Suitability of existing and novel spectral indices to remotely detect water stress in Populus spp.

Abstract

Undetected water stress within poplar plantations can result in high economic losses. Although remotely sensed spectral measures associated with variations in plant water status have been developed for non- Populus species, little is known about the suitability of such spectral indices to identify water stress in Populus species. Our experiments assessed whether such spectral indices acquired at the leaf and canopy level were suitable for detecting water stress in Populus. Relationships between four spectral indices and four common measures of plant water status were analyzed for both low to moderate, and highly water stressed trees. The proposed maximum difference water index (MDWI) exhibited significant relationships ( P < 0.001) with changes in plant water status at both the leaf and canopy levels. At the leaf level, statistically significant, though poorer relationships were obtained between each of the normalized differential water index (NDWI), the red edge inflection point (REIP), the water index (WI), and the plant water status measures. At the canopy level, statistically significant relationships were obtained between MDWI and the measure of relative water content (RWC), and the equivalent water thickness (EWT) ( r 2 > 0.56, P < 0.001), while WI produced reasonable relationships with these water status measures ( r 2 > 0.42, P < 0.001). On exclusion of the highly stressed plants, only MDWI exhibited low sensitivity to RWC ( r 2 = 0.37, slope = 0.64) and leaf water potential at the canopy level ( r 2 = 0.42, slope = 9.89) whereas no other indices exhibited sensitivity to changes in plant water status. MDWI, which incorporates short-wave infrared (SWIR, 1300–2500 nm) spectral bands, is more suitable to detect more changes in plant water status as compared to NDWI, REIP, and WI that incorporate near infrared (NIR, 700–1300 nm) spectral bands. These results suggest that such remotely sensed indices are not a viable option to detect low and moderate levels of water stress at a scale of a Populus spp. plantation, although further research is warranted to assess the broader applicability of MDWI and other SWIR wavelength incorporating indices to detect water stress in other species.