Vulnerability to cavitation is linked to home climate precipitation across eight eucalypt species

Vulnerability to cavitation in leaves is the result of highly adaptive anatomical and physiological traits that can be linked to water availability in a species' climate of origin. Despite similar gross leaf morphology, eucalypt species are often confined to specific climate envelopes across the variable rainfall environments of Australia. In this study, we investigate how the progression of cavitation differs among eucalypts and whether this is related to other hydraulic and physical leaf traits. We used the Optical Visualisation technique to capture cavitation progression across the leaves of eight eucalypt species (Angophora crassifolia, Corymbia tessellaris, Eucalyptus atrata, Eucalyptus grandis, Eucalyptus laevopinea, Eucalyptus longifolia, Eucalyptus macrandra, Eucalyptus tereticornis) from a wide range of climates and grown in a common garden setting. Vulnerability to cavitation, represented by the leaf water potential required for 50% cavitation of leaf vessels, varied significantly among species (-3.48 MPa to -8.25 MPa) and correlated linearly with home climate precipitation and leaf SLA (R2 of 0.64 and 0.75, respectively). P12-P88, the range of water potentials between which 12% to 88% of cavitation occurs, was decoupled from P50 but also correlated with leaf SLA (R2 of 0.72). We suggest the magnitude of P12-P88 may be representative of a species' drought strategy - a large P12-P88 signifying leaves that exhibit drought tolerance (retention of leaves under drought conditions) and a small P12-P88 signifying drought avoidance (leaf shedding after a threshold of drought is reached). Our results agree with other studies that highlight these cavitation metrics as genetically fixed traits. Turgor loss point, on the other hand, may be more plastic, as evidenced by the low variability of this trait across these eucalypt species grown in a common garden environment. Further study will help to establish the SLA-related anatomical traits that impart cavitation resistance and to extend these conclusions to a greater number of species and home climates.

Red List assessment of widespread and long-lived species

The use of criterion A of the IUCN Red List to categorize species as threatened that have undergone recent decline can lead to the listing of relatively common and widespread species. Loss of habitat through deforestation is a common cause of decline throughout much of the world but is often not incorporated into assessments because of uncertainty about the magnitude of change. A recent assessment of eucalypt species in Australia subject to deforestation provides a method for assessment under criterion A and has implications for listing of long-lived, widespread species affected by deforestation. Scenarios for two widespread eucalypt species subject to extensive deforestation are used to demonstrate how the threat status of a species may be recategorized in a lower threat category as declines resulting from a threatening process are mitigated. I argue that criterion A indicates an appropriate assessment of extinction risk and I provide a simple function based on predicted diminishment of the population decline to identify when a species could be disqualified from a threat category under subcriterion A2 (past decline).

Early silvicultural performance of eucalypt species grown in environmental conditions of the tropical dry forest of Colombia

The establishment of new forest plantations requires making important decisions starting with the selection of the species to be planted, as well as the choice of an adequate planting spacing to guarantee the maximization of wood production. The aim of this study was to evaluate the performance of nine eucalypt species planted with three different spacings under the environmental conditions of the tropical dry forest of the inter-Andean valleys of Colombia. A split-plot arrangement with two replicates was used as the experimental design. The planting spacing factor (three planting spacings) was assigned to the main plots and the species factor (nine eucalypt species) was assigned to the subplots. Survival and growth were evaluated at twenty-four months of age. The study identified the Brazilian provenances of Eucalyptus camaldulensis Dehnh, Eucalyptus urophylla S.T. Blake, Eucalyptus grandis W. Hill, and Eucalyptus pellita F. Muell. and the Colombian provenance of Eucalyptus pellita F. Muell. as promising for commercial reforestation programs in areas with a water deficit in the tropical dry forest. Planting spacings of 3×2 m (1666 stems·ha-1) and 3×2.5 m (1333 stems·ha-1) maximized the production of basal area and the volume for the species evaluated. Finally, no interaction was detected between species and planting spacing factors, therefore, the species identified had a better performance regardless of the planting spacing used.

Chloroplast Distribution in the Stems of 23 Eucalypt Species

Small diameter branchlets and smooth barked stems and branches of most woody plants have chloroplasts. While the stems of several eucalypt species have been shown to photosynthesise, the distribution of chloroplasts has not been investigated in detail. The distribution of chloroplasts in branchlets (23 species) and larger diameter stems and branches with smooth bark (14 species) was investigated in a wide range of eucalypts (species of Angophora, Corymbia and Eucalyptus) using fresh hand sections and a combination of bright field and fluorescence microscopy. All species had abundant stem chloroplasts. In both small and large diameter stems, the greatest concentration of chloroplasts was in a narrow band (usually 100–300 μm thick) immediately beneath the epidermis or phellem. Deeper chloroplasts were present but at a lower density due to abundant fibres and sclereids. In general, chloroplasts were found at greater depths in small diameter stems, often being present in the secondary xylem rays and the pith. The cells of the chlorenchyma band were small, rounded and densely packed, and unlike leaf mesophyll. A high density of chloroplasts was found just beneath the phellem of large diameter stems. These trees gave no external indication that green tissues were present just below the phellem. In these species, a thick phellem was not present to protect the inner living bark. Along with the chlorenchyma, the outer bark also had a high density of fibres and sclereids. These sclerenchyma cells probably disrupted a greater abundance and a more organised arrangement of the cells containing chloroplasts. This shows a possible trade-off between photosynthesis and the typical bark functions of protection and mechanical strength.