Asset retirement with infinitely repeated alternative replacements: Harvest age and species choice in forestry

At what age should productive assets be retired? How should replacements be chosen when they differ in their uncertain ability to generate future incomes? As a particular version of that problem, we study the tree harvesting decision with two possible replacement species whose values as timber are stochastic and whose growth functions are deterministic. In the single-rotation (Wicksell) problem starting with a bare piece of land (an empty shop), it is optimal to choose and plant one species immediately if its current value is sufficiently high relative to that of the other species (the alternative equipment). However, if the species are insufficiently price-differentiated, it is preferable to leave the land vacant (the shop empty) despite the opportunity cost of doing so. In the repeated version of the problem, it is never optimal to leave the land bare provided the cost of replacement is null. Furthermore, the optimal harvest (tree retirement) age not only depends on the price and current productivity of the trees in place but also on the price and productivity of the other species, because it may replace the current one. The harvest age reaches a peak at some critical threshold of the relative price that signals the necessity to switch to the alternative species; indeed this is when the opportunity cost of choosing one alternative replacement over the other is the highest. The land value (and also the value of the firm) is similar to an American option with free boundary, infinite expiry period, and endogenous payoff. The paper highlights the opportunity cost of alternative replacement options, and the central role of their volatility in both asset-retirement and replacement-choice decisions. All results are derived analytically; a numerical treatment by the penalty method completes the resolution.