Climate and climate-driven vegetation change impacts on the hydrology of a Manoomin (wildrice) watershed

Manoomin (wild rice, Zizania palustris) is an ecologically important plant and cultural staple for Native American tribes in the Upper Great Lakes region. Manoomin has been declining since Euro-American settlement due to multiple environmental stressors, including changes in water level in the shallow lakes and slow-flowing streams that serve as its natural habitat. Future climate is expected to further affect water levels through precipitation changes. Less attention has been paid to additional hydrological impacts that may occur due to climate-driven changes in the surrounding forests. Understanding the interactions among climate, vegetation type, and hydrological processes is important for integrated ecosystem stewardship in Manoomin watersheds, as well as other vulnerable wetland systems. Here we used the Community Land Model 4.5 to investigate the joint impact of climate and vegetation change on hydrological process. We conducted multiple scenario experiments considering climate variability (current versus future climate) and warming-triggered vegetation shifts (conifer-dominated evergreen versus broadleaf deciduous forest) in a Manoomin watershed on the Lac du Flambeau reservation (northern Wisconsin). Model results demonstrate that vegetation type changes alone have a minor effect on annual total runoff, which is consistent with recent field-based studies in water-abundant environments. However, due to differences in canopy structure, we found that deciduous forests yielded higher runoff and earlier peak flow during the spring season. This shift in timing and intensity could adversely affect early life stages of Manoomin, including germination/establishment failure and uprooting. The comparison between current and future climate simulation shows that a projected wetter climate in the region will lead to increases in total runoff and earlier peak flow, raising overall water levels and further threatening Manoomin. Our modeling results indicate that climate change and climate-induced vegetation change could generate excess water levels and fast flow fluctuations that harm Manoomin. Our findings will provide decision-making support for tribes in mitigating impacts on Manoomin. Our results also provide general insights into temporal ecohydrological processes that may be difficult to discern from field-based studies alone.