Vegetation Cover since the 1980s
Knowing the history of a piece of land can help us understand issues around greenhouse gas accounting, bio-diversity, soil health and much more. The July map of the month looks at how often fields in Ireland have been bare of vegetation, due to harvesting or re-seeding since the 1980's.
The management history of any parcel of land is an important piece of information.
This month we present an interim output from a study looking to see how often fields are bare of vegetation, due to harvesting or re-seeding. Using the Teagasc archive of satellite imagery going back to the 1980's we can use vegetation indices to give an indication of whether vegetation is present or not.
Because of cloud cover each field is only imaged perhaps 4 times a year, so the analysis can only give an indication of how often the parcel has had low vegetation cover.
The archive used had 317 Images over 35 years up to 2019, each parcel had on average 210 observations. Each image was used to populate the Prime2 vegetation polygons (from 2017), with the average NDVI* value for each pixel in the image. The archive is quality controlled for atmospheric and cloud issues, so only good quality pixels are used. A cut-off value for bare fields was established, referenced against total image means. The number of time the reference value was met was counted for each field and this is the number of times the field has appeared bare in our satellite record.
The dark green fields in the map have been covered with vegetation almost permanently in this record, with fewer than 2 low observations. They would potentially be sites of botanical interest as they may less intensely managed than other grasslands in the area. The pink fields have been apparently bare in more than 10% of the observations. These fields are mostly tillage but some are intensely managed grassland. A few of the pink areas around Limerick city are due building in the city recently and some may be capturing a record of flood events. The rest of the fields have a range of counts of being bare- we chose to only highlight the two extremes in the map.
This is a partial, interim output- the full data set, with complete time-signatures of bare soil should be available at the end of the year.
*NDVI or Normalised Difference Vegetation Index, is a long established method to monitor vegetation cover from satellites. It relies on the fact that vegetation deeply absorbs red light but strongly reflects the very near infra-red (a wavelength slightly longer than visible red light). The ratio of the two values is the vegetation index and the higher the number, up to 1, the greater the amount of vegetation.