A long-standing national order, the arrêté du 16 mars 1955, requires dogs to be kept on a lead inside French forests and woods for a fixed seasonal window every year: 15 April through 30 June. Outside forest roads (allées forestières), an unleashed dog in a forest during this period is against the rule, regardless of how well-trained it is off-lead the rest of the year.
The purpose is straightforward: this window covers the main nesting and breeding season for forest wildlife, and an off-lead dog, even a calm one, can disturb ground-nesting birds and young animals simply by moving through the undergrowth. It's the same underlying logic as the UK's spring lambing-season countryside rule, applied to French woodland instead of farmland.
Separately, France's broader rule against dogs "in divagation" — effectively, out of the owner's control, including being more than 100 metres away — applies year-round in open fields, meadows, vineyards and orchards under a different part of the same 1955 order, carrying a fine of up to 750 euros. Some mayors layer their own additional leash requirements on top of either rule in busier local areas.