C. Timothy Lindstrom (Director of Protection & Staff Attorney, Jackson Hole Land Trust) has published A Guide to the Tax Aspects of Conservation Easement Contributions, 7 Wyo. L. Rev. 441 (2007).  Here is the Summary:

There are five types of tax benefits available to easement donors and their families, all of which can be enjoyed in combination:

• Income Tax Deduction: The gift of a permanent conservation easement to a qualified organization or governmental agency constitutes a charitable contribution and the value of the easement (generally, the difference in the value of the property subject to the easement before and after the easement is put in place) may be deducted from the donor’s income for purposes of calculating federal income tax and, in many states, state income tax.
• Income Tax Credits: In some states (e.g. Virginia and Colorado), conservation easements generate credits against state income tax liability. Credits are more powerful incentives than deductions because they represent a direct offset against tax due rather than a reduction of the income against which tax is assessed.
• Reduction in Taxable Estate: The restrictions imposed by a conservation easement reduce the value of real property in a decedent’s estate. This reduction in value results in estate tax savings.
• Exclusion from Taxable Estate: Section 2031(c) of the Internal Revenue Code allows the executor of a decedent’s estate to exclude 40% of the restricted value of land subject to a qualified conservation easement (i.e., the value of the land after subtracting the value of easement). The maximum amount that may be excluded under this provision is $500,000 per estate.
• Reduced Real Estate Tax Assessment: Under the provisions of many state and local laws, land subject to a conservation easement is entitled to a lower real estate tax assessment to reflect the restrictions of the easement. This can result in substantial local real estate tax savings.