Unravelling Changes in the <i>Pinus radiata</i> Root and Soil Microbiomes as a Function of Aridity

Increased aridity is emerging as a key impact of climate change in terrestrial ecosystems globally. Forest biomes are particularly vulnerable to the impacts of changing environmental conditions due to their long-lived and sessile nature. Microbiomes have coevolved with plants under changing environmental conditions with shared fitness outcomes. However, both the movement of plants via domestication and rapid pace of environmental change may impact the ability of plants to recruit microbial symbionts that support environmental stress tolerance. This study investigates the effects of aridity on tree-root microbiome symbiosis, focusing on the widely planted Pinus radiata. By sampling a broad geographic range and diverse environmental gradients, we reveal how aridity, soil and climatic variables shape microbial communities in P. radiata roots and soils. Our findings highlight that while aridity significantly predicts microbial community assembly, other environmental variables such as soil pH and organic carbon, strongly influence bacterial diversity. Groups of both bacterial and fungal taxa were identified as conditionally present with aridity, underscoring their importance in P. radiata resilience under increasingly environmental stress. Based on the transition of current mesic ecosystems to arid conditions under climate change, we found these arid associated taxa vary in their frequency in bulk soils projected to become arid. These results highlight the risk that these taxa will need to be recruited by other means. Ecological filtering by the host and environmental conditions fosters a "friends with benefits" relationship, wherein certain microbial taxa provide key benefits, such as extension of phenotypic tolerance to water limitation, to the host. Both bacterial and fungal communities are shaped more by stochastic than deterministic assembly processes, suggesting a complex interplay of host and environmental factors in community structure formation. The insights gained have implications for understanding the resilience of tree species and the ecosystem services they provide under future climate scenarios.