Using existing agricultural infrastructure for restoration practices: Factors influencing successful establishment of Populus fremontii over Tamarix ramosissima

Dryland riparian restoration often requires the use of large amounts of surface water. Retired agricultural fields located on floodplains associated with shallow water tables may minimize water requirements for restoration because irrigation could be ceased when target species become phreatophytic. Using existing agricultural infrastructure, we implemented two irrigation treatments over two years to promote native Fremont cottonwood (Populus fremontii) establishment over non-native tamarisk (Tamarix ramosissima): (1) frequent shallow irrigation; and (2) infrequent deep irrigation. We assessed how these treatments: (a) contributed to above- and below-ground growth of native and non-native species; (b) promoted native species survival; and (c) supported a competitive advantage of P. fremontii over T. ramosissima. Overall growth rates were highest one year after seeding, and average P. fremontii growth rates were higher than T. ramosissima in both years. Overall P. fremontii mortality was lower than T. ramosissima in both years. P. fremontii crown cover consistently increased and was ultimately the most abundant species in the overstory. Our results suggest that P. fremontii can out-compete T. ramosissima in these retired agricultural settings. In addition, this study suggests that irrigation frequency can be further decreased while reducing irrigation input, thus minimizing both effort and surface water required for restoration.