Incipient reshaping of the critical zone of native mountain grasslands caused by exotic pines

Background and aims The way in which roots distribute throughout the critical zone influences soil and critical zone formation, as well as water, nutrient and energy fluxes. Root distributions, however, are also influenced by how the critical zone functions. Rapid vegetation changes offer a valuable setting to disentangle these reciprocal effects, and to observe the early changes that occur in the critical zone and how this affects its evolution. Methods We took advantage of recent exotic Pinus elliottii stands established over native mountain grasslands, to compare how roots of contrasting plant life forms occupy the soil-saprolite-rock continuum. We combined root count and biomass measurements across 2-m-deep observation pits into the saprolite, coupled with topsoil removal plots describing the coarse root transport system introduced by pines. Results Pine stands had four-fold more root biomass than grasslands (1.74 vs 0.41 kg of dry matter per square meter) and allocated a greater proportion of their fine roots into the saprolite. Pines also developed a coarse root transport system with profuse superficial lateral roots and irregularly distributed sinker roots occupying deep cracks for the first time. Conclusion The establishment of exotic pines over grasslands reshaped and expanded the root zone in only four decades, highlighting the capacity of vegetation to rapidly change the occupancy and dynamics of the critical zone.