As van der Borght implies, a further significant reason for the conclusion of the Permanent Forestry Contract lay in the need to ensure the water supply for the rapidly growing population of Berlin (a fourfold increase between the Proclamation of the German Empire in 1871 and 1910 from around 900,000 to over 3,700,000). The forest areas acquired included many lakes with very good water quality, such as Schlachtensee or Krumme Lanke – lakes which today are understood to be part of Berlin, but at that time lay far outside the city limits. To improve water retention in the soil, a change in forestry policies was planned which envisioned more natural mixed woods, those existing at the end of the 19th century mainly comprising pine monoculture.