Seed-based treeline seedlings are vulnerable to freezing events in the early growing season under a warmer climate: Evidence from a reciprocal transplant experiment in the Sergyemla Mountains, southeast Tibet

Seedling mortality is important to the formation and dynamics of alpine treeline. There is a need to understand the mechanisms governing seedling mortality at and above treelines under a warmer climate. We tested the hypothesis that under a warmer climate, seed-based treeline seedlings are especially vulnerable to freezing events in the early growing season. Using space-for-time substitution, we conducted a 5-yr reciprocal transplant experiment for >10 yr-old seedlings of seed-based fir and root-sprouting juniper between north-facing and south-facing slopes of a valley with elevations of 4200-4600 m in the Sergyemla Mountains, and additional experiments of seed germination and younger seedling transplants (with ages of 3-5 yr) at and above the fir treeline. Between both slopes, annual precipitation was similar but annual mean air-temperature above the treeline differed by 2.0 °C, being comparable to the temporal difference of 2.3 °C between the warmest and coldest years and the unchanged trend of precipitation during 1960-2008 at Nyingchi station nearby the study sites. The frequency, intensity and duration of growing-season freezing events were much higher under the warmer climate on the south-facing slope. Across years and non-forested sites above both treelines, annual mean air-temperature was well correlated with the early-season (April-June) freezing events. In pooled data across years and sites, annual mortality increased in fir seedlings but varied little in juniper seedlings with increasing freezing events in the early growing season. Similar patterns were also found in their annual growth rates. Partial correlation analysis indicated that the early-season freezing event was the major limiting factor determining annual mortality of fir seedlings, while that of juniper seedlings varied little with all the microclimate factors. Harsh environments above the treeline did not limit fir seed germination. The finding that the early-season freezing events under a warmer climate increased fir seedling mortality can explain the cause for the unique distribution pattern of fir and juniper treelines on opposite slopes of a valley in southeast Tibet, and suggests an explanation for the phenomenon that the world's highest fir-treeline position did not advance with climatic warming in past 200 years.