SEASONAL TRENDS IN LEAF-AREA OF HONEY MESQUITE TREES - DETERMINATION USING IMAGE-ANALYSIS

Black-and-white photographs were used to estimate seasonal trends in whole plant leaf area of honey mesquite (Prosopis glandulosa var. glandulosa Torr.) trees occurring on a site with limited subsurface water. Height and canopy width of the trees ranged from 1 to 5 m and 1 to 7 m, respectively. Images consisted of profile-view angles of trees occurring on flat terrain. Four image variables, height, width, canopy profile perimeter length, and canopy profile area were obtained from the photographs and used to predict leaf area of unharvested trees. Predictive equations were based on adjacent trees which were photographed and harvested for actual leaf area determination. Canopy profile area was evaluated as the most accurate image variable for predicting leaf area. Whole plant leaf area of unharvested trees varied within and between growing seasons and was dependent on precipitation patterns. During the 1987 growing season, leaf area declined significantly by 14.6% from 17.1 m2 (1 leaf surface) in May to 14.6 m2 in August, in conjunction with a mid-summer dry period. Leaf area increased in September 1987 in response to late-summer precipitation. Leaf area was less in the spring of 1988 than the spring of 1987 because of lower precipitation during the winter prior to the 1988 than the 1987 growing season. Leaf area did not decline significantly from spring to mid-summer in 1988 as it did the previous year because of atypically high precipitation in July 1988. Leaf area did not increase in September of 1988 as it did in 1987 because of lack of late-season rains in 1988. These results suggest mesquite on this study site used partial leaf shedding to augment drought resistance.