Raised levels of marine aerosol deposition owing to increased storm frequency; a cause of forest decline in southern Sweden?

Abstract

Since the 1960s an increasing frequency of gales (winds exceeding 21 m s −1 t ) has been observed on the Swedish west coast. The increase has been especially pronounced during the last 30 years and might be related to a globally changing climate. Simultaneously, the health of the forests of southern Sweden has declined dramatically, which implies a possible connection to global change. The deposition of marine aerosols is directly correlated with westerly wind speeds. Aerosol droplets are ejected from the sea surface as waves break and the smaller fractions can be transported a couple of kilometres up into the atmosphere and travel very long distances before they are deposited on land. As high concentrations of the salts in marine aerosols (mainly NaCl) are toxic to many plants, an increasing deposition would cause damage, especially in exposed sites. Damage appears both from salt entering the needles and from changes in soil water chemistry disturbing the sensitive balance between potassium (K) and sodium (Na) as well as causing episodic acidification and raised Al concentrations. Studying the distribution of damage reveals that it is most severe at the west-facing edges of forest stands. Because marine aerosols contain many important nutrients, such as boron and magnesium, essential to higher plants, they also have a fertilising effect. The most damaged species in southern Sweden is the Norway spruce ( Picea abies L.), introduced to the area in the beginning of the twentieth century. Norway spruce is a continental species with low salt tolerance. This project presents empirical models for determination of dry deposition to vertical surfaces of marine aerosols in coastal areas. Results from isochronic measurements across southern Sweden at high westerly wind speeds performed at six meteorological stations are presented, as well as a discussion on the possible impact of salt deposition on spruce trees.