Another hypothesis could be that in colder climates people get less Vitamin D synthesized due to less sun exposure. There have been some links made between PD and low Vitamin D levels, but obviously that doesn't mean causality.

Vitamin D has also been shown to increase tyrosine hydroxylase, which is a rate limiting step in conversion of tyrosine to l-dopa.

Vitamin D increases expression of the tyrosine hydroxylase gene in adrenal medullary cells

I plan to have my mom tested for her 25-OH Vitamin D levels and increase her supplementation of Vitamin D accordingly. There is a lot of good literature out there arguing that the RDA recommendations on Vitamin D are still outdated and daily intake should be significantly higher. Basically the RDA level prevents us from getting rickets but it is not the ideal intake level for optimal body function.