A transfer to a Trust will be subject to estate taxes at full market value, with market value being defined as "highest and best use." In other words, subdivision for those lands in the intermountain west that are critical wildlife habitat, yet the place people would pay a premium to have their ranchette.
A conservation easement would be valued the same way - highest and best use. But, with a properly designed CE, the land cannot be subdivided, so the highest and best use would exclude subdividing and development. Thus, an estate transfer to an heir or a Trust for the heirs will be at a much lower value and likely escape any estate taxes that often requiring selling the land to pay the taxes.