Changes in Upper Canopy of Farm North Woodlot in 2019

The previous post summarized the lack of evidence that Beech Leaf Disease was having an effect on the upper canopy of a woodlot at CWRU’s University Farms in Hunting Valley, Ohio.   Continuing observations in the spring and early summer of 2019, however, suggest that the forest canopy is changing.  During the winter of 2018/2019, several large beech trees in the Farm’s north woodlot fell out of the canopy.  These oldest beeches had survived a number of pathological stresses and continued the pattern of gradual losses of the oldest beech trees.  The purpose of this post is to document the changes in the forest canopy of the north woodlot observed during drone surveys in the spring and early summer of 2019.

Drone surveys of the north woodlot during 2019 concentrated on imagery from a multispectral sensor.  The Sentera Double 4K Multispectral Sensor has two cameras, the RGB camera captures reflectance in three narrow visible light bands of red, blue, and green and the the NIR camera captures red edge and near infrared bands.  Figure 1 shows the spectral characteristics and quantum efficiency of the sensors.

Figure 1.  Manufacture’s spectral specifications for the Sentera Double 4K Multispectral Sensor, showing the quantum efficiency of the five spectral bands captured by the sensor’s two cameras.

The images from the sensor cameras capture light reflectance from the landscape.  Plants absorb and reflect light in the visible spectrum and reflect light at wavelengths greater than 700 nm (Gates et al. 1965, Gao et al. 2000).  For this work, I have chosen to use the normalized difference red edge index (NDRE):

NDRE = (NIR – RE)/(NIR + RE)

Figure 2 is an NIR orthoimage of the north woodlot on July 19, 2019 that I created with OpenDroneMap, which is an open source drone mapping software application.  Figure 2 is a composite image of the red edge and near infrared bands from the Sentera Double 4K NIR camera.  Using the raster calculator in QGIS, I created the NDRE orthoimage in Figure 3 from the red edge and near infrared bands in Figure 2.  Because the quantum efficiency of red edge is greater than near infrared, the NDRE index has negative values.

Figure 2.  OpenDroneMap orthomosaic of images taken on July 19, 2019 with the Sentera Double 4K Multispectal Sensor’s NIR camera, which captures Red Edge and NIR reflectance spectra.  Red dots represent the locations of recent tree falls, which created openings in the canopy.

Figure 3.  NDRE index raster calculated from the red edge and near infrared bands in Figure 2.  White areas correspond to high values of the NDRE index and darker areas to lower values of the index.  Red dots indicate position of large tree falls.

Figure 4.  Raster histogram of the NDRE index raster in Figure 3. 

Although the composite image from RGB camera of the Sentera sensor is not true color, the orthomosaic from the RGB images provides a composite visible color reflectance of the north woodlot (Figure 5).  The red dots in Figure 5 show locations where large tree falls occurred and the canopy gaps appeared.  These same areas appear as “white” areas in the NDRE index raster (Figure 3).

Figure 5.  Composite orthomosaic of the north woodlot from RGB camera images on July 19, 2019.  The orthomosaic location corresponds to the location of the orthomosaic in Figure 2.

The pattern of variability of the NDRE index in Figure 3 is quite different from the patterns observed in 2018.  Figure 6 shows the NDRE index pattern observed in June 2018, and Figure 7 shows the pattern observed in August 2018.  The raster histograms are also different with those from both 2018 dates showing markedly less skew toward higher values (Figure 8).

Figure 6.  NDRE index raster calculated from the red edge and near infrared bands of the NIR orthomosaic from sensor images taken on June 19, 2018.

Figure 7.  NDRE index raster calculated from the red edge and near infrared bands of the NIR orthomosaic from sensor images taken on August 10, 2018.

Figure 7.  NDRE index raster calculated from the red edge and near infrared bands of the NIR orthomosaic from sensor images taken on August 10, 2018.

Preliminary Conclusions

In contrast to observations in 2018, it appears that the north woodlot upper canopy is beginning to show substantial changes in NDRE index values in 2019.  Apart from gap openings associated with the loss of large beech, the increase in the “light” colored areas is an indication of widespread deterioration in the apparent health of the canopy in the beech dominated areas of the north woodlot.  Future posts will provide more detailed analysis.

References
Gates, D. M., H. J. Keegan, J. C. Schleter, and V. R. Weidner.  1965.  Spectral Properties of Plants.  Applied Optics, 4(1):11-20.

Gao, X., A. R. Huete, W. Ni, and T. Miura.  2000.  Optical-Biophysical Relatonships of Vegetation Spectra without Background Contamination.  Remote Sens. Environ. 74:609-620.