Tree growth and regeneration response to climate and stream flow in a species-rich southwestern riparian forest

We studied the influence of climate variables, stream flow, and topography on regeneration and growth of several riparian tree species (Acer negundo, Alnus oblongifolia, Fraxinus velutina, Juglans major, Platanus wrightii, Populus fremontii, Salix spp.) at an unregulated perennial stream, West Clear Creek, in central Arizona. A pulse of seedling regeneration occurred for Alnus, Fraxinus, Platanus, Populus, and Salix in 1995 and 1996 following high winter and spring surface flows in 1993 and high spring surface flow in 1995. In contrast, little regeneration occurred for Acer and Juglans under these conditions. Most seedlings occurred at the active channel topographic location, and few seedlings occurred at abandoned channel, gravel-boulder bar, and bench locations. Relationships between environmental variables and annual radial stem growth varied among species and between constrained and unconstrained reaches. High spring or winter surface flows were negatively related to growth of Acer, Alnus, and Platanus, whereas high spring surface flow was positively related to growth of Fraxinus. Positive relationships between precipitation and growth occurred only for Fraxinus and Juglans, suggesting greater use of surface soil water by these species. Annual radial growth was high for Platanus and Alnus, medium for Acer and Fraxinus, and low for Juglans. Overall, the tree species in our study responded individually, rather than collectively, in regeneration and growth to changes in stream flow and climatic variables.