12 years of assembly patterns in saproxylic beetles suggest early decay wood as ephemeral resource patch.

The ephemeral resource patch (ERP) concept provides a framework for understanding how finite, short-lived resources shape community assembly processes at both patch and landscape scale. Some of these theories and principles can be applied to intermediate-lived resources, such as deadwood, but this remains largely unexplored. We tested three ecological mechanisms of community assembly (more-individuals hypothesis, habitat-heterogeneity hypothesis and habitat filtering) to investigate whether beetle assemblages in deadwood fit the ERP concept. We tracked saproxylic beetle communities in experimental logs of Norway spruce (Picea abies), European silver fir (Abies alba) and beech (Fagus sylvatica) in the temperate mountain forest of the Bavarian Forest National Park over a 12-year decomposition period, from the early decomposition stage until near-complete resource depletion. Beetle abundance and number of species declined consistently in all tree species until the 4th year but increased again in spruce after ~8 years. Species richness (number of species controlled for abundance) showed inconsistent patterns over time: U-shaped for spruce, weakly hump-shaped for fir and no temporal effect for beech. Habitat filtering was more pronounced in the early stage as functional diversity was initially low but increased for all tree species up to 4 years, then plateaued and increased again after ~10 years for both conifers. Conditional inference tree identified two temporally distinct beetle assemblages (years 1-3 and 4-12), and strong differences within the first 4 years. Our findings suggest that the more-individuals hypothesis and habitat filtering are key mechanisms driving community assembly in saproxylic beetles. Early decomposition stages supported functionally similar assemblages, highlighting this phase as a critical period for decomposer community structuring. Synthesis. The consistency of the early successional trajectories of beetles suggests that the early stages of deadwood decomposition up to the 3rd year in the temperate zone follow ephemerality theories similar to those of short-lived ERPs, while the advanced stages provide a habitat for a more random combination of beetle species. Furthermore, our findings highlight the need for temporally continuous deadwood input, via natural processes or staggered retention during logging operations, to provide coarse woody debris for wide range of saproxylic beetles.