Co-development of physical and biological variables on glacial hillslopes over millennia

• Multi-disciplinary chronosequence analysis reveal synchronous physical–biological change. • Hillslope Development Index (HDI) quantifies long-term landscape evolution. • Key proxies enable efficient assessment of landscape maturity across timescales. Glacial retreat, volcanic eruptions, and erosion by landslides, rivers, and humans create barren landscapes where soils and vegetation develop over decadal to millennial timescales. Previous studies have documented the development of individual physical, chemical, and biological parameters in post-disturbance landscapes. However, the rates governing the co-evolution of soils, vegetation, and biogeochemical cycling remain obscure. Here, we present a multi-disciplinary analysis of 39 variables that reflect the storage and release of energy and matter on chronosequences of morainal hillslopes ranging in age from 30 to 14,000 years in two glacier forelands. We find a remarkable similarity in the timescales of physical and biological changes on these moraines. Based on this finding, we propose a ‘hillslope development index’ (HDI) that represents and quantifies the integrated hydrological, biogeochemical, and ecological state of development. In our glacier forelands, the HDI increases rapidly in the first 1000 years and then starts to asymptote, achieving a quasi-steady state at 5000 years. We find that a few key variables closely follow the HDI. These proxies allow us to assess the integrated hydrological, biogeochemical, and ecological state of hillslope development with limited experimental effort.