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What does a tree do in response to defoliation?

Christina Borzak
PhD Candidate
School of Plant Science
University of Tasmania

I am collaborating with Karen Barry (TIAR) and Libby Pinkard (CSIRO) who are in CRC programme 1.2.2.  We are investigating the genetic basis to variation in growth and physiological responses of E. globulus seedlings to artificial defoliation.  

My PhD research so far has shown clear population differences in E. globulus seedling recovery after severe artificial defoliation.  I conducted an experiment on two populations of E. globulus from St Helens (northeastern Tasmania) and Bluegum Hill (southern Tasmania).   The seedlings were artificially defoliated and split into a pot trial and a field trial where they were protected from further damage.  Growth traits were scored regularly up to 12 months after defoliation.  For further background information on this experiment see related article in Biobuzz 7.

Seedling recovery was linked to greater stored resources (i.e. lignotubers).  The population from St Helens developed larger lignotubers and produced more shoots in response to defoliation. The  Bluegum Hill population, on the other hand, developed smaller lignotubers with relatively few shoots, but in the longer term grew as tall as the St Helens population.  These results led to the hypothesis that the E. globulus populations from St Helens and Bluegum Hill use different mechanisms to cope with defoliation.  Previous studies by Karen and Libby have shown that E. globulus seedlings can compensate for herbivory through increased photosynthetic capacity and efficiency, but so far it is unkown if these responses have a genetic basis.

We will conduct a pot trial on E. globulus seedlings from different populations (i.e., different genetic stock), to look at variation in photosynthetic rates, chlorophyll content and resource allocation to above-ground biomass before and after a defoliation treatment.  Chemicals used for defense against herbivory (e.g., sideroxylonals and macrocarpals) will also be analysed in regrowth tissue to provide a better understanding of any induced effects of defoliation and trade-offs between plant recovery and chemical resistance to herbivory. Three E. globulus populations will be included in the pot trial: St Helens (relatively low chemical resistance to mammalian browsers), Bluegum Hill (relatively high defensive chemistry) and Jeeralang (from Victoria; intermediate levels of defensive chemicals).

The experiment will begin this summer.  Growth responses will be measured one week, three weeks, six weeks and nine weeks after defoliation.

Biobuzz issue thirteen, December 2010