Effects of Climate Change on Northeastern American Forests: Comparison
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Forests and forestry-related industries and ecosystem services play a critical role in the daily life of all societies, including in cultural, ecological, social, economic, and environmental aspects. Globally, there are about 4.1 billion hectares of forestland. In the United States, there are about 304 million hectares of forestland, covering about 34% of the total land area, and the forest product industry produces over USD 200 billion worth of forestry products annually. Evidence suggests these precious resources may be negatively impacted by climate change via direct and indirect processes, including wildfires, insect/pest pressure, drought, extreme storm events, increased air temperature, solar radiation, vapor pressure deficit, and other factors and variables that can be detrimental.

  • Northeastern US forests
  • climate change
  • forest

1. Introduction

Forests are an important component of the environment and ecosystem services and are sources of raw materials, woody biomass, etc. Forests play a critical role in providing food resources and habitats for wildlife [1]; maintaining biodiversity; providing refuge for fauna and flora [2]; protecting land and water resources [3]; minimizing or eliminating flooding [4], soil erosion, and landslides [5]; regulating/moderating air and soil temperatures [6]; and mitigating climate change by acting as a carbon sink [7] through several processes, including photosynthesis; removing carbon dioxide from the atmosphere; reducing the impacts of extreme heat and solar radiation on flora and fauna by intercepting solar radiation at the forest canopy, sheltering the understory vegetation and wildlife habitat; serving as a buffer for improving water quality and reducing the run-off of chemicals that cause environmental degradation; providing biomass for renewable energy production; providing wood materials for heat, construction, and many other purposes; and providing many other important environmental services [8].
Currently, 304 million hectares of the US (approximately 34% of its land area) are covered by forests. Forest lands have gradually changed with time, and these changes in their history cannot be overemphasized [9]. The original American forests covered about 404 million hectares out of the 971 million hectares of the US, approximately 46 percent of the United States’ total land area [10]. Since the migration of Native Americans to America after the Ice Age about 10,000 years ago, humans have been fully involved in impacting the American forests (and vice versa) through plant domestication for food, medicine, heat, hunting, and construction. The increase in the human population and development of various industrial and residential complexities has resulted in deforestation that also encouraged settlement, building construction, farm equipment, shipping construction, domestication, and food gathering [11]. All these activities profoundly influenced the American forest ecosystems, affecting the soil, tree species, landscape, and wildlife, and gradually molding them into what they have become today [12]. There was an extreme decline in American forests from about 404 million hectares to approximately 283 million hectares by the 1970s, followed by a cessation of decline. After World War II, there was a gradual increase in forestland, which has continued until today [10]. Therefore, American forest ecosystems are known to be resilient, with a lifelong ability to renew their potency, complexity, and diversity. However, past, current, and projected climate change can have significant negative impacts on forest health, productivity, and its many ecosystem services; these negative impacts can vary substantially with tree species and geographic location and other factors, and the Northeastern American forests are no exception to these variations. The Northeastern America region is a heavily forested area that provides important forest-based services to society and a great contribution to the overall economy. Because of its high population, this region of the US has an increasing demand for energy, and the forests are promising resources for fulfilling this need.

2. Distribution of Forests in United States

The American forests are astounding resources, comprising approximately 323 million hectares of natural forests, planted forests, and woodlands, with about 16.2 million hectares being virgin old-growth forests. Due to this large area of forestland, there are vastly diverse vegetation species as well [13]. Because of human activities, significant changes in forest tree species have occurred. European settlement into the US was accompanied by the introduction of various trees species, either deliberately or accidentally, some of which became established, some being invasive while others are benign [14]. The US Forest Inventory and Analysis (FIA) identified more than 400 different tree species in the US forests. According to this identification, the majority of the hardwood trees are found in the midwestern and northeastern forests, while softwood trees are predominant in the southeastern and western forests of the US [13]. The distribution of the US forest is a result of various factors, including climatic change, dispersal and disturbances, and other natural causes. Temperate, tropical, and taiga or boreal forests are the main forests found in the US Tropical forests, which are evergreen with lots of rainfall, are only found in Puerto Rico and Hawaii [15]. The majority of the forests in the US are known as temperate forests, stretching from the northeastern region to the western US. Taiga or boreal forest, characterized by cold and snowfall, is found on the mountains in the north central to the pacific northwestern region [17][16]. The Northeastern US forest spans nine states, including Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont. This region contains temperate forest, which is dominated by deciduous and evergreen trees [18][17]. The northeastern temperate forests are composed of oaks, hickories, tulip poplar, American beech, hard maples, and basswood. Hardwood and coniferous tree species are prevalent in the northeastern forest.
 

