6
Forests cover approximately 42 km2 in tropical, temperate and boreal lands, approximately 30% of the land surface. Forests store approximately 45% of terrestrial carbon and contribute approximately 50% of terrestrial primary productivity. Complex and nonlinear forest–atmosphere interactions can dampen or amplify anthropogenic climate change.
Gordon B. Bonan, 2008
6.1Introduction
In the previous chapter, environmental and biological factors that affect tree and forest health, vitality, growth, carbon sequestration, and survival were considered. How different types of forests respond to interactions of these factors depends on their composition and location. The nature of the world’s natural and urban forests affects their function and role in carbon sequestration and in atmospheric cooling and warming. It seems appropriate to consider the nature, composition, function, and future of the forests of the world.
Most of the forested areas of the Earth are found in tropical areas and in the Northern Hemisphere, between 35° north and the Arctic. NPP in these regions constitutes the main land sink for carbon. Major differences between types of forests relate to extent of foliar biomass, determined by light, water, and nutrient availability, and growing season length (Perry et al., 2008). Distance from the equator and rainfall are general criteria for distinguishing between forest types. A more detailed presentation of the nature and function of forests of the world was developed, using the following references for source material (Bonan et al., 1992; University of California, Berkeley, Museum of Paleontology, 2006; Bonan, 2008; Perry et al., 2008; Gauthier et al., 2015; Baccini et al., 2017).
6.2Forest Types by Latitude
6.2.1Tropical and Subtropical Forests
Tropical and subtropical forests are found between the Tropic of Cancer (23.5° north) and the Tropic of Capricorn (23.5° south). They occupy 7.55% of the Earth’s land surface. High temperatures (average 27 °C), rainfall, and 12-hour day length affect tree growth. The temperature prevents winter, resulting in only wet and dry seasons. Their tree diversity is the most extensive of all forests. Many species are evergreen. Soils are acidic, nutrient-poor, often with low phosphorus, and easily leached. Fallen leaves and branches decompose quickly.
More than half of tropical forests have been lost since pre-industrial times. Drought and fires are increasing. Deforestation for logging and agriculture causes forest fragmentation and lowers the carbon sink. Destructive human influence is rapidly increasing (Lewis et al., 2015).
An example of a tropical forest is shown in Figure 6.1.
Figure 6.1
Tropical forest, Brazil.
Credit: Jean-Yves Grospas / Biosphoto / Getty Images
Tropical and subtropical forests have been subdivided by seasonal rainfall and the length of wet and dry seasons. The evergreen rainforest has no dry season. Seasonal rainforest of evergreen trees is in a wet region with a short dry period. Semi-evergreen forest has a deciduous tree overstory and an evergreen understory, and occurs in a region with a longer dry season. In deciduous forests, the length of the dry season increases in relation to rainfall. The composition and function of these forest types is very different, but all are often considered to be tropical or semitropical forests.
6.2.1.1Climate Forcing by Tropical Forests
Tropical forests have high rates of evapotranspiration. Transpiration from leaves especially cools the air in the vicinity of the leaves. It is usually assumed that transpiration from all tropical forests together is a negative force that cools the atmosphere, neutralizing the warming effect of trees with low albedo. Trees in tropical forests have been estimated to contain 25% of stored terrestrial carbon, further providing negative forcing (Bonan, 2008). Tropical forests have generally been assumed to be large, reliable, expanding natural sinks for carbon from carbon dioxide and to partially mitigate global warming. This assumption is open to question.
Conditions in tropical forests have been changing rapidly, with widespread deforestation and extensive tree deaths (Hansen et al., 2013). This has reduced carbon sequestration and transpirational cooling. Carbon dioxide is released from decomposing tree residues (from logging and land clearing) and fires. This suggests that tropical forests may be approaching net carbon-neutrality (Bonan, 2008) or becoming small net carbon sources (Grace et al., 2014).
6.2.1.2Methods of Assessment
The long-running question of whether tropical forests are effective carbon sinks, are net carbon-neutral, or are net carbon sources has not had a conclusive answer. Assessment is usually indirect, and the various methods of assessment have produced different answers.
