When a tree falls in the forest - exploring the science behind rainforest revegetation

Article and images by Michael Reif, conservation partnerships officer, Sunshine Coast Council

When helping landowners with rainforest revegetation projects I am often asked why we advise planting at high densities, dominated by fast growing species. The short answer is competition, by planting at high densities the plants will grow faster to get sunlight and a closed canopy will be reached as soon as possible to reduce the growth of sunlight-loving weeds. The longer answer though delves deeper into the ecological processes that occur in rainforests when the dense canopy is broken by single tree falls or during larger disturbances such as cyclones and landslides.

One of the first books on rainforest revegetation I read was Rob Kooyman’s “Growing Rainforest”. After describing the types of rainforest it explains “succession”, the way rainforests naturally recover from disturbances to the forest canopy. Put simply there are 3 main groups of plants involved in the process; pioneers germinate from the soil seed bank and grow quickly in response to sunlight and create a “temporary” low canopy. They grow quickly and have a short lifespan. Kangaroo apple (Solanum aviculture) is a good example of a native pioneer. Secondary species also germinate from the soil seed bank and grow into the gap created by the disturbance. These can be small fast growing trees like poison peach (Trema tomentosa) or larger trees like black wattle (Acacia melanoxylon). Finally mature phase rainforest species establish in the shade created by the earlier phases and grow in time to replace the canopy. These mature species generally don’t germinate from the soil seed bank and are brought into a regenerating area from other nearby rainforests by fruit eating birds. It’s worth mentioning that this process can take hundreds if not thousands of years to complete so in the relatively short space of a human lifetime it’s not always easy to come to grips with.

Each phase creates the microclimate suitable for the next phase to take over, with the pioneer species having a much shorter lifespan than the mature phase species. The concept of lifespan though is also up for debate because after the pioneer and secondary species have been replaced by mature phase species their seeds lie dormant in the soil. Just how long they remain viable is not accurately known as it can be decades or hundreds of years between disturbances. So while you might not be able to see these species when you’re in a mature rainforest it doesn’t mean they are not there dormant under your feet waiting patiently for the sunlight.

When ex-TC Debbie passed over the Sunshine Coast in March 2017 the winds proved too much for one maiden’s blush (Sloanea australis) along the creek at my property in Mapleton. This tree snapped about 1.5 metres from the ground and left quite a large opening in the forest canopy. Suddenly sunlight was getting a clear path to the ground during the day for the first time in many decades if not longer.

The old saying that “a picture tells a thousand words” is applicable to the photo that was taken about 12 months after the tree fell. This is an area underneath the gap in the canopy. There was no disturbance of the soil here, the only obvious environmental change has been that sunlight can now reach the forest floor. In this small area of only a few square metres the species germinating from the soil seed bank include bleeding heart (Homalanthus populifolius), poison peach (Trema tomentosa) and black wattle (Acacia melanoxylon). One species though is quite dominant and it took a while for the seedlings to get large enough for me to identify them as pink ash (Alphitonia petriei). In this photo alone there are over 50 seedlings in just a single square metre! Now the race is on and the strongest ones will grow towards the light. Whilst most will not survive in the long run it does make me ponder just how long they have been sitting in the soil waiting for sunlight that triggers them to germinate? In the meantime I’ll keep an eye on this patch of forest to see how the race pans out. Stay tuned for further updates in the years to come.