Easier Way to Record Yearly Mean NDVI

Question

For my Google Earth Engine project, I am finding the mean NDVI for each year between 2000 and 2020 in protected areas a part of the World Database on Protected Areas (WDPA). As an example, I have included code written to find the yearly mean NDVI values in Salonga National Park, which is located in the Democratic Republic of the Congo (DRC). I am using the MODIS vegetation index produced on 16-day intervals.

The code finds the mean NDVI value within the park boundaries and then places it into a dictionary with the years as keys and mean NDVI as values. Is there an easier/more concise way to write this code? This example finds the mean NDVI for 2000 and 2001 so far.

var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');
var drcongo = countries.filter(ee.Filter.eq('country_co', 'CG'));
var styleParams = {
  fillColor: 'b5ffb4',
  color: '868686',
  width: 2.5,
  opacity: 0.6,
};
//Map.addLayer(drcongo, styleParams, 'DRC');
Map.setCenter(23.5, -2.5, 6);

var salonga = ee.FeatureCollection('WCMC/WDPA/current/polygons')
.filter(ee.Filter.and(
    ee.Filter.bounds(drcongo.geometry()),
    ee.Filter.eq('ORIG_NAME', 'Parc national de la Salonga')
  ));
//Map.addLayer(dataset, {}, 'Parc national de la Salonga');

var modisNDVI = ee.ImageCollection('MODIS/006/MOD13Q1')
              .select('NDVI')
              .map(
                      function(img) {
                        var rescaled_NDVI = img.select('NDVI')
                                                .multiply(0.0001)
                                                .rename('NDVI_rescaled');
                        return img.addBands(rescaled_NDVI);
                   
  }
);
                    
var modisNDVI_2000 = modisNDVI.filter(ee.Filter.date('2000-01-01', '2000-12-31'));
var modisNDVI_2000_clip = modisNDVI_2000.mean().clip(salonga);
var mean_dict_2000 = modisNDVI_2000_clip.reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: salonga.geometry(),
  scale: 500,
});

var modisNDVI_2001 = modisNDVI.filter(ee.Filter.date('2001-01-01', '2001-12-31'));
var modisNDVI_2001_clip = modisNDVI_2001.mean().clip(salonga);
var mean_dict_2001 = modisNDVI_2001_clip.reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: salonga.geometry(),
  scale: 500,
});

var dictionary = ee.Dictionary({
  2000: mean_dict_2000.get('NDVI_rescaled'),
  2001: mean_dict_2001.get('NDVI_rescaled')
});
print('Parc national de la Salonga Annual NDVI', dictionary);

Answer 1

Make a list of years and filter the modisNDVI:

var years = ee.List.sequence(2000, 2019, 1).map(function(year){
  var start = ee.Number(year);
  var end = start.add(ee.Number(1));
  var ndviMeanYear = modisNDVI.select('NDVI_rescaled')
                              .filter(ee.Filter.calendarRange(start, end, 'year')).mean();
  return ndviMeanYear.set('year', start)
});

Then, get the ndvi mean for each PA and set the year:

    var ndviMean = ee.ImageCollection(years).map(function(img){
      var rR = img.reduceRegions({collection: pa,
                                reducer: ee.Reducer.mean(),
                                scale: 500,
                                tileScale: 4})
      return rR.map(function(feat){
        return feat.set('year', img.get('year'))
      })
}).flatten()

You will end up with a FeatureCollection with the 'mean', and 'year' properties for each PA.

https://code.earthengine.google.com/355b8a95f609f06529eadd73b4c2ae74