Is the growth of birch at the upper timberline in the Himalayas limited by moisture or by temperature?

Abstract

Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf timberline species that survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper timberlines between 3900 and 4150 m above sea level is primarily limited by moisture availability rather than by low temperature. To examine this assumption, a total of 292 increment cores from 211 birch trees at nine timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross‐dating and a common climatic signal in the tree‐ring width variations. From March to May, all nine tree‐ring‐width site chronologies showed a strong positive response to total precipitation and a less‐strong negative response to temperature. During the instrumental meteorological record (from 1960 to the present), years with a high percentage of locally missing rings coincided with dry and warm pre‐monsoon seasons. Moreover, periods of below‐average growth are in phase with well‐known drought events all over monsoon Asia, showing additional evidence that Himalayan birch growth at the upper timberlines is persistently limited by moisture availability. Our study describes the rare case of a drought‐induced alpine timberline that is comprised of a broadleaf tree species.

The Coupling of Treeline Elevation and Temperature is Mediated by Non-Thermal Factors on the Tibetan Plateau

“…on the southeastern Tibetan Plateau, whereas fir and spruce treeline elevations are usually located at 4300-4500 m [12,15]. By contrast, the elevation of birch treelines is less than 4100 m in the Himalayas [4]. In this study, tree species also reached different treeline elevations on the Tibetan Plateau which may be attributed to their different traits (e.g., growth and phenology thresholds, vulnerability to freezing damage, evergreen and deciduous leaf habit, vegetative vs. sexual reproduction, recruitment limitation), which have far-reaching influence on treeline responses to climate at local and landscape scales (e.g., [24,56]).…”

“…There is growing evidence that tree growth at treeline is mainly limited by summer temperatures in this region [16,21,22,[39][40][41][42][43][44][45][46][47], thereby suggesting a coupling of thermal conditions and treeline dynamics. In some sites and species, moisture availability also constrains tree growth at treelines [4,48]. Furthermore, regardless of non-thermal factors, elevation of treelines on the different parts of Tibetan Plateau has shifted upwards during the past century, indicating that shifts of treeline elevation are largely warming-driven, albeit species interactions slow warming-induced upward shift rate [12].…”

“…However, treeline studies at the regional and local scales are needed to give further support to this idea, because often few treeline sites represent some biomes which could affect the validity of this global pattern [1]. For instance, in Mediterranean or some temperate biomes, drought, in addition to low temperatures, constrains tree growth and determines the treeline elevation [3][4][5]. In addition, based on new data from New Zealand and Chile, it was found that southern temperate treelines are driven by similar thermal thresholds as are northern treelines, thus refuting the postulated taxon-specific limitation hypothesis and confirming that southern treelines are not climatically depressed [6].…”

Complex climate constraints of upper treeline formation in the Pyrenees

“…In the aforementioned cases the effect of drought stress on growth was due to low soil moisture availability induced by warm summer temperatures, which indicates that local factors such as soil depth and stoniness or slope steepness may lead to drought stress in cold climate treelines (Villalba et al 1994;Galván et al 2014). In many cases local factors override climatic factors as drivers of growth in mountain environments (Bunn et al 2005; Treml temperatures at treeline under subtropical (Morales et al 2004), Mediterranean (Lloyd and Graumlich 1997) and continental (Kress et al 2010;Liang et al 2014;Yu et al 2014) conditions. This also has bio-geographical implications since constrains on tree growth due to warming-related drought stress may nullify any beneficial effect on treelines due to the rising temperatures.…”

Moisture-Limited Tree Growth for a Subtropical Himalayan Conifer Forest in Western Nepal

“…In recent decades, several studies have investigated forest radial-growth responses to changing climate in the central Himalayas and surrounding regions using tree-ring data [4][5][6][7][8][9][10][11][12][13][14]. However, most studies focused on subalpine forests, showing variable growth responses to climate across different species, sites, and topographical conditions.…”