Precipitation change, functional traits and stand structure jointly control the spatiotemporal variability of radial growth in alpine treeline ecotones across northern China

Abstract

Tree growth and survival at alpine (arctic) treelines are generally limited by temperature, but it is increasingly observed that hotter drought is threatening tree growth at treelines. How treeline growth variability in response to climate change varies at broad scale, and how it is jointly modulated by functional traits, stand structure and climate factors remains unclear. We sampled tree-ring cores from 11 treeline sites along longitude (precipitation) gradients across northern China, and examined two new aspects of radial growth response to climate change: temporal growth variability and spatial growth synchrony. We explored how climate conditions, climate trends and temporal variability, functional traits [specific leaf area (SLA), leaf nitrogen concentration (LN)], and stand factors affect growth variability and synchrony, and assessed the major drivers with mixed-effects models. 1) Contrary to our expectation, growth variability decreased from humid to arid regions. This pattern was partly because water availability increased in the west (arid) while decreased in the east (humid) regions since 1950. 2) Hotter drought and higher climate variability led to higher growth variability and synchrony. 3) Species with fast-growing strategies (higher SLA and LN) showed higher growth variability and synchrony due to experiencing drastic hotter drought and having lower drought tolerance. 4) Taller and denser stands revealed higher growth variability and synchrony due to stronger hydraulic limitation and competition, respectively. 5) Functional traits and stand factors are indispensable modulators of growth spatiotemporal variability even at broad scales, because their variable importance was often higher than climate factors. These results caution against predicting forest vulnerability solely from climate factors. We conclude that forests at many treeline sites were threatened by hotter drought, especially in humid regions of northern China. But the importance of stand factors and functional traits found also suggests forest management as an effective way to alleviate climate change vulnerability.