Digital image analysis of Old World bluestem canopy cover and leaf area

WW-B.Dahl Old World bluestem [Bothriochla bladhii (Retz) Blake] is a well-adapted perennial forage grass in the semi-arid Texas High Plains. WW-B.Dahl yields well with limited irrigation where Ogallala Aquifer levels are diminishing. This research aims to estimate ground cover, percentage light interception, and leaf area index from digital image analysis (DIA), which potentially allows low-labor measurements of plant canopy development. Successful application of DIA will aid in developing a simulation model that predicts growth and water use of WW-B.Dahl under limited irrigation conditions. This research used an established pasture of WW-B.Dahl at the Texas Tech University research station at New Deal. Samples were taken biweekly from 12 randomly selected plots during two growing periods: Period 1 was May 22 to July 16, and Period 2 was July 31 to October 16. Field samplings included irradiance, which was measured above and below canopy for calculating light interception; and overhead photos, which were taken before clipping biomass at 8-cm stubble height for biomass and leaf area determination. Overhead photos were converted by ImageJ® software into two color groupings corresponding to green tissue cover and non-green (dead) cover plus bare ground for ground cover calculation. Growth rate was greater in the first growing period. Light interception was linearly related to green ground cover as determined from DIA with greater R2 value in the first period. Leaf area index was curvilinearly related to green ground cover in both periods. Grass growth in Period 1 consisted almost exclusively of leaves. Stem elongation and seedhead development introduced high variation in light interception and biomass yield in the latter part of the Period 2, which likely explains the lower R2 values. The use of DIA with an inexpensive digital camera and free (public domain) software was effective at rapidly and economically recording and measuring plant ground cover in largely vegetative (leafy) canopies.