oh dear-- is it a full size tree or a sapling--- doesn't sound too good

Monkey Puzzle trees or Chilean Pines ( Araucaria araucana) as they grow naturally lose leaves near the trunk and often lose their lower branches which turn brown, die and eventually fall off.
Do you water your tree regularly? You need to, especially during the hot season.
Having said that, if this is happening on a young tree (10 years old or less) water now and regularly and you might save the tree, however it may be disfigured by the leaf/branch loss. By the time you notice the leaves turning brown, it's already too late to save those leaves.
These trees naturally live in the temperate forests of Chile, so will require much water in sunny Spain.
I hope this helps.