Diurnal changes in needle gas exchange in alpine Pinus pumila during snow-melting and summer seasons

Pinus pumila (Pallas) Regel. is a dominant dwarf tree in alpine regions of Japan. The possible factors limiting the net photosynthetic rate (P-n) of the needles of P. pumila were examined in the snow-melting (May) and the summer (August) seasons. In August, in situ maximum P-n was 20 mu mol kg(-1) needle s(-1) in the current-year needles and 25 mu mol kg(-1) needle s(-1) in the 1-year-old needles. Diurnal trends of P-n in August were positively related to fluctuations in photosynthetic photon flux density (PPFD) and no midday depression of P-n was found, indicating that a decrease in PPFD rather than an increase in needle-to-air vapor pressure deficit (DeltaW) might cause the reduction of P-n. Both stomatal conductance (g(s)) and P-n were lower in May than in August. In May, P-n and g(s) were almost zero in the morning, but gradually increased with decreasing DeltaW, reaching maximum P-n values (4 mu mol kg(-1) needle s(-1)) and g(s) (60 mmol kg(-1) needle s(-1)) at 16.00 hours. The daytime P-n in May was positively related to g(s). Relative water content in the exposed needles above the snow in May was 83%, which was far above the lethal level. This indicates that the water flow from stems or soils to needles was enough to compensate for a small amount of water loss due to the low g(s) in May, although the water supplied to needles would be impeded by the low temperatures. Thus, the reduced g(s) in May would be important for avoiding needle desiccation, and would result in a reduced P-n.