Canopy shade and the successional replacement of tamarisk by native box elder

Summary 1. Tamarisk species (Tamarix ramosissima Ledeb., T. chinensis Lour., T. galilea L. and hybrids) have invaded riparian areas throughout western North America, resulting in expansive control efforts. Tamarisk is a relatively recent addition to North American plant communities, and competitive and successional processes are still developing. Box elder (Acer negundo L. var. interius (Britt.) Sarg.) is a native competitor found in canyons throughout western North America. We investigated the establishment chronology, competition and comparative shade tolerances of tamarisk and box elder to determine the superior competitor and to predict successional trajectories in mixed stands. 2. Competition was studied through neighbourhood analysis, and successional trends were analysed through dendrochronology in the canyons of Dinosaur National Monument, CO, USA. The shade tolerance of mature tamarisk was studied by enclosing individual plants in shade cloth for two growing seasons. The comparative shade tolerances of juvenile tamarisk and box elder were studied using five levels of shading in a glasshouse experiment. 3. Box elder and tamarisk established concurrently, or tamarisk establishment preceded box elder establishment. Box elder was the superior competitor; the presence of canopy box elders within 1 and 2 m was significantly related to tamarisk mortality but not box elder mortality. The presence of canopy tamarisk trees was not related to box elder or tamarisk mortality. 4. In the field-based experiment, mature tamarisks were killed when subjected to one to two growing seasons of 98% shade. In the greenhouse-based experiment, box elder had superior shade tolerance to tamarisk, maintained positive growth and survived under higher shade than tamarisk. 5. Synthesis and applications. Box elder is a superior competitor to tamarisk, and is capable of establishing under dense tamarisk canopies, overtopping and eventually killing tamarisk. Superior shade tolerance appears to be the mechanism for the successional replacement of tamarisk by box elder. The manipulation of competitive and successional processes through the promotion of box