Seeing the wood for the trees: research reveals the survival secrets of forest trees

Species extinction or `biodiversity loss` has accelerated at an alarming rate over the past century. Although much of the blame has been laid at the door of human activity, biologists are looking at the factors that influence how species-particularly similar species-co-exist, in their efforts to better understand how the balance of species can be maintained.

New research into forest trees by Dr Colleen Kelly of the Division of Ecology and Biodiversity at the University of Southampton has shown that trees of similar species growing together in a forest will follow different cycles of success in achieving maturity. In successful forests these cycles complement each other allowing both species to grow without one displacing the other.

`We know a lot about how different species co-exist in nature, but the means by which similar species can also co-exist-without one necessarily dominating the other-is a central question in ecology,` says Dr Kelly. `It is also one of growing importance as humans become increasingly responsible for the construction and maintenance of `natural` areas, in cities and countryside.`

The report of Dr Kelly`s study, written in collaboration with Dr Michael Bowler of the University of Oxford, has been published in the journal Nature this month.

Dr Kelly`s research team looked at trees in one of the most diverse deciduous forests in the world, near Puerto Vallarta on the Pacific coast of Mexico. They found that while one species might be more sensitive to some fluctuating factor in the environment, such as drought or pests, another would grow less rapidly but be better at withstanding adverse conditions.

`The slower growing tree has to spend energy to produce a larger, deeper root system to stave off drought effects, or to make chemical defences that repel insects, explains Dr Kelly. `If the young of the two species meet, the faster growing species will be able to outgrow and essentially displace the other.`

`In times of plentiful rainfall or low levels of pests, the young of the fast growing species survive and the others do not. But when conditions deteriorate the seedlings and saplings of the “slow but sure” species are the ones that prosper.`

The study also showed that although there will be periods when no young are able to survive because the trees that make it to adulthood are so long-lived, the total numbers of adults in the forest as a whole does not fluctuate.

However, the cycles of seedling and sapling survival do leave their mark in the adult population. `There will be gaps in the ages of the adults to show the times when no young trees managed to make through the gauntlet of enemies and catastrophes,` adds Dr Kelly. `This process is called “storage” dynamics because the ability to recover from the bad years when the young trees don`t survive is “stored” in the reproductive ability of the standing adults.`