Forests regulate global and local climates in ways that impact human well-being. In this Review, we discuss the scale-dependent mechanisms through which forests regulate climate, highlighting their contributions to global mitigation and local adaptation. Locally, forests tend to buffer temperatures, cooling in warm conditions and warming in cold ones. In regions that naturally support dense forest cover, trees contribute to global cooling primarily through carbon uptake, with some offsetting from albedo-related warming. By enhancing rainfall interception, evapotranspiration, and cloud formation, forests also influence the hydrological cycle, lowering flood risks in humid regions but often reducing downstream water availability, especially in drier climates. Collectively, these interacting processes show that the greatest climate benefits occur where forests are native, highlighting their importance for both climate adaptation and mitigation.

An issue of global concern is how climate change forcing is transmitted to ecosystems. Forest ecosystems in mountain landscapes may demonstrate buffering and perhaps decoupling of long‐term rates of temperature change, because vegetation, topography, and local winds (e.g., cold air pooling) influence temperature and potentially create microclimate refugia (areas which are relatively protected from climate change). We tested these ideas by comparing 45‐year regional rates of air temperature change to unique temporal and spatial air temperature records in the understory of regionally representative stable old forest at the H.J. Andrews Experimental Forest, Oregon, USA. The 45‐year seasonal patterns and rates of warming were similar throughout the forested landscape and matched regional rates observed at 88 standard meteorological stations in Oregon and Washington, indicating buffering, but not decoupling of long‐term climate change rates. Consideration of the energy balance explains these results: while shading and airflows produce spatial patterns of temperature, these processes do not counteract global increases in air temperature driven by increased downward, longwave radiation forced by increased anthropogenic greenhouse gases in the atmosphere. In some months, the 45‐year warming in the forest understory equaled or exceeded spatial differences of air temperature between the understory and the canopy or canopy openings and was comparable to temperature change over 1,000 m elevation, while in other months there has been little change. These findings have global implications because they indicate that microclimate refugia are transient, even in this forested mountain landscape.

AbstractDespite much interest in relationships among carbon and water in forests, few studies assess how carbon accumulation scales with water use in forested watersheds with varied histories. This study quantified tree growth, water use efficiency, and carbon‐water tradeoffs of young versus mature/old‐growth forest in three small (13–22 ha) watersheds in the H.J. Andrews Experimental Forest, Oregon, USA. To quantify and scale carbon‐water tradeoffs from trees to watersheds, tree‐ring records and greenness and wetness indices from remote sensing were combined with long‐term vegetation, climate, and streamflow data from young forest watersheds (trees ∼45 years of age) and from a mature/old‐growth forest watershed (trees 150–500 years of age). Biomass production was closely related to water use; water use efficiency (basal area increment per unit of evapotranspiration) was lower; and carbon‐water tradeoffs were steeper in young forest plantations compared with old‐growth forest for which the tree growth record begins in the 1850s. Greenness and wetness indices from Landsat imagery were not significant predictors of streamflow or tree growth over the period 1984 to 2017, and soil C and N did not differ significantly among watersheds. Multiple lines of evidence show that mature and old‐growth forest watersheds store and accumulate more carbon, are more drought resistant, and better sustain water availability compared to young forests. These results provide a basis for reconstructions and predictions that are potentially broadly applicable, because first‐order watersheds occupy 80%–90% of large river basins and study watersheds are representative of forest history in the Pacific Northwest region.

Globally,forests play a critical role in regulating the hydrologi-cal cycle,supplying water to ecosystems and people.However,both forests and this cycle are undergoing rapid changes[1,2].Future states of forests and the associated hydrological cycle remain highly uncertain,influenced by altered disturbance regimes such as increasingly frequent and severe wildfires,droughts,and hurricanes,as well as gradual changes such as sea level rise,warming temperatures,and shifts in the amount,sea-sonal pattern,and form of precipitation[3].

