Arbor Age

Green Media Technology and Software Special Edition - Aug'14

For more than 30 years, Arbor Age magazine has been covering new and innovative products, services, technology and research vital to tree care companies, municipal arborists and utility right-of-way maintenance companies

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16 Green Media / August 2014 B y D r . K e n C a r e y T urfgrass managers can tell a lot about turf just by look- ing at it — and the more experienced they are, the better their judgment. Nutrient status, pest damage, abiotic stresses (drought, traffi c, etc.) are all visible to the trained eye. Sometimes, however, it's good to have some tools to help; the highly trained manager may not be avail- able to see everything, or the problem may produce very subtle effects. This article discusses some recent innovations in assessing turfgrass, developed and widely used in turf research, which might be useful to the turfgrass manager. What we see when we look at a turfgrass sward could be termed "canopy refl ectance"; it's just the ambient sunlight re- fl ected off the leaves in the full visible light spectrum. A trained researcher or turfgrass manager learns to record and interpret the details of what they see, whether it's the off color of nutri- ent defi ciency or spray damage, or the darkening of drought stress. However, both in research and in practical management situations, we work with less well-trained helpers, and will benefi t from techniques that remove the subjectivity and ob- server bias, and reduce the need for training. One very familiar tool is a camera, and with improved digital cameras this is a very useful adjunct to assessing prob- lems. However, even though they can form an important permanent record, the digital photos still need to be inter- preted. Researchers are working on improving software to analyze digital images to document and quantify turf charac- teristics (weed and disease infestation, drought and nutrient stress), but these full-spectrum techniques are still relatively early in development for widespread turf use. A more mature, and somewhat simpler, technology for assessing turf involves restricting the wavelengths observed to ones that we have learned through experience are indica- tive of turfgrass problems. Photosynthesis in plants involves chlorophyll absorbing light to power the plant, and the wave- lengths that chlorophyll absorbs are a subset of the sunlight hitting the plant (Fig. 1). Light that chlorophyll absorbs is not refl ected, and the light hitting the plant looks different from that refl ected. Of the visible wavelengths, chlorophyll absorbs red light, generally, so the light refl ected is white minus red = green. The wavelengths that chlorophyll absorbs are often termed photosynthetically active radiation or PAR. Various sensors have been developed that all function TURFGRASS MANAGEMENT TURFGRASS MANAGEMENT USING CANOPY REFLECTANCE TOOLS IN TURFGRASS MANAGEMENT Figure 1. The peaks show the wavelengths of visible light that are absorbed by chlorophyll (Photosynthetically active radiation). Figure 3. Measurement (660 nm) and reference (770 nm) wave- lengths used by the Greenseeker to calculate NDVI. Figure 2. Measurement (ρVIS) and reference (ρNIR) waves of light refl ecting off bare soil and turf.

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