Sink limitation of plant growth determines tree line in the arid Himalayas

Abstract

Understanding what determines the high elevation limits of trees is crucial for predicting how tree lines may shift in response to climate change. Tree line formation is commonly explained by a low‐temperature restriction of meristematic activity (sink limitation) rather than carbon assimilation (source limitation). In arid mountains, however, trees face simultaneously low temperature and drought, both potentially restricting their growth and thus setting range limits. However, the mechanisms of tree line formation in high arid mountains are largely unknown. We studied Myricaria elegans, one of the world’s highest growing winter‐deciduous woody species, endemic to the arid Himalayas. We hypothesized that the upper elevation limit of Myricaria is associated with low temperatures during the early growing season affecting earlywood formation, while later in the season drought is constraining earlywood maturation and latewood formation. To test this hypothesis, we studied the quantitative anatomy of tree rings at different developmental stages across the entire species elevation range (3,200–4,400 m). We also explored daily stem increment and rehydration rates, seasonal dynamics of non‐structural carbohydrates and stable C isotopes as a proxy for possible drought constraints. Both earlywood and latewood increments decreased towards the tree line, while NSC in leaves, twigs and stem sapwood did not change, indicating a sink limitation as a main driver of the tree line. At tree line, low temperatures restricted earlywood formation more than latewood formation. Tree line individuals had—compared to individuals from lower elevations—smaller and fewer earlywood vessels, frequent frost rings and shorter periods with positive daily increments, but comparable night‐time stem rehydration rates and latewood density. All these results suggest a sink limitation as a main mechanism behind the tree line formation in high arid mountains. In the arid Himalayas, the tree line is set by the drastic growth reduction (sink limitation) caused by low temperature and short growing season under otherwise sufficient carbon and water supply. At the tree line, spring freezing and high summer temperatures further constrain annual stem increment. The mean 6.9°C temperature during the growing season places the Myricaria tree line within the thermal range of other high elevation tree lines worldwide and supports a common mechanism of alpine tree line formation. A plain language summary is available for this article.

Relationships between population traits, nonstructural carbohydrates, and elevation in alpine stands of Vaccinium myrtillus

“…Starch greatly increased at mid‐high elevations where, in the same sample areas, a strong reduction of V. myrtillus also occurred (Boscutti et al., ). The NSC pattern observed in V. myrtillus shows the same trend already observed in Himalayan populations of Myricaria elegans (Dolezal et al., ), although V. myrtillus population evidenced a stronger effect of altitude. Moreover, similar to what has been described for the vegetative organs of bilberry collected at different latitudes (Pakonen et al., ), our results revealed that when sucrose decreased, products of its hydrolysis (fructose and glucose) increased.…”

“…It is widely recognized that elevation affects plant growth and physiology by generating more severe environmental conditions, namely, increased exposure to low temperatures (Körner, ; Schulze et al., ; von Arx et al., ), and consequent reduction of the growing period also due to prolonged snow cover (Körner, ; Jonas et al., ). Nonstructural carbohydrates have been shown to respond to environmental stress associated with altitude (Fajardo et al., ; Dolezal et al., ).…”

“…The GLH has mainly been proposed for trees, whereas its validity for shrubs and perennial forbs is uncertain (Körner, ). Recently, in branch wood of the shrub Myricaria elegans a nonlinear relationship between NSCs and elevation was observed (Dolezal et al., ), but in smaller plants like dwarf shrubs no clear pattern of NSC in relation to elevation has been so far observed. Nevertheless, some studies have examined the effect of latitude, snow cover, and frost on resistance to drought stress, showing that NSCs are used by plants in response to environmental stress, especially in relation with snow cover (e.g., Palacio et al., ; Wheeler et al., ).…”

Alpine plant growth and reproduction dynamics in a warmer world

“…While the studied alpine and subalpine plants were controlled by summer (mostly July–August) temperatures during the colder 1970s–1980s period, they have become dependent on spring (March) temperature and summer (current July and previous September) precipitation during the warmer 1990s–2000s period. The reduced growth with the occurrence of warm and dry weather during the previous September may reflect increased plant respiration and reduced carbohydrate reserve, with both factors causing fewer assimilates to be stored throughout the rhizome, thus resulting in a lower growth potential in the following vegetation season (Dolezal et al ., 2019). The increased sensitivity to summer precipitation is intriguing because R. alpinus is usually bound to sites with high groundwater levels (Stachurska‐Swakoń, 2009) and is even capable of tolerating temporary waterlogging and low soil oxygen levels through the formation of conspicuous intercellular spaces in the rhizome cortex (schizogenous aerenchyma, Schweingruber et al ., 2020).…”

Transcriptomic Analysis Reveals the Mechanism of Picea crassifolia Survival for Alpine Treeline Condition

“…Although the decrease in temperature with increasing elevations has been recognized as a major reason for treeline formation, and the mechanism by which low temperature affects the growth of treeline trees is unclear. The concordance of this treeline isotherm suggests a common low-temperature effect on plant metabolism, either related to carbon (C) gain (source activity; photosynthesis) or sink activity (tissues formation; meristems) [4].…”