Tree lines rank among the most prominent vegetation boundaries worldwide. The upper limit at which trees can continue to grow in uncultivated territory in the Alps is restricted primarily by the availability of heat during the summer months. For this reason, the warming of the climate is likely to trigger a significant rise in the altitude of tree lines. Apart from temperature, a series of other factors also restrict the growth of trees at the tree line. Among these are snow fungi which, given favourable conditions, can decimate entire tree populations. At the tree line in Davos, SLF researchers have used a long-term data series to investigate the effect of such snow fungi on one tree line species, the Swiss pine (Pinus cembra).
Young Swiss pine particularly affected
On the Stillberg site (NW side of the Dischma valley, altitude 2000-2230 m), 92,000 larch, Swiss pine and mountain pine were planted in a systematic pattern in 1975. At the same time, a comparative plantation (Lucksalp) observing the same pattern was established on the opposite, SW-facing side (Fig. 1). A survey conducted after 30 years has shown that only around 5% of all P. cembra trees have survived in both plantations. The die-back was caused principally by two species of snow fungus or fungal diseases, namely "scleroderris canker" (Gremmeniella abietina, Fig. 2) and "white snow blight" (Phacidium infestans, Fig. 3). The fungal infestation was critical for 10 to 20-year-old saplings (Fig. 4) in particular, whose crowns occupy the snow cover fluctuation range. The infestation often occurred over a large area of the needles, which are sparse on the Swiss pine. These trees, which are still small at this age at the tree line, died just two to three years after first becoming infected by the snow fungi in most cases.
Duration of snow cover plays a key role
P. cembra was infected by scleroderris canker in particular in locations where the snow cover persists for a long time in the spring. The rate of infection was especially high in summers with heavy precipitation and relatively low temperatures. White snow blight, in contrast, spread above all in locations where the snow melted relatively early and where numerous larger trees grew, which were able to transmit the fungus to smaller trees. The study thus confirms that the web of factors influencing the growth of trees at the tree line can be very complex. It further indicates that snow fungi can already, and will probably continue to, significantly restrict the propagation of Swiss pine at the upper limit of the tree line.
2012 - 2014