The carbon balance of European larch (Larix decidua) at the alpine timberline

The annual course of the carbon (C) balance of a 65-year-old larch tree (Larix decidua Mill.) growing at the timberline was determined, as based on continuous measurements of the net photosynthesis and dark respiration rate of sun and shade twigs, the stem and branch respiration rate and on assessments of the light and biomass distribution within the crown. Response curves of photosynthesis to light, temperature, and air humidity provided the basis for modelling net photosynthesis and dark respiration of the various crown parts. The dry matter increment of needles and radial stem growth were followed throughout the year, and newly formed stem, branch and coarse-root material was examined microscopically at the end of the growing season. Daily and seasonal C balances were calculated from the C gain, respiratory C release and dry matter increment of the various tree organs. The annual C gain by net photosynthesis of the whole tree was 12.19 kg (100%), with the needles (including foliated twigs) releasing 1.77 kgC (14.5 %) by dark respiration. 3.28 kgC (26.9 %) were invested into the annual increment of wood and bark in stem, branches, and coarse roots, with the annual respiration of these organs amounting altogether to 2.89 kgC (23.7 %). 0.88 kgC of the 1.95 kgC (16.0 %) invested into needles originated from C reserves. From November through mid-June, the period of a permanently negative C balance, 1.87 kgC (15 %) were supplied from reserves. Assuming new C storage similar to the amount used during the current year, a surplus of 2.30 kgC (19.0%) was calculated to cover the C demand of the fine roots. The C balance of larch at the timberline was compared with findings from other tree species and habitats. Annual C gain of trees from subalpine sites was lower in absolute terms but in relation to the C gain, allocation to aboveground woody organs was higher, while respiration per unit dry matter produced was lower than at low elevation sites. Besides temperature and length of growing season, the aboveground C demand of rapidly growing, light-demanding pioneer species is reflected in the C balance of larch relative to the late-successional, evergreen competitors even under harsh timberline conditions.