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.
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There are several types of tree lines defined in ecology and geology:
Some typical Arctic and alpine tree-line tree species (note the predominance of conifers):
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:
Location | Approx. latitude | Approx. elevation of tree line | Notes | |
---|---|---|---|---|
(m) | (ft) | |||
Sweden | 68°N | 800 | 2600 | |
Norway | 61°N | 1100 | 3600 | Lower near the coast |
Olympic Mountains WA, USA | 47°N | 1500 | 5000 | Heavy winter snowpack buries young trees until late summer |
Swiss Alps | 46°N | 2100 | 7000 | Higher on the southern side of the Alps. |
Alps of Piedmont, Northwestern Italy | 45°N | 2100 | 7000 | |
Rila | 42°N | 2300 | 7700 | Mountain Pine is the most common tree line species |
Sierra Nevada, Spain | 37°N | 2400 | 8000 | Precipitation low in summer |
Mount Katahdin, Maine, USA | 46°N | 1158 | 3800 | |
New Hampshire, USA | 44°N | 1220 | 4000 | Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua. |
Wyoming, USA | 43°N | 3000 | 10000 | |
Wasatch Mountains, Utah, USA | 40°N | 2900 | 9500 | Higher (nearly 11,000 feet) in the Uintas |
Rocky Mountain NP, USA | 40°N | 3500 | 11500 | On warm southwest slopes |
3250 | 10800 | On northeast slopes | ||
Japanese Alps | 39°N | 2900 | 9500 | |
Yosemite, USA | 38°N | 3200 | 10500 | West side of Sierra Nevada |
3600 | 11800 | East side of Sierra Nevada | ||
Popocatepetl, Mexico | 28°N | 4000 | 13000 | |
Himalaya | 28°N | 4400 | 14400 | |
Hawaii, USA | 20°N | 2800 | 9000 | Precipitation low above the trade winds |
Costa Rica | 9.5°N | 3400 | 11200 | |
Mount Kilimanjaro, Tanzania | 3°S | 3000 | 10000 | 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 |
New Guinea | 6°S | 3900 | 13000 | |
Andes, Peru | 11°S | 3900 | 13000 | East side; on west side tree growth is restricted by dryness |
Andes, Bolivia | 18°S | 5200 | 17000 | Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana) |
4100 | 13000 | Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate) | ||
Sierra de Córdoba, Argentina | 31°S | 2000 | 6500 | Precipitation low above trade winds, also high exposure |
Australian Alps, Australia | 36°S | 2000 | 6500 | West side of Australian Alps |
1700 | 5500 | East side of Australian Alps | ||
South Island, New Zealand | 43°S | 1200 | 4000 | Strong maritime influence serves to cool summer and restrict tree growth |
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:
Location | Approx. longitude | Approx. latitude of tree line | Notes |
---|---|---|---|
Norway | 24°E | 70°N | 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 | 75°E | 66°N | |
Central Siberian Plateau | 102°E | 72°N | 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) | 160°E | 60°N | The Oyashio Current and strong winds affect summer temperatures to prevent tree growth. The Aleutian Islands are almost completely treeless. |
Alaska | 152°W | 68°N | 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 | 132°W | 69°N | Reaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures. |
Nunavut | 95°W | 61°N | Influence of the very cold Hudson Bay moves treeline southwards. |
Quebec | 72°W | 56°N | Very strong influence of the Labrador Current on summer temperatures. In parts of Labrador, the treeline extends as far south as 53°N. |
Greenland | 50°W | 64°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. |
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.