Dead wood is bursting with life. Decomposition sets in slowly but surely as soon as a tree is felled. Dust to carbon. But what determines the speed of decomposition? And how much does dead wood contribute to the carbon cycle? Two special graveyards for trees should provide answers to these questions.
It is Thursday morning in the first really autumnal week of the season. A group of students of Forest and Nature Management are on a field trip to the Schovenhorst estate in Garderen on the Veluwe. The ground is still wet after a heavy rain shower; there is a tang in the air. On a tree stump a bright orange, antler-shaped toadstool peeks out from a bed of pine needles. ‘The yellow stagshorn,’ says teacher Ute Sass- Klaassen (Forest Ecology and Forest Management). The atmosphere is serene. As you would expect of a solemn procession – because that is pretty much what this is. Today, Sass-Klaassen is leading the second-year students to a graveyard. All at once, there we are. On a fenced plot of about 10 by 15 metres. A few dozen saplings one metre long and 20 to 25 centimetres in diameter lie neatly arranged in the forest soil. As if laid to rest, they wait exposed to wind and weather for the inevitable decline, the unstoppable decomposition process. Because that is what Loglife, as this research project is officially called, is all about. ‘The tree graveyard is a long-term study of the breakdown of wood at different locations,’ explains Sass-Klaassen. ‘Wood behaves differently in different environments. So we have two graveyards: one here on the Veluwe moorland and one in the Flevo polder.’
The tree graveyard is the perfect spot for fungi. These organisms are real grave robbers, feeding on death. Juul Limpens (Nature Management and Plant Ecology) and her fungus team are keeping a close record of which fungi are present, in what combinations, numbers and in what order they set in. The graveyard is a paradise for them. ‘The interesting thing is that while some of the trees are in their native environment, others are on each others’ soil. So there is a very clear contrast in the spores present. You can see it as a kind of ongoing war. Which spores will win and which will lose?’ The first inspection, a year ago, revealed about 30 different species of fungus after only one year in the ground. There will be a lot more than that in due course, the researchers expect. ‘The diversity is greatest after two to three years. First you get the species whose spores are already present in the living wood, as a kind of pathogen,’ explains Limpens. ‘Under the bark, in little holes and old veins in the wood. They do nothing until the tree weakens or is felled. Then they attack. At a later stage you get the really pure wood-eaters that get blown in or come out of the soil.’ Limpens expects to see clear differences between deciduous trees and conifers. Each tree trunk carries its own unique cargo of spores. ‘The turkey tail is typical of deciduous trees, for instance, and silver leaf of conifers. But because these tree trunks are close together you get crossover too. Which is interesting.’
Duration fifteen years
Loglife is a joint project of the universities of Amsterdam (UvA), Utrecht and Wageningen, the NIOO and SHR Wood Research. Between them, thinks Sass-Klaassen, these organizations offer an ‘armada of different disciplines’ all directed at one goal: identifying what happens when wood disintegrates and what effect it has on the soil and the environment. An area in which there is much to be discovered. ‘Most of what we know about the breakdown of wood comes from wood technology. There are standard tests for the breakdown of sawn and dried wood: one example is placing wood-destroying fungi on sterile blocks of wood. But the rotting process in wet tree trunks in the forest is a dynamic process; these are ecological questions, not technological ones.’ The graveyard was established two years ago. It was a massive task, recalls Sass-Klaassen. Eighteen tons of wood was felled, sawn and transported. There is a reason the tree trunks here are no more than one metre in length: even they weigh at least 30 kilos. A total of 600 tree trunks of 10 different species were divided over the graveyards on the Veluwe and in the polder and left to rot away. Some of the trees are on ‘foreign’ soil: Veluwe trees in the polder and vice versa. What effect that has remains to be seen over the next 15 years. Loglife is an unusual project, not least because it is primarily a labour of love. ‘There is not a cent in funding,’ says Sass-Klaassen. ‘The whole project was born of curiosity. We did the field work ourselves with the help of many students and staff at Unifarm. You can’t get funding for these sorts of projects very easily. They are just too long-term. Longlife goes on for 15 years, whereas PhD students have four years. So it is difficult to involve them from the start.’
After two years in the ground most of the logs still look relatively unscathed. The bark is working loose on some of them and some are overgrown with moss. Discoloration of the top ends betrays the activity of fungi. ‘Fungi penetrate the wood grain at the ends of the trunks,’ explains Sass-Klaassen. ‘The speed at which they penetrate varies per species. Once the bark drops off, the process speeds up because the fungi can they get in through the sides.’ This process is monitored by sawing out a block of wood lengthways from the trunk at set intervals and putting it under a scanner. The first time this was done last year, it cost almost 120 tree trunks. ‘So we won’t be harvesting every year because if we do we’ll get through the wood in no time.’
The whole trial is a compromise between what is ideal and what is feasible. But that does not matter, says Sass- Klaassen: ‘The point is the relative changes: all the wood of different species is laid out under the same conditions. The experiment’s strength lies in comparison. How do different species rot under the same climatic conditions? We think you can deduce a lot from that. You can find links between wood density, as an indicator of speed of decay, and the processes that influence it. The results that will produce are of importance to climate change research, among other things, for improving models of the carbon cycle. There are still many gaps on the knowledge of these kinds of process.’
Photo: Guy Ackermans