Watersheds in layers: Landform influences on tree growth and understory species richness

Abstract

AbstractQuestions:1. Does tree growth differ among bedrock‐controlled and depositional (floodplains and alluvial fans) riparian landforms? 2. Will the elemental composition of tree cores reveal long‐term differences in nutrient availability among bedrock‐controlled and depositional landforms? 3. Is under‐story vascular plant species richness higher on depositional landforms than bedrock‐controlled landforms, and highest yet on floodplains?Location:Lower Indian Creek watershed, a tributary of Turnagain Arm of Cook Inlet, Alaska, USA.Methods:Sampling of riparian forests was stratified by land‐form type. Forest structure, tree growth characteristics, and the elemental composition of tree cores were assessed within point‐centered‐quarter plots (n= 30). Plots were oriented along transects bisecting the valley floor, along which 50 m × 2 m plots (n= 35) were also sampled for understory species composition and richness.Results:Bedrock‐controlled and depositional landforms supported mature trees that did not differ in age, yet structural differences were significant. Forests on depositional landforms were less dense, radial tree growth (cm.a‐1) and annual basal area increments were significantly higher, and tree cores revealed lower C: N ratios than trees growing on bedrock‐controlled landforms. Species richness of vascular plants was higher on depositional landforms at the landform and plot (100 m2) scale. Floodplains supported higher species richness than fans at the landscape scale yet differences at the plot scale were insignificant.Conclusions:Stratification of sampling by landform showed that structural differences between landforms may strongly influence the ecology of riparian plants, and that studies conducted without regard to landform structure may overlook fundamental influences on the ecology of riparian forests.