3. Effects of Climate Change on Forests

Climate change influences several natural disturbances (insect outbreaks, invasive species, wildfires, storms, etc.) that threaten forest health. These disturbances can be direct or indirect impact(s) of climate change through increased drought [83][18], warmer air temperatures [84][19], extreme precipitation/storm events [85][20], increased incoming shortwave radiation, and increased duration, as well as the severity of some of these variables/factors [86][21]. Based on the level, frequency and duration of unusual weather events, the damage on forested ecosystems can be different. Climate change affects forest composition and productivity by influencing many factors such as tree growth and development, flowering times and seed quality and quantity, distribution, etc. Global climate models have been developed to predict changes in future climate based on greenhouse gas emissions. Forest health and productivity response to climate change can exhibit variations spatio-temporally as well as by tree type. For example, forests in the Northeastern US may not respond to climate change in the same way as forests respond in the western part of the country due to differences in interactions and productivity response of different tree species to the same climate variables (temperature, radiation, vapor pressure deficit, drought, precipitation, etc.). Even if the same tree species are considered, the same tree species grown in the Northeastern US and the western part, these same species can respond differently to the changes (both magnitude and duration) in the same climate variables due to differences and dynamics involved in genetic vs. environment and geolocation interactions. Different physiological, biophysical, evapotranspiration, photosynthesis, and productivity responses of forests to climate change have been studied. Mohan et al. (2009) stated that exceedances of United States and Canadian ozone air quality standards are apparent and offset CO2-induced gains in biomass and predispose trees to other stresses [87][22]. The accumulation of nitrogen and sulfate in the Northeastern US changes forest nutrient availability and retention, negatively impacting the reproductivity of trees and frost hardiness, which can cause damage to the leaves and can also negatively influence the ability of trees to defend themselves against forest pests and diseases. These important stresses may cause declines in certain tree species and ecosystem health during the modulation to a warmer climate. For example, responses of tree species to climate change in New England and the northern New York region were examined by two forest impact models under two contrasting greenhouse gas emission (high and low) climate scenarios [88][23]. Based on this assessment, the researchers observed that tree species with ranges that extend to the south may increase. These include red maple, northern red oak, black cherry and American basswood. They also observed that the tree species associated with boreal forests may decrease, which include balsam fir, black spruce, white spruce, red spruce, quaking and bigtooth aspens, and white birch and gray birch. (Janowiak et al., 2018) also suggested that a loss of coastal forests may occur and tree species with low adaptive ability may decrease, which include black ash, white ash, balsam fir, butternut, and eastern hemlock [88][23]. Mohan et al. (2009) stated that climate change will restructure forests of the Northeastern US over the coming century, although the details of this restructuring remain uncertain. They further showed that climate change could bring some additional species into the Northeastern US, but more importantly, there is a potential for expansion of area and importance for species that are in the region but have relatively minor prominence [87][22]. Based on the interpretation of the modeling results, Mohan et al. (2009) suggested that “it is logical that many southern species, especially ones that are driven largely by climate (especially air temperature), would have suitable habitat appear or increase in the Northeastern US” [87][22]. The effects of non-climate variables, such as disturbance regimes, dispersal mechanisms, and fragmentation, add complexity and uncertainty to the final outcomes. Besides the possibility that there will be more habitat for less-common species, the habitat of some of the very common northern species, such as balsam fir, paper birch, red spruce, bigtooth and quaking aspen, and black cherry, will likely shrink. The models thus suggest a retreat of the spruce–fir zone back into Canada, as seen in the past [89][24]. Rusted et al. (2009) synthesized climate observations and modeling results and suggested that the Northeastern US and eastern Canada show that the climate of the region has become warmer and wetter over the past 100 years and that there are more extreme precipitation events and projections indicating significant declines in suitable habitat for spruce–fir forests and the expansion of suitable habitat for oak-dominated forests [90][25]. They further stated that climate change affects the distribution and abundance of many wildlife species in the region through changes in habitat, food availability, thermal tolerances, species interactions such as competition, and susceptibility to parasites and disease. They recommended that with the accumulating evidence of climate change and its potential effects, forest stewardship efforts would benefit from integrating climate mitigation and adaptation options in conservation and management plans. It is important to note that while climate change can regulate or influence forest species response to change in climate variables, the important role of the forest soil structure and soil’s influence in the forest response to climate cannot be ignored. Lafleur et al. (2010) showed that the projected global warming and alteration of the precipitation regime will influence tree physiology and phenology and is likely to promote northward migration of tree species [91][26]. In addition to air temperature, solar radiation, vapor pressure deficit and precipitation, the coupled impact of hot climate as well as the increase in atmospheric CO2 concentration, the impact(s) of climate change on forest health, productivity, and responses become more sophisticated. For example, through a complex and comprehensive modeling study, Ollinger et al. (2008) indicated a wide range of predicted future growth rates for Northeastern US forests [92][27].
Natural disturbances may increase the distribution and abundance of invasive plants and trees. Invasive species could reduce some plants and tree species that are vulnerable to climate change and cause decreases in forest biomass. Since invasive species are tolerant to changes, they are expected to spread more with climate change. This effect may vary depending on the region. Japanese honeysuckle (Lonicera japonica), kudzu (Pueraria montana var. lobata), autumn olive (Elaeagnus umbellata), garlic mustard (Alliaria petiolata), Japanese stiltgrass (Microstegium vimineum), mile-a-minute vine (Polygonum perfoliatum), tree of heaven (Ailanthus altissima), and wavyleaf basketgrass (Oplismenus hirtellus spp. undulatifolius) are some examples of the most commonly observed invasive plant/tree species in Northeastern US forests. The utilization of an excess amount of damaged or dead trees as well as invasive plant and tree species in forests play an important role in mitigating the negative impact of climate change by removing these carbon rich biomass materials from lands and transitioning to sustainable energy.

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