(plaintiff challenged defendant’s plan to increase herbicide spraying to control invasive plants in Wallowa-Whitman National Forest on basis that defendant failed to properly consider cumulative effects of spraying in conjunction with other actions that might be taken; court determined that plaintiff, environmental group, that defendant had, in part, violated National Environmental Policy Act in its analysis of cumulative impacts of herbicide project; case remanded to defendant for further development of analysis of cumulative impacts, but defendant’s other decisions concerning project not arbitrary or capricious).