As I understand it, carbon offsets and such must be new or additional sources. So, maintaining existing trees, parks, etc. would not qualify. Donations made to expand parks, or to re-green areas of parks that might have been damaged, would seem like projects that park sponsors might be able to offer as available offsets.
It seems like what you are talking about would be some kind of a credit program where there is some reward for preservation and maintenance of existing resource. A system by which those responsible for the preservation can, for the lack of a better word, "license" a "credit" to some sponsor. Such a credit would, in theory, provide the sponsor with some benefit.
I like the concept, but the key question is what would be the incentive for entities to purchase such credits? Those who want to donate financially and/or volunteer can generally do so. There would need to be some kind of benefit, beyond the existing environmental benefits, to encourage this type of exchange. Generally, that means the threat of some kind of regulatory or economic penalty to be avoided, or market-based reward to be gained, by sponsoring these credits. I'm not sure that good PR would be sufficient to entice private sector entities to increase funding for preservation and maintenance since donations may already be tax-deductible and things like the carbon-offset programs have tangible benefit. There needs to be some creative thought given to possible ways in which such incentive might be created.
Maybe some kind of property tax credit system where corporations could get tax reductions for paying property taxes on protected lands, or for funding maintenance costs. The key is that this would need to be as attractive, or more so, than the tax breaks available simply through charitable donation.
Maybe some kind of trade-off system where, under certain conditions, corporations are allowed to expand in one area by sponsoring offsetting protection and/or maintenance in another area. Obviously, the net trade-off would need to be a positive for such a system to make sense. But, if it were dealing with situations where expansion in one area was going to be inevitable, then any offsetting would be positive by definition.
These ideas have large and obvious holes in them. I'm afraid I don't have much for solutions right now. I think it will take a lot of creative brainstorming, but the concept of creating a reward/offset system to help fund the protection of resources seems like a good idea. Maybe others out there have already started thinking about such a system.
Brian