International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.5, 229-239 http://ecoevopublisher.com/index.php/ijmec 23 3 humidity gradient, the humidity at the top layer of the tree canopy varies greatly, and most epiphytic plants have water-retaining structures and dormancy during the dry season and other adaptations. When the humidity in the lower part of the tree canopy is persistently high, epiphytes tend to maintain active growth and rely on stable humidity. For example, many mosses remain in a water-absorbing state for 24 hours and are quite sensitive to dryness (Murakami et al., 2022; Landeros-Lopez et al., 2025). 4 The Interaction Relationship Between Epiphytes and Their Hosts 4.1 Positive effects Although epiphytes mainly utilize their hosts to provide space and support, an increasing number of studies have shown that epiphytes can also have a positive impact on host trees and the entire canopy environment. Epiphytes promote the nutrient cycling of their hosts by intercepting and accumulating nutrients. Sometimes, host trees even directly benefit from the nutrients accumulated by epiphytes. Epiphytic plants also provide physical protection. The dense epiphytic moss covering the tree trunks and branches helps to reduce direct exposure of the bark to strong light and sudden temperature changes, and reduces the heat stress and sunburn risk of the host bark (Santos-Escamilla et al., 2024). In addition, some epiphytic plants can secrete compounds that inhibit pathogenic microorganisms, which may have indirect benefits for the host. From the perspective of biodiversity, the existence of epiphytes can also increase the overall environmental capacity of the host tree, attract beneficial insects and birds to stay, and thereby indirectly help the host control pests and spread seeds, etc. Experimental studies also support the positive effects of epiphytes: Garcia-Garcia et al. (2025) found through greenhouse experiments that when the dead leaves of the common epiphytic Tillandsia recurvata were added to the host soil, the survival and growth of the host seedlings could be significantly improved. This indicates that epiphytic plants enrich the soil through their litter and have a positive impact on the host. Epiphytic plant communities can also buffer the impact of acid rain or pollution on the host. 4.2 Neutral and negative effects Although in most cases, epiphytes do not pose obvious harm to the host tree, in some situations, a large number of epiphytes may cause certain stress or even negative effects on the host. Mechanical load is the main potential problem. When epiphytes accumulate and grow to a considerable biomass on the host tree, they will increase the load-bearing capacity of the host's branches. Epiphytes may compete with their hosts for space and light. When epiphytes completely cover the branches, especially when epiphytic vines spread rapidly, they will block the light from the host's leaves or cover the treetops, thereby hindering the growth of some branches of the host (Ceballos et al., 2025). Under natural conditions, epiphytes usually do not directly draw resources from their hosts, but indirect competition may occur, such as for rainwater. Dense epiphytic lichen traps a large amount of precipitation, and host branches may reduce the chance of water absorption through bark (Murakami et al., 2022). Another potential negative effect is the risk of host infection. Epiphytes themselves are not pathogenic, but their presence may alter the microbial community and humidity on the surface of the host tree, thereby affecting the development of pathogenic fungi and mosses (Mitchell et al., 2021). For instance, the bark of the host is more prone to mold growth in an overly humid environment. Epiphytic moss that remains saturated with water for a long time and adheres closely to the bark may lead to poor ventilation of the bark, which in turn can cause some fungal infections. 4.3 The preference mechanism of epiphytes for host selection The colonization of epiphytes does not occur randomly in any tree species and location. They have certain "preferences" or selection mechanisms for the host (Shen et al., 2022). This preference is related to the physical and chemical properties of the host tree on the one hand, and is also influenced by the propagation process on the other. The bark properties of different host trees are very crucial for epiphytes. Epiphytic plants often prefer tree species with rough bark and wrinkles and cracks, as rough surfaces are more likely to retain seeds, spores, water and nutrients. The Prosthechea karwinskii orchid mainly grows on the trunks and branches of Quercus plants, presenting a drooping epiphytic form. It has a high dependence on microclimatic conditions (such as humidity and
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