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Sampling and analyses of wood and soil on European level

It was hard to compare the 27 sites sampled because of their large variety in soil type, groundwater level and age. It can be concluded that wood degrading bacteria exist in a wide range of environmental conditions and is active in timbers from different ages ranges from 65 - 3000 years. Although the process of degradation is not fully understood it is clear that velocity is environmental and species depended. From the long-term measurements site we learn that in two different Amsterdam environments (one more active then the other one) pine sapwood is not degraded by bacteria within a period of 18 months. Whereas degradation in a lower level is found only after 300 years under salty conditions in site 12 or after 600 years in Venice.

Regarding the foundation sites only, the following general trends can be seen:
1. There seems to be no relation between the degree of decay and the surrounding environment;
2. There seems to be a relation between the nitrogen content in the wood (and phosphors content) and the degree of degradation;
3. The transport of nitrogen (and phosphorus) to the inside of the wood seems to be crucial for active bacterial wood degradation. The origin of nitrogen could be explained by (or a combination of) A) already available in the wood before installation of the piles; B) active accumulation by bacteria (e.g. nitrogen fixating bacteria); C) water flux through the piles; D) diffusion
4. Because bacterial wood degradation is found under oxygen free circumstances and without a "continuous" stream of oxygen supply, it seems that oxygen is not a crucial factor to initiate and support the process of bacterial wood degradation. However an increasing oxygen concentration seems to speed up the process of degradation as showed in the microcosm experiment (chapter 4. But this could be caused by secondary wood degrading fungi, which feed themselves with bacterial debris and stimulate in this way the bacterial activity. Wood under these circumstances shows more "colonising fungi", which could be actually secondary wood degraders. Therefore the role of oxygen is unclear, but it is probably of minor importance while high degradation velocities were found in deep soil layers, where no oxygen can be expected.
5. It is hypothesed that a water-flux through the wood supports the process of bacterial wood degradation because of the processes as explained at point 3 and 4. The higher degree of degradation in pine compared to spruce under the same conditions (e.g. site 3 Amsterdam) can be explained by the flux theory. But also the high degradation in relative short pine piles, in situations without a difference over longer time in water pressure between the deeper and shallower groundwater, shows that specific events (e.g. temporary lower groundwater, heavy rain) causes a water flux in the wood which is probably enough to support bacterial wood degradation.

6. The degradation velocity within Spruce piles is lower than that of pine but within 100 years a peal of 1-20mm is weakly degraded by bacteria. These differences are clear by comparing the degree of bacterial wood degradation of those sites were whole piles were extracted. The pine-line: In Amsterdam (site 3, pile length 11m), Zaandam (site 6 pile length (6) and Haarlem (site 4, pile length 5m) pine piles were extracted and all piles are degraded over the whole length and only in the sapwood. Haarlem: severe degradation both at the head and at the tip; Amsterdam severe to weak degradation both at the head and at the tip; Zaandam severe to weak degradation at the head and weak to sound at the tip. The Spruce line: Amsterdam (site 3, pile length, 11m) weakly degraded in the outmost 20mm in both the head and the tip; Rotterdam (site 5, length 16m) weakly degraded in the outermost 0-10mm in both the head and the tip.

Regarding the archaeological sites site only, the following statements can be made:
1. Because of the large diversity no specific conclusions could be made;
2. Because the sites were chosen with the expectation that bacterial wood degradation would occurs, sites with oxygen were ignored. If oxygen in measurable concentrations occurs fungal activity will appear and destroy wood structures in relative short periods. At all sites, except for Bryggen, the main cause of the degradation was bacterial activity, although often-colonising hyphes were observed;
3. Almost all samples, ranging in age from 300 - 2000 years, were over full diameter degraded by bacteria. Although the degree degradation is age independent, in most of the samples the wood degrading bacteria were still active. This means that not the infection, but the velocity of bacterial wood degradation is regulated by the conditions under which the wood is storied. As no environmental (soil chemistry) dependency was found it is also here supposed that the presence of a water flux could be crucial.

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Stichting Hout research
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