The tree line or timberline is the edge of the habitat at which trees are capable of growing. Beyond the tree line, they are unable to grow because of inappropriate environmental conditions (usually cold temperatures, insufficient air pressure, or lack of moisture).
At the tree line, tree growth is often very stunted, with the last trees forming low, densely matted bushes. If it is caused by wind, it is known as krummholz formation, from the German for 'twisted wood'.
The tree line, like many other natural lines (lake boundaries, for example), appears well-defined from a distance, but upon sufficiently close inspection, it is a gradual transition. Trees grow shorter towards the inhospitable climate until they simply stop growing.
Types of tree lines
There are several types of tree lines defined in ecology
- Alpine tree line The highest elevation which sustains trees; higher up, it is too cold or snow cover persists for too much of the year, to sustain trees. Usually associated with mountains, the climate above the tree line is called an alpine climate, and the terrain can be described as alpine tundra. Mountains of the Pacific Northwest of North America exhibit lower treelines on north-facing slopes than south-facing slopes, because increased shade means the deep snowpack takes longer to melt, which shortens the growing season for trees.
- Desert tree line The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees. These tend to be called the "lower" tree line and occur below about 5000 ft (1500 m) elevation in the Desert Southwestern United States. The desert treeline tends to be lower on pole-facing slopes than equator-facing slopes, because the increased shade on a pole-facing slope keeps those slopes cooler and prevents moisture from evaporating as quickly, giving trees a longer growing season and more access to water.
- Desert-Alpine tree line In some mountainous areas, higher elevations above the condensation line or on south-facing in the northern hemisphere and north-facing in the southern hemisphere, or leeward slopes can result in low rainfall and increased exposure to solar radiation. This dries out the soil, resulting in a localized arid environment unsuitable for trees. The slopes of Mauna Loa above 10,000 ft in Hawaii are an example of this. Many south-facing ridges of the mountains of the Western U.S. have a lower treeline than the northern faces because of increased sun exposure and aridity.
- Double tree line Different tree species have different tolerances to drought and cold. Mountain ranges isolated by oceans or deserts may have restricted reportoires of tree species with gaps that are above the alpine tree line for some species yet below the desert tree line for others. For example several mountain ranges in the Great Basin of North America have lower belts of Pinyon Pines and Junipers separated by intermediate brushy but treeless zones from upper belts of Limber and Bristlecone Pines.
- Exposure tree line On coasts and isolated mountains, the tree line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because strong winds reduce tree growth. In addition, the lack of suitable soil, such as along talus slopes or exposed rock formations prevent trees from gaining an adequate foothold and expose them to drought and sun.
- Arctic tree line The furthest north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees. Extremely cold temperatures can result in freezing of the internal sap of trees, killing those trees. In addition, permafrost in the soil can prevent trees from getting their roots deep enough for the necessary structural support.
- Antarctic tree line The furthest south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees. It is a theoretical concept that does not have any defined location. No trees occur on Antarctica or the sub-antarctic islands, and there are no land masses to the north that have a true treeline.
- Other tree lines The immediate environment is too extreme for trees to grow. This can be caused by geothermal exposure associated with hot springs, such as at Yellowstone, or near volcanoes, high soil acidity near bogs, high salinity associated with playas or salt lakes, or ground that is too saturated by ground water which excludes oxygen from the soil, which most tree roots need for growth. The margins of muskegs and bogs are common examples of these types of open areas. However, no such line exists for swamps, where trees, such as Bald cypress and the many mangrove species, are adapted to growing in permanently waterlogged soil.
Some typical Arctic and alpine tree-line tree species (note the predominance of conifers):
Alpine tree lines
The alpine tree line at a location is dependent on local variables, such as aspect
of slope, rain shadow
and proximity to either geographical pole
. In addition, in some tropical or island localities, the lack of biogeographical access to species that have evolved in a sub-alpine environment, can result in lower tree lines than one might expect by climate alone.
