Responses of photochemical efficiency and shoot growth of alpine dwarf-pine Pinus pumila to experimental warming, shading, and defoliation in Japan

Abstract

Global warming accelerates shrub expansion in high-latitude and high-elevation ecosystems. Over the last several decades, alpine dwarf-pine Pinus pumila has expanded its range in northern Japan because of enhanced shoot growth under warm climatic conditions. In alpine regions, local environmental conditions and the length of the growing season, vary depending on the topography, elevation, and snowmelt time. This leads to spatially varying shoot performances that are co-affected by climatic change. We applied a warming, shading, and defoliation treatment to assess how temperature and carbon relations in interaction with habitat type (elevation and snowmelt time) affect shoot growth and photochemical efficiency of needles in this species. Photochemical efficiency (Fv/Fm) was maximized during peak growth in the middle of growing season (mid-July–mid-August), and it increased in the shading and warming treatments especially in the early and late season. Shoot growth increased only in the warming treatment, and was not affected by shading and defoliation. These results indicate that shoot growth of alpine dwarf-pine is limited by low temperature, but not by carbon assimilation, i.e., growth is sink- rather than source-limited. Furthermore, the seasonal trend of photochemical efficiency shifted to the late season at higher elevations, and the recovery time of photochemical efficiency took longer in the late-snowmelt habitat, where the growing season was short. Therefore, warmer summers and longer snow-free periods are likely to enhance the growth and areal expansion of alpine dwarf-pine at the expense of the adjacent, species-rich, low-stature alpine plant communities.