Timber Management Improvement Act of 1991

Timber Management Improvement Act of 1991 - Directs the Secretaries of Agriculture and of the Interior, acting through the Forest Service and the Bureau of Land Management, respectively, to prepare a report displaying the yields and costs (financially and environmentally) of timber management investment opportunities for Federal lands in the States of California, Oregon, and Washington. Requires the Secretary of Agriculture to include in such report reasons why certain lands were classified as unsuitable for timber production. Requires both Secretaries to establish advisory boards to review and comment on the investments examined.

Amends the Cooperative Forest Assistance Act of 1978 to allocate appropriations for private forest lands in the States of California, Oregon, and Washington to further second growth management of timber resources to offset the expected decline in timber harvests from Federal lands as a result of listing the northern spotted owl as a threatened species under the Endangered Species Act of 1973. Requires financial assistance to be concentrated in those counties containing Federal lands designated as habitat conservation areas for the purposes of the protection and recovery of the northern spotted owl.

Amends specified Federal law (the Knutson-Vandenberg Act) to assure that amounts from the fund established by such Act are available for timber management investments on Federal lands consistent with the Act's original intent.