Given this caveat, here is a list of approximate tree lines from locations around the globe:
||Approx. elevation of tree line
||Lower near the coast |
|Olympic Mountains WA, USA
||Heavy winter snowpack buries young trees until late summer |
|| Higher on the southern side of the Alps. |
|Alps of Piedmont, Northwestern Italy
|| Mountain Pine is the most common tree line species |
|Sierra Nevada, Spain
||Precipitation low in summer |
|Mount Katahdin, Maine, USA
|New Hampshire, USA
|| Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua. |
|Wasatch Mountains, Utah, USA
||Higher (nearly 11,000 feet) in the Uintas |
|Rocky Mountain NP, USA
||On warm southwest slopes |
||On northeast slopes |
| Yosemite, USA
||West side of Sierra Nevada |
||East side of Sierra Nevada |
||Precipitation low above the trade winds |
|Mount Kilimanjaro, Tanzania
||Example of a tropical location lacking biogeographical access to species that are evolved for living in a subalpine environment. Thus, the tolerance of the indigenous species is lower and it results in a lower tree line |
||East side; on west side tree growth is restricted by dryness |
||Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana) |
||Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate) |
|Sierra de Córdoba, Argentina
||Precipitation low above trade winds, also high exposure |
|Australian Alps, Australia
||West side of Australian Alps |
||East side of Australian Alps |
|South Island, New Zealand
||Strong maritime influence serves to cool summer and restrict tree growth |
Arctic tree lines
Like the alpine tree lines shown above, polar tree lines are heavily influenced by local variables such as aspect
of slope and degree of shelter. In addition, permafrost
has a major impact on the ability of trees to place roots into the ground. When roots are too shallow, trees are susceptible to windthrow and erosion. Trees can often grow in river valleys
at latitudes where they could not grow on a more exposed site. Maritime influences such as ocean currents
also play a major role in determining how far from the equator trees can grow. Here are some typical polar treelines:
||Approx. latitude of tree line
||The North Atlantic current makes Arctic climates in this region warmer than other coastal locations at comparable latitude. In particular the mild winters prevents permafrost. |
|West Siberian Plain
|Central Siberian Plateau
||Extreme continental climate means the summer is warm enough to allow tree growth at higher latitudes, extending to 72°30'N at Ary-Mas (102° 27' E) in the Novaya River valley, a tributary of the Khatanga River. |
|Russian Far East (Kamchatka and Chukotka)
||The Oyashio Current and strong winds affect summer temperatures to prevent tree growth. The Aleutian Islands are almost completely treeless. |
||Trees grow north to the south facing slopes of the Brooks Range. The mountains block cold air coming off of the Arctic Ocean. |
|Northwest Territories, Canada
||Reaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures. |
||Influence of the very cold Hudson Bay moves treeline southwards. |
||Very strong influence of the Labrador Current on summer temperatures. In parts of Labrador, the treeline extends as far south as 53°N. |
||Determined by experimental tree planting in the absence of native trees because of isolation from natural seed sources; a very few trees are surviving, but growing slowly, at Søndre Strømfjord, 67°N. |
Antarctic tree lines
Kerguelen Island, Île Saint-Paul, South Georgia, and other Sub-Antarctic islands are all so heavily wind exposed and marginal in climate, that none have any indigenous tree species, although many such islands receive enough rainfall that they would otherwise be capable of hosting temperate rain forest. However, these are not directly related to the Antarctic tree line, but are related to exposure.
- Ecotone: a transition between two adjacent ecological communities
- Edge effect: the effect of contrasting environments on an ecosystem
- Massenerhebung effect
- Tundra: an area where tree growth is inhibited by low temperatures and short growing seasons
- Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
- Ødum, S. 1979. Actual and potential tree line in the North Atlantic region, especially in Greenland and the Faroes. Holarctic Ecology 2: 222-227.
- Ødum, S. 1991. Choice of species and origins for arboriculture in Greenland and the Faroe Islands. Dansk Dendrologisk Årsskrift 9: 3-78.
- Beringer, J., Tapper, N. J., McHugh, I., Lynch, A. H., Serreze, M. C., & Slater, A. 2001. Impact of Arctic treeline on synoptic climate. Geophysical Research Letters 28 (22): 4247-4250.