Forests play a crucial role in regulating the global climate. Yet, forests also influence the local climate conditions through biophysical processes that directly impact human wellbeing. With growing policy emphasis on these climate adaptation effects, we review the scale dependent impacts of forests on climate conditions and their implications for human wellbeing. Generally, existing forests buffer local temperatures, with warming effects in cold regions and cooling effects in hot regions. At a global scale, trees are more conducive to cooling in regions where dense forests would naturally exist. Additionally, forests generally reduce water runoff, which can reduce flooding in wet areas, but it can also limit water availability downstream, especially in drier regions. Together, these findings suggest that climate positive tree effects tend to be most frequent in regions where forests naturally occur, and highlight the growing consensus around the importance of natural forests for climate adaptation.

This study examined the 70-year history of clearcutting of old-growth forest and associated road construction, floods, landslides, large wood in rivers, and channel change in the 64 km(2) Lookout Creek watershed in western Oregon, where forestry practices began in 1950 and largely ceased by the 1980s. Responses differed among three zones with distinctive geomorphic processes within the watershed: a glacially sculpted zone, an earthflow-dominated zone, and a debris slide and debris flow-dominated zone. Watershed response to floods was more related to the timing of road construction and clearcuts, past geomorphic events, and forest dynamics than to flood magnitude. Even small (1-3 year) floods generated geomorphic responses in the period of initial road construction and logging (1950-1964) and during ongoing logging in the early part of a 30-year period between large flood events (1966-1995). The floods of 1964/65, 15 years after the onset of logging, produced much larger geomorphic responses than the flood of record (1996), more than a decade after logging ceased. Geomorphic response was negligible for the third largest event on record (2011) during the last period (1997-2020), when former clearcuts were 20 to 70-year-old forest plantations. Watershed response in each of five distinct time periods depended on conditions created during prior periods in the three zones. Understanding of watershed response to forestry requires integrated observation of forestry practices, floods, landslide susceptibility, wood delivery and movement, and channel change on time scales that capture responses to past and ongoing management practices and geophysical and biological factors and events.

The 22-23 April 2015 eruption of the Calbuco volcano (Southern Andes, Chile) led to extensive pyroclastic density currents (PDCs) interactions with vegetation. We seek to describe the PDCs which affected both Tepu and Frío rivers, northern Calbuco, from their timing and deposition to cooling and erosion, as well as their impacts on forests. Our investigation is based on field stratigraphy, forest disturbance assessment, and geothermometry from degassing pipes and charcoal. These PDCs reached at least ~540-603 °C, as estimated from fumaroles, and consisted of both concentrated and dilute PDCs during the first pulse (22 April) at Tepu and mainly during the second pulse (23 April) at Frío. Effects of PDCs on forest vegetation recorded in Tepu consisted of heating, abrasion, burial, and impact force. On the valley floor, trees were buried with up to 4 m of deposits from the concentrated PDCs, and all trees in this deposition zone died with no subsequent sprouting. Conversely, in the margins of the valley, defoliated fallen trees and standing shrubs indicate scorching due to the passage of dilute PDCs, and some of them were later sprouting. Estimated impact forces required to produce toppling range from 1.5 to 3.7 kPa, and PDC velocities reached up to 36 m s-1. Charring of the buried wood involved an emplacement temperature of 400-550 °C within PDC deposits. The rapid watershed formation may have facilitated infiltration, decreasing the temperature in the basal part within the deposits at the Tepu river. Runoff during the subsequent months triggered lahars and caused the rivers to incise the deposits and transport sediment downstream. This set of observations provides valuable insights into how the interaction between volcanic phenomena and margine forest on the valley floors informs eruptive processes, dynamics, and impacts. Our study is also relevant to interpret the thermal history and potential hazards of PDCs.

