Climate Controls Cold- But Not Warm-Adapted Sphagnum Distribution and Carbon Sink Capacity

Despite the critical role of Sphagnum mosses in carbon (C) sequestration, it remains unclear how climate governs their distribution and C sink function. To address this, a reciprocal transplant experiment was performed over 2 years in two peatlands in the Changbai Mountains, utilizing moss monolith mesocosms from low-elevation (S. imbricatum), high-elevation (S. fuscum), and wide-elevation-range (S. magellanicum) Sphagnum species. We found that when low-elevation Sphagnum populations were transplanted upward, both the non-structural carbohydrates (NSC) content and photosystem II efficiency (Φ_PSII) increased, with concomitant reduced ecosystem respiration (ER) and enhanced C sequestration at high elevation. In contrast, for high-elevation populations, downward transplant significantly weakened gross primary productivity (GPP), primarily by reducing Sphagnum photosynthesis and physiological performance, accompanied by slightly enhanced peat decomposition, ultimately suppressing net ecosystem productivity (NEP) by more than 50%. These results indicate that the 4°C temperature difference associated with the elevational gradient does not restrict the upward distribution of warm-adapted S. imbricatum and S. magellanicum populations, but it severely restricts the downward distribution of cold-adapted S. fuscum and S. magellanicum populations. Altered vegetation composition and microbial community coincided with NEP changes in ex-situ transplants. Our study demonstrates that geographical origin, rather than genetic divergence, strongly affects the short-term performance of Sphagnum under both cooling and warming conditions. Climate thus controls the distribution and C sink capacity of cold- but not warm-adapted Sphagnum, highlighting the vulnerability of cold-adapted Sphagnum to warming.