finding 6.2 : key-message-6-2

It is very likely that climate change will decrease the ability of many forest ecosystems to provide important ecosystem services to society. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperatures, more frequent drought, and increased disturbances (medium confidence). The onset and magnitude of climate change effects on water resources in forest ecosystems will vary but are already occurring in some regions (high confidence).

This finding is from chapter 6 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Lead authors, chapter authors, and technical contributors engaged in multiple technical discussions via teleconference between September 2016 and March 2018, which included a review of technical inputs provided by the public and a broad range of published literature as well as professional judgment. Discussions were followed by expert deliberation on draft Key Messages by the authors and targeted consultation with additional experts by the authors and technical contributors. A public engagement webinar on May 11, 2017, solicited additional feedback on the report outline. Webinar attendees provided comments and suggestions online and through follow-up emails. Strong emphasis was placed on recent findings reported in the scientific literature and relevance to specific applications in the management of forest resources. 

Description of evidence base:

Altered forest conditions caused by a changing climate are likely to influence the quantity and quality of many of the ecosystem services that humans derive from forests, and climate change is expected to increase the frequency and severity of natural disturbances in the coming decades and to reduce forest growth in most places.71c75d19-f2ad-4bf1-9cb8-b9a08f8c3ef0 Extreme high temperatures can also cause heat-related stress in vegetation and exacerbate drought conditions, potentially increasing tree mortality and reducing forest productivity.abe49f4d-90c4-40e2-a4b9-a58158c00560,811ef6d7-304b-40e0-8b90-433d80cdb5f0 Positive effects of carbon dioxide (CO2) on growth will be negated in some species and locations by low soil fertilityb0de50e3-d2ce-49db-94f1-5c7225f4c3cc and by air pollutants such as ground-level ozone, where concentrations of those pollutants are high enough to cause toxic effects in plants.307a94b5-6aaa-4a65-a8ad-19173443a633

Most evidence suggests that increased carbon sinks (caused by higher growth rates and more forest area in some regions) will not be sufficient to offset higher emissions from increased disturbances and enhanced release of carbon from decomposition in the future.6bd92a32-feef-4b92-a124-49555ada8b5d,56615fa3-710a-4667-aaa2-347fa27fb133,ad121807-c933-445c-ac86-eb346f6177d1,a25c9b98-c0f8-49ba-98f0-7e1cb81fd43e U.S. forests are projected to continue to sequester carbon but at declining rates caused by land-use change and aging forests.71c75d19-f2ad-4bf1-9cb8-b9a08f8c3ef0 In the western United States, the aging of forests, coupled with disturbance dynamics, is projected to diminish carbon sequestration to negligible levels by around 2050, and some forests (for example, dry western forests with frequent fire and some eastern hardwood forests) will likely become a carbon source.71c75d19-f2ad-4bf1-9cb8-b9a08f8c3ef0 Younger productive forests in the eastern United States portend high carbon uptake rates, although harvest-related emissions substantially reduce the net effect on atmospheric carbon.

Land-use change that increases forest cover (such as cropland converted to forestland) is a major contributor to reductions in atmospheric CO2,1c5cb83e-eb5f-4e99-8d1a-3f044b57381a but this conversion is expected to slow in the near future.44fcdc28-381a-41b6-b267-c4fd63ebc0d5 The estimated net carbon flux in the United States associated with forestland conversion is approximately zero, with gains in forestland constituting +23 teragrams (Tg) of carbon per year and losses resulting in emissions of −23 Tg carbon per year over the last decade. The estimated emissions constitute decades, and in some cases centuries, of accumulated carbon within forest ecosystems, which is abruptly or gradually released to the atmosphere during conversion from forest to nonforest land. In contrast, gains in forestland represent carbon sequestration only from new growth of live biomass and the accumulation of newly dead organic matter over the 20 or so years since the renewal of forest cover.

Economic conditions and population growth will affect national and global production and consumption of wood products, which can temporarily sequester carbon (currently 189 Tg carbon per year, or 8% of the global forest sink).ffa77675-22b0-499b-a6d5-5108b3595472 Increases in wood products carbon are contingent on a sustained or increasing rate of harvest removals of forest carbon or on a shift toward forest products that exist for long periods of time before they are no longer suitable for reuse or recycling. In the United States, 76% of the annual domestic harvest input to the wood products pool in 2015 (110 Tg carbon) was offset by release processes (84 Tg carbon), yielding a corresponding net increase in wood products of 26 Tg carbon.81430bfc-5d67-4109-982a-4cfd344f057c However, if harvest rates decline (as they did in 2007–2009, during the last economic recession), net additions to wood products will likely be lower than emissions from wood harvested in prior years.81430bfc-5d67-4109-982a-4cfd344f057c Looking ahead, carbon storage in wood products is expected to increase by 7–8 Tg carbon per year over the next 25 years.70bc9ca3-4303-4dca-af68-4fc733395664

Snowfall amount, timing, and melt dynamics are affecting water availability and stream water quality in the western United States, where less precipitation is falling as snow and more as rain in winter months, leading to longer and drier summer seasons.e450ba2c-db69-43c8-8af4-e0c8ce7c8f2f Furthermore, rapid opening of forests in the western United States by wildfire has caused faster spring snowmelt through increased solar radiation and decreased reflectivity of radiation from charcoal,4d98fcfd-e95f-4060-923d-40e8453ce930 leading to drier summer conditions that offset increased water yield following a disturbance.b86c02d3-167a-4c9e-a17c-be4cba9283db The persistence of winter snowpack in the northeastern United States has declined over the last few decades; mid-winter thaws have become more common, and snowmelt flushing of mobilized soil nutrients into streams has become less common, although increased variability in climate–hydrology interactions can alter flushing.8c6e713b-c943-4d9f-a3a9-2333218d6587

New information and remaining uncertainties:

It is difficult to identify geographically specific changes in forest conditions at fine scales because of high spatial variability in forest structure and function and variability in projections of climate change and how it will affect large disturbances (drought, wildfire, insect outbreaks). Uncertainties about the rate and magnitude of climate change effects on carbon sequestration are moderately high, because it is difficult to project future trends in forest cover and socioeconomic influences on forest management (for example, demand for wood products, bioenergy). Although empirical evidence for young trees indicates that atmospheric enrichment of CO2 can enhance tree growth, few long-term data on mature trees are available on which to base inferences about long-term forest productivity.bac5e675-3edc-4fe0-8981-7379e1597b39 Temporal patterns and magnitude of carbon sequestration, especially after 2050, will be affected by uncertainties related to future land-use conversions (from forests to other uses and vice versa) and the production of wood products.

Assessment of confidence based on evidence:

Because of variability in forest structure and function and species-level variation in adaptive capacity to climate change, it is difficult to project future changes in forest conditions at smaller than regional scales. Hence, there is medium confidence about how ecosystem services will be affected in different forest ecosystems, including effects on tree growth and carbon storage, as a function of higher temperature, more frequent drought, and increased disturbance. Observations from recent droughts and changing snowfall/snowmelt dynamics provide high confidence that climate change effects on water are already occurring in some regions, although the onset and magnitude of future effects will vary regionally.

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5

Regions Covered

Related NASA GCMD keywords

References :

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