Forests strongly influence Earth’s climate by exchanging carbon with the atmosphere. Understanding how carbon is stored in forest ecosystems, and how logging and fire change that storage, is essential for explaining human-driven climate change.

Forests as carbon reservoirs and air-quality buffers

Forests affect the atmosphere in two major ways emphasized in AP Environmental Science:

• They absorb pollutants (e.g., ozone precursors and particulate matter can be intercepted by leaves and deposited to surfaces).

• They store carbon dioxide by converting atmospheric CO$_2$ into plant and soil carbon through growth.

Key term: carbon sink

Forests are often carbon sinks when they are expanding in biomass or recovering after disturbance, but they can become carbon sources during intense disturbance, decay pulses, or widespread burning.

How forests store carbon dioxide

Carbon storage in forests is not only in standing trees; it is distributed across several carbon pools:

This forest carbon-cycle schematic shows how atmospheric $CO_2$ is converted to biomass through photosynthesis, then partitioned among living trees, litter/dead organic matter, and soil carbon. It also illustrates carbon returning to the atmosphere through respiration and decay, plus carbon leaving the ecosystem when timber is harvested. The figure reinforces that disturbance or removal can shift a forest from net carbon storage to net carbon release. Source

• Aboveground biomass: trunks, branches, leaves/needles

• Belowground biomass: roots

• Dead organic matter: leaf litter, fallen logs, standing dead trees

• Soil organic carbon: carbon stored in soils after plant material is decomposed and stabilized

Biological mechanism

• Through photosynthesis, trees take in atmospheric CO$_2$ and build carbohydrates, which become wood and other tissues.

• Some carbon enters soils via root growth, root exudates, and litterfall; decomposition returns part of this carbon to the atmosphere, while some becomes longer-lived soil carbon.

This diagram summarizes the major carbon pathways in a terrestrial ecosystem. It highlights how $CO_2$ enters biomass through photosynthesis, then returns to the atmosphere via plant and soil respiration, with additional transfers into soils through litterfall and losses via runoff. Seeing these arrows together helps explain why net carbon storage depends on the balance between uptake and release. Source

The net climate benefit depends on whether the forest, across all pools, is gaining carbon faster than it is losing carbon.

Cutting and burning: why CO$_2$ rises

The syllabus highlights that cutting and burning trees releases carbon dioxide and contributes to climate change. This occurs through several linked pathways.

Carbon released by combustion

When forests are burned (wildfire, land clearing, or burning slash), carbon in biomass is rapidly oxidized and emitted primarily as CO$_2</strong> (with smaller amounts of other carbon-containing gases and particulates). This creates an immediate increase in atmospheric greenhouse gases.</p><h3 class="editor-heading"><strong>Carbon released after cutting (even without fire)</strong></h3><p>Even if trees are cut and not burned, forest carbon storage can decline because:</p><ul><li><p><strong>Removed biomass</strong> is no longer storing carbon on-site.</p></li><li><p><strong>Residual debris</strong> (branches, roots, disturbed litter) decomposes, releasing <strong>CO$_2 over time.

Lost future uptake

Cutting reduces photosynthetic capacity, so less atmospheric CO$_2$ is removed in subsequent years. This “missed sequestration” can be a major part of the climate impact, especially where regrowth is slow or repeated harvesting prevents full recovery.

Climate-change connections and feedbacks

Forests influence climate beyond the immediate emissions pulse:

• Greenhouse effect amplification: Added atmospheric CO$_2</strong> increases heat retention, raising global temperatures.</p></li><li><p><strong>Fire–climate feedback:</strong> Warmer and drier conditions can increase fire frequency/intensity in some regions, which can further reduce forest carbon storage and increase emissions.</p></li><li><p><strong>Timing matters:</strong> Emissions from burning are fast, while re-accumulating carbon in regrowing forests is typically slower, creating a period of higher atmospheric CO$_2$that contributes to near-term warming.</p></li></ul><h2 class="editor-heading" id="interpreting-storage-versus-release"><strong>Interpreting “storage” versus “release”</strong></h2><p>For AP Environmental Science, focus on the direction of carbon movement:</p><ul><li><p>Intact, growing forests tend to <strong>remove and store CO$_2 (sink behavior).

• Cutting and especially burning tend to transfer stored carbon to the atmosphere (source behavior), strengthening climate change drivers.