Transpiration is the process by which plants release water vapor into the atmosphere through tiny pores in their leaves. On the AP Enviro exam, it shows up as a step in the hydrologic cycle (Topic 1.7) and as a source of atmospheric water vapor (Topic 9.3).
Transpiration is water moving up through a plant and escaping as vapor through pores in the leaves called stomata. Roots pull water from the soil, the xylem carries it up the plant, and most of it evaporates off the leaves into the air. Think of it as a plant slowly sweating, except the "sweat" leaves through leaf pores instead of skin.
In the bigger picture, transpiration is one way liquid water on land becomes gaseous water vapor in the atmosphere. Per EK ERT-1.G.1, the hydrologic cycle is the sun-powered movement of water between sources and sinks in its solid, liquid, and gaseous phases. Transpiration is part of the "liquid to gas" leg of that cycle, working alongside evaporation to return water from the land to the sky.
Transpiration sits at the intersection of two units, which is exactly why it's worth knowing well. In Unit 1 (Topic 1.7), it supports learning objective AP Enviro 1.7.A, explaining the steps and reservoir interactions in the hydrologic cycle. It's how plants pump water from the soil reservoir back into the atmosphere. In Unit 9 (Topic 9.3), the water vapor that transpiration releases ties into greenhouse gases. Per EK STB-4.C.1, water vapor is one of the principal greenhouse gases. So a single process connects ecosystem water movement to global climate, which is the kind of cross-unit link the exam loves to reward.
Keep studying AP Environmental Science Unit 1
Evaporation and the Hydrologic Cycle (Unit 1)
Evaporation turns liquid water into vapor off any wet surface, like oceans, lakes, and soil. Transpiration does the same thing, but specifically through plants. Together they're often lumped into "evapotranspiration" because both return water to the atmosphere in the same leg of the cycle.
Water Vapor as a Greenhouse Gas (Unit 9)
The vapor transpiration releases is a greenhouse gas (EK STB-4.C.1). But EK STB-4.C.2 says water vapor doesn't drive long-term climate change much because it has a short residence time, meaning it falls back out as precipitation quickly instead of building up like CO2.
Deforestation and Watershed Changes (Units 1 and 9)
Cut down a forest and you remove the plants doing the transpiring. Less transpiration means less moisture returning to the air locally, which can shift precipitation patterns and how water cycles through that watershed.
Transpiration usually shows up in MCQs about the hydrologic cycle, where you trace the path water takes after falling as precipitation in a forested ecosystem. Expect questions asking what happens to a watershed when deforestation removes transpiring plants, or how warming intensifies the water cycle by speeding up evaporation and transpiration. On FRQs, transpiration supports answers about ecosystem water movement and the effects of habitat loss or urbanization (like the 2022 urban heat island FRQ, where less vegetation means less transpiration and hotter cities). Be ready to place it correctly in a cycle diagram and to explain cause-and-effect, not just define it.
Both turn liquid water into atmospheric vapor, so they're easy to mix up. Evaporation happens off any open surface (oceans, soil, puddles). Transpiration happens specifically through plant leaves via the stomata. If a question mentions plants, leaves, or vegetation, it's transpiration; if it mentions open water or bare surfaces, it's evaporation.
Transpiration is water vapor escaping from plant leaves through stomata, after roots pull it up and xylem carries it.
It's part of the liquid-to-gas leg of the hydrologic cycle (Topic 1.7), working alongside evaporation to return water to the atmosphere.
The water vapor it releases is a greenhouse gas (EK STB-4.C.1), but vapor has a short residence time so it doesn't drive long-term warming much (EK STB-4.C.2).
Removing plants through deforestation reduces transpiration, which changes how water cycles through a watershed.
Evaporation comes off any wet surface; transpiration comes specifically through plants, so watch the wording in MCQs.
It's the process where plants release water vapor into the atmosphere through pores in their leaves called stomata. On the AP exam it's a step in the hydrologic cycle (Topic 1.7) and a source of atmospheric water vapor (Topic 9.3).
Both turn liquid water into vapor, but evaporation happens off any open surface like oceans and soil, while transpiration happens specifically through plant leaves. Scientists often combine them into one term, evapotranspiration.
Yes, water vapor is one of the principal greenhouse gases (EK STB-4.C.1). But per EK STB-4.C.2 it doesn't significantly drive global climate change because it has a short residence time in the atmosphere and falls back out as precipitation quickly.
Removing trees removes the plants that transpire, so less water vapor returns to the air. This can alter local precipitation and shift how the watershed cycles water, a common FRQ and MCQ scenario.
Yes. Warmer temperatures intensify the hydrologic cycle by speeding up both evaporation and transpiration, putting more water vapor into the atmosphere. This connects Unit 1 water cycling to Unit 9 global change.