Why Do Trees Stop at a Certain Height? The Edge of Where Wood Works

The limit is set primarily by growing season temperature rather than by winter cold, and the relationship is surprisingly consistent worldwide. Measurements of soil temperature at tree line across many mountain ranges and latitudes converge on a mean growing season temperature of around six or seven degrees, which holds in the tropics and in the far north despite enormous differences in everything else. The proposed mechanism is not that photosynthesis fails, since trees can still make sugars at those temperatures, but that tissue growth fails, because building new cells requires warmth more than making sugars does. A tree unable to build wood and roots cannot maintain the structure its height requires, so the growth form rather than survival is what the cold defeats.

Local factors move the line up or down and produce the variation seen on any mountain:

• •Aspect, with slopes facing the sun carrying trees higher than shaded ones
• •Wind, which desiccates and damages and lowers the line on exposed ridges
• •Snow, which protects buried tissue in winter and shortens the season when it lies late
• •Avalanches and landslides, which clear corridors down through the forest
• •Grazing and burning, which have lowered the line substantially in many inhabited regions
• •Soil depth and stability, since a young slope may have nowhere for roots to hold

The transition is a zone rather than a line and the forms found within it are distinctive. Trees become progressively shorter and more widely spaced. Growth becomes deformed by wind, producing flagged trees with branches only on the sheltered side and, at the extreme, stunted matted growth lying low enough to stay under the winter snow, which is the form known by a German term meaning crooked wood. Individual trees in that zone can be very old despite being small, since growth is slow and the conditions that limit them also limit competition and disturbance. Reproduction frequently fails there, with trees persisting vegetatively and seedlings establishing only in unusually warm years, which means the upper edge of the forest reflects past conditions rather than present ones.

Tree line is one of several boundaries stacked up a slope and separating them avoids confusion. The forest line, where continuous forest ends, sits below the tree line, where the last individual trees stand, and the zone between is where the stunted deformed growth occurs. The timber line, a term used in forestry, marks where trees large enough to be worth cutting stop, which is lower still. The snow line marks where snow persists year round and is far above the tree line in most ranges. In deserts and dry mountains a lower tree line also exists, below which trees fail for lack of water rather than for cold, so a range in a dry region carries forest as a band between two limits. Each line responds to different factors and they move independently.

Tree lines have advanced upslope and northward at many sites over the past century, which is among the more visible ecological signals of warming and is complicated to interpret. Studies across many mountain ranges find advance at a majority of sites and no change or retreat at others, with the response depending on whether temperature is the actual limiting factor locally or whether moisture, wind or land use dominates. Abandonment of upland grazing has allowed forest to recover ground it was cleared from, which produces advance that has nothing to do with climate and must be separated from it. Establishment lags well behind the climatic possibility, since seedlings need a sequence of favourable years. The advance matters beyond the boundary itself, since forest is darker than tundra and absorbs more heat.

Growing season soil temperature around six or seven degrees marks the limit worldwide, and the constraint is on building new tissue rather than on photosynthesis. Aspect, wind, snow, grazing and burning move the line locally. Lines have advanced at many sites over the past century, though abandonment of upland grazing produces advance that must be separated from warming.