Summary of environmental and stand characteristics of thinleaf alder sites along the Tanana River floodplains from 2006-2007.

Methods Terrace height: The relative heights of terraces above the river level were determined through differential leveling at 5 m intervals along each transect, then averaged for each site. Hourly Tanana River gage height readings from the U.S. Geological Survey (USGS) site in Fairbanks (#15485500) were used as benchmark elevations (waterdata.usgs.gov/nwis/). The mean terrace height of each site is reported as the elevation above the gage datum (121.92 m above sea level) for the Fairbanks USGS site. Soil properties: The surface litter layer was removed prior to soil sampling in each site. Five soil cores (1.9 cm diameter by 10 cm deep) were collected and pooled from each site. The thicknesses of both surface and buried organic layers were recorded before organic and mineral layers were combined for chemical analyses. Soils were oven dried at 60 ºC, passed through a sieve, homogenized, and ground. Soil pH was measured in a 5:1 slurry of deionized water and 10 g soil. Total soil C and N were analyzed with a LECO TruSpec CN Analyzer , and total P was analyzed with a Bran+Luebbe AA3 Autoanalyzer following acid digest . Duplicate analyses of each sample were conducted. Results are expressed on a concentration basis and on a pool-size basis to a depth of 10 cm. Foliar chemistry: In mid-July 2007, alder leaf samples were collected from a randomly chosen subsample of 38 sites. Leaves from 5 trees (pooling 4 leaf samples per tree) were sampled in each site (n = 190 pooled leaf samples). Whole leaves, with petioles removed, were oven-dried at 60 ºC, ground, and analyzed for C, N, and P. Total C and N were analyzed with a LECO TruSpec CN Analyzer, and total P was analyzed with a Bran+Luebbe AA3 Autoanalyzer following acid digest. Duplicate analyses of each sample were conducted. Stand age: 27 sites were subsampled for alder age distributions by collecting disks at ground level from up to 10 live alder stems for each of three size classes based on dbh (0 - 2.9 cm, 3 - 4.9 cm, and ? 5 cm). Disks were oven-dried and sanded with 400-600 grit sandpaper, and growth rings were counted and measured on a sliding bench micrometer to a precision of 0.01 mm. Three to four radii were sampled from each disk and averaged to ensure accuracy of dating and growth measurements. Disks which could not be dated consistently were excluded from subsequent analyses (n = 547 disks). Stand age was estimated by the age of the oldest alder in each site. Canopy cover: Canopy cover measurements were taken from 3 evenly spaced points within a site with a spherical densiometer. Stand structure: The diameter at breast height (dbh) of all trees and shrubs > 2 m tall were measured and the stem density and basal area of each species were calculated. Disease survey: Each standing alder stem was classified as either: 1) live without canker – no visible cankers; 2) live with branch canker – visible cankers on branches; 3) live with main ramet canker: visible cankers on the main ramet; or 4) dead with canker – standing dead with visible cankers.