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After the Napoleonic Wars, Sweden sought to secure long-term naval timber supplies amid concerns over resource depletion. A delegation (essentially acting in an exploratory/survey capacity) was sent to identify ideal planting sites. They selected the island of Visingsö in Lake Vättern for its suitable conditions. Over the next decade or so, around 300,000 oak trees were planted there. Foresters carefully pruned them to encourage straight, tall growth ideal for naval use—oaks were prized for strong, durable ship components, while straight trunks supported masts and spars.
atlasobscura.com
The trees matured over ~150 years. In 1975, when the forest was ready for harvest, the navy noted that ships were now made of steel, so the oaks were largely left standing or repurposed (some for other high-quality timber needs). This "forest planted for a fleet that never came" remains a notable case of deliberate, long-term arboriculture for maritime purposes.