Sink limitation of plant growth determines treeline in the arid Himalayas

1. Understanding what determines the high elevation limits of trees is crucial for predicting how treelines may shift in response to climate change. Treeline 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 treeline formation in high arid mountains are largely unknown. 2. 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. 3. To test this hypothesis, we studied the quantita...