There are two I make: Summer Garden Jam and Hedgerow Jelly.
The fruit for the Summer Garden Jam is whatever is ready at the time, but certain combinations don't work well and the proportion of the different fruits is important. For example too many blackcurrants in the mix is overpowering, strawberries tend to disappear, and plums are wasted as plum jam on its own is wonderful. Rhubarb can be a good base also gooseberries, but not too much of either. I also prefer blackberries on their own but you could try them with other fruit.
For the Hedgerow jelly, which I think might be close to your forest fruits but not as jam, I just pick whatever I can forage, which at this time of year would include brambles (blackberries), elderberry, crabapples, rose hips, rowan berries, hawthorn berries, maybe some autumn raspberries or late gooseberries, bullaces. Whatever I pick I cover with water in the pan and bring it to the boil, then cook until very soft. I then poor it through a jelly bag and make the jelly with 1lb sugar to 1 pint of juice.
The sugar for the Summer Garden Jam is a bit more complicated, as I tend to work out how much I need for each fruit then add them all together. I think it would probably be just as successful using 1 lb sugar per 1lb fruit, and maybe some lemon juice if I thought there might not be enough pectin in that particular mix.
Really, it's worth experimenting with what's available then choosing which one you prefer. Every forest is different and every day what's available is different, so unless the jam you are imitating has an ingredients list and isn't based on turnips, then there is probably no recipe which will reproduce what you want exactly.