Cryptic Degradation in Urban Forests: Decoupled Collapse of Soil Methane Sink and Understory Regeneration

Urban forests are typically assessed by canopy metrics, assuming that tree persistence indicates ecosystem health. We present evidence of "cryptic degradation" (a functional decoupling between canopy biomass and belowground processes) using 25 years of data from the Baltimore Ecosystem Study (BES). We document two concurrent phenomena: (1) a 59% reduction in soil methane (CH4) uptake beginning abruptly circa 2008 (p < 0.001), and (2) a 70% decline in forest floor regeneration (seedlings and saplings) between 1998 and 2015, despite a 29% increase in tree basal area over the same period. We propose that these patterns reflect a common theme: degradation of soil physical structure reducing both gas diffusivity and seedling establishment. Drawing on invasion ecology literature, we hypothesize that the replacement of deep-burrowing earthworms (Lumbricus spp.) by surface-feeding invasive earthworms (Amynthas spp.) has altered soil architecture, though we acknowledge this mechanism remains untested at these sites. This is a hypothesis requiring field validation, and we invite collaboration from soil ecologists, invasion biologists, and the BES research community to test these predictions.