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
]. It is widely acknowledged that low temperature at high altitudes inhibits tree growth above the treeline [3
]. Although many studies have focused on how low temperature constrains tree growth and survival at the alpine treeline [4
], 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
]. A multi-stemmed shrubby tree (either multi-stemmed shrubby tree or a group of single stems) has two main advantages relative to their upright counterparts under cold conditions. 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
]. The best evidence for this comes from an infra-red thermal imagery study in which upright trees always represent “cold fingers” surrounded by warmer air [2
]. The second advantage is that shrubby trees are more flexible in adjusting their xylogenetic processes under cold conditions than their upright counterparts [2
]. These advantages explain why shrubby trees are more commonly found at higher altitudes in comparison to upright trees.
Given that low temperature is the leading factor determining tree form at the alpine treeline, once temperature condition becomes more favorable for plant growth, seedlings do not need to first grow in a creeping form but can directly form a single stem stature with upright growth [7
]. Studies on the Ural Mountains have found that, with the climate warming over the past centuries, the dominant tree forms changed from multi-stemmed creeping to multi-stemmed upright stature, and further to a single-stemmed tree form [7
]. This implies that, over a longer time span, trees’ forms may adapt to the environment in which they are growing.
Height (H)-diameter (D) relationship, as one of the most important stature characteristics [2
], has been recognized as one manifestation of the many ways in which trees adapt to changes in environment, and this process of stature change may last over decades or centuries. Stature change of trees with increasing altitude has been attributed to both heat deficiency and plants’ responses to cold environments [4
]. Tree stature change along altitudinal gradients can be represented by the relationship between height and diameter growth because height and diameter growth have different temperature sensitivities [14
]. Although results have been insufficient to show exactly how the stature of treeline trees responds to changing climate, it has been reported that more biomass appears to be allocated to radial growth under colder climatic conditions [14
], showing a decreasing H/D ratio with increasing altitude [2
]. A large-scale study in the cold Northeast China also found that a lower proportion of biomass is allocated to height growth than to radial growth under conditions of intensified winter cold [16
]. Thus, it may be hypothesized that warmer climatic conditions may lead to a more tapered tree stature rather than a stunted growth form.
When taking into account the fact that the stature of different sized trees may exhibit different responses to climate [17
], the above hypothesis deserves more consideration. Saplings with a height of less than 0.5 m in the alpine treeline ecotone can become taller, since their height growth is mainly influenced by the near ground surface temperature associated with microclimate, while height growth of those trees taller than 3 m tends to be constrained by low temperature associated with meso-climate at high altitude [14
]. In other words, smaller trees invest more carbon to height growth and taller trees use more carbon for diameter growth in the alpine treeline ecotone [14
]. This result also implies that the current rapid climate warming may modify or shift these breakpoints (i.e., 0.5 m and 3 m) to a higher level. However, considering that trees have their own adaptive responses to climate, and shrubby and/or upright growth forms of treeline trees may have many functional differences, as noted earlier, there is still a need to precisely understand how the stature of treeline trees will change with a warming climate. A closer focus on height-diameter relationship dynamics can therefore help us to better understand the adaptive mechanisms of tree stature in response to climate and climate change.
Erman’s birch (Betula ermanii
Cham.) is widely distributed across Northeast Asia, including in China, Japan, and eastern Russia [18
]. On Changbai Mountain in northeastern China, this species is distributed from low altitude to high altitude and forms the alpine treeline at ~2050 m above see level (a.s.l.). It has been reported that radial growth of this species was sensitive to temperature [20
]. However, no studies have focused on the responses of height growth or the allometric relationship of height-radial growth of Erman’s birch to temperature. In this study, fixed monitoring sample plots were established and investigated in the alpine treeline ecotone on Changbai Mountain in 2006, and reinvestigated in 2013. Using these data, we aimed to explore the changes in stature of birch in the ecotone over the 8-year study period. As references, we compared stature of birch to those of three other dominant species including spruce (Picea jezoensis
Siebold & Zucc. Carr.), larch (Larix olgensis
A. Henry) and fir (Abies nephrolepis
Trautv. ex Maxim.) at six altitudes below the treeline on Changbai Mountain, using data investigated in 2006. These comprehensive analyses allow us to better understand the adaptive strategy of treeline birch to harsh environmental conditions and provide insight into the formative mechanisms of the alpine treeline.