ARTIFICIAL DEFOLIATION EFFECT ON POPULUS GROWTH, BIOMASS PRODUCTION, AND TOTAL NONSTRUCTURAL CARBOHYDRATE CONCENTRATION

The impact of artificial defoliation on Populus growth, biomass produc tion, and total nonstructural carbohydrate concentration was examined Four Populus clones were field planted and artificially defoliated. As signed defoliation levels (0, 25, 50, or 75%) were applied to leaves o f leaf plastochron index 0 through 8 during a 6-d period in a 3-step i ncremental manner to simulate cottonwood leaf beetle, Chrysomela scrip ta F., larval feeding patterns. Artificial defoliations were timed to coincide with the outbreaks of natural beetle populations in adjacent areas. After 2 growing seasons, trees were measured for height, diamet er, and biomass accumulation Root samples were collected from 0 and 75 % defoliation treatments and analyzed for total nonstructural carbohyd rate content. Height, diameter, and above ground biomass decreased sig nificantly with increasing defoliation for all clones. There was a sig nificant difference in root and total biomass between 0 and 75% defoli ation treatments for each clone. Biomass was reduced an average of 33% as defoliation level increased from 0 to 75%. As defoliation level in creased from 0 to 75%, a consistent allocation ratio of biomass to 2/3 above and 1/3 below ground components continued in all clones. An ove rcompensation response occurred in above ground biomass when a defolia tion level of 25% was applied. Between 25 and 75% a strong Linear tren d of decreasing biomass as defoliation increased was indicated. Vitali ty of the tree, as indicated by total nonstructural carbohydrate conte nt, was affected only slightly by increasing defoliation.