Newswise – The findings from this study, led by Professor Eryuan Liang (Institute of Tibetan Plateau Research, Chinese Academy of Sciences), were published as a research article in the journal National Science Review. The study also involved researchers from CREAF, CSIC, Global Ecology Unit CREAF-CSIC-UAB, Instituto Pirenaico de Ecología (IPE-CSIC), Spain, and Université du Québec à Chicoutimi, Canada.
Climate warming is altering the structure and function of alpine ecosystems, including shifts in vegetation boundaries. The upward shift of alpine treelines, the highest limit of tree growth that forms the boundary between montane forest and alpine communities, is strongly modulated by shrub–tree interactions. But little is known about the responses of such symbionts to a warming climate.
Based on 8- and 10-year unique observations of cambial phenology in sympatric trees and shrubs along two treelines in the southeastern Tibetan Plateau, the researchers found contrasting patterns of cambial phenology between shrubs and trees in response to warming. At +1°C spring heat, xylem regrowth in trees progresses by 2-4 days, but in shrubs it is delayed by 3-8 days. Shrubs are 3.2 times more sensitive than trees to cold storage due to the different phenological response to warming.
“Under the same degree of cooling, shrubs require more forced accretion than trees, leading to a delay in cambial phenology. “For trees, however, the forcing was less dependent on cold and therefore resulted in earlier cambial phenology during warmer spring,” says Liang.
Researchers combined a process-based growth model to explore the drivers of phenological growth shifts in response to warming from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Based on growth model-derived cambial phenological records, they further hypothesized differential effects of spring temperature on cambial phenology between trees and shrubs at treelines in the Northern Hemisphere. Model outputs indicate that trees show a higher sensitivity to spring warming than shrubs, as expected. Shrubs are more affected by cold accumulation than trees. “Our meta-analysis confirmed the same mechanism across continental scales,” says Liang.
Regardless of whether shrubs and trees in alpine treelines interact synergistically or competitively, climate warming is likely to induce differential changes in their phenology. This includes shortening the growing season for shrubs and extending it for trees. Consequently, warming-induced phenological mismatch may give trees a competitive advantage over shrubs by increasing growth, carbon gain, and improving resource availability, promoting upward treeline shifts in cooler mountain regions (Figure 1).