Few logistically feasible techniques exist for monitoring changes in the biomass of willow and other woody riparian species. In this ongoing project, we are attempting to develop a method for monitoring changes in willow biomass which is based on evaluating percent visual obstruction and incorporates the use of scanned 35mm images. The relationship between visual obstruction and willow biomass is determined using a sequential removal technique. Harvested willow branches are placed in a holding device such that they are oriented perpendicular to the ground and located in front of a 70 x 50cm photoboard. The leaves obstructing view of the photoboard are then incrementally removed, with each successive removal representing an approximately 25% decrease in visual obstruction of the photoboard. A photo is taken, before and after each removal, and harvested leaf material is dried and weighed. Camera placement is 3.5 meters from the photoboard with a lens focal length of 80mm. Slides are then scanned and cropped to encompass the dimensions of the photoboard. Visual obstruction is estimated for all scanned images, using Sigma Scan 5.0 computer software, by determining the amount of the photoboard visible in the image and comparing that to its' actual area. The relationship between percent visual obstruction and leaf biomass is evaluated by regressing the amount of leaf biomass covering the photoboard against percent visual obstruction. Preliminary results indicate a strong relationship between visual obstruction and willow leaf biomass.
We are currently exploring the idea of using permanent photo monitoring stations to evaluate changes in willow biomass over time. Each monitoring station will consist of two 2 x 12" boards placed behind a willow clump (Figure 1).
The boards will be placed at roughly 1/2 and 2/3 the height of the willow clump. If the clump is immature, an average willow height of nearby mature willows will be used to determine height placement of boards. Annual photographs will be taken from a permanent photo point located perpendicular to the visual obstruction boards. We will use a minimum focal length of 50mm to minimize distortion of scale, and the photo-point will be located just far enough from the willow clump to frame the mounting posts in the photograph. Photos will be scanned and visual obstruction of each board will be determined. This setup will facilitate determination of visual obstruction at two levels in the tree canopy. Changes in these readings from year to year can be used to imply changes in biomass of the willow clump. The boards will be of known length and can be used as scale references for determining the height and width of the clump.
If you are interested in this study, contact Chad Boyd at this location.