Which of Thoreau's later essays exemplifies his qualities as a scientist?

Henry David Thoreau's interest in science was in that of the natural world; though not formally educated, he was an avid observer of plants and animals and kept meticulous records on his walks, hikes, and canoe trips as he camped in New England, Canada, New York, and as far west as Minnesota. 
"The Succession of Forest Trees" (1860) is an analysis of the ecology of forests; in this essay, Thoreau makes a case for why farmers should plant trees in a natural pattern of succession. It describes in scientific detail how seed propagation occurs, assisted by natural forces of animals, wind, and water movement, and what man's role can and should be in this process.
"Wild Apples" (1862) is an appreciation for the titular fruit, and does offer a homegrown botanist's view of the fruit's origin and spread as a species throughout the world, making the case for ungrafted and uncultivated apples' superiority.