Ungulate impact on rowan (Sorbus aucuparia L.) and Norway spruce (Picea abies (L.) Karst.) height structure in mountain forests in the eastern Italian Alps

Abstract A new approach for studying browsing impact on the regeneration of rowan (Sorbus aucuparia L.) and Norway spruce (Picea abies (L.) Karst.) is presented. This approach can be a useful, complementary tool to damage surveys because it helps to identify possible underestimation of damage in cases where the most palatable species are likely to have completely disappeared due to browsing. The impact of wild ungulates on the height structures of the populations of these two species was studied at four sites in the Trentino area (Italy). The recorded height structures (affected by ungulate browsing) were compared with predicted structures and then residuals were calculated from power function models. The residuals of the most palatable species (rowan) showed that in all study sites there is a decrease in regeneration individuals in the height classes most affected by browsing. Indeed, the greater the density of wild ungulates, the greater the decrease, and in the site with the highest density (Paneveggio), there is a total absence of rowan individuals with a height between 100 and 160 cm. On the contrary, among the Norway spruce we did not observe a high number of residuals in the height classes affected by browsing. In order to better define the temporal dimensions of the browsing, a dendroecological study was conducted. Abrupt growth releases in the tree rings indicate exactly when the leader of a single tree escapes from intense browsing. An abrupt growth release chronology for each site and each species was thus constructed and the differences, in terms of the length and intensity of browsing, were evidenced. The dendroecological study did not show particular differences in the temporal distribution of abrupt growth releases in the Norway spruce, whereas it did show a significant difference for the rowan between the first three study sites and the Paneveggio site. After 1985, no further releases from suppression were observed in Paneveggio and we can therefore hypothesize that following that date, the browsing was so intense that it prevented any rowan individuals from growing beyond browsing height. The fact is that in Paneveggio, the impact from wild ungulates is splitting the rowan population in two: one part established and grew above browsing height before the recent wild ungulate population increase, while the second is made up of those trees that established after the wild ungulate population explosion and which at the time of measurement had either not yet reached browsing height or had been kept suppressed in the lower vegetation layers by browsing.

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