Mitchard (2018) looked at annual carbon fluxes in and out of tropical forests, comparing results from different methods of assessment, including forest inventory plots, remote sensing in an El Nino year or in an ordinary year, and combined methods (Figure 6.2). Except in the hot dry conditions of an El Nino year, Mitchard concludes that tropical forests, with carbon dioxide emissions from deforestation and degradation, are approximately carbon-neutral. Continuing forest loss and warmer drier conditions could result in tropical forests becoming sources of carbon dioxide.
Figure 6.2
The effects of climate and land-use change on the intact forest carbon sink.
Potential contrasting effects of climate change and different trajectories of land-use change on the size of the intact forest carbon sink. Arrows pointing up show how climate change and policy could increase the magnitude of the sink, whereas arrows pointing down show how it will be reduced. All processes will occur to some extent, so predicting how the sink size will change is very difficult.
Adapted by permission from Springer Nature: Springer Nature Ltd, Nature, The tropical forest carbon cycle and climate change, E. T. A. Mitchard © (2018)
Baccini et al. (2017) directly determined changes in above-ground carbon density (above-ground carbon) of live woody vegetation across tropical America, Africa, and Asia (between 23.45° N and 23.45° S), using a combination of LiDAR data and NASA MODIS satellite imagery, from 2003 to 2014. Forest growth provided carbon gains of 436.5 ± 31.0 Tg carbon per year (1 teragram = 1 million metric tonnes of carbon dioxide equivalents). Losses from deforestation (degradation and disturbance) were 861.7 ± 80.2 Tg C yr−1. The net carbon source was 425.2 ± 92.0 Tg C yr−1. Forest degradation and disturbance accounted for 68.9% of the overall forest carbon loss. Baccini et al. concluded that they had provided direct evidence that the world’s tropical forests are a net carbon source. The future role of tropical forests as significant carbon sinks should not be automatically assumed. Tropical forests in the Amazon, and tropical forests in general, are at risk, as most of the 40,000 tropical tree species are threatened with decline or extinction (ter Steege et al., 2015).
6.2.1.3Effects of Agriculture and Deforestation on Climate Forcing by Tropical Forests
Mitchard (2018) has provided a schematic diagram (Figure 6.3) to indicate how the tropical forest sink might be affected by increase or reduction of forest size and the influence of climate and atmospheric carbon dioxide. Climate includes the influence of higher temperatures, fires, and drought. Mitchard concludes that integrating all these interacting factors and predicting their effects on the size of the tropical forest sink “is very difficult.”
Figure 6.3
Annual fluxes into and out of tropical forests.
Fluxes (in Pg C yr−1) are shown for different overlapping time periods (a–c) and for a recent El Niño year (d). The net intact forest flux is shown in turquoise, the net flux in regrowing forest is in orange, and the deforestation and forest degradation flux (including fire) is shown in pink. Panels a–c show that there is broad agreement that the tropics have made an approximately neutral contribution to atmospheric carbon stocks in the recent past, but d shows that intact forest can become a carbon source in hot and dry years, leading to considerable net emissions from the tropics. Data in a are from networks of forest inventory plots (see original paper for references), combined with forest area data from country surveys. In b, values are obtained from annual 463-m resolution optical satellite data, calibrated using LiDAR data and field plots from the mid-2000s. Intact and regrowth fluxes are not separated in this method. The figures in the study have been grossed up from biomass to total carbon stock change (that is, including dead wood, litter, soil). Data in c are derived from looking for overlap between atmospheric inversion, modeling and field-plot estimates. In d, data are obtained from satellites sensitive to atmospheric CO2 concentrations for the 2015 El Niño year, which contrast sharply with the other estimates shown. Change in land use could not be divided into separate regrowth and loss fluxes in this method.
Adapted by permission from Springer Nature: Springer Nature Ltd, Nature, The tropical forest carbon cycle and climate change, E. T. A. Mitchard © (2018)
Rosa et al. (2016) reconstructed deforestation rates at 5-year intervals from 1950 to 2009 for tropical forests in the Amazon, southeast Asia, and the Congo Basin. They estimated that, during that time, approximately 2.27 million square kilometers were deforested (Figure 6.4). Cumulative emissions of carbon dioxide during this period were 28–66% for the Amazon, 25–60% for southeast Asia, and 9–14% for the Congo Basin.
