Annual and intra-annual growth dynamics of Myricaria elegans shrubs in arid Himalaya

To disentangle complex drivers of Myricaria elegans growth in arid Himalaya, we combined tree-ring analysis with detailed dendrometer records. We found that the combination of winter frost, summer floods, and strong summer diurnal temperature fluctuations control annual and intra-annual growth dynamics. The relative importance of these drivers is, however, changing with ongoing climate change. High-mountain areas are among the most sensitive environments to climate change. Understanding how different organisms cope with ongoing climate change is now a major topic in the ecology of cold environments. Here, we investigate climate drivers of the annual and intra-annual growth dynamics of Myricaria elegans, a 3-6 m tall tree/shrub, in a high-elevation cold desert in Ladakh, a rapidly warming region in the NW Himalayas. As Myricaria forms narrow stands around glacier streams surrounded by the desert, we hypothesized that its growth between 3800 and 4100 m will be primarily limited by low temperatures and summer floods. We found that warmer and less snowy conditions in April and May enhance earlywood production. Latewood formation is mostly driven by the June-July temperatures (T). The positive effect of warmer summers on both annual and intra-annual growth is related to fluctuating daily T (from +30 to 0 A degrees C). In particular, dendrometer measurements over a 2-year period showed that net daily growth increments increased when the summer night T remained above 6 A degrees C. While high night T during generally cold desert nights promoted growth, high daytime T caused water stress and growth inhibition. The growth-temperature dependency has gradually weakened due to accelerated warming since the 1990s. In addition, positive latewood responses to high March precipitation during the colder 1960s-1980s have become negative during the warmer 1990s-2000s, reflecting an intensification of summer floods. Latewood width increased while earlywood width decreased from the 1990s, indicating a prolonged growing season and a higher risk of drought-induced embolism in earlywood vessels. Due to a multiplicity of environmental drivers including winter frost, intensified floods and strong summer diurnal T fluctuations, Myricaria growth is not controlled by a single climate parameter. Similar results are increasingly reported from other Himalayan treelines, showing that ongoing climate change will trigger complex and probably spatially variable responses in tree growth. Our study showed that these complex climatic signals can be disentangled by a combination of long-term data from tree-rings with detailed, but short-term, records from dendrometers.