Weakening of Hydrologic Episodes Leads to Biogeochemical Disruptions in Northern Forests

Abstract:

Hydrologic processes are cardinally affected by climate change. In northern forests, climate change is leading to a loss of winter and a weakening of hydrologic high flow events. Hydrological high flow events, or episodes, are pulses of water the promote hydrologically driven terrestrial export of biogeochemical constituents (such as carbon and nitrogen) to proximate waters, influencing the productivity of downstream waters (streams, rivers and lakes). Hydrological intensification (i.e., the increasing strength of hydrological cycles due to increased energy inputs that manifests in the intensification of dryness in dry locales and/or periods, and of wetness in wet locales and/or periods) is leading to hydrologic disruptions (i.e., shifts in the timing, magnitude, and flowpaths contributing to hydrologic episodes). Here, the effects of climate change on hydrologic episodes on biogeochemical processing are explored over a 30-year (1980s to 2010s) period. Climate changes characterized by warmer and drier conditions lead to (1) earlier onset and longer duration of the vernal window (transition from spring to summer), (2) a lengthening of forest growing season and increase in forest productivity; and (3) more subtle changes in the autumnal window (transition from autumn to winter). These catchment changes are associated with hydrologic episodes during spring melt and autumn storms that are coming earlier that are smaller in magnitude in narrower in timing, and a reduction in biogeochemical export to surface waters. A time series in catchment discharge-solute concentration (Q-C) plots reveal hysteresis patterns that show a systematic shift from source-limited (less resistance, faster flow of water, where nutrients in the water are less processed) to transport-limited (more resistance, slow flow of water, where constituents in the water are more processed). Catchment export of nitrogen is more susceptible to these hydrologic shifts than carbon. The loss of winter, weakening of hydrological episodes, and reduction in terrestrial export of nutrients will have cascading effects on downstream waters, including the likely loss of productivity of downstream surface waters.