Abstract
Height-diameter relationship is one of the most important stature characteristics of trees. It will change with climatic conditions because height and diameter growth displays different sensitivities to climatic factors such as temperature. Detecting and understanding changes in the stature of trees growing along altitudinal gradients up to their upper limits can help us to better understand the adaptation strategy of trees under global warming conditions. On Changbai Mountain in northeastern China, height-diameter datasets were collected for 2723 Erman’s birch (Betula ermanii Cham.) in the alpine treeline ecotone in 2006 and 2013, and for 888 Erman’s birch, spruce (Picea jezoensis Siebold & Zucc. Carr.), larch (Larix olgensis A. Henry), and fir (Abies nephrolepis Trautv. ex Maxim.) along an altitudinal gradient below the alpine treeline in 2006. These datasets were utilized to explore both changes in the stature of birch at the alpine treeline over time and variations in tree stature of different tree species across altitudes at a given time point (2006). Results showed that birch saplings (<140 cm in height) became stunted while birches with a height of >140 cm became more tapered in the alpine treeline ecotone. The stature of birch along the altitudinal gradient became more tapered from 1700 to 1900 m above see level (a.s.l.) and then became more stunted from 1900 to 2050 m a.s.l., with 1900 m a.s.l. being the altitudinal inflection point in this pattern. The treeline birch, due to its great temperature magnitude of distribution, displayed higher stature-plasticity in terms of its height-diameter ratio than the lower elevation species studied. The stature of birch is strongly modulated by altitude-related temperature but also co-influenced by other environmental factors such as soil depth and available water, wind speed, and duration and depth of winter snow cover. The high stature-plasticity of birch makes it fare better than other species to resist and adapt to, as well as to survive and develop in the harsh alpine environment.
Highlights
The alpine treeline ecotone, constituting the transition zone from uppermost closed montane forest to treeless alpine vegetation, commonly represents the most obvious forest boundary [1,2]
From 2006 to 2013, the overall population of treeline birch displayed a mean ∆H of 0.56 m, mean ∆diameter at breast height (DBH) of 1.2 cm, and mean ∆basal ground diameter (BD) of 0.6 cm
The stature of multiple tree species in several prominent mountain ranges was found to become dramatically stunted across a relatively small altitudinal range of only 50 to 100 m below the treeline [26,27]. These results suggest that tree stature change is much faster or even abrupt in the alpine treeline ecotone or close to the alpine treeline
Summary
The alpine treeline ecotone, constituting the transition zone from uppermost closed montane forest to treeless alpine vegetation, commonly represents the most obvious forest boundary [1,2]. Many studies have focused on how low temperature constrains tree growth and survival at the alpine treeline [4,5,6], the specific ways in which trees respond or adapt to the cold environment at high altitudes have received less attention. In the alpine treeline ecotone, two tree forms—upright (single-stemmed) and shrubby tree (multi-stemmed)—are generally observed, distributed as patches and/or along an altitudinal gradient. This is considered as trees’ direct response to low temperature in combination with wind, edaphic conditions, and seasonal snow cover patterns [7,8,9]. The first advantage is that the bulk of their mass is located closer to the ground, which helps to create their microclimate to decouple from the cold ambient air, and to achieve higher heat accumulation in their leaf canopy than taller individuals [2,10,11,12]
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