Canopy wetting and drying has a variety of effects on the function of plant foliage, ranging from increased risk of pathogenic infection to reduced diffusion of gases to enhanced leaf water status in plants capable of foliar water uptake (FWU). Projected shifts in rainfall regimes and increases in summertime vapor pressure deficit will likely change the timing and duration of canopy wetting, yet current patterns of wetting are poorly understood. In this study, we investigated patterns of wetting by source (rain, dew, or frost), at different canopy heights, and at annual, seasonal and diurnal time scales using leaf wetness sensor data collected over a 4-year period in an old growth Douglas-fir tree in a temperate wet forest. We found that canopy layers were wet for roughly half the year with strong seasonal variation, staying wet 83% of the cold winter season but only 1.9% of the dry season. Upper canopy layers experienced higher wetting frequency and shorter wetting duration in all seasons compared to lower canopy layers. Outside of the dry season, wetness was predominantly caused by rain, while in the dry season the predominant source was dewfall. Throughout the year and particularly in the dry season, dewfall was restricted to the upper canopy, occurring on 28.5% of dry season nights. Multiple models which use meteorological variables to predict dewfall timing and length were developed and evaluated. Using in-tree observations, dry season dewfall was best predicted with a logistic model using dewpoint depression as a predictor. Using observations from a nearby weather station in a clearing, dry season dewfall was best predicted with the Penman equation, a biophysical model. The most important determinant of dry season dewfall in our study was sufficient nighttime cooling of the air, suggesting that increasing nighttime temperatures will lead to a decrease in dew formation frequency in the future.

In this article marking the 40th anniversary of the US National Science Foundation's Long Term Ecological Research (LTER) Network, we describe how a long-term ecological research perspective facilitates insights into an ecosystem's response to climate change. At all 28 LTER sites, from the Arctic to Antarctica, air temperature and moisture variability have increased since 1930, with increased disturbance frequency and severity and unprecedented disturbance types. LTER research documents the responses to these changes, including altered primary production, enhanced cycling of organic and inorganic matter, and changes in populations and communities. Although some responses are shared among diverse ecosystems, most are unique, involving region-specific drivers of change, interactions among multiple climate change drivers, and interactions with other human activities. Ecosystem responses to climate change are just beginning to emerge, and as climate change accelerates, long-term ecological research is crucial to understand, mitigate, and adapt to ecosystem responses to climate change.

Mountain breezes, including katabatic and anabatic flows, and temperature inversions are common features of forested mountain landscapes. However, the effects of mountain breezes on moisture transport in forests and implications for regional climate change are not well understood. A detailed, instrumented study was conducted from July to September 2012 in an even‐aged conifer forest in the Oregon Cascade Range to investigate how temperature profiles within the forest canopy influenced atmospheric surface layer processes that ventilate the forest. Subcanopy inversion strength has a bimodal relationship to subcanopy wind speed and moisture flux from the forest. On days with relatively modest heating of the top of the canopy and weak subcanopy inversions, above canopy winds more efficiently mix subcanopy air, leading to greater than average vertical moisture flux and weaker than average along‐slope, subcanopy water vapor advection. On days with strong heating of the top of the canopy and a strong subcanopy inversion, vertical moisture flux is suppressed, and daytime downslope winds are stronger than average under the canopy. Increased downslope winds lead to increased downslope transport of water vapor, carbon dioxide, and other scalars under the canopy. Increasing summer vapor pressure deficit in the Pacific Northwest will enhance both processes: vertical moisture transport by mountain breezes when subcanopy inversions are weak and downslope water vapor transport when subcanopy inversions are strong. These mountain breeze dynamics have implications for climate refugia in forested mountains, forest plantations, and other forested regions with a similar canopy structure and regional atmospheric forcings.

