Representing Plant Litter Insulation in Land Surface Models: A Critical Process for Simulating the Soil Thermal-Hydrological-Ecological Nexus in Cold Regions

The plant litter layer, a critical interface between the atmosphere and soil, regulates energy, water, and carbon exchanges, yet its thermal insulation effects are poorly represented in Earth System Models (ESMs). This omission hampers our ability to accurately simulate the climate-hydrology-ecosystem nexus, particularly in cold regions where soil thermal regimes control freeze-thaw processes, hydrology, and biogeochemical cycles. To address this gap, we integrated a dynamic litter layer with explicit thermal properties into the Noah-MP land surface model. Validation against global flux tower sites confirms significant improvements in simulating soil temperature and moisture.
Our results reveal that litter insulation creates a strong seasonal asymmetry in soil temperatures, inducing a net annual cooling (up to –0.69 °C) by providing stronger summer cooling than winter warming. Furthermore, it fundamentally alters soil freeze-thaw processes (FTP), but with divergent impacts: it delays the freezing end date in permafrost regions while advancing it in seasonally frozen ground, with shifts up to 40 days. The strongest modulation of freezing duration (~100 days) occurs in regions with a mean annual temperature near 10°C. We identify six distinct FTP response modes, controlled by the non-linear interplay between climate, litter thickness, and snow depth. The altered thermal and hydrological states feedback to ecosystem processes, offsetting the greening-driven gains in gross primary productivity by 20.57 ± 3.65 g C m⁻² yr⁻¹ while enhancing forest soil organic carbon stocks by 2.08 ± 0.24 kg C m⁻².
These findings demonstrate that the litter layer is a key biogeophysical mediator, directly coupling vegetation dynamics with soil thermal-hydrological states. Explicitly representing this process in ESMs is therefore essential for advancing the simulation of the carbon-water-energy nexus, improving projections of permafrost thaw, ecosystem feedbacks, and hydrological changes under vegetation greening and climate warming.