• Technical note: Estimating aboveground plant biomass using a photographic technique

      Paruelo, J. M.; Lauenroth, W. K.; Roset, P. A. (Society for Range Management, 2000-03-01)
      We present a non-destructive, photographic method to estimate biomass in semiarid grasslands. Though the method needs to be calibrated, it allows for a dramatic increase in the number of samples compared with the clipping method. The method is based on a relationship between the percentage or "green pixels" in a digital image and green biomass. We identified "green pixels" as those satisfying the following condition: G/B > 1 and G/R > 1, where G, B and R are the intensities of a particular pixel in the green, blue, and red bands respectively. The percentage of green pixels of the image and green grass biomass showed a correlation of 0.87 (n = 36, p < 0.001) when data were pooled from 3 sample dates. The relationship was slightly curvilinear and a log transformation of green biomass yielded a better correlation (r = 0.91, n = 36, p < 0.001). The percentage of green pixels showed a lower correlation with total green biomass than with grass biomass (r = 0.59) for the linear model and 0.73 for the log transformed model). The relationship between the percentage of green pixels and either green grass or total green biomass changed during the growing season. Both the slope and the Y-intercept of the model differed significantly among dates. Correlation coefficients for different dates ranged between 0.76 and 0.95.
    • Ungulate herbivory on Utah aspen: Assessment of long-term exclosures

      Kay, C. E.; Bartos, D. L. (Society for Range Management, 2000-03-01)
      The role of livestock grazing and big-game browsing in the decline of aspen (Populus tremuloides Michx.) in the Intermountain West has long been questioned. All known aspen exclosures (n=8) on the Dixie and Fishlake National Forests in south-central Utah were measured during late summer of 1995 and 1996 to determine aspen stem dynamics, successional status, and understory species composition. Five of the exclosures were of a 3-part design with a total-exclusion portion, a livestock-exclusion portion, and a combined-use portion which permitted the effects of deer (Odocoileus hemionus) and elk (Cervus elaphus) herbivory to be measured separately from those of livestock. Aspen within all total-exclusion plots successfully regenerated and developed multi-aged stems without the influence of fire or other disturbance. Aspen subjected to browsing by wildlife, primarily mule deer, either failed to regenerate successfully or regenerated at stem densities significantly lower (2,498 stems ha(-1)) than that on total-exclusion plots (4,474 stems ha(-1)). On combined wildlife-livestock-use plots, most aspen failed to regenerate successfully, or did so at low stem densities (1,012 stems/ha(-1)). Aspen successfully regenerated on ungulate-use plots only when deer numbers were low. Similarly, ungulate herbivory had significant effects on understory species composition. In general, utilization by deer tended to reduce shrubs and tall palatable forbs while favoring the growth of native grasses. The addition of livestock grazing, however, tended to reduce native grasses while promoting introduced species and bare soil. Thus, communities dominated by old-age or single-age trees appear to be a product of ungulate browsing, not a biological attribute of aspen as has been commonly assumed. There was no evidence that climatic variation affected aspen regeneration. Observed differences are attributed to varied histories of ungulate herbivory.