How to exclude certain elevation ranges from the extracted image

[Huanglh1001]

Hello Professor Wu, I want to subtract the NDVI calculated from my own computation by areas with elevation above 135. However, the expected effect is not achieved after applying the masking formula. Could you please tell me why? Here is the code (kindly advise on necessary modifications):

# 初始化
import ee
import geemap
geemap.ee_initialize()
ee.Initialize()
# ROI
Map = geemap.Map(center=[36 , 120], zoom=8)
region = ee.Geometry.BBox(120.69, 36.4, 122.33, 37.4)
fc = ee.FeatureCollection(region)
style = {'color': 'ffff00ff', 'fillColor': '00000000'}
Map.addLayer(fc.style(**style), {}, 'ROI')
Map
# get image
collection = ee.ImageCollection("COPERNICUS/S2_SR_HARMONIZED") \
    .filterDate('2019-06-01', '2020-09-01') \
    .filterBounds(fc)\
    .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE',5))
image = collection.median().clipToCollection(fc) 
vis = {
  'min': 0.0,
  'max': 3000,
  'bands': ['B4', 'B3', 'B2'],
}
Map.add_layer(image,vis,'s2')
Map.centerObject(fc)
Map
# NDVI
NDVI = image.normalizedDifference(['B8', 'B4']).rename('NDVI')
visParams = {
  min: -1,
  max: 1,
  'palette': ['blue', 'white', 'green']
};
dem = ee.Image('USGS/SRTMGL1_003').clipToCollection(fc)
Map.addLayer(NDVI, visParams, 'NDVI');
vis1 = {
  'min': 0,
  'max': 6000,
  'palette': ['006633', 'E5FFCC', '662A00', 'D8D8D8', 'F5F5F5']
}
Map.addLayer(dem, vis1, 'SRTM')
# 提取植被
threshold = 0.4
NDVI_1 = NDVI.gt(threshold).selfMask()
Map.addLayer(NDVI_1, {'palette': 'green'}, 'NDVI_1');
# 查看大于135m的山地
threshold = 135
DEM = dem.gt(threshold).selfMask()
Map.addLayer(DEM, {'palette': 'purple'}, 'DEM');
# 去除植被中海拔大于135的山地植被z
area = NDVI_1.unmask().subtract(dem.unmask()).gt(135).selfMask()
Map.addLayer(area, {'palette': 'white'}, 'area');

[giswqs]

mask = dem.lt(135)
area = NDVI_1.updateMask(mask)
Map.addLayer(area, {'palette': 'white'}, 'area')

[Huanglh1001]

thanks！