Long-term vegetation changes in an Abies alba forest: natural development compared with response to fertilization

A fertilization experiment was set up in 1969 in a mature fir forest (Abies alba Mill.) in the Vosges Mountains in France, at an elevation of 800 m, on poor granites. It consists of 65 plots (250 m2), divided into 8 treatments: control, N, P, Ca, N+P, N+Ca, P+Ca, N+P+Ca. The quantities of fertilizers were: 200 kg.ha-1 N, 150 kg.ha-1 P2O5, 1500 kg.ha-1 CaO. The humus type and the ground vegetation were described in 1969, and descriptions were repeated in 1989. Statistical analyses show that: 1. Liming, and liming only, has led to considerable alterations of the flora; the control plots are now dominated by Vaccinium myrtillus, a very acidophilous species, while the limed plots are dominated by Festuca altissima and numerous nitrophilous species. 2. Similarly, the humus layer of limed plots has changed from a mor or moder type to an acid mull type. That process might reflect an improvement of the biological activity and of the mineral nutrient cycle. 3. During the past 20 years, the unfertilized ground vegetation has changed, almost as expected from a moderate liming. Two possible causes of such a phenomenon are suggested: (a) a change in the light and temperature microclimate at the ground level, as a consequence of needle losses during the recent 'forest decline', which in turn could have improved organic matter mineralization; and/or (b) the chronic nitrogen deposition.