Forest restoration aims to increase forest cover, structure, function, and/or species composition, and it influences hydrology through the partitioning of precipitation into evapotranspiration and streamflow. This paper provides a conceptual framework for forest restoration and hydrology, reviews the literature on forest hydrology that is relevant to forest restoration, and assesses practical forest restoration approaches, their hydrologic effects, and tradeoffs. The hydrologic effects of three types of forest are assessed: mature and old-growth forests, which often are the reference model for restoration; managed forest plantations, which dominated early efforts for forest restoration; and the early stages of native forest succession, an increasingly popular, ecologically-oriented or nature-based approach to forest restoration. This review indicates that mature and old-growth forests have high evapotranspiration and consistent water yield, provided by moderated peak discharges and sustained low flows, while water yield is low from managed forest plantations, especially during dry periods. The early stages of native forest succession may provide greater water yield and increased low flows compared with managed plantations. Inclusion of native species and natural processes in forest restoration can increase some hydrological benefits relative to other forest restoration approaches. Although forest restoration affects hydrology, few studies examine the hydrologic effects of specific forest restoration practices such as choice of species, silvicultural practices, legacies of past land use, and geographic setting. Forest managers and ecologists can play valuable roles by designing studies that explore the hydrologic effects of forest restoration approaches on time scales relevant to ecological succession and forest management under a changing climate.

Climate change can directly affect forest hydrology by altering precipitation, evapotranspiration, and streamflow generation, or indirectly by changing disturbance regimes and forest structures at multiple scales. Climate change impacts on the forest-water nexus across biomes are pervasive characterized by a great complexity and uncertainty, significantly impeding the design of adaptive forest watershed management to mitigate climate change risks. This paper reviews our current knowledge on the interactions between climate change and the forest-water nexus at the scales of individual tree, stand, and watershed. We found that climate change dramatically altered watershed hydrology in many parts of the world, with varying hydrological responses at multiple scales of tree species, forest types, climate types, and hydrological regimes. The streamflow response was often more pronounced in snow-dominated or water-limited watersheds, especially in watersheds with increasing droughts due to climate change and intensively managed plantations of either non-native tree species (e.g., Eucalyptus plantations in Brazil, Chile, Uruguay, and Australia) or young coniferous species. Climate change impacts can be compounded or offset by forest changes (i.e., deforestation, and forestation) through forest-climate interactions and feedbacks. Forest management can mitigate or aggravate the negative hydrologic impacts of climate change. Adaptive forest management is a prerequisite for managing the forest-water nexus in the face of climate change. Various forest management strategies aiming at maintaining optimal forest structure and high species diversity are recommended to enhance forest resistance and resilience to climate change and sustain water provision services from forests and other beneficial ecosystem services while minimizing negative impacts and risks of climate change.

Forest and freshwater ecosystems are tightly linked and together provide important ecosystem services, but climate change is affecting their species composition, structure, and function. Research at nine US Long Term Ecological Research sites reveals complex interactions and cascading effects of climate change, some of which feed back into the climate system. Air temperature has increased at all sites, and those in the Northeast have become wetter, whereas sites in the Northwest and Alaska have become slightly drier. These changes have altered streamflow and affected ecosystem processes, including primary production, carbon storage, water and nutrient cycling, and community dynamics. At some sites, the direct effects of climate change are the dominant driver altering ecosystems, whereas at other sites indirect effects or disturbances and stressors unrelated to climate change are more important. Long-term studies are critical for understanding the impacts of climate change on forest and freshwater ecosystems.

