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A com­par­ison of spectral indices for mea­suring the water status of conifer foliage using hyper-​​spectral sensing.

Paper. (200kb .pdf)

Overview:

We col­lected needles from piñon pine (Pinus edulis) and oneseed juniper (Juniperus monosperma) bushes and trees around the Earth and Environmental Sciences Division at Los Alamos National Laboratory. The needles were racked onto slides and their spectral reflectance sig­na­tures was recorded with a high-​​sensitivity field spec­trom­eter. Water status of the sampled trees was measured using pre-​​dawn water poten­tial sampling, and water content of the measured needles was found using dry-​​weight/​wet-​​weight com­par­isons. We extracted a series of spectral indices from the recorded sig­na­tures, and compared their cor­re­la­tion with the water status mea­sure­ments. For the most part, the winner was good ol’ NDVI, which had con­vincing cor­re­la­tion with water content and also water poten­tial, espe­cially for the piñon.

Abstract:

Many fun­da­mental ecosystem prop­er­ties and dynamics are deter­mined by plant water stress, par­tic­u­larly in dryland ecosys­tems where water is usually limiting. Indeed, under severe drought, plant water stress and asso­ci­ated insect infes­ta­tions can produce landscape-​​scale mor­tality. Despite the fun­da­mental impor­tance of plant water stress in deter­mining prop­er­ties and dynamics at ecosystem and land­scape scales, approaches for remotely sensing plant water stress are largely lacking, par­tic­u­larly for conifers. We eval­u­ated the remotely sensed detec­tion of foliar drought stress in two conifer species, Pinus edulis and Juniperus monosperma, which are co-​​dominants of exten­sive– juniper wood­lands in North America, the first of which expe­ri­enced exten­sive mor­tality in asso­ci­a­tion with a recent drought. Needle spectra were made on these species in the field using an inte­grating sphere and portable spec­trom­eter. Two indices of foliar water con­di­tion, plant water content (% of dry mass) and plant water poten­tial, were compared to five spectral analyses: con­tinuüm removal of the 970 and 1200 nm water absorp­tion features, the Normalized Difference Water Index (NDWI), the Normalized Difference Vegetation Index (NDVI), and the red edge wave­length position. For P. edulis, plant water content was sig­nif­i­cantly cor­re­lated with four of the five indices: NDVI (R2 =0.71) and NDWI (R2 =0.68) which exhib­ited stronger rela­tion­ships than 970 nm con­tinuüm removal (R2 =0.57) or red edge position (R2 =0.45). All five indices were sig­nif­i­cantly cor­re­lated with P. edulis water content when trees under­going mor­tality were included in analyses (R2 =0.60–0.93). Although the cor­re­la­tions were weaker than for plant water content, plant water poten­tial was sig­nif­i­cantly cor­re­lated with NDWI (R2 =0.49), 970 nm (R2 =0.44), NDVI (R2 =0.35), and red edge (R2 =0.34); again all five indices had sig­nif­i­cant rela­tion­ships when trees under­going mor­tality were included (R2 =0.51–0.86). The rela­tion­ships were weaker for J. monosperma: water content was sig­nif­i­cantly related to 970 nm (R2 =0.50) and 1200 nm (R2 =0.37) con­tin­uums and NDVI (R2 =0.33), while water poten­tial was related only to 1200 nm (R2 =0.40). Our results demon­strate a critical link between plant phys­i­o­log­ical char­ac­ter­is­tics tied to water stress and asso­ci­ated spectral sig­na­tures for two exten­sive co-​​occurring conifer species.

Keywords:

Pinus edulis; Juniperus monosperma; Foliar water content; Water poten­tial; Spectral remote sensing; Drought stress; Spectral indices

Citation:

Hugh C. Stimson, David D. Breshears, Susan L. Ustin, Shawn C. Kefauver. Spectral sensing of foliar water con­di­tions in two co-​​occurring conifer species: Pinus edulis and Juniperus monosperma. Remote Sensing of Environment 96 (2005) 108–118. doi:10.1016/j.rse.2004.12.007