From what I've seen at least in California recreation_ground is used on general open spaces that are used for recreational purposes. Since landuse=park doesn't really cut it. I think the tag fits in this case to because it's a mountain biking area, which would qualify as a pitch. Although obviously a larger and less well defined one then say a basketball court, but a pitch none the less. It's unfortunate that there isn't a better tag but it is what it is. 

I don't think tagging it as a nature reserve works because the area isn't in it's natural state due to it being turned into a BMX course. An extensive network of BMX trails isn't natural, obviously. It's also not a place set aside for any kind of natural conservation. Since again, it's full of mountain biking trails. And multiple refuse piles from when the area was heavy mined in the 20s BTW. So it's not an ecological preserve. Nor would I say it's an area of critical environmental concern or a research area. So I don't really see what would qualify it to be tagged as leisure=nature_reserve.