Some Thoughts on Mountain Forests: Their Benefits and Sustainability

KEYWORDS:

• Mojuntains tree lines
• sacred mountains
• forests

Mountain forests occur on most of the continents on this planet. Definitions of what constitutes as a mountain forest can be arbitrary. A reasonable operational definition is “forests on land with an elevation of 2500 m a.s.l. or higher, irrespective of slope, or on land with an elevation of 300–2500 m and a slope with sharp changes in elevation within a short distance” (Price et al., 2011). However, where they start and end on a particular mountain depends on its climate, soils, topography, other organisms that live in the forest, and random factors. To paraphrase the Greek philosopher Heraclitus no person climbs the same mountain twice because the mountain is not the same and the person is not the same. Mountain forests comprise about 20% of the world’s forests and provide many essential services such as preventing erosion and serving as water sheds (Price et al., 2011). Trees take up carbon dioxide (a greenhouse gas) and give off oxygen. The carbon dioxide is converted into woody tissue that sequesters the carbon and helps mitigate global warming for long periods of time. The oxygen released is required for all aerobic life. Like other forest types, mountain forests are a significant part of these processes. As I approach my nonagenarian years, I look back at 70 years of my close relationship with the mountain forests and I call for their sustainable management that can support and improve the capacity of mountain forests to provide environmental services (Gratzer & Keeton, 2017).

Forests are complex systems. Each tree in a forest supports a diversity of life including insects, birds, mammals, mosses, and lichens (Körner, 2004; Perrigo et al., 2020). The complexity of a forested system is beyond what the eye can see as trees of the forest communicate both above and below ground in many ways. In the soil, the trees communicate by sharing soil resources with the help of a fungal network with the tree roots termed mycorrhizae. Above ground, the trees give off volatile compounds if they are attacked by pathogens and these stimulate neighboring trees to synthesize protective compounds like polyphenols. Some deeper-rooted trees can bring up water from great depths and exude it at shallower depths permitting other organisms to take it up. Tree roots also exude a variety of chemicals into the surrounding soil. Some of these attract beneficial soil microorganisms and create a special environment around the roots termed the rhizosphere. Other exudates given off by the roots of one species inhibit other species in a process called allelopathy. This gives a competitive advantage to the excreting species over other species in the competition for scarce resources. In the initial stage of forest formation from seedlings the tree stems are very dense, but as the forest develops the number of trees per hectare decreases as there is not enough space and resources for all the trees to grow large. This competition is especially evident at higher elevations when tree density decreases with elevation as resources become increasingly limited. Mountain forest trees grow in a fragile ecosystem because of their steep slopes and often-extreme climates and weather events. Therefore, the sustainable management of mountain forests needs to consider the higher risk environment presented by mountains.

Mountains are dynamic systems. As one goes up in elevation the temperature goes down due to what is called the adiabatic lapse rate. This decrease in temperature averages about −6°C per 1000 meters of elevation. Precipitation also increases with elevation as air masses cool and condense as they rise in elevation. In arid regions like the Sky Islands (isolated mountains) in the southwestern U.S. and northern Mexico, the increase in moisture with elevation results in more temperate vegetation in the higher elevation (Poulos et al., 2008, 2020). Some mountain forests have bright sunny environments (e.g. Sierra Nevada in California) while others have diminished light in cloud forests (e.g. Blue Mountain peak in Jamaica and El Yunque in Puerto Rico).

Mountain forests obviously end at tree line (the limit of erect tree life), but these tree lines vary in elevation depending on the local climate. Frequently, the upper tree lines follow the July 10°C isotherm (the point where the mean July temperature is 10°C), but there is great variation because temperature is just one of the many interconnected factors that affect tree life. Many other factors influence tree life like water availability, sun exposure, physiology, wind, slope, aspect, soil depth, and rockiness. The climate can vary markedly at a given altitude even on the same mountain and over short distances. Sun exposure (aspect) is an important factor as northern and eastern slopes tend to be cooler and drier than western and southern slopes. Wind tends to increase with elevation and this increase in wind is greater on isolated mountains like Mt. Washington (New Hampshire, USA) than on large mountain massifs. Factors that affect tree health like the length and quality of the growing season, the sum and distribution of temperatures, and insect and disease also influence tree line.

Trees that cannot fully complete their annual growth cycle at given elevations for whatever reason are much more vulnerable to the stresses of high elevation than those that can fully complete their annual cycle of growth at a given point in space and time. High elevation forests outside of the wet tropics tend to be dominated by conifers (pine, spruce, fir, larch, cedar, etc.), but deciduous trees like birches and aspens are also present in some mountain forests. Polylepis rugulosa, a small tree of the rose family, can grow at elevations over 4000 m above sea level (asl) in parts of the Andes mountains of South America and thus grows at the highest elevations of any tree in the world.

Because growing conditions change with elevation, there is a change in the species and density of trees and other associated organisms along the elevational gradient. This series is often described as the lower, middle, and upper montane zones. With increasing elevation these zones are followed by the sub-alpine zone. As elevation increases, dense mountain forests become less dense until tree line is reached. Tree lines can be gradual or abrupt and this can vary even on the same mountain. Tree lines can be as low as a few hundred meters in Alaska of North America or as high as 4000 m in the Himalayas of Asia and the Andes of South America. As one travels north from the equator, the lower and upper limits of the mountain forests decrease in elevation. At the upper tree line, the erect trees of the forest often give way to stunted and twisted trees that continue in a patchy way and these are called Krummholz (crooked woods). This community eventually gives way to alpine tundra. Northern mountain forests are affected by seasons, but tropical mountains are essentially aseasonal. Day and night temperature differences can be greater than seasonal differences in the tropics. Elevational gradients along mountain slopes are like natural experiments in climate change and much can be learned about climate change by studying them.

