Regional variation of change in crown density in spruce forests: a comparative study of data from three projects in southern Norway

age air pollution boreal forests canopy color colour copa couleur couvert crown cubierta de copas defoliation densidad density edad environmental factors facteur du milieu factores ambientales feuille forest decline forest stands hojas houppier leaves marchitez del bosque measurement mesure monitoring noruega norway peuplement forestier picea abies rodales site factors viabilidad viability vigilancia