Climate change effects on riparian tree seed dispersal phenology, flood timing, and seedling establishment

Background/Question/Methods

Riparian forests in the arid and semiarid western US may be particularly vulnerable to climate change because climate change affects streamflow, on which riparian organisms depend, as well as local climate.  On western US rivers, spring snowmelt peak flows now occur several weeks earlier than they did a century ago, and climate-driven hydrologic models predict that this trend will intensify over the next century.   Earlier snowmelt peak flows may reduce seedling establishment of dominant, native riparian cottonwood and willow trees.  Cottonwood and willow seed dispersal typically occurs during or just after snowmelt peak flows, which increases the likelihood that their small, short-lived, non-dormant seeds will land on favorable, bare, moist surfaces created along rivers by flooding and exposed by flood recession.  Climate change is already altering the timing of snowmelt peak flows and may also alter the timing of riparian tree seed dispersal, because spring phenology of temperate plants is largely controlled by temperature.  Depending on the relative magnitude of these effects, climate change may decouple the timing of cottonwood and willow seed dispersal from the timing of snowmelt peak flows, potentially hindering seedling establishment.

Results/Conclusions

To predict effects of climate change on the relative timing of riparian tree seed dispersal and snowmelt peak flows, we linked phenological models of seed dispersal with a hydrologic model of snowmelt peak flows and forced the models with historical and future climate scenarios.  The PRMS hydrologic model was calibrated for the upper South Platte River Basin in northcentral Colorado using historical streamflow records.  Phenological models of Populus deltoides spp. monilifera, Salix amygdaloides, and Salix exigua seed dispersal, structured similarly to budburst phenology models, were parameterized using field observations over two years from 14 riparian sites in the Platte River Basin.  Model projections under historical and future climate scenarios suggest that both seed dispersal phenology and snowmelt peak flow timing in the upper South Platte River Basin are likely to shift under climate change.  The relative magnitude of these shifts varies among climate scenarios and species.  Changes in the relative timing of seed dispersal and snowmelt peak flows, together with greater water stress due to projected lower summer streamflows and warmer temperatures, may reduce the relative abundance of native cottonwoods and willows, and favor species less tightly linked to streamflow hydrology, in riparian corridors in the arid and semiarid West.