Forest restoration aims to increase forest cover, structure, function, and/or species composition, and it influences hydrology through the partitioning of precipitation into evapotranspiration and streamflow. This paper provides a conceptual framework for forest restoration and hydrology, reviews the literature on forest hydrology that is relevant to forest restoration, and assesses practical forest restoration approaches, their hydrologic effects, and tradeoffs. The hydrologic effects of three types of forest are assessed: mature and old-growth forests, which often are the reference model for restoration; managed forest plantations, which dominated early efforts for forest restoration; and the early stages of native forest succession, an increasingly popular, ecologically-oriented or nature-based approach to forest restoration. This review indicates that mature and old-growth forests have high evapotranspiration and consistent water yield, provided by moderated peak discharges and sustained low flows, while water yield is low from managed forest plantations, especially during dry periods. The early stages of native forest succession may provide greater water yield and increased low flows compared with managed plantations. Inclusion of native species and natural processes in forest restoration can increase some hydrological benefits relative to other forest restoration approaches. Although forest restoration affects hydrology, few studies examine the hydrologic effects of specific forest restoration practices such as choice of species, silvicultural practices, legacies of past land use, and geographic setting. Forest managers and ecologists can play valuable roles by designing studies that explore the hydrologic effects of forest restoration approaches on time scales relevant to ecological succession and forest management under a changing climate.

Cold-air drainage and pooling can have wide-ranging impacts, including affecting ecosystem processes and agricultural crops, and contributing to decreased air quality associated with temperature inversions. Future climate changes may alter both the frequency and intensity of cold-air drainage. This study estimates the response of nocturnal cold-air drainage to warming resulting from anthropogenic greenhouse gases, specifically CO 2 , considering radiative and thermodynamic effects but not changes in background air flow (dynamic effects). A simple index is proposed to represent the propensity for clear-sky nocturnal cold-air drainage as a function of air temperature and humidity near dusk. Decreases in this index with increasing atmospheric emissivity due to increasing anthropogenic greenhouse gase concentrations imply a weakening of cold-air drainage. The magnitude of the decrease in the index is positively related to the initial background temperature and humidity: Warm regions are more sensitive than cold regions, and humid regions are more sensitive than dry regions, implying that warm and/or humid regions are more at risk of decreases in cold-air drainage. Under atmospheric CO 2 concentrations consistent with Representative Concentration Pathway (RCP) 8.5, the magnitude of decrease in the index indicates that nocturnal cold-air drainage intensity may decline by at least 10% by 2100 CE (compared to 1979–1990) with larger decreases in warm and humid regimes. The index should be tested with intentionally designed field or lab experiments, and the relative effects on cold-air drainage of changes in radiative, sensible, and latent heat fluxes, and atmospheric circulation, should be compared.

Global increases in intensive forestry have raised concerns about forest plantation effects on water, but few studies have tested the effects of plantation forest removal and native forest restoration on catchment hydrology. We describe results of a 14‐year paired watershed experiment on ecological restoration in south central Chile which documents streamflow response to the early stages of native forest restoration, after clearcutting of plantations of exotic fast‐growing Eucalyptus, planting of native trees, and fostering natural regeneration of native temperate rainforest species. Precipitation, streamflow, and vegetation were measured starting in 2006 in four small (3 to 5 ha) catchments with Eucalyptus globulus plantations and native riparian buffers in the Valdivian Coastal Reserve. Mean annual precipitation is 2500 mm, of which 11% occurs in summer. Streamflow increased, and increases persisted, throughout the first 9 years of vigorous native forest regeneration (2011 to 2019). Annual streamflow increased by 40% to >100% in most years and >150% in fall and summer of some years. Streamflow was 50% to 100% lower than before treatment in two dry summers. Base flow increased by 28% to 87% during the restoration period compared to pre‐treatment, and remained elevated in later years despite low summer precipitation. Overall, these findings indicate that removal of Eucalyptus plantations immediately increased streamflow, and native forest restoration gradually restored deep soil moisture reservoirs that sustain base flow during dry periods, increasing water ecosystem services. To our knowledge this is the first study to assess catchment streamflow response to native forest restoration in former forest plantations. Therefore, the results of this study are relevant to global efforts to restore native forest ecosystems on land currently intensively managed with fast‐growing forest plantations and may inform policy and decision‐making in areas experiencing a drying trend associated with climate change.

