Leaf phloem alterations in two provenances ofAbies alba Miller (silver fir) during ageing

The phenomenon of premature needle senescence in two Italian provenances of silver fir was studied as part of a more general research project on decliningAbies alba. The two provenances showed a different degree of sensitivity to atmospheric pollutants. Comparative observations on 1-, 3-, 5-, 7- and 9-year-old needles from both provenances highlighted a faster loss of function in the phloem of needles from the Vallombrosa provenance. This fact appears to be related more to a loss of function in the cambium, than to a different rate of degeneration of sieve cells. This phenomenon may help explain why needles of the Vallombrosa provenance tend to be shed sooner than those of the Serra San Bruno provenance.