THE RELATIONSHIP OF SOME POPULATIONS INVOLVING EUCALYPTUS CYPELLOCARPA AND E. GLOBULUS TO THE PROBLEM OF PHANTOM HYBRIDS

Abstract

Summary A taxonomically anomalous population of eucalypts at Mallacoota, Victoria, is mapped and studied in detail. Measurements of adult and seedling morphology show the population to be intermediate between Eucalyptus globulus and E. cypellocarpa. However, on leaf terpenoid data the population is indistinguishable from E. cypellocarpa. The very restricted distribution of the population, and the high variability of its adult morphology suggest that it arose by hybridization between the two species named above. As the nearest stand of E. globulus is 6.4 km away, the population may represent a phantom hybrid stand. It is inferred from distribution data that E. globulus stands may have been present in the area before submergence by rising sea levels. If any pure E. globulus trees remained, a preponderance of E. cypellocarpa pollen may have prevented the continued production of more preponderant E. globulus progeny.

Eucalypt decline in Australia, and a general concept of tree decline and dieback

“…Norton (1887) saw no evidence of pre-European tree decline in the landscape, and the natural distribution of eucalypt ecosystems in south-eastern Australia has been relatively stable for the last 5000-10,000 years (e.g. Kirkpatrick et al, 1973;Barlow, 1981;Singh et al, 1981). These ecosystems have persisted through countless short-term climatic extremes as well as longer term climatic fluctuations such as the Medieval Warm Period and the Little Ice Age (e.g.…”

Assessing a Bayesian Approach for Detecting Exotic Hybrids between Plantation and Native Eucalypts

“…Seven putative E. cypellocarpa × globulus hybrids from Victoria are as follows: one was collected from a mature native tree in mixed E. globulus/cypellocarpa forest; two were tentatively identified as "possible hybrids?" (with a high degree of uncertainty based on morphology) from beside an E. globulus plantation; and four were from a population that has been speculated to be a phantom hybrid zone [46,47]. Four putative E. viminalis × globulus samples were collected in Tasmania, one identified on the basis of seedling morphology in open-pollinated progeny from a native E. globulus tree; the other samples were collected from mature native trees with intermediate bud and capsule morphology.…”

“…After morphological and chemical analysis, a previous study concluded that the population is most likely of hybrid origin and represents a genetic remnant of the past distribution of E. globulus [46]. The current population is at sea level and it was hypothesised that the E. globulus source population was probably flooded when sea level rose after the last glacial maximum [46]. This situation would result in the population being made up mainly of backcrosses (to E. cypellocarpa) or 2 hybrids.…”

“…All trees in the population appear to be intermediate to varying degrees between E. cypellocarpa and E. globulus, but despite E. cypellocarpa occurring nearby, the nearest native E. globulus tree is 6.4 km away-hence the "phantom" hybrid zone [88]. After morphological and chemical analysis, a previous study concluded that the population is most likely of hybrid origin and represents a genetic remnant of the past distribution of E. globulus [46]. The current population is at sea level and it was hypothesised that the E. globulus source population was probably flooded when sea level rose after the last glacial maximum [46].…”

“…For example, the putative E. cypellocarpa × E. globulus hybrids collected at Mallacoota come from one of the first reported examples of a phantom hybrid zone [46,47,87]. All trees in the population appear to be intermediate to varying degrees between E. cypellocarpa and E. globulus, but despite E. cypellocarpa occurring nearby, the nearest native E. globulus tree is 6.4 km away-hence the "phantom" hybrid zone [88]. After morphological and chemical analysis, a previous study concluded that the population is most likely of hybrid origin and represents a genetic remnant of the past distribution of E. globulus [46].…”