Variability among five riparian cottonwood (Populus fremontii Wats.) populations: An examination of size, density and spatial distribution

Various abiotic and biotic factors are known to affect tree size, including age, genetics, and environment. Knowledge of size variation within natural riparian tree populations has both ecological and restorative importance. We determined tree sizes, basal area densities, and spatial distributions o f 5 Populus fremontii Wats. populations within the Rio Grande watershed in New Mexico. At each site 10 randomly spaced plots, perpendicular to the riv er and extending from the river to the end of the forest, were established. Diameter at breast height (DBII) and distance to the river were determined for 1803 trees within the 5 populations, and stand cover (measured as basa l area [BA] per hectare) was determined for each population. Significant va riation in tree size and basal area density existed among sites. Mean DBH p er site ranged from 11.7 to 58.4 cm and differed significantly (P < 0.0001) among sites. Mean BA per hectare also varied significantly (P < 0.0001) am ong sites and ranged from 13.2 to 28.9 m(2) ha(-1). Spatial distribution of trees in I-elation to the river also differed among sites. Mean distance f rom the river ranged from 50 to 353 m and differed significantly (P < 0.000 1) among sites. Tree size was both linearly and nonlinearly related to dist ance from the river, with models varying among sites. Patterns of distribut ion in these New Mexico P. fremontii populations may be influenced by diffe rences in water availability across a site; trees farthest from water sourc es mag experience greater water stress and, therefore, growth limitation. I ncreasing BA cover with increasing tree size indicated no real thinning of mature trees within a population. Recruitment and establishment of cottonwo od seedlings and saplings was evident only at sites with newly formed flood plains. For these 5 populations tree size appeared to Le affected by enviro nmental factors.