Drivers of Productivity Trends in Cork Oak Woodlands over the Last 15 Years

Abstract

Abstract:Higher biodiversity leads to more productive ecosystems which, in turn, supports more biodiversity. Ongoing global changes affect ecosystem productivity and, therefore, are expected to affect productivity-biodiversity relationships. However, the magnitude of these relationships may be affected by baseline biodiversity and its lifeforms. Cork oak (Quercus suber) woodlands are a highly biodiverse Mediterranean ecosystem managed for cork extraction; as a result of this management cork oak woodlands may have both tree and shrub canopies, just tree and just shrub canopies, and just grasslands. Trees, shrubs, and grasses may respond differently to climatic variables and their combination may, therefore, affect measurements of productivity and the resulting productivity-biodiversity relationships. Here, we asked whether the relationship between productivity and climate is affected by the responses of trees, shrubs, and grasses in cork oak woodlands in Southern Portugal. To answer this question, we linked a 15-year time series of Enhanced Vegetation Index (EVI) derived from Landsat satellites to micrometeorological data to assess the relationship between trends in EVI and climate. Between 2000 and 2013 we observed an overall decrease in EVI. However, EVI increased over cork oaks and decreased over shrublands. EVI trends were strongly positively related to changes in relative humidity and negatively related to temperature. The intra-annual EVI cycle of grasslands and sparse cork oak woodland without understorey (savannah-like ecosystem) had higher variation than the other land-cover types. These results suggest that oaks and shrubs have different responses to changes in water availability, which can be either related to oak physiology, to oaks being either more resilient or having lagged responses to changes in climate, or to the fact that shrublands start senesce earlier than oaks. Our results also suggest that in the future EVI could improve because the rate of increase in minimum EVI is greater than the rate of decrease in maximum EVI, and that this is contingent on management of the shrub understorey as it affects the rate of decrease in maximum EVI. This will be the challenge for the persistence of cork oak woodlands, their associated biodiversity and social-ecological system.

Long-Term Monitoring of Cork and Holm Oak Stands Productivity in Portugal with Landsat Imagery

“…Cork oak trends were estimated in southern Portugal for a 20 km 2 area in Serra de Grândola [27]. Landsat-derived EVI time series between 2000 and 2013 were extracted for five land cover types: dense cork oak stands with and without understory, sparse cork oak stands with and without understory, and grasslands.…”

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

“…This trend, identified using a summer NDVI (July and August) time series obtained from Landsat and MODIS imagery, did not result from short-term events, such as droughts or debarking, but reflects a long-term evolution of the forest stands health. A 15-year Enhanced Vegetation Index (EVI) time series and micrometeorological data was used by Santos et al [10] to evaluate if the relationship between cork oak woodlands productivity, in Southern Portugal, and climate is affected by the combined response of trees, shrubs and grassland. EVI trends, obtained also from Landsat data, showed differences in oaks and shrubs productivity that were positively related with relative humidity and negatively with temperature, suggesting that cork oak seems to be more resilient or having lagged responses to climate change than shrubs and grassland.…”

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

“…This trend, identified using a summer NDVI (July and August) time series obtained from Landsat and MODIS imagery, resulted not from short-term events, such as droughts or debarking, but reflects a long-term evolution of the forest stands health. A 15-year Enhanced Vegetation Index (EVI) time series and micrometeorological data was used by [10] to evaluate if the relationship between cork oak woodlands productivity, in Southern Portugal, and climate is affected by the combined response of trees, shrubs and grassland. EVI trends, obtained also from Landsat data, showed differences in oaks and shrubs productivity that were positively related with relative humidity and negatively with temperature, suggesting that cork oak seems to be more resilient or having lagged responses to climate change.…”