Tree lines also varied in the past due to climate changes over the last several centuries. The medieval warm period occurred from 900-1300AD. During this time tree lines rose in elevation. Following this, there was the “little ice age” that ran from 1300 to about 1850 AD. The effect of this is that tree lines decreased in elevation. Some trees can survive for hundreds of years via long life spans and asexual reproduction in places they cannot reproduce sexually. The result from these climate changes is that trees along the mountain slopes may not represent the sustainable forest at these points along the elevational gradient. This is because trees have much greater survival capacity than sustainable capacity. As Aldo Leopold (1949) said, “a sustainable forest is one that can reproduce itself.” This difference between survival and sustainability can greatly complicate the interpretation of the ecology of mountain forests. The climatic tree lines may have no special economic value but need to be protected as they play important environmental roles, such as habitat for endemic species.

Mountain forests provide critical ecosystem services like protection from erosion and flood control, soil stabilization, and reduction of landslides, and rockslides. They also provide habitats for organisms like elk, mountain goats, mountain sheep, mountain lions, rodents (e.g., marmots), lichens, and birds to name a few. Humans have used mountain forests for grazing of domesticated animals for thousands of years. Mountain forests provide mountain peoples with forest products like wood for heat and cooking. There are also a host of non-timber forest products like mushrooms, fruits, honey, and medicinal plants. In glaciated regions, the mountain forests are very different from the surrounding lowland forests. As the glaciers moved south, the trees and other flora also moved south and lived together in the unglaciated areas. As the warmth returned, the glaciers retreated and the plants followed. They climbed the mountains and as the warmth continued became isolated on the mountains (cf. Darwin, 1859). The uniqueness of these forests did not stop with their isolation as species have hybridized and continued to adapt (Berlyn et al., 1989, 1990; DeLucia & Berlyn, 1984; Richardson & Berlyn, 2002; Richardson et al., 2001). These fragile forests are vulnerable to pollution like acid rain. They are also subject to overuse from grazing by domestic animals and wildlife, over harvesting of trees, and recreation (e.g., skiing, snow mobiles, and development like resorts, ski lodges, etc.). Some recreational uses like mountain hiking can provide aesthetic value to people with minimal detriment to the forests.

Management of mountain forests creates many dilemmas. The Central Himalayas of India presents a case in point. Like many mountain regions, indigenous people use wood as a main fuel source. Where the cutting down of trees is prohibited, the villagers lop off branches to supply their fuel needs and the forests decrease in vigor and in their ability to survive and sequester carbon that moderates global warming. Villagers also harvest fodder from the forest for their domestic animals. One solution for the damage or deforestation due to fuel wood harvesting is to introduce sustainably managed fuel-wood plantations. The problem with this solution, despite its effectiveness, is that wood smoke is very deleterious to human health. A better solution may be to introduce a rural electrification program that supplies electricity from solar, wind, and hydro sources. A similar rural electrification program in the United States in the 1940s significantly improved rural life. Another benefit would be to provide carbon payments. Mountain people really need assistance. Mountain peoples suffer from hunger more than any other of the World’s peoples; in developing countries, poverty rates in mountains tend to be higher than those in lowland communities, with half of the population in mountainous rural areas experiencing food insecurity (Romeo et al., 2020).

Mountains and mountain forests also provide spiritual values to many peoples, and these are invaluable in many ways. These values are understood and expressed in diverse ways among local and indigenous peoples (e.g., Zeng, 2018). In many cases, these local peoples set aside groves and areas of ritual activity in relation to mountains. Their preservation ensures protection for birds and other wildlife and maintains biological diversity along with spiritual values. Thus, managing these sites sustainably can aid in promoting diversity, water and nutrient cycling, carbon sequestration, and air quality.

Sacred mountains occur on every continent and are designated by many religions (see Bernbaum, 2022). In the border between India and Tibet, in the Central Himalayas, there is Mount Kailash that is sacred to four religions (Hinduism, Buddhism, Jainism, and Bon-Pon). The Kailash range is the source of several major rivers of the Himalayas. The Navajo nation in North America has four especially sacred mountains that surround their tribal homeland. In Europe, the ancient Greeks had sacred mountains like Mount Olympus, the home of the Gods. Mount Sinai in the Middle East is sacred to all three Abrahamic religions (Judaism, Christianity, and Islam). There is a majesty in mountains and their forests that somehow inspires the human mind. The main policy issue is how to sustainably manage mountain forests to preserve the people, other organisms, and the environmental services they provide.

Editor’s note

As Graeme approaches his nonagenarian years, in this writing, he looks back at 70 years of his close relationship with the mountain forests and I call for their sustainable management (Figure 1).

Figure 1. Professor Graeme Berlyn, founding editor of the Journal of Sustainable Forestry, began his close connection to Mountain Forests (a) as a forest fire fighter fighting fires for the California Division of Forestry in the foothills of the Sierra Nevada in 1951. (b) Graeme Berlyn on the Ochoco collecting spruce budworms in Oregon in 1954.

Acknowledgments

The author thanks Dr. Andrew D. Richardson of Northern Arizona University, USA and Dr. John Grim Senior Lecturer and Research Scholar at the Yale School of the Environment, USA, Dr. Uromi Manage Goodale and managing editors of the Journal of Sustainable Forestry at Xi’an Jiaotong Liverpool University, China, and anonymous reviewers for helpful suggestions.

Disclosure statement

No potential conflict of interest was reported by the author.