## 3 Results and Discussion
### Morphological Decay Pattern
The degradation pattern, e.g., the intact MLs and unspecific orientation of fungal hyphae, suggests that white-rot and soft-rot fungi seemed not to be involved in the decomposition of this archaeological poplar [21; 49]. The hyphae observed in the sections are present only in cell lumina and colonize adjacent cells passing through the pits. This clearly indicates that these fungi are not ligninolytic or are less so. Some brown-rot fungi can only change the lignin structure [55; 56] and could therefore also fall under this category. Unfortunately, no clear evidence for a brown-rot infestation could be found. It is also possible that the hyphae have colonized the wood only after salvage, e.g., due to inadequate proper storage. A trace of bacterial degradation was observed in a single cell of the tangential section with the help of polarized light (Figure 4e), by showing the rhombic-shaped cell wall losses. This degradation pattern is a typical sign of erosion bacteria in wood from wet and near-anaerobic environments, e.g., waterlogged archaeological wood and foundation piles [9; 21; 22; 29; 55]. As it was observed by light microscopy, the SEM micrographs of the archaeological elm also confirmed that the severe degradations occurred in wood cells (Figure 5a), although no evidence of fungal hyphae was found. The cells were mainly degraded, and no sound cell was found in the cross and tangential sections, which might be due to the shrinkage of the sample during the drying process [50]. This might be related to the deformation of cells during sectioning. The residual degradation materials in the cell lumens are obvious in Figure 5b,c (blue arrow). Grainy structures, as a sign of bacterial erosion, are visible in the ray parenchyma cells (white arrow in Figure 5c and blue arrow in Figure 5d). The ray parenchyma contains a higher quantity of sugars in comparison with fibers and vessels, as they are part of the nutrient reserves of the trees [21], and it is expected that those cells are susceptible to various degrading agents. Very recently, [PERSON] et al. [56] revealed that the degradation of the parenchymal cell wall is delayed relative to its adjacent tracheas when they are exposed to erosion bacteria. However, it did not exhibit entire resistance to bacterial decay. The degradation patterns of the archaeological elm illustrated that the erosion bacteria were the main degrading agent. Although it was assumed that the fungi might have been involved in the degradation, i.e., according to the results obtained from chemical analysis, there were no fungal hyphae detected by microscopical analyses to approve it. The scanning electron microscopy (SEM) micrographs of archaeological poplar are shown in Figure 6a-d. These images clearly illustrate the high degree of degradation and deformed cell walls, as indicated by white arrows. The hyphae are obvious in the cell lumens penetrating through the cells (blue arrow in Figure 6a). As observed by the light microscope, the SEM micrograph of the cross-section with higher magnification also showed that the MLs are intact (Figure 6b, blue arrows). The tangential section of archaeological poplar demonstrated that, despite the degradation of the cell structure, the intercellular pits (white arrow in Figure 6c) and perforation plates (blue arrow in Figure 6c) are still clearly visible. The grainy structured residues, a typical pattern of bacterial degradation [38], were found on the outer part of the cells (yellow arrow in Figure 6b). The hyphae, which are penetrating through the vessels (yellow and white arrows in Figures 6c and 6d, respectively), and grainy structured residues are also obvious in the tangential section (blue arrow in Figure 6d). Investigation of the morphological decay pattern together with the chemical analyses confirmed that the degradation in the archaeological poplar was most likely due to erosion bacteria.