**3. Results**

### *3.1. Spectral Signatures from Ground-, Drone-, and Airplane-Based Sensors*

Spectral signatures for all three sensors showed similar characteristics but nuanced differences. The prickly pear, cholla, and barrel cactus spectral signatures, as measured from the ASD spectroradiometer, all exhibit a dip in reflectance centered at 972 nm and a peak around 1072 nm (Figure 2). The non-cacti classes do not have this dip. The prickly pear and cholla signatures show a similar dip at 972 nm and a peak at 1072 nm when extracted from the NEON AVIRIS data. However, the absolute reflectance values are lower and the dip is shallower. From the drone-mounted Nano Hyperspectral sensor, the same water absorption dip at 972 nm is present in the barrel and prickly pear cacti signatures, but the absolute reflectance values are also lower than the ASD.

**Figure 2.** Average spectral signatures of mesquite (orange line), creosote (grey line), bare ground (brown line), herbaceous (pink line), barrel cactus (purple line), cholla (blue line), and prickly pear (red line), as measured from by the ASD spectroradiometer (**A**), drone-mounted Nano Hyperspectral sensor (**B**), and airplane-mounted NEON AVIRIS (**C**). The three dark grey lines represent the three portions of the electromagnetic spectrum used to calculate the two cacti indices.

### *3.2. Range of Cacti Indices by Plant Type*

Using the field spectroradiometer, we sampled bare ground (N = 26), barrel cactus (N = 9), cholla (N = 5), creosote (N = 9), herbaceous (N = 28), mesquite (N = 32), and prickly pear (N = 11). CI1 values calculated from the field spectroradiometer show a separation of the cacti from non-cacti vegetation and bare ground (Figure 3). The majority of CI1 values for the cholla samples range from 0.173 to 0.186, with a mean value of 0.133. The majority of CI1 values for prickly pear samples range from 0.095 to 0.177, with a mean value of 0.103. The majority of CI1 values for the barrel cactus samples range from 0.101 to 0.257 with a mean value of 0.125.

**Figure 3.** This graph shows the range of Cacti Index 1 (862 and 972 nm) values for cacti, non-cacti vegetation, and bare ground computed from the Field Spectrometer (**A**), Nano Hyperspectral (**B**), and NEON AVIRIS (**C**) collections. The red hatched boxes represent prickly pear, the blue hatched boxes represent cholla, and the purple hatched boxes represent barrel cactus. The square in the middle of each box is the mean for that series of samples. The whiskers represent the minimum and maximum values within the interquartile range. Any triangles outside the whiskers are outlier values.

We identified bare ground (N = 10), barrel cactus (N = 10), herbaceous (N = 10), mesquite (N = 7), and prickly pear (N = 11) using the drone-mounted Nano hyperspectral sensor. CI1 values calculated using the Nano hyperspectral data for the selected samples show a separation of the barrel cactus and prickly pear (cacti) from non-cacti vegetation and bare ground. The majority of values for the barrel cactus samples range from 0.205 to 0.390, while the majority of values for the prickly pear samples range from 0.264 to 0.482. The mean value for the barrel cactus samples is 0.289 and the mean value for the prickly pear samples is 0.385 (Figure 3).

The greater coverage of the NEON AVIRIS data made it possible for us to recognize bare ground (N = 300), creosote (N = 418), herbaceous (N = 100), mesquite (N = 105), palo verde (N = 100), and prickly pear (N = 325). CI1 values calculated using the NEON AVIRIS data for the selected pixels show a separation of the cholla and prickly pear (cacti) from non-cacti vegetation and bare ground. The majority of values for the cholla samples range from 0.056 to 0.109, while the majority of values for the prickly pear samples range from 0.039 to 0.083 on the CI1. The mean value for the cholla samples is 0.083 and the mean value for the prickly pear samples is 0.062. The only other series of samples that overlaps the two cacti boxes are those for palo verde with a range from 0.032 to 0.045 and a mean of 0.040 (Figure 3).

We used the same bare ground (N = 26), barrel cactus (N = 9), cholla (N = 5), creosote (N = 9), herbaceous (N = 28), mesquite (N = 32), and prickly pear (N = 11) samples pulled from the field spectroradiometer to investigate CI2. The CI2 values show a separation of cacti from the other land cover types (Figure 4). The mean value for the cholla samples is 0.083 with the majority of values falling between 0.173 and 0.186. The mean value for the prickly pear samples is 0.143 with the majority of values falling between 0.948 and 0.177. The majority of barrel cactus samples have values between 0.101 and 0.257, with a mean value of 0.134.

**Figure 4.** This graph shows the range of Cacti Index 2 (972 and 1072 nm) values for cacti, noncacti vegetation, and bare ground computed from the Field Spectrometer ( **A**) and NEON AVIRIS (**B**) collections. The red hatched boxes represent prickly pear, the blue hatched boxes represent cholla, and the purple hatched boxes represent barrel cactus. The square in the middle of each box is the mean for that series of samples. The whiskers represent the minimum and maximum values within the interquartile range. Any triangles outside the whiskers are outlier values.

Using the same examples of bare ground (N = 300), creosote (N = 418), herbaceous (N = 100), mesquite (N = 105), palo verde (N = 100), and prickly pear (N = 325) extracted from the NEON AVIRIS data we computed CI2. The calculated CI2 values for the selected pixels show a separation of the cholla and prickly pear (cacti) from non-cacti vegetation and bare ground. The majority of values for the cholla samples range from 0.078 to 0.106, while the majority of values for the prickly pear samples range from 0.074 to 0.097. The mean value for the cholla samples is 0.092 and the mean value for the prickly pear samples is 0.086 (Figure 4).
