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Late developer reaches maturity ... after 29 years

Dynamics of a mixed mating system

Rod Griffin - image 1

Canopies of 29 year old outcrossed E. regnans trees in the Mount Worth trial.

Mark Reynolds (contractor to HVP) measuring tree diameters.  The density of trees was 441 stems/ha and the density of leeches was estimated at 10 /m2.

­Rod Griffin - Image 3

A younger Rod Griffin assessing his trial in 1988.

­Rod Griffin
School of Plant Science
University of Tasmania

'Swamp gum' or 'mountain ash' (Eucalyptus regnans) is one of the main eucalypts of the production forests of Tasmania and Victoria. In 1979 I planted a trial (EP27) at Mount Worth in Gippsland, Vic. on land now owned by Hancock Victorian Plantations Pty Ltd (HVP). The primary aim was to quantify the effects of inbreeding on growth and survival, which interested me as a breeder and seed orchard manager. I and my collaborators in CSIRO [Griffin A.R & Cotterill P.P (1988) Silvae Genetica 37:124-131]  and UTas (Hardner and Potts 1997) have published various results.

We made a set of controlled selfs and outcrosses among 13 trees from natural populations in Victoria at Narracan and Thorpdale and also collected open-pollinated seed from the parents. An incomplete block design with 12 replicates of 3-tree non-contiguous plots was used to compare the growth and survival of the families.   Inbreeding depression for volume growth was 37% by age 4. The open-pollinated seedlots were intermediate, consistent with an isozyme outcrossing estimate of 0.55 (1= fully outcrossed, 0 = fully self fertilised) across the Narracan and Thorpdale samples. Eucalyptus regnans, in common with all the other eucalypts since studied, thus has a mixed mating system (i.e. both selfing and outcrossing occurs), which is interesting as most of the published models of mating system evolution suggests that this should not be a stable condition. Once genes for selfing have appeared in a species we are told that it should evolve to fixation for either selfing or outcrossing. So, in all these species are we observing transition to one of these states, or are there in fact mechanisms which actively maintain stable mixed mating systems?

I have always been more comfortable with empirical observation and experimentation than theoretical modelling (“don’t let data get in the way of a good model” was a comment from one of my eminent colleagues that really says it all), so I revisited this trial, and also the fairly substantial literature on the regeneration and reproductive ecology of E. regnans, for evidence.

Eucalyptus regnans regenerates after disturbance by fire at a mean interval of between 75-150 years [McCarthy M.A., Gill A.M. and Lindenmayer D.B. (1999)  Forest Ecology and Management 124: 193-203] so early and prolific seed production will have little selective advantage. However strong vegetative growth should enhance the chances that a genet will be one of the few survivor­s as the stand self-thins from several hundred thousand germinants per hectare to about 150 at maturity.  Maybe this self thinning favours the more vigorous outcrosses so that complete outcrossing is normal in the effective breeding population at regeneration, but the ability to self does have a selective advantage under exceptional conditions where outcross pollination is inadequate for some reason? In EP27, by age 29, the stocking has reduced to 441 live stems/ha, about what is expected under natural conditions. Self-thinning has not been at random with respect to mating type. All but 3% of the selfs have died, while 48% of the outcrosses from the same parents are still alive and 26% of those trees carried capsules. A dominant position in the canopy increases the chances of flowering, but is not itself sufficient to guarantee it. The diameter range of the living trees is between 185 and 670 mm. Of the 34 trees with diameters greater than 500 mm, only 16 had capsules (47%).  None of the surviving selfs have flowered, so they would not contribute to the seed rain if a fire came through tomorrow. The same competition will have occurred in the naturally mixed open pollinated progenies (22% of survivors now with capsules), so we have pretty strong evidence that even at this relatively young age the reproducing population is effectively outcrossed.  The remaining question is “what mating system is represented in the seed carried by these trees?”

The canopy of the stand is closed and individual crowns small and it seems unlikely that  the 1 in 4 trees which carried any flowers would have attracted a strong array of pollinators. The chances of outcrossing would have been further reduced by phenological variations. My prediction therefore is that the outcrossing rate in this seed crop could be even  lower than in the original seed collections from the previous generation at at Narracan and Thorpdale.  If so, we have demonstrated the process whereby the population cycles between mixed mating types at the seed stage and fully outcrossed at reproductive maturity as a result of competitive elimination of the less vigorous inbreds. Such a system would prevent the build up of inbreeding in the local populations of E. regnans but allow reproduction when mates for cross-pollination are scarce, such as in the later successional  stages of these wet eucalypt forests.

We plan to sample the seed when the stand is felled in the next year and determine the outcrossing rates using molecular markers to test this prediction. Anyone want to help with the modelling?