ECOLOGY AND GEOMORPHOLOGY OF EPISODICALLY ACTIVE INLAND DUNEFIELDS OF THE GREAT PLAINS AND NORTHERN CHINA

Recent work on dunefields in the Great Plains and north China has demonstrated that episodic dune activity occurred frequently in the late Quaternary, and has linked this activity to climate change (especially change in effective moisture). The ecological processes that are critical in determining dune mobility remain poorly understood, however. Transitions between dune activity and stability are of particular interest. Sequential Landsat imagery of dunefields in north China indicates that patches of active dunes stabilize from the edges inward. This makes sense intuitively because seed sources are at the margins of the active dunes (especially where the dunes border on lowlands with a high water table). Recovery of the Nebraska Sand Hills from widespread dune activity during the Medieval Climatic Anomaly may have followed a different pattern (although admittedly at quite different temporal and spatial scales). Optical ages from this episode become younger from the core to the margin of the Sand Hills, suggesting later stabilization near the margin. Superimposed linear and parabolic dunes, attributed to late Holocene dune activity, are much better developed around the margin of the Sand Hills than in the core. This geomorphic evidence is also consistent with more rapid stabilization (and possibly less complete activation) in the dunefield core, allowing less time for superimposed dune development. We propose that these observations can be explained by the abundant wet interdunes in the central Sand Hills, some of which persisted through severe drought, providing “refugia” for plants that quickly colonized the dunes in wet years. These persistent wet interdunes themselves result from the geomorphic processes that built the high-relief dunes of the central Sand Hills. Dune stabilization and subsequent reactivation may also be affected by a feedback involving geomorphic, ecological, and pedogenic processes. Dense vegetation cover during prolonged periods of stability will result in accumulation of organic matter, nitrogen, and fine-grained dust in dune surface soils, probably enhancing drought resistance. When the climatic threshold for activation is ultimately crossed, these soils are deflated, possibly slowing stabilization and increasing the likelihood of reactivation in the short term.