For a small but growing number of Oregon forestland buyers, timber output is no more than a potential byproduct. Their purchases are driven less by a desire for logs than for clean, drinkable water. …city governments have long drawn their drinking water from surrounding forests, but experts say more are now actually buying the tracts encompassing those crucial streams and rivers. …The prospect of hotter, drier weather diminishing summer stream flows — even as populations keep growing — is spurring cities to assert more control over their water supplies, experts say. …Apart from water quality considerations, cities are buying forested watersheds to encourage old growth characteristics, with the intent of actually boosting water supplies over the long term, experts say. …Though municipal ownership of forest watersheds is intended to pre-empt disputes between cities and timber operators, the arrangement can still lead to tension over management decisions.