Setting a value of the timber is a sophisticated process. The board feet mentioned by cbslee are necessary because you need to measure what was available versus what was cut. Calculation of board feet as lumber is irrelevant because calculation of board feet as logs is 10-15% less because of what the sawmill figures they will lose in bark, knots, and irregularities in shape. Walnut is more expensive than most other wood species - up to 4X as valuable as pine. Today walnut is worth more than red oak, 15 years ago the reverse was true. Also, typically a landowner is paid 50% as much as what the logger can sell to a sawmill. This percentage drops as the contour of the land becomes extreme and difficult to transport the logs away.
Real estate appraisers know the timber is there, but they are not necessarily qualified to assess it, so it is often ignored. Other complicating factors: over time, timber grows and creates more board feet than in younger years - but this is deceptive if the timber was mature and if so it reaches a point of maximum value and then begins to deteriorate. Timber is something that if you don't value it at the critical point in history, you lose the opportunity forever to have an accurate appraisal.
Another difficulty is that timberland (forested land) is not worth more than pastureland just because it contains harvestable logs. Cattle cannot graze in forested land, nor can any row crops be raised.