Figure 6.4
Modeled annual deforestation rates from 1950 to 2009 in five-year intervals.
Rates are shown in km2/yr.
Reprinted from Current Biology 26, 2161–2166, The environmental legacy of modern tropical deforestation, I. M. D. Rosa et al. © (2016), with permission from Elsevier
6.2.2Temperate Forests
Temperate forests occur between 25° and 50° N in eastern North America, northeast Asia, and western and central Europe. They are approximately 20% of the terrestrial land mass and contain about 10% of the stored terrestrial carbon. Seasons are well-defined and all occur. Precipitation (rain or snow) occurs in all seasons at 75–150 cm all year. Winter temperatures may be as low as −30 °C. Summer temperatures range to 30 °C and are increasing in southern regions. The forests have fertile soil with decomposing litter. The growing season is 140–200 days, with four to six frost-free months. Canopy density allows enough light penetration for understory vegetation development. Three or four species may dominate per square kilometer. Broadleaved deciduous trees may predominate: examples include oak, beech, maple, poplar, and hickory. Spruce, pine, hemlock, and fir are examples of conifers.
Seasonal rainfall determines forest subdivisions as follows.
· Mixed moist conifer/deciduous forests: mild wet winters, dry summers.
· Dry conifer forests: higher elevations, low rainfall.
· Mediterranean forests: rain in winter only, less than 100 cm per year.
· Temperate coniferous: mild winters, rain greater than 200 cm per year.
· Temperate broadleaved rainforests: frost-free winter, even rainfall.
A typical deciduous temperate forest is shown in Figure 6.5.
Figure 6.5
Temperate forest, Northeast United States.
Pixaby Pictures
6.2.2.1Climate Forcing by Temperate Forests
Deforestation for agricultural crops and pastures can have two possible effects on climate forcing by temperate forests (Bonan, 2008).
1.Net climate effect: neutral or slight. Land clearing decreases sink strength for carbon dioxide, and emissions increase. Cleared land has higher albedo values than darker forested land. Albedo negates effects of carbon dioxide.
2.Net climate effect: cooling effect reduced. Land clearing decreases transpiration and shade from trees, decreasing atmospheric cooling.
Deforestation and commercial use of forests as sources for wood are increasing globally. This reduces carbon dioxide capture and sequestration in wood. Forest reduction decreases the sink capacity of forests.
European forests are often managed for wood production, and long-term data on tree growth and production are readily available. Using available forest growth data, Nabuurs et al. (2013) determined that long-term stem volume increments in European forests are declining. They considered this to be an early indication of signs of forest sink saturation in European forests.
Declines in forest carbon sinks for the United States have been projected by Wear and Coulston (2015). Forests in the northeast region will gradually decline over a 25-year period. Fire, insect infestations, and drought could cause more rapid forest decline in the Rocky Mountain region. In the cooler Pacific Northwest, forests might become stable.
6.2.3Boreal Forests
These forests occur between 50° and 60° N and with difficulty to 70° N. Their distribution is circumpolar in Alaska, Canada, Siberia, and Scandinavia. A boreal forest area in the Yukon Territory of Canada is shown in Figure 6.6. Many are extensively managed for wood production. Winters are long and severe, and freezing temperatures are present for 8 months of the year. Snow is the principal form of precipitation (40–100 m annually). An example of a boreal forest area in winter is shown in Figure 6.6. The summer growing season is warm and short (130 days). Soils are acidic and nutrient-limited. Light penetration through canopies is low. Mixed forests include cold-tolerant conifers, such as spruce, pines, and fir, and cold-tolerant broadleaved deciduous trees, such as poplar, birch, and alder. Conifers may dominate at higher latitudes.
Figure 6.6
Boreal forest, Yukon Territory, Canada.
Good Free Photos
Despite a short growing season, boreal forest trees capture and sequester carbon in wood. Annual carbon gain by older, more mature trees, however, may be low (Bonan, 2008).
Fires are increasing in boreal forests. The extent of loss relates to forest composition in a given forest. Where conifers such as black spruce (Picea mariana) are dominant, loss from fires is more likely. Leaves of deciduous trees contain more moisture than conifer needles. When pure stands are compared, deciduous forests are 24 times less likely to burn than conifer forests (Astrup et al., 2018).
