Consistent response of crown transparency, shoot growth and leaf traits on Norway spruce (Picea abies (L.) H. Karst.) trees along an elevation gradient in northern Italy

The significance of crown transparency as forest health indicator has been frequently challenged. Here we examined crown transparency together with damage, crown productivity (shoot length) and leaf traits (needle weight, O and C stable isotopes, chlorophyll a fluorescence) on 9–27 Picea abies (L.) H. Karst. trees along an elevation gradient in Trentino, northern Italy. The gradient was assumed to represent several environmental stressors. A fully randomized sampling design was adopted and data were analyzed by means of one-way ANOVA and partial correlation. Results show that (i) crown transparency and visible damage on trees increase with elevation; (ii) such a pattern is consistent with a reduction of shoot length, needle weight, photosynthetic potential, and δ18O. Even when the driving effect of elevation is controlled, significant relationships remain between crown transparency and needle weight and δ18O. These results provide evidence that crown transparency reflects changes in tree condition and health, and are supportive of the use of crown transparency as a rapid method to monitor tree and forest health