Encountering Fungal Crossroads: How the Ambrosia Symbiosis Influences Wood Decay and Fungal Communities in Diverse Hardwood Trees

Trees store the majority of living terrestrial biomass. When trees die, specialized wood-decaying fungi release the carbon stored in their wood. This release of organic carbon to atmospheric carbon dioxide impacts carbon cycles, nutrient cycles, and climate. However, the rate of wood decomposition depends on fungal competition and on which fungi colonize the deadwood first. Fungus farming beetles known as ambrosia beetles are among the first organisms to colonize dying trees. They bore into wood and inoculate it with a variety of fungal symbionts. Their fungal partners have been thought to increase tree decay, but recent research suggests they may actually slow decay by competing with wood-decaying fungi in the wood of conifers. I hypothesized that non-native fungi associated with recently introduced and hyperabundant non-native bark beetles suppress wood decay and compete with native fungi. I found that the majority of ambrosia beetles in Williamsburg, Virginia were non-native, and ambrosia beetles readily colonized all 4 tested hardwood tree species (American Beech [Fagus grandifolia], Red Maple [Acer rubrum], Bitternut Hickory [Carya cordiformis], and Southern Magnolia [Magnolia grandiflora]). My study utilizes Lindgren Funnel collections, a beetle exclosure experiment, and DNA metabarcoding to observe the ambrosia symbiosis and measure its impact on wood decomposition in hardwood tree species. I found that ambrosia beetles significantly increased fungal diversity in wood. However, contrary to my hypothesis and previous work in pine wood, hardwood logs exposed to ambrosia beetles experienced more mass loss than logs without ambrosia beetle exposure. My results show that the effects of introduced ambrosia beetles and their symbiotic fungi on carbon cycling in forest ecosystems is more complex than previously thought because the effect changes direction with forest type (i.e., conifer versus angiosperm). As more non-native symbioses invade, it is imperative that we understand under which conditions the ambrosia symbiosis enhances or reduces wood decay and the native ecosystem’s unexplored response.