Scots pine needle longevity and gradation of needle shedding along pollution gradients

Scots pine (Pinus sylvestris L.) needle longevity and gradation of needle shedding (the time span with 90-10% needle survival) in the middle and northern boreal vegetation zones were studied. The duration of the first half of gradual shedding (the time span with 90-50% needle survival) was of special interest, because it represents the number of youngest cohorts that have lost needle biomass. Branches of adult trees along transects with an air pollution gradient were sampled. Mean longevity (the sum of survival proportions) decreased 15-40% as pollution sources were approached. To rank or compare the values of a single needle age parameter, we prefer to use mean longevity. It gives an estimate of the whole needle biomass, and it was the parameter with the least variation. The first half of gradual shedding proved to be a valid indicator of changes in needle age structure. The most marked variation in needle age structure was seen in slightly polluted areas, which continued to pose a risk of acute pollution injuries. This study confirmed the feasibility of the needle age structure of branch samples as an objective and reliable vitality indicator for Scots pine.