Wood is a natural, sustainable and organic composite characterized by its hygroscopic behaviour, orthotropic composition, and variable natural durability (Verbist et al., 2019) . Recently, the use of wood has extended social interest, which has led to a global growth in its demand (Zhu et al., 2016) . Hitherto, this demand is often more than not covered by the production of wood of low natural durability (due to biological deterioration). According to the European Standard EN 350 (CEN, 2015) , wood durability is the “resistance of wood to destruction by wood-destroying organisms”. Wood durability can vary widely depending on the species, the age of the tree from which the wood was derived, the geographical site on which the tree is growing, and the tree growing conditions (Mazela & Popescu, 2017) .

In Kenya, Eucalyptus wood is processed for sawn timber, used in building and construction and other secondary processed products like furniture. The species is also used for utility poles, which are largely absorbed in the electricity and telecommunication transmission industry. Other uses include fencing, palisade components, decking supports, railway sleepers, soil stabilization, soil terracing, and terrestrial and aquatic foundation pilings. All these uses expose wood to degradation agents at different levels, which then influences the level of durability of wood.

The use of wood is subsequently limited due to its susceptibility to wood-degrading organisms (Edlund & Nilsson, 1998) . Decomposition of wood in terrestrial and marine environments, in general, involves many biotic agents including fungi, bacteria, insects, nematodes, molluscs, and crustaceans (Gelbrich, 2009; Marais et al., 2022) . According to Juan and Rosana (2023) , the main biological agents capable of reducing the structural integrity of wood are wood-decay fungi, saproxylic beetles and termites. Biotic degradation agents such as subterranean insects and marine borers occur overshadowing the effect of fungal and bacterial decay but are restricted in geographical spreading and aquatic submissions, respectively (Clausen, 2010) .

The natural durability of wood is, therefore, key to the estimation of the service life of wooden products. The wood properties involved in natural durability relate to the structure of lignin in the cell wall, the anatomy of the xylem, nutrient availability, the quantity and composition of heartwood extractives, and the occurrence of moisture-regulating components (Juan & Rosana, 2023) . For instance, the wood moisture content influences the establishment of wood-deteriorating organisms (Juan & Rosana, 2023) while high lignocellulosic substance makes the wood cell more resistant to deteriorating agents (Andlar et al., 2018) . During cell wall invasion, wood-decay fungi combine both enzymatic and non-enzymatic mechanisms built on low-molecular-weight compounds. Often, the grabs of saproxylic beetles digest wood cell components, causing deterioration of the wooden structure (Juan & Rosana, 2023) .

Insect and fungi survival rates are also directly affected by climate change, which also affects the population of the bio-agents. Increasing winter temperatures in certain locations have been associated with changes in the occurrence and effects of a range of biotic agents because rates of overwinter survival may confine epidemics and breakouts (McAvoy et al., 2017; Martín & López, 2023) . Apart from termites and fungi, the other economically important insects causing wood degradation include beetles (Coleoptera) in the families Cerambycidae, and Curculionidae (Ulyshen, 2016) , termites in the families Kalotermitidae and Rhinotermitidae (Reinprecht et al., 2016) and carpenter bees (Sreedevi et al., 2022) . Beetles and carpenter bees can bore large galleries in wood (Sreedevi et al., 2022) . However, they normally attack debilitated or freshly felled trees and more rarely occur on wood in service (Rodríguez-Barreal, 1998) . Attack by different wood deteriorating agents is illustrated in Figure 1 .

Several bark beetles such as Ips typographus and Dendroctonus ponderosae, have been documented to have earlier emergences and prolonged flight season lengths in warming conditions (Bentz et al., 2019) . The damage by the bark beetles on wood is demonstrated in Figure 2 .

Treatment of wood with different preservatives is aimed at enhancing the durability of otherwise non-durable or less durable wood or parts of the wood. The most used preservatives in Kenya and to some extent in the East African region include water-based chromated copper arsenate (CCA) and oil-based creosote. CCA is usually formulated to contain 18.5% Copper (CuO), 47.5% Chromium (CrO₃), and 34.0% Arsenic (As₂O₅) compounds. These chemicals are pressure-impregnated into the wood targeted for outdoor purposes, to protect it from insects, borers and wood-deteriorating microbes including fungi. Copper

as a component in CCA protects wood against fungi and bacteria decay while arsenic is the insecticidal component, protecting wood from wood-deteriorating insects like termites and marine borers. Wood pressure treated with CCA wood preservative is commonly used for outdoor construction materials, transmission poles and fencing posts. Despite its potential against pests and microbes, the CCA efficacy may vary depending on the environmental conditions (temperature, precipitation) of the area from which the wood is sourced, the wood (tree species), and the wood type (heartwood or sapwood). Many wooden poles treated with wood preservatives have been observed to fail across eco-regions in Kenya and other regions possibly due to malpractices during pre-treatment and treatment procedures, in addition to a combination of abiotic and biotic factors responsible for decay (Muthike & Ali, 2021; Schultz et al., 2007) . In high-humidity or rainy regions, a small percentage of CCA compounds may gradually be released by leaching and weathering (Williams et al., 2003) . Release of these compounds can be minimized by applying finish to the surface of the treated wood, but little is known about how or to what degree finish properties affect the release of different preservatives (Hill, 2007) .

The objective of this study was to provide information on the cause of lifespan reduction of CCA-treated Eucalyptus wood which has led to questioning the efficacy and quality of CCA preservatives, and variability in wood deteriorating agents in four eco-regions of the country.

