure–volume analysis showed that E. gunnii ssp. divaricata had lower values for osmotic potential at turgor loss point (−2.33 ± 0.06 MPa) than E. pauciflora (−2.13 ± 0.03 MPa), suggesting that E. gunnii ssp. divaricata may be more drought tolerant than E. pauciflora, in contrast to the prevailing paradigm that it is more susceptible to drought than E. pauciflora.

Understanding the regeneration niche: Microsite attributes and recruitment of eucalypts in dry forests

“…Other microsite studies (Lamont et al, 1993;Sanger et al, 2010) have concluded that the relative safeness of a seedling recruitment microsite depended on its effect on water availability. Our results support and expand on this principle, indicating that, seedling microsites within Tasmanian dry eucalypt forests are characterised by ashbeds with: soft soil that facilitates root growth and water capture; soil that is less water repellent than the general forest floor; low cover of grass (limiting competition between grass and eucalypt seedlings for soil water); charcoal with possible water retention and insulation properties and coarse woody debris that provides shelter and an adjacent reservoir of stored soil moisture.…”

“…These protected microsites were described as physical microhabitats, or ''protected sites created by, for example, stones, depressions or fallen logs'' (Potts, 1986). Sanger et al (2010), found that the regeneration of Eucalyptus gunnii ssp. divaricata, an endangered species from the relatively wet and cold Central Plateau of Tasmania, appeared to be shifting to microsites of increased water holding capacity in response to an observed 25% reduction in summer rainfall.…”

Applying an animal‐centric approach to improve ecological restoration

“…The climate is becoming hotter, with rainfall more seasonal and distinct dry periods (e.g. Sanger et al ). Eucalypt genetic research (Gauli ) has shown a strongly heritable variation in physiology and morphology in response to the maximum temperature of the warmest month.…”

Climate adaptation and ecological restoration in eucalypts

“…For example, at the lowest elevation trial, survival diminished in the local E. gunnii provenance compared with a provenance originating from an even lower (warmer) elevation, but this trend did not become evident until 25 years after planting (Potts 1985;Figure 8). Maximum temperatures in the region of the field trial have increased by 0.21˚C per decade over the last 50 years and late summer/autumn rainfall has decreased by 25% (Sanger et al 2011), which may have contributed to the observed change in provenance performance. This shift coincides with extensive dieback in the local E. gunnii in this region and a shift in its regeneration niche to deeper soils (Calder & Kirkpatrick 2008;Sanger et al 2011).…”

“…Maximum temperatures in the region of the field trial have increased by 0.21˚C per decade over the last 50 years and late summer/autumn rainfall has decreased by 25% (Sanger et al 2011), which may have contributed to the observed change in provenance performance. This shift coincides with extensive dieback in the local E. gunnii in this region and a shift in its regeneration niche to deeper soils (Calder & Kirkpatrick 2008;Sanger et al 2011). …”