Long-term trends in leaf level gas exchange mirror tree-ring derived intrinsic water-use efficiency of Pinus cembra at treeline during the last century

The ability of treeline conifers in the Central European Alps to cope with recent climate warming and increasing CO₂ concentration is still poorly understood. We determined basal area increment (BAI) and tree ring stable carbon isotope ratios (δ¹³C) of Pinus cembra trees from 1925 through 2013. Stable isotope ratios and BAI were compared with leaf level gas exchange measurements carried out in situ between 1934 and 2012, and thus, provided new insights into long-term trends of tree-ring derived intrinsic water-use efficiency (iWUE). Mature P. cembra trees at treeline responded to increasing C_a and air temperature with a parallel increase in maximum net CO₂ uptake rate at ambient CO₂ (A_max) and tree-ring-derived intercellular CO₂ partial pressure (C_i). A_max tripled and was positively correlated to BAI and C_i. The latter increased in parallel with ambient CO₂ concentration and stomatal conductance. In contrast to the instantaneous gas exchange parameters, δ¹³C derived iWUE informs about the long-term changes in the carbon water relations. These data showed three changes in the iWUE chronosequences, which could be identified with different long term gas exchange patterns: (1) from stomatal controlled functioning from 1925 to 1981, to a situation where (2) both net CO₂ fixation (A) and leaf conductance for water vapour (g_w), responded to the environment from 1982 to 1997, and (3) back to a stomata controlled pattern over iWUE from 1998 onwards. This temporal pattern was also mirrored in leaf level gas exchange assessments, suggesting a parallel increase of A and g_w of P. cembra at treeline during the last nine decades.