6.2.3.1Climate Forcing by Boreal Forests
Boreal forests constitute approximately 30% of all global forested land area. Given their large size and geographic distribution, there is considerable interest in their nature, how they function, and whether this will have warming or cooling effects on global climate. It has been suggested that boreal forests have the largest biophysical influence on average annual global temperatures (Bonan, 2008). Much has been published about their probable influence. Some of it is examined here, and a more extensive discussion is found in Chapters 7 and 9.
The discussion in the literature focuses on tree color and deforestation and their effects on surface albedo. Forests that are dominated by conifers, especially black spruce, are dark in color. They do not reflect light (radiant energy) well and have low albedo numbers. Deciduous trees and forests reflect somewhat more light and have higher albedo values (Astrup et al., 2018). Clean fresh snow effectively reflects light and can have an albedo close to 1.00 (the highest value). This can be diminished by the low albedo values of dark forests, resulting in a small increase in air temperature during the long winter and more in summer (Bonan et al., 1992). Deforestation, caused by humans, insects, and fires, results in areas of open ground that have higher albedo values than dark forests. This may result in atmospheric cooling (Bala et al., 2007; Bonan, 2008). Much of what is known about albedo values and atmospheric warming or cooling in boreal forests has been obtained from results from models. Empirical ground-based data are much less available.
6.2.4Urban Forests
As indicated in Chapter 5, the number and size of core cities and periurban areas is increasing dramatically worldwide. In some places, cities have become so large that accurate population data are not available. Interest in maintaining existing trees, in parks and along streets, and planting new trees in core cities is increasing correspondingly. Part of the rationale is to influence urban climate by reducing gaseous and particulate air pollution, and lowering air temperature by sequestering carbon, providing shade and transpirational cooling to alleviate urban heat-island effects. Trees also provide aesthetic value through their appearance, while introducing urban dwellers to some semblance of the natural world.
A city is an unnatural environment and a challenge for trees, especially for native species that grew there before the city was developed (Johnston et al., 2011). Infrastructure, concentrated buildings of different sizes, streets, and development result in limited soil area for root growth, and soil water may be limited. Higher temperatures can inhibit or enhance growth. Microclimates are common, particularly involving temperature and light availability.
Except for any relic natural forest areas, all desired city trees must be planted. This is expensive, as nursery-grown trees, often specific selections or hybrids, are planted. At the same time, invasive tree species, such as Tree-of-Heaven (Ailanthus altissima) readily colonize almost any open area, grow quickly, and provide shade. A. altissima is considered to be a messy, undesirable tree that should be eradicated. Invasive plant species, however, are becoming the default flora of cities.
Traditionally, a small number of tree species have been used in cities – those that were available in nurseries and were well-liked. In the northeastern United States, oaks (Quercus), maples (Acer), ash (Fraxinus), and linden (Tilia), and in the past elm (Ulmus), were commonly used. They were planted in large single-species plantings, especially along streets. Then, as now, many newly planted trees from nurseries died within 2–3 years after planting from poor management: failure to establish, neglect, drought, and vandalism. Survival of replacement trees can be even less than that of originals (Gilbertson and Bradshaw, 2012). Tree planting on a large scale is expensive, and it became evident that perhaps different tree species might be more successful and provide new benefits. New tree species and clones, and hybrids of more common trees, better adapted to cities, have become available as a result.
In architecture, the original law was that form must follow function. This should also be applied to selection and use of trees for city plantings. Given the cost and effort needed to purchase, plant, and maintain city trees, it is essential that they function in ways that allow them to survive, grow to appropriate size, and enhance environmental quality. Aesthetic function is desirable, but it is not enough. A combination of environmental and aesthetic functions is ideal.
Databases for selection of appropriate trees for cities by categories are available. The City of New York developed a list of appropriate tree species (New York City Parks Approved Tree Species List, 2005) to plant in New York before embarking on a million-tree planting program. They used ultimate size and other criteria to select trees for specific locations, following the arboricultural dictum of “right tree in the right place”. McPherson et al. (2016) established a similar urban tree database. Yang et al. (2015), however, conclude that despite interest in new tree species for cities, the tree composition of many cities worldwide remains remarkably homogeneous.
