Growth responses of picea mongolica saplings to goat browsing

Picea mongolica W. D. Xu is an endemic and endangered species that is only found in semiarid areas in north China. It has also been widely used as the forestation tree in China's "Three-North" shelterbelts construction. Although a National Natural Reserve has been set up for conserving the rare species, it always suffers from goat browsing, especially during the winter. Dendrochronology analysis is applied to understand the consequences of goat browsing on Picea mongolica's growth. Two sites with different types of trees were chosen side by side to compare the differences between shape, radial growth, height growth, and age of spruce saplings: (1) stunted and heavily browsed spruce, shorter than the browsing limit and (2) escaped spruce that were taller than the browsing limit but still browsed in their lower part. Under repeated and intense browsing, the shape in stunted spruce was compact and heavily ramified, and the same phenomenon was shown below the browsing limit in escaped spruce (1.05 ± 0.06 m), distinctly different from the shape of escaped spruce above the browsing limit. The results showed that the release of browsing pressure, once the tree reached the browsing limit, was characterized by an abrupt increase in radial growth. Before release, trees showed a growth stagnation characterized by annual rings (0.5 mm per year) and annual height (<5 cm per year) increments. After release, the increments of rings and height were 2 mm and 14 cm each year, respectively. We use this pattern to estimate the release ages and their possible variation over time. Age differences between stunted and escaped spruce were highly significant and indicate that, despite browsing, most, if not all, trees will ultimately reach the browsing limit and escape. Heavy goat pressure delayed spruce sapling recruitment by about 10 years. This delay varied in relation to site quality and seemed to have increased over time, suggesting an increase in browsing pressure.