Exploring Drivers of Leaf Bacterial Community Dynamics Across Temperature-Driven Phenology in Two Temperate Tree Species

Abstract

Leaf microbiomes are vital to host and ecosystem functions, yet their interplay with host phenology remains ill-resolved in temperate forests, where climate change is shifting phenological timing. We investigate how cumulative growing degree days (GDDs)—an index of the active, useable heat energy directing phenology—shape leaf bacterial dynamics and ecological strategies, comparing two functionally similar but phylogenetically distant trees: Grey birch (Betula populifolia) and trembling aspen (Populus tremuloides). 16S rRNA amplicon sequencing captured variation across seven GDDs from June–October 2018 and relative impacts of host identity and GDDs on bacterial diversity, taxonomic enrichment, and niche breadth were tested. Host differences in amplicon sequence variant (ASV) richness were primarily shaped by GDDs, increasing non-linearly and non-monotonically with GDDs in birch but remaining stable in aspen; Shannon diversity was unaffected by GDDs and remained consistently higher in aspen. Beta diversity diverged seasonally; birch microbiomes underwent marked restructuring at higher GDDs, while aspen exhibited compositional overlap. Overall, birch hosted more GDD-limited ASVs, with disproportionate enrichment of generalists like Endobacter and 1174-901-12, whereas aspen hosted more even communities of specialists like Nocardioides and Pelomonas. The prominence of unclassified ASVs emphasizes a need to resolve microbial “dark matter”. Niche analysis showed that ~81% of ASVs were thermal specialists, but species-specific patterns emerged: birch leaves supported communities with wider niches, while aspen maintained narrower niches. Though typically used to predict phenology, our results suggest that the cumulative thermal environment—as tracked by GDDs—imposes ecological filtering on leaf microbiomes based on thermal niche compatibility. Together, this study advances our understanding of microbial ecology in situ and expands the utility of the GDD metric.