Long-term watershed experiments provide the opportunity to understand forest hydrology responses to past logging, road construction, forest regrowth, and their interactions with climate and geomorphic processes such as road-related landslides. We examined a 50-year record from paired-watershed experiments in the H. J. Andrews Experimental Forest, Oregon, USA in which 125 to 450-year-old conifer forests were harvested in the 1960s and 1970s and converted to planted conifer forests. We evaluated how quickflow and delayed flow for 1222 events in treated and reference watersheds changed by season after clearcutting and road construction, including 50 years of growth of planted forest, major floods, and multi-decade reductions in snowpack. Quickflow runoff early in the water year (fall) increased by up to +99% in the first decade, declining to below pre-harvest levels (-1% to -15%) by the third to fifth decade after clearcutting. Fall delayed flow responded more dramatically than quickflow and fell below pre-treatment levels in all watersheds by the fifth decade, consistent with increased transpiration in the planted forests. Quickflow increased less (+12% to 70%) during the winter and spring but remained higher than pre-treatment levels throughout the fourth or fifth decade, potentially impacted by post-harvest burning, roads, and landslides. Quickflow remained high throughout the 50-year period of study, and much higher than delayed flow in the last two decades in a watershed in which road-related changes in flow routing and debris flows after the flood of record increased network connectivity. A long-term decline in regional snowpack was not clearly associated with responses of treated vs. reference watersheds. Hydrologic processes altered by harvest of old-growth conifer forest more than 50 years ago (transpiration, interception, snowmelt, and flow routing) continued to modify streamflow, with no clear evidence of hydrologic recovery. These findings underscore the importance of continued long-term watershed experiments.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Research and Observatory Catchments: Promoting the Sites Behind the Rich Legacy of Knowledge Discovery and InnovationAuthorsStephenSebestyeniDJamesShanleyTheresaBlumeiDJonathanDuncanJuliaJonesCatalinaSeguraSee all authors Stephen SebestyeniDCorresponding Author• Submitting AuthorUSDA Forest ServiceiDhttps://orcid.org/0000-0002-6315-0108view email addressThe email was not providedcopy email addressJames ShanleyUSGSview email addressThe email was not providedcopy email addressTheresa BlumeiDGFZ German Research CentreiDhttps://orcid.org/0000-0003-3754-7571view email addressThe email was not providedcopy email addressJonathan DuncanPennsylvania State University Main Campusview email addressThe email was not providedcopy email addressJulia JonesOregon State Universityview email addressThe email was not providedcopy email addressCatalina SeguraOregon State Universityview email addressThe email was not providedcopy email address

The H. J. Andrews Experimental Forest (HJA) encompasses the 6400 ha Lookout Creek watershed in western Oregon, USA. Hydrologic, chemistry and precipitation data have been collected, curated, and archived for up to 70 years. The HJA was established in 1948 to study the effects of harvest of old-growth conifer forest and logging-road construction on water quality, quantity and vegetation succession. Over time, research questions have expanded to include terrestrial and aquatic species, communities and ecosystem dynamics. There are nine small experimental watersheds and 10 gaging stations in the HJA, including both reference and experimentally treated watersheds. Gaged watershed areas range from 8.5 to 6242 ha. All gaging stations record stage height, water conductivity, water temperature and above-stream air temperature. At nine of the gage sites, flow-proportional water samples are collected and composited over 3-week intervals for chemical analysis. Analysis of stream and precipitation chemistry began in 1968. Analytes include dissolved and particulate species of nitrogen and phosphorus, dissolved organic carbon, pH, specific conductance, suspended sediment, alkalinity, and major cations and anions. Supporting climate measurements began in the 1950s in association with the first small watershed experiments. Over time, and following the initiation of the Long Term Ecological Research (LTER) grant in 1980, infrastructure expanded to include a set of benchmark and secondary meteorological stations located in clearings spanning the elevation range within the Lookout Creek watershed, as well as a large number of forest understory temperature stations. Extensive metadata on sensor configurations, changes in methods over time, sensor accuracy and precision, and data quality control flags are associated with the HJA data.