Human impact on size, age, and spatial structure in a mixed European larch and Swiss stone pine forest in the Western Italian Alps Renzo Motta and Emanuele Lingua Abstract: Spatiotemporal development and human impact on dynamic processes were investigated in the mixed European larch (Larix decidua Mill.) and Swiss stone pine (Pinus cembra L.) subalpine forest of Lago Perso (Piedmont, Italy). We mapped and measured all 295 trees (DBH ≥4 cm) and 914 saplings (>10 cm height, <4 cm DBH) in a permanent plot (1 ha). One core per tree was extracted upslope at 50 cm height, and dendrochronological techniques were applied to reconstruct age structure and growth patterns. All of the data collected were stored in a GIS, and tree and stem crown maps were generated and analysed to quantify spatial patterns. Ripley’s K(t) univariate and bivariate point pattern anal- yses were employed to assess the degree of spatial autocorrelation. Documentary research was conducted to reconstruct human land use. The stand is uneven-aged, and there were no obvious age cohorts or other evidence of major distur- bances in the past. Stone pine saplings and trees and larch saplings exhibited a clumped structure. The same clumping was not so evident in larch trees. The observed structural changes are mainly related to human land use and grazing regime. Although human influence is still manifest, in the recent decades natural dynamics have become the predomi- nant influence on the forest’s structure and processes. Resumé : La dynamique spatio-temporelle et le rôle des impacts humains ont été étudiés dans un peuplement subalpin de mélèze (Larix decidua Mill.) et de pin cembro (Pinus cembra L.) à Lago Perso (Haute Vallée de Suse, Piémont, Italie). Dans une placette permanente d’un hectare, 295 arbres (DHP ≥4 cm) ont été mesurés et cartographiés, ainsi que 914 recrûs (DHP <4 cm et hauteur >10 cm). Une carotte par arbre a été prélevée côté amont à 50 cm de hauteur, et une analyse dendrochronologique a permis de reconstruire les modalités de croissance et les structures d’âge. Toutes les données ont été intégrées dans un système d’information géographique pour quantifier la structure spatiale des tiges et des couronnes. Les tests K(d) uni- et bivariés de Ripley ont permis d’évaluer le degré d’autocorrélation spatiale entre les tiges. Parallèlement, une étude des documents d’archive a été conduite pour connaître l’histoire de l’utilization des terres au cours des derniers siècles. Le peuplement est inéquienne et il n’y a pas de cohorte bien individualizée ni d’autre évidence de perturbation majeure. Le changement de structure observée est principalement à mettre en relation avec la réduction importante du parcours du bétail. Les recrûs des deux espèces ainsi que les pins adultes présentent un structure agrégée évidente. Pour les mélèzes adultes en revanche, cette agrégation est moins évidente. Les résultats montrent que même si les impacts humains sont encore très marqués, la structure du peuplement et les processus dynamiques qui s’y déroulent sont principalement influencés par la dynamique naturelle. Motta and Lingua 1820 Introduction Subalpine forests in the Western Italian Alps play important social and ecological roles. In the past these forests were used mainly as grazable forestland (Motta and Dotta 1995); other important functions included a protective role against natural hazards like avalanches (Motta and Haudemand 2000; Brang et al. 2001) and rock falls (Dorren et al. 2003) as well as the production of timber (Farrell et al. 2000). Throughout human presence in the Alps, the natural disturbance regimes have been replaced by various human interventions linked to changing economic and social conditions (Motta et al. 2002). The main effect of human influence has been the disappear- ance of part of the subalpine forests and the modification of the composition, structure, and spatial patterns of those that remain. In the subalpine forests, trees form more or less defined groups of differing size according to species, fertility, and al- titude (Ott et al. 1991; Brang et al. 2001). This tendency be- comes more marked as altitude increases until the pattern becomes one of small clumps of trees interspersed with many openings in the upper subalpine belt (Mayer and Ott 1991; Bebi et al. 2001; Varga and Klinka 2001). This structure is the consequence of a “collaboration” at crown (Zeller 1993) and root levels via grafting (Bormann and Graham 1959), which gives single trees better protection against harsh envi- ronmental conditions. In most subalpine forests in the Western Alps, human modifications were carried out both as a function of forestry uses and practises and to increase the grazing area available Can. J. For. Res. 35: 1809–1820 (2005) doi: 10.1139/X05-107 © 2005 NRC Canada 1809 Received 20 December 2004. Accepted 12 May 2005. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 2 September 2005. R. Motta 1 and E. Lingua. Department Agroselviter, University of Turin, Via Leonardo Da Vinci 44, 10095 Grugliasco (TO), Italy. 1 Corresponding author (e-mail: renzo.motta@unito.it).