Factors relevant to degradation are important in every wood application. For wood used in ground and water contact; well documented abiotic factors (or agents) include exposure to temperature and moisture linked to the physiological requirements of biotic degradation agents such as wood-decaying fungi and bacteria. Other biotic degradation agents such as subterranean insects and marine borers occur overshadowing the effect of fungal and bacterial decay, but are restricted in geographical distribution and to aquatic applications, respectively. This review focusses on decay specific to soil exposure. The inherent material characteristics are important to durability in that heartwood and sapwood show differences in resistance to degradation between species, provenance, and individual trees. Wood durability testing methods and classification, as well as a summary of prominent models and variables suitable for regional-level modelling of in-ground wood durability are presented.

This article presents the results from two separate studies investigating the decay of wood in ground contact using adapted versions of laboratory-based terrestrial microcosm (TMC) tests according to CEN/TS 15083-2:2005. The first study (A) sought to isolate the effect of soil water-holding capacity (WHC_soil [%]) and soil moisture content (MC_soil [%WHC_soil]) on the decay of five commercially important wood species; European beech (Fagus sylvatica), English oak heartwood (Quercus robur), Norway spruce (Picea abies), Douglas-fir heartwood (Pseudotsuga menziesii), and Scots pine sapwood (Pinus sylvestris), while keeping soil temperature (T_soil) constant. Combinations of soil mixtures with WHC_soil of 30%, 60%, and 90%, and MC_soil of 30%, 70%, and 95%WHC_soil were utilized. A general trend showed higher wood decay, measured in oven-dry mass loss (ML_wood [%]), for specimens of all species incubated in soils with WHC_soil of 60% and 90% compared to 30%. Furthermore, drier soils (MC_soil of 30 and 70%WHC_soil) showed higher ML_wood compared to wetter soils (95%WHC_soil). The second study (B) built on the first’s findings, and sought to isolate the effect of T_soil and MC_soil on the decay of European beech wood, while keeping WHC_soil constant. The study used constant incubation temperature intervals (T_soil), 5–40 °C, and alternating intervals of 10/20, 10/30, and 20/30 °C. A general trend showed drier MC_soil (60%WHC_soil), and T_soil of 20–40 °C, delivered high wood decay (ML_wood > 20%). Higher MC_soil (90%WHC_soil) and T_soil of 5–10 °C, delivered low wood decay (ML_wood < 5%). Alternating T_soil generally delivered less ML_wood compared to their mean constant T_soil counterparts (15, 20, 25 °C). The results suggest that differences in wood species and inoculum potential (WHC_soil) between sites, as well as changes in MC_soil and T_soil attributed to daily and seasonal weather patterns can influence in-ground wood decay rate.

This study investigated the effect of phenol-formaldehyde (PF) resin treatment on the weathering stability and biological durability of birch plywood. Silver birch (Betula pendula) veneers were vacuum-pressure impregnated with four different PF resins with average molecular weights (M_w) of 292 (resin A), 528 (resin B), 703 (resin C), and 884 g/mol (resin D). The aging properties of PF resin modified birch plywood were analyzed using artificial weathering with ultraviolet (UV) light, UV and water spray, and weathering under outdoor conditions. The same combinations of PF-treated plywood specimens were then tested in soil-bed tests to determine their resistance against soft-rot wood decay. It was not possible to compare weathering processes under artificial conditions to processes under outdoor conditions. However, the weathering stability of birch plywood treated with PF resins A, B, and C, scored better than plywood treated with commercial resin D (regardless of solid content concentration [%]). Results from unsterile soil bed tests showed improvements in resistance to soft-rot wood decay compared to untreated plywood and solid wood. Mass loss [%] was lowest for birch plywood specimens treated with resin of highest solid content concentration (resin D, 20%). Provisional durability ratings delivered durability class (DC) ratings of 2–3, considerably improved over untreated solid wood and untreated birch plywood (DC 5).

To evaluate the performance of new wood-based products, reference wood species with known performances are included in laboratory and field trials. However, different wood species vary in their durability performance, and there will also be a within-species variation. The primary aim of this paper was to compare the material resistance against decay fungi and moisture performance of three European reference wood species, i.e., Scots pine sapwood (Pinus sylvestris), Norway spruce (Picea abies), and European beech (Fagus sylvatica). Wood material was collected from 43 locations all over Europe and exposed to brown rot (Rhodonia placenta), white rot (Trametes versicolor) or soft rot fungi. In addition, five different moisture performance characteristics were analyzed. The main results were the two factors accounting for the wetting ability (k_wa) and the inherent protective properties of wood (k_inh), factors for conversion between Norway spruce vs. Scots pine sapwood or European beech for the three decay types and four moisture tests, and material resistance dose (D_Rd) per wood species. The data illustrate that the differences between the three European reference wood species were minor, both with regard to decay and moisture performance. The results also highlight the importance of defined boundaries for density and annual ring width when comparing materials within and between experiments. It was concluded that with the factors obtained, existing, and future test data, where only one or two of the mentioned reference species were used, can be transferred to models and prediction tools that use another of the reference species.

Construction using timber has seen a resurgence in light of global climate mitigation policies. Wood is a renewable resource, and engineered wood products are proving to be competitive against concrete and steel while having several advantages. However, while the renewable nature of wood in construction is a beneficial property for climate mitigation policies, the process of biodegradation introduces a challenge for service life planning. A review of hazard mapping is presented while developing contemporary hazard maps, occurrence maps and projected hazard maps for 2050 using representative concentration pathways (RCP) 2.6 and 8.5. The risk of timber decay is expected to increase in most of Europe as the temperatures rise, with a decrease expected in dryer regions. Termites are likely to experience a range expansion as more areas become suitable, while human activity and an increase in extreme weather events like floods are expected to facilitate dispersion. Marine borer species already present a risk in most European coastal regions; however, the effect of changes in water temperatures are likely to shift the boundaries for individual borer species. Overall, warmer climates are expected to increase the metabolic activity of all of these organisms leading to a general reduction in service life.

In this article a dose–response model was developed to describe the effect of soil temperature, soil moisture content, and soil water-holding capacity, on the decay of European beech (Fagus sylvatica) wood specimens exposed to soil contact. The developed dose–response model represents a step forward in incorporating soil-level variables into the prediction of wood decay over time. This builds upon prior models such as those developed within the TimberLife software package, but also aligns with similar modeling methodology employed for wood exposed above ground. The model was developed from laboratory data generated from terrestrial microcosm trials which used test specimens of standard dimension, incubated in a range of soil conditions and temperatures, for a maximum period of 16 weeks. Wood mass loss was used as a metric for wood decay. The dose aspect of the developed function modelled wood mass loss in two facets; soil temperature against wood mass loss, and soil water-holding capacity and soil moisture content against wood mass loss. In combination, the two functions describe the wood mass loss as a function of a total daily exposure dose, accumulated over the exposure period. The model was deemed conservative, delivering an overprediction of wood decay, or underprediction of wood service-life, when validated on a similar, but independent dataset (R² = 0.65). Future works will develop similar models for outdoor, field-trial datasets as a basis for service-life prediction of wooden elements used in soil contact.