Why are trees taller on top of hills?

That’s the question we asked our star intern from France, Emma Leroux, when she came to work with us in Canberra at the start of February. With her time in Australia sadly coming to an end, we asked Emma to tell us about the results of her scientific research.

Hi Emma, before we start talking about trees and hills, can you tell us what brought you to Australia, and how you ended up interning with Greening Australia?

Before my last year of study I had the opportunity to do a gap year in order to get work experience and improve a foreign language. I wanted to live in an English-spoken country as English might be compulsory now to find a job in some companies. The choice of Australia was quite easy as I have wanted to go there since I was 8. I looked for internships in Australia, applied to an offer posted by Greening Australia on my school’s website, and luckily got the job.

Now to your research – what exactly have you been trying to find out?

Visually, Greening Australia has observed a different growth rate of trees (Acacia and Eucalypts) after direct seeding between the top and the bottom of the hill. Trees are taller at the top of the hill and I dedicated my months with you to figure out why.

For those of us not in the know, what exactly is direct seeding?

Direct seeding consists in planting seeds where we want to revegetate, rather than pre-grown plants. There are different ways to place the seeds but Greening Australia uses mechanical planting which means seeds are placed by machine into a prepared site. One of the big advantages is that direct seeding is more cost-efficient than using pre-grown tubestocks.

Can you tell us about the techniques you’ve used to get the answers?

Different assumptions have previously been raised to explain this phenomenon, but I only focussed on two of them – soil characteristics and frost. First, I collected data about height, number of stems, genus and species in the studied transects with the help of volunteers. Then, thanks to a soil corer I was able to do soil analysis to define the texture, and with data logger thermometers I recorded the temperatures at the top and the bottom of the hill from May to August to study frost-effects.

And what have the results shown?

I noticed that trees are smaller when the clay content in soil is higher. It seems that seeds germinate better in a lighter soil and a delay of germination at the bottom of the hill might explain why trees are taller at the top. Furthermore, high clay content generates bad drainage, and waterlogging can occur after episodes of rain. Both Acacia and Eucalypts require a well-drained soil so that could have an impact. However, the frost analysis is not very significant. It is logically colder at the bottom of the hill but the difference is not big enough to explain the phenomenon.

What are you hoping Greening Australia and others will be able to do with the results?

Some techniques exist to improve the drainage but it will not always be cost-effective to do it as these techniques are quite expensive. However, by determining soil texture before direct seeding, they might be able to modify the scheme of direct seeding by avoiding sowing where the soil is too heavy.

That sounds like some really practical advice. And now that you’ve nearly finished your internship, what’s next for you?

I have to go back to France in early September and restart my school straight away. Then I have a last 6-month internship to do from March 2015, before graduating and hopefully getting a job afterwards. I am already thinking about coming back to Australia.

Thanks Emma, make sure you come and say hello if you do come back!

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And for more information about Emma’s research contact Graham Fifield at  or 02 6253 3035.

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Emma Leroux and Dean Ansell Emma Leroux and Dean Ansell

For more information about Emma’s research contact Graham Fifield at  or 02 6253 3035.