6.2.4.1Climate Forcing by Urban Forests
Urban trees sequester carbon from carbon dioxide. Through the shade that they provide, they also reduce carbon emissions by reducing energy demand for cooling, and thus reducing power-plant generation of electricity (Nowak and Crane, 2002). One hundred common tree species were evaluated for size at maturity, life span, growth rate, and maintenance/care requirements, to determine which characteristics would be best for urban trees to function well in carbon capture and sequestration. Nowak and Crane concluded that long-lived, low-maintenance, moderate- to fast-growing, large-at-maturity, deciduous tree species would function well, provided they were in or were planted in appropriate sites that would promote their growth. Empirical evidence for carbon dioxide reduction by urban trees is not well developed.
Cities all over the world are characterized by higher ambient temperatures due to heat island effects and periodic heat waves. Tree populations in cities could provide some localized cooling, by shading or transpirational cooling. Large parks and relic forest areas might offer larger-scale localized cooling. Empirical evidence for air cooling is available only from observations and measurements from small groups of trees (Bowler et al., 2010; Linden et al., 2016).
Warmer city temperatures and other stresses increase emissions of the small hydrocarbon isoprene from leaves. Isoprene can increase the formation of ozone in the photochemical oxidant cycle in cities, affecting tree and human health, and global warming. In response to high temperatures, some species such as oaks, especially northern red oak (Quercus rubra), commonly used as city trees in temperate zones, emit large quantities of isoprene in summer, which is when ozone formation in cities occurs. Isoprene emissions should be considered when selecting tree species for planting in cities (Donovan et al., 2005; Simpson and McPherson, 2011), and high-emitting species should not be planted there in large numbers.
6.3Summary
“Forest” is often used as a generic term in large process models to assess environmental effects on the present and future condition and extent of the forests of the world. The major forests of the world, however, differ widely in their location, composition, subdivisions, and functions, and should be assessed individually.
Forest area is decreasing worldwide, and this is decreasing the forest carbon sink. Deforestation for wood and agriculture, degradation, fires, insects, and drought are the major causes.
Deforestation in tropical forests increases carbon dioxide emissions and decreases transpirational cooling. Increased albedo in cleared land may partially mitigate the increased warming. Extensive deforestation may turn tropical forests from net carbon sinks to net carbon sources.
Deforestation in temperate forests may have only a slight negative effect on warming.
Vast boreal forests are dark in color and have low albedo values, resulting in warmer winter and summer temperatures. The high albedo value of snow for cooling can be reduced by low albedo values for trees. Deforestation by timber harvesting, fires, and insects opens up lighter-colored land surfaces to sunlight. Open surfaces have higher albedo than dark trees, and this may result in cooler air. It has been speculated that global climate might be cooler if there were no trees in the boreal regions.
Tree planting is increasing in cities to reduce carbon dioxide and urban temperatures. Shade and transpirational cooling can reduce air temperatures, but this may occur only in localized areas. More attention is being paid to selecting trees for cities that offer positive functions for environmental quality. Trees, such as oaks, that are high isoprene emitters should not be planted extensively.
REFERENCES
Astrup, R., Bernier, P. Y., Genet, H., Lutz, D. A. and Bright, R. M. 2018. A sensible climate solution for the boreal forest. Nature Climate Change 8: 11–12. doi: 10.1038/s41558-017-0043-3.
Baccini, A., Walker, W., Carvalho, L. et al. 2017. Tropical forests are a net carbon source based on aboveground measurements of gain and loss. Science. doi: 10.1126/science.aam5962.
Bala, G., Caldeira, K., Eickett, M. et al. 2007. Combined climate and carbon-cycle effects of large-scale deforestation. Proceedings of the National Academy of Sciences 104: 6550–6555. doi: 10.1073/pnas0608998104.
Bonan, G. 2008. Forests and climate change: forcings, feedbacks, and the climate benefits of forests. Science 320: 1444–1449. doi: 10.1126/science.1155121.
Bonan, G., Pollard, D. and Thompson, S. L. 1992. Effects of boreal forest vegetation on global climate. Nature 359: 716–718.
