Assessing the Use of Sentinel-2 Time Series Data for Monitoring Cork Oak Decline in Portugal

Abstract

In Portugal, cork oak (Quercus suber L.) stands cover 737 Mha, being the most predominant species of the montado agroforestry system, contributing to the economic, social and environmental development of the country. Cork oak decline is a known problem since the late years of the 19th century that has recently worsened. The causes of oak decline seem to be a result of slow and cumulative processes, although the role of each environmental factor is not yet established. The availability of Sentinel-2 high spatial and temporal resolution dense time series enables monitoring of gradual processes. These processes can be monitored using spectral vegetation indices (VI) as their temporal dynamics are expected to be related with green biomass and photosynthetic efficiency. The Normalized Difference Vegetation Index (NDVI) is sensitive to structural canopy changes, however it tends to saturate at moderate-to-dense canopies. Modified VI have been proposed to incorporate the reflectance in the red-edge spectral region, which is highly sensitive to chlorophyll content while largely unaffected by structural properties. In this research, in situ data on the location and vitality status of cork oak trees are used to assess the correlation between chlorophyll indices (CI) and NDVI time series trends and cork oak vitality at the tree level. Preliminary results seem to be promising since differences between healthy and unhealthy (diseased/dead) trees were observed.

Continuous Detection of Small-Scale Changes in Scots Pine Dominated Stands Using Dense Sentinel-2 Time Series

“…In forest disturbance studies using the Sentinel-2 data, Puletti and Bascietto [49] used the NDVI series to separate cut from uncut forests, with high accuracy. The potential of the Sentinel-2 series was confirmed in the study of Navarro et al [51] in the monitoring of cork oak decline. Puletti et al [67] used Sentinel-2 time series to analyze the vegetation condition following extreme drought events and showed a reduction of trees photosynthetic activity.…”

“…Furthermore, the Sentinel-2 imagery is available for free, which, in combination with the variety of open-source software, creates opportunities relating to the successful application of this approach in operational forest health monitoring. So far, the Sentinel-2 time series have been used for the assessment of forest cuttings [49] and treefall gaps [50], as well as the monitoring of cork oak decline [51]. Sentinel-2 was also used in the mapping of bark beetle green attack stages [30], Scots pine stands defoliation [31], and red-edge bands were assessed in forest decline [26].…”

Sentinel-2 Data in an Evaluation of the Impact of the Disturbances on Forest Vegetation

“…Few articles have been written for the TS of the Sentinel-2 focused on forest vegetation, e.g., that dealt with the recognition of unhealthy cork oaks (Quercus suber) [43]. Based on a multitemporal comparison (using vegetation indices), researchers found that while in the wetter part of the year, the differences were small, and, during the drier part of the year (September and October), the differences between the healthy and unhealthy cork oak stands were more significant.…”

Using Sentinel-2 Images to Estimate Topography, Tidal-Stage Lags and Exposure Periods over Large Intertidal Areas

“…The application of a major axis regression visibly reduced this unwanted source of variability in the NIR values (see Figure 3), ultimately improving the estimation of the inflection point of the logistic function, from which the height of each pixel is calculated. This linear correction is very easy to implement and it is likely to be of use in several other remote sensing techniques involving the use of time-series of satellite images, such as crop monitoring [50] or annual monitoring of tree vitality [51].…”