Wood is susceptible to biotic and abiotic deteriorating agents. A high proportion of degradation was observed in dryland eco-regions. This degradation was isolated to the untreated parts of the treated wood and was highly attributed to termites and in some cases, fungi decay. These findings corroborate with other earlier studies where fungi decay and termites have been considered as major

bioagents involved in the decomposition of wooden poles (Martín & López, 2023; Liu et al., 2018) .

Reportedly, climate conditions have a major influence on the biological disintegration of wood (Brischke & Thelandersson, 2014) . Despite the influence of climate conditions, the nature of soil and microorganisms present in the matter pose a higher threat compared to climate conditions (Hietala et al., 2014) . Temperature and precipitation are important climatic variables that can significantly influence the wood deteriorating agent community development, especially fungi and the decomposition of dead wood (Edman et al., 2021; Wijas et al., 2024) . This study highlighted that termite diversity and fungal populations, and their abundance are influenced by temperature, elevation and rainfall. The percentage of attack and observed wood deteriorating agents (termites and fungi) showed variations across the eco-regions. This can be attributed to climatic variations of these eco-regions in terms of temperature and rainfall. For example, eco-regions such as highlands with high rainfall and low temperatures recorded significantly high damage instigated mainly by fungi while wood poles in coastal, lakeside and dryland regions characterized by low rainfall and high temperature were prone to termite attacks. This result corroborates the findings of Palin et al. (2011) who identified Ascomycetes and Basidiomycetes as fungal classes predominantly causing decomposition on CCA-treated wood. Several other factors have been shown to influence wood degradation. For instance, Chen and Olsen (2016) have shown that some soil fungi are capable of modifying wood preservatives like CCA, hence, enabling secondary decomposition of wood by various organisms. Additionally, the complex mutualistic or parasitic influences with symbionts or natural enemies can regulate the populations of many insects and fungal flora in the soil (Kandasamy et al., 2016) . Soil contact promotes fungal colonization due to the easy accessibility and greater moisture content of wood (Schultz & Nicholas, 2002) , while heavy rainfalls or flooding, accompanied by elevated temperatures have been known to increase termite activity (Oberst et al., 2019) .

Some termite activities were hidden, and some samples that were seemingly undamaged from external surface had massive termite attacks on the inside presenting completely worn-out wood on the above ground. This was an indication of dry-wood termites following their attack on already fungi-infested wood samples. Similar observations have been reported from other studies where attacks by termites were often discovered too late, leaving the outer wood surface intact while causing substantial destruction to the interior wood material (Goodell, 2001; Woodpress.com) .

Heartwood is found in the larger and established transmission poles and not easily in fencing posts. Reduced treatability in heartwood has been identified to cause reduced penetration and retention of preservatives. According to Wang and De Groot (1996) , this can reduce the durability of heartwood compared to the treated sapwood. Coinciding results were obtained from this study, where termite attack was seen to manifest on heartwood compared to sapwood. The key wood properties involved in wood durability include the amount and composition of heartwood extractives, i.e., secondary low-molecular-weight organic compounds produced during heartwood development, though other factors such as the wood anatomy, density, lignin and moisture content are also intricate (Martín & López, 2023) . The heartwood is considered more durable than the sapwood due to the higher concentration of toxic extractive compounds, such as alkaloids, phenols, and terpenes. In general, extractive content increases from the pith towards the outer heartwood and reaches a maximum at the transition zone between heartwood and sapwood; thus, as the tree ages, the extractive content increases (Hillis, 2012) and the decay susceptibility decreases (Taylor et al., 2002) . This agrees with the results from this study where the fencing posts seemed more protected from wood deteriorating agents as manifested by minimal damage. Longitudinally, extractive concentration decreases with tree height. In many species, the outer heartwood at the base of the tree is the most resistant to decay (Cornelissen et al., 2012) . This did not agree with the results from this study since in Kibwezi and Maseno sites, termites were imposing their attack on the untreated heartwood and in some cases on the transition region between heartwood and sapwood. Wang and De Groot (1996) highlighted that for species with naturally durable heartwood such as Cedars, treatment of heartwood may not be critical. This is not the case for wood species with a less durable heartwood and treatment of heartwood is paramount for bridge construction.

The current study underlined that for effective wood treatment, the preservative must be able to penetrate the heartwood and the transition region, to ensure wood adequate wood durability. The premature failure of CCA-treated poles observed across climatic regions may be associated with poor quality of treatment, where the heartwood and the transition zone are not chemically impregnated (Lebow et al., 2003) and the heartwood may fail to provide effective durability due to the wood cells being immature (lacking full transformation into heartwood cells with enough extractives).

Termites and fungi were the main wood deteriorating agents of the utility wood poles. Termites were found in all the eco-regions with exception of highland eco-regions while fungal attacks were observed only in Highland and Lake eco-regions. This indicated that attacks by termites and wood rot fungi vary based on climatic conditions of the regions. The study highlights that the concentration of CCA is not a major factor influencing wood durability. The study, however, shows that the natural durability of Eucalyptus wood is questionable. To increase utility wood durability, treatment should target to include the heartwood part of the wood material, which was previously known to have natural durability, but seems to be prone to destruction. Further, treatment could be adjusted by the eco-region in which the poles or posts will be used. Wood species with lesser durable heartwood should be treated to the core to enhance wood durability. Higher pressure on such species is recommended during wood treatment for all uses. We recommend further studies to evaluate the mechanisms used by wood-deteriorating organisms, their ecology and mode of dispersion, as well as soil types and wood traits affecting durability of wood in service.

The authors acknowledge the financial support provided by the Government of Kenya through the National Research Fund (NRF) and KEFRI, which facilitated the study. We are grateful for the technical assistance in data collection by Mr Mogire, a staff at KEFRI—National Forest Products Research Programme, and other staff from the respective eco-regions.