Role of climate, crown position, tree age and altitude in calculated ozone flux into needles of Picea abies and Pinus cembra: a synthesis

Abstract

Ozone (O3) flux into Norway spruce (Picea abies) and cembran pine (Pinus cembra) needles was estimated under ambient conditions at six rural sites between 580 and 1950 m a.s.l. We also assessed age-related differences in O3 flux by examining changes in leaf conductance across the life span of Norway spruce. At the leaf level O3 flux into the needles was effectively controlled by stomatal conductance and, hence by factors such as temperature, irradiance and humidity, which control stomatal conductance. Seasonal variations in O3 flux were mainly attributed to the course of the prevailing temperature. During the growing season, however, data have emphasised leaf–air vapour pressure difference as the environmental factor most likely to control stomatal conductance and O3 flux into the needles. In the sun crown stomatal conductance averaged over the growing season decreased with increasing tree age from 42.0±3.5 mmol O3 m−2 s−1 in 17-year-old trees to 7.1±1.0 mmol O3 m−2 s−1 in 216-year-old trees, indicating that O3 concentration in the substomatal cavities is higher in young than in old trees. Independent from tree age stomatal conductance and O3 flux were approximately 50% lower in shade needles as compared to sun-exposed needles. Stomatal conductance was also greater in the current flush (24±5.6 mmol O3 m−2 s−1) and in 1-year old needles (16±4 mmol O3 m−2 s−1) than in older needle age classes (12±1 mmol O3 m−2 s−1, averaged across the four older needle age classes). In trees similar in age (60–65 years old) average O3 flux into sun needles increased from 0.55±0.36 nmol m−2 s−1 at the valley floor to 0.9 nmol m−2 s−1 in 1950 m a.s.l. Cumulative O3 uptake during the vegetation period increased from 11.4±1.7 mol m−2 in the valley to 14 mol m−2 at the alpine timberline. Although stomatal conductance provides the principal limiting factor for O3 flux, additional field research is necessary in order to improve our understanding concerning the quantitative ‘physiological threshold dose’ which internally can be active and can have adverse effects of O3 on forest trees.