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

Ana Navarro,Joao Catalao,Joao Calvao

doi:10.3390/rs11212515

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.

Highlights

Cork oak (Quercus suber L.) woodlands are one of the most representative forest ecosystems characterizing the Western Mediterranean region. This species occurs spontaneously in Portugal mainland, usually forming low density stands with open heterogeneous canopies in silvo-pastoral ecosystems, named montado
In Portugal, the importance of cork oak has been recognized by law since the 13th century
The main goal of this research was to evaluate the potential of using Sentinel-2 imagery for the annual monitoring of cork oak vitality

Summary

Introduction

Cork oak (Quercus suber L.) woodlands are one of the most representative forest ecosystems characterizing the Western Mediterranean region. This species occurs spontaneously in Portugal mainland, usually forming low density stands with open heterogeneous canopies in silvo-pastoral ecosystems, named montado. According to the Portuguese National Forest Inventory, eucalyptus (mainly Eucalyptus globulus) is the dominant species (812 Mha; 26%), but cork oak (737 Mha; 23%) and maritime pine (714 Mha; 23%) stands are representative [1]. In Portugal, the importance of cork oak has been recognized by law since the 13th century. Legislation protecting cork oak stands forbids the felling of trees, meaning that only dead or diseased trees can be felled but only with a permission from the National Nature Conservation and Forest Authority (ICNF)

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Results