MARCESCENCE AND SENESCENCE IN A SUBMEDITERRANEAN OAK (QUERCUS-SUBPYRENAICA DELVILLAR,E.H.) - PHOTOSYNTHETIC CHARACTERISTICS AND NUTRIENT COMPOSITION

We have studied some characteristics of marcescent leaves (withered re tained leaves) and senescent leaves in Quercus subpyrenaica, a tree sp ecies that plays a major role in the climatic transition forests betwe en temperate and mediterranean environments in north-eastern Spain, Le aves were taken from the upper and lower halves of the tree crown, bot h in the south- and north-exposed parts of the tree, Leaves receiving low photosynthetic photon flu density (PPFD) undergo autumnal senescen ce, which is associated with decreases in photosynthetic pigments and decreases in the chlorophyll a to chlorophyll b ratio, Leaves receivin g higher PPFD underwent senescence at a later date, Leaves situated in the upper, south-exposed part of the tree, which receive the highest PPFD, showed no signs of senescence and remained photosynthetically ac tive for a longer period of time, marcescence occurring suddenly at th e end of the season, Marcescence is unlikely to cause an improvement i n nutrient recycling, since the removal of nutrients proceeded similar ly in marcescent and senescent leaves, Marcescence may increase the ti me-span of the assimilation capacity for a significant period of time during September and October, when high light intensities and mild tem peratures occur, The phenomena triggering marcescence may be related t o low temperatures.