How allogenic factors affect succession in glacier forefields

In glacier forefields, the chronosequence approach is used to investigate ecological, biogeochemical and physical features of autogenic successional change as a function of time. Chronosequences rely on the central assumptions that all sites were subjected to the same initial environmental conditions and have undergone the same sequence of change, and thus sites only differ by their age. In many cases, these two assumptions can be challenged by the fact that allogenic factors (initial environmental conditions and geomorphological disturbances) may affect the rates and/or trajectories of successional sequences in a spatially and temporally heterogeneous manner. Here, we emphasize that the patterns of glacier forefield successional sequences should be interpreted as the result of (1) autogenic changes (equivalent to time since deglaciation), (2) initial site conditions and (3) geomorphological disturbances. We provide an original and up-to-date synthesis of knowledge from various fields on how initial local conditions (climate, substrate properties and resources availability) and geomorphological (hillslope, fluvial, periglacial and aeolian) disturbances may affect the evolution of glacier forefield ecosystems. Further, we present a conceptual model for glacier forefield ecosystem development whereby stochastic and allogenic factors are important in early successional stages but gradually decline thereafter, while the relative importance of autogenic processes increases over the course of successional sequences. Lastly, we summarize how biota may provide biogeomorphological feedbacks to the major types of geomorphological disturbances taking place in glacier forefields.