Bowler, D., Buyung-Ali, L., Knight, T. M. and Pullin, A. S. 2010. Urban greening and cooling to towns and cities: a systematic review of the empirical evidence. Landscape and Urban Planning 97: 147–155.
Donovan, R. G., Stewart, H. E., Owen, S. M., Mackenzie, A. R. and Hewitt, C. N. 2005. Development and application of an urban tree air quality score for photochemical pollution episodes, using the Birmingham, United Kingdom area as a case study. Environmental Science and Technology 39: 6730–6738. doi: 10.1021/es050581y.
Gauthier, S., Bernier, P., Kuuluvainen, T., Shvidenko, A. Z. and Schepaschenko, D. G. 2015. Boreal forest health and global change. Science 349: 819–822. doi: 10.1126/science.aaa9092.
Gilbertson, P. and Bradshaw, A. D. 2012. The survival of newly planted trees in inner cities. Arboricultural Journal 14: 287–309. doi: 10.1080/03071375.1990.9746850.
Grace, J., Mitchard, E. and Glook, E. 2014. Perturbations in the carbon budget of the tropics. Global Change Biology 20: 3238–3255. doi: 10.1111/gcb.12600.
Hansen, M. C., Potapov, P. V., Moore, R. et al. 2013. High-resolution global maps of 21st-century forest cover change. Science 342: 850–853. doi: 10.1126/science.1244693.
Johnston, M., James, S. and Nail, S. 2011. ‘Natives vs Aliens’ – the relevance of the debate to urban forest management in Britain. Trees in Britain 181–191. doi: 10.13140/2.1.1340.3844.
Lewis, S. L., Edwards, D. P. and Galbraith, D. 2015. Increasing human dominance of tropical forests. Science 349: 827–831.
Linden, J., Fonti, P. and Esper, J. 2016. Temporal variations in microclimate cooling induced by urban trees in Mainz, Germany. Urban Forestry and Urban Greening 20: 198–209. doi: 10.1016/j.ufug.2016.09.001.
McPherson, E. G., van Doorn, N. S. and Peper, P. J. 2016. Urban tree database and allometric equations. US Forest Service General Technical Report PSW-GTR-253.
Mitchard, E. T. A. 2018. The tropical forest carbon cycle and climate change. Nature 559: 527–534. doi: 10.1038/s41586-018-0300-2.
Nabuurs, G.-J., Lindner, M., Verkerk, P. J. et al. 2013. First signs of carbon sink saturation in European forest biomass. Nature Climate Change 3: 792–796. doi: 10.1038/nclimate1853.
New York City Parks Approved Tree Species List. 2005. www.nycgovparks.org/trees/street-tree-planting/species-list (accessed 05/04/2018).
Nowak, D. J. and Crane, D. E. 2002. Carbon storage and sequestration by urban trees in the USA. Environmental Pollution 116: 381–389.
Perry, D. A., Oren, O. and Hart, S. C. 2008. Forest Ecosystems. Baltimore: Johns Hopkins University Press.
Rosa, I. M. D., Smith, M. J., Wearn, O. R., Purves, D. and Ewers, R. M. 2016. The environmental legacy of modern tropical deforestation. Current Biology 26: 2161–2166.
Simpson, J. R. and McPherson, E.G. 2011. The tree BVOC index. Environmental Pollution 159: 2088-2093. doi: 10.1016/j.envpol.2011.02.034.
ter Steege, H., Pitman, N. C., Killeen, T. J. et al. 2015. Estimating the global conservation status of more than 15,000 Amazonian tree species. Science Advances 1: e1500936.
University of California, Berkeley, Museum of Paleontology. 2006. The forest biome. www.ucmp.berkeley.edu/exhibits/biomes/forests.php (accessed 05/04/2018).
Wear, D. N. and Coulston, J. W. 2015. From sink to source: regional variation in US forest carbon futures. Scientific Reports 5: article 16518. doi: 10.1038/srep16518.
Yang, J., La Sorte, F. A., Pysek, P. et al. 2015. The compositional similarity of urban forests among the world’s cities is scale dependent. Global Ecology and Biogeography 24: 1413–1423. http://wileyonlinelibrary.com/journal/geb.