In 2007, Utah State University, in cooperation with landowners in Northern Utah, initiated a forest manipulation study to examine the efficacy and consequences of various forestry treatments to regenerate aspen (Populus tremuloides) stands encroached by conifers. Treatments include control, selective harvest of merchantable timber, clear-felling, chaining, and burning arranged in a split-plot design replicated four times on three north-facing hillslopes. Each block consists of eight 1-ha treatment units. Treatments are scheduled to begin summer 2008; pre-treatment data were collected in summer 2007. Sampling was designed to (1) assess stand composition of tree species, (2) quantify understory biomass and species composition, (3) indicate the presence of non-native species, and (4) determine soil bulk density and C and N status. To this end, sample points were arranged every 20 m along a series of five transects (20 m apart) within each treatment unit. At each point, a nested-frequency plot (0.5 x 0.5 m) was used to assess presence of different understory species and each plot was clipped for determination of biomass. At four of the five points on each transect a soil core sample was collected to a depth of 15 cm. Within each unit, four of the sample points were randomly selected for stand assessment with fixed and variable-radius plots.
Results/Conclusions
Blocks differed considerably in canopy species composition, ranging from nearly uniform cover by aspen to virtually no aspen detected. Units with substantial aspen cover (> 80%) also had substantial aspen regeneration (> 2000 individuals ha-1). Over 120 species were observed in the understory and only eight of these were non-native. Blocks differed in species composition; each block contained only ca. 50% of the species detected among all blocks and intrablock variation in understory biomass was high. Two non-native species were detected with high frequency: prostrate knotweed (Polygonum aviculare), and dandelion (Taraxacum officinale); both are widely distributed in North America. There was a weakly significant (P < 0.08) decline in understory woody biomass with increasing cover by aspen. Combined with samples from related work, our data suggest higher soil organic content in soils with an aspen canopy compared to soils with a conifer canopy. Changes in these variables will be monitored following manipulation.
