Establishment and Persistence of Trees Growing in the Channel of an Intermittent Stream in a Temperate, Karst Environment

Trees, mostly sycamore (Platanus occidentalis L.), are growing in the channel of the lower 7 km of Sinking Creek in the Appalachian Mountains of Virginia. Sinking Creek is a gravel-bedded stream that disappears into a subsurface cave network and for its lower 7 km, flows intermittently throughout the year, exposing a dry streambed. We hypothesize that there have been low-flow periods in the past conducive to tree-seedling establishment. We describe this system and test our hypothesis through an analysis of hydrologic, dendrochronological, and geomorphic data to better understand the conditions leading to tree establishment and persistence in the stream channel. To our knowledge, this is the first study on trees growing in the channel of an intermittent stream in a temperate environment. Most riparian vegetation studies along intermittent rivers and ephemeral streams focus on dryland areas, where trees have also been observed growing in the channel. Our results show that sycamores are the oldest trees in the channel, that they established during periods of low precipitation, and they are located at the upstream end of bars. In their wake, on bars, other younger trees have established. Analysis of tree roots suggest that scour of the base of some trees occurred during thalweg migration. Additionally, we present a conceptual model of the flow-tree-sediment dynamics occurring in Sinking Creek that can be further tested as part of future work. This work highlights the conditions for tree establishment in streams, which is an important threshold for any streams under projected future drying.