Effects of an Environmental Flow Release and Active Revegetation on Riparian Seedling Establishment along the Bill Williams River
Patrick B. Shafroth*1 and Eduardo González2
1U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO, USA; shafrothp@usgs.gov
2Colorado State University, Department of Biology, Fort Collins, CO, USA; edusargas@hotmail.com
Restoration of dryland riparian ecosystems in the western USA is often synonymous with promoting the establishment of trees and shrubs in the Salicaceae family (cottonwoods and willows). Natural establishment of Salicaceae taxa is understood to be linked to components and functions of natural flow regimes, and managing streamflows to provide these functions is an often-advocated restoration approach (an example of implementing “environmental flows”). Active revegetation, such as planting of Salicaceae seedlings, is another restoration approach that has been implemented across the West and can be important where propagules are limited and/or other requirements for seedling establishment are lacking. We studied seedling establishment responses to both an environmental flow release and seedling planting along the Bill Williams River in western Arizona. The flow release occurred in early April 2018 and included a high flow peak designed to prepare seedbeds, and a gradual flow recession designed to expose sites suitable for Salicaceae seed germination and early seedling establishment. Seed release of dominant Salicaceae species (Populus fremontii and Salix gooddingii) occurs from March to May in the study area; however, recent mortality of many of the mature Salicaceae trees along the Bill Williams R. raised concerns that there would be very little natural seed production. To overcome this potential lack of propagules, some greenhouse grown P. fremontii seedlings were planted in conjunction with the environmental flow release. We designed a field experiment in four ~1 km-long reaches of the Bill Williams (two high flow channels, and two low flow channels), each differing in hydrogeomorphic properties that we hypothesized could lead to different seedling responses. Within each reach, we established 12-30 experimental plots, each of which consisted of a pair of 10 m2 sub-plots located on a distinctive landform (e.g., bar, floodplain). In one of the sub-plots, we planted 20 rooted cuttings (ca. 15 cm tall), while the other sub-plot (“control”) was unplanted. We assessed the survival and growth of planted and naturally-established seedlings, as well as other vegetation and several environmental variables one month and six months after planting. Survival of planted seedlings six months after planting was 53-80%, except in the driest of the four study reaches, where it was 8%. Volunteer P. fremontii established in all but the driest of the four reaches, with an average density of 0.1-0.7 seedlings/m2. Volunteer density was considerably lower along high flow channels. Density of volunteer seedlings was much lower in the planted plots, likely due to trampling effects during planting. Growth of both planted and volunteer seedling was impressive; the tallest individuals grew 300-400 cm in 6 months. The percent foliar cover of P. fremontii in planted sub-plots was from 2 to 13 times (depending on reach) greater than that of volunteer seedlings in control sub-plots, with the exception of the driest reach where survival of planted seedlings was low. In summary, planting P. fremontii in conjunction with an environmental flow release resulted in greater density and growth relative to control plots. However, there was still considerable volunteer seedling establishment along low flow channels where disturbance from the high flow release prepared suitable seedbeds. This suggests that future planting efforts (and associated costs) could be focused on areas such as high flow channels, where environmental flow releases are less likely to be effective on their own.