Climate and altitudinal limits of beech woods in the Northern Apennines

The spatial pattern of the beech wood altitudinal limit on the northern Apennines and its correlation with the heat deficiency of Fagus sylvatica is under study. On the whole northern side of the Northern Apennines, including a tree line, the pattern of the wood limit was digitized on a scale of 1: 25 000. This limit stretches for 354 km. Adjacencies between beech woods and natural plant communities were measured to identify the present length of the natural wood limit. This limit has a length of 163 km and lies entirely 1600 m a.s.l. Adjacencies to Vaccinium dwarf shrublands (71%) and Brachypodium genuense grasslands (21%) prevail. The values of the thermal parameters were calculated using fifty years (1951-2000) data from five weather stations. Linear regressions of values vs elevation showed that the present highest beech limit on the chain (1875 m a.s.l) corresponds to a mean temperature of the coldest month (January) of -2.5 °C, to a mean annual temperature of 4.2 °C, to a summer mean of 11.0 °C, to a mean of the warmest month (July) of 12.7°C and to 132 days with maximum temperatures above 10°C. These results are only a first approximation to the lowest beech heat limits, as they derive from an adiabatic interpolation of temperature data from stations which are far from the local altitudinal limits of woods. In addition, geomorphology can locally affect the altitudinal distribution of trees and, in turn, different corresponding isothermes. A deeper insight into the beech heat constraints can be provided by dendrochronological and dendroecological analyses. Two study areas were considered: Mt. Giovo-Rondinaio and Mt. Prado-Cusna. The wood limits and the isolated trees above such limits were georeferenced. Samplings were made along a transect composed of three plots (10 x 30 m) at different altitudes: the first plot at the tree line, the second one at the wood limit and the third plot 50 m below the second one. For each plot 12 trees were chosen and for each tree two samples were taken with an increment borer. Ring width data were correlated to temperature and precipitation data from 1851 to the present. In general a high autocorrelation was found: the annual growth of beech heavily affects its growth in the two following years. On Mt. Giovo-Rondinaio there are significant correlations between ring width and precipitations rather than with temperatures. On Mt Prado the annual width seems more correlated to the temperatures of the coldest months. Every tree was dated. On Mt. Giovo-Rondinaio most stems date back to the first decades of the 20th century and there are no significant differences between the age of trees at the wood limit and the trees at a lower altitude. A different situation occurs on Mt. Prado where at the wood limit stems date back to the second half of the 20th century. In this area trees above the wood limit are younger than trees sampled at a lower altitude. These results lead us to suppose that on the Mt Prado-Cusna the present altitudinal wood limit at 1677m a.s.l is far from a natural condition, perhaps due to a greater human impact until a few decades ago. On Mt. Giovo-Rondinaio the wood limit at 1738 m a.s.l. could be nearer to its local upper altitude.