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

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 degrees 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.