Altitudinal variation in leaf construction cost and energy content of Bergenia purpurascens

It is still unknown if the variation of leaf energy content across altitudinal gradients is caused by varying climatic factors or species replacement. We test whether there is an altitudinal increase in leaf energy properties within a single species, which is mainly due to the decreasing air temperature with altitude. We collected samples for Bergenia purpurascens Engl. (an evergreen herb) at every 10 m in altitude from outside the timberline forest (4320 m) to the hilltop (4642 m) in the Sergyemla Mountains, southeastern Tibet. We measured mass- and area-based leaf construction cost (CCm, CCa) and their components: nitrogen concentration (Nmass), ash concentration (AC), the heat of combustion (HC), and specific leaf area (SLA), as well as leaf lignin concentration (LC) and new leaf dry mass per plant (NLDM). As altitude increased, CCm, CCa, HC, and LC increased, whereas Nmass, SLA, AC, and NLDM decreased. CCm and CCa were positively correlated with HC and LC but negatively with Nmass, SLA, AC and NLDM. CCm, CCa and HC were negatively correlated with mean air temperature. The data indicated that some high-HC constituents like lignin rather than protein contributed to the observed pattern of leaf energy properties. For high-altitude plants, having relatively high leaf CCm and HC can be regarded as a growth strategy for sustaining carbon gain and maximizing nitrogen-use efficiency. Since CC tends to decrease with increasing air temperature, evergreen herbs at high altitude are expected to construct relatively “cheaper” leaves in response to global warming.