Analysis Of Kap Results

Digital kite aerial photographs clearly revealed the impact of D. elongata on salt cedar at the USBR Pueblo study site. In August, defoliated salt cedar exhibited a distinctive reddish-brown color that was visually obvious and could be separated digitally for quantitative analysis. However, some caveats must be taken into account. The visible color associated with defoliated salt cedar also appeared in the open prairie vegetation around salt cedar thickets. Presumably this color indicated dead or senescent vegetation. For the most part, this color occurred as individual pixels or quite small pixel groups in the prairie zone and could be removed by setting a minimum size threshold (1 dm ). Most of the suspect pixels were removed from the prairie portion in this manner. However, some small pixel groups may have been eliminated from the salt cedar portions also.

Salt cedar grows to a typical height of 3-4 m. D. elon-gata consume some leaves, but they kill more foliage than they actually consume. Their method of feeding disrupts water transport to non-consumed foliage and causes leaves to die and turn reddish brown. Adult beetles move about the tree, laying eggs, and larvae move also as they grow. Their feeding appears to be driven somewhat randomly following oviposition patterns. A salt cedar bush under attack, thus, may contain a 3D patchwork of healthy and dead portions.

As seen from above, healthy branches near the top may obscure defoliated lower branches. On the other hand, defoliated upper branches could allow healthy lower portions to show through and be visible from above. Thus, defoliation must be nearly continuous in the vertical profile of the salt cedar bush in order to appear clearly in vertical aerial photographs. Furthermore, dead leaves that fall off would not be included in the scene. Based on these factors, quantitative analysis of vertical kite aerial photographs was considered to provide a minimum areal estimate for the amount of defoliation in salt cedar (Aber et al., 2005). The actual volume of salt cedar defoliation could be substantially greater, which might be indicated to better advantage in oblique photographs.

The amount of impacted foliage displayed in vertical images in early August amounted to only a few percent of the study area. This low result reflects the relatively early stage of defoliation exhibited at this phase in the growing season. Within two weeks of taking airphotos, ground checking revealed substantially more defoliation, and by the end of the growing season many salt cedars had no live foliage (Fig. 15-3). KAP could be used to monitor this progression at frequent intervals during the latter part of the growing season.

Salt cedar trees defoliated by beetle attack are not dead and could recover and green up the following year. However, repeated beetle attacks over several (at least 3) years would eventually kill salt cedar. Thus, multi-year monitoring of beetle release sites would be necessary to document the long-term impact of this biocontrol effort. Such monitoring on the ground is time-consuming and labor intensive. Furthermore ground observations are limited by dense thickets of salt cedar that are difficult to penetrate for direct study. KAP offers a bird's-eye view that allows rapid acquisition of large-scale imagery over dense thickets covering several hectares.

The initial cost of equipment and software would be recouped by repeated use at multiple study sites during a period of years. For example, Pitt and Glover (1993) did a detailed cost-benefit comparison of traditional ground survey versus small-format aerial photography (SFAP) from a tethered blimp for evaluating forestry research plots in eastern Canada. They found that low-height aerial photography was not only less expensive, but also produced more objective results and provided a permanent record of seasonal vegetation conditions.

Based on this preliminary success, KAP was adopted by the USBR as a means to document biocontrol study sites. KAP also was utilized by scientists conducting similar biocontrol research at New Mexico State University. KAP proved effective for acquiring the type of low-height, high-resolution imagery necessary to monitor the effects of insect defoliation at the level of individual salt cedar trees. Relatively low cost of equipment and operation combined with flexible scheduling, convenient transportation to distant sites, and ease of use in the field make KAP a technique that could be employed widely for similar biological investigations elsewhere.

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