Human land-use, forest dynamics and tree growth at the treeline in the Western Italian Alps

Abstract

-Three plots were sampled along an altitudinal gradient in the upper Susa Valley (Piedmont, Italy) on a northeastern slope from 1800 to 2300 m a.s.l. In order to reconstruct recent dynamics at this altitudinal range various techniques were used. Dendroecological methods were used to reconstruct the age structures of tree populations. Growth dynamics were investigated both by observing Basal Area Increment (BAI) in old and dominant trees and by comparing the BAIs within a given cambial age class in different time periods. Historical documents were analyzed as an independent data source to explain changes in establishment rate. As far as the tree establishment at the forestline and at the treeline is concerned we observed three distinct periods: during the first one (1850-1930) larch establishment was reduced or prevented because of heavy grazing and the stone pine establishment was almost null because of the grazing and of the human anthropogenic removal. During the second one ) the past heavy grazing followed by periods of moderate grazing favored the larch establishment; stone pine establishment was still prevented both by grazing and by anthropogenic removal. Finally the third period (1960-present) has been the period of massive stone pine regeneration. The growth rates of stone pine and larch have increased in the last decades: individuals in the 100-, 150-and 200-year age classes grow more rapidly in present times as compared to the previous two centuries. In the same time younger trees (1-50 years old) showed a decline in growth because the current stands are denser and the young and suppressed trees have worse growth conditions respect the previous open stands. An analysis of all the data taken together in the present study argues in favor of the fact that the tree establishment, and more in general the forest dynamic, has been mainly controlled by human land-use and that the tree growth has been mainly climatically controlled. dendroecology / Larix decidua / Pinus cembra / tree-rings / basal area increment (BAI) / herbivory Résumé -Histoire de l'occupation du sol, et dynamique forestière à la limite supérieure des arbres dans la vallée de Suse (Piémont, Italie). Afin de reconstruire la dynamique récente de l'étage subalpin, la présente étude a mis en oeuvre plusieurs techniques. Des méthodes dendrochronologiques ont permis de reconstruire la structure d'âges des populations d'arbres. La croissance a été étudiée à la fois par l'observation de l'accroissement en surface terrière des vieux arbres dominants et en comparant les accroissements en surface terrière des classes d'arbres avec la même classe d'âge cambial à différentes périodes de temps. De plus, des documents d'archives ont été étudiés. On a observé trois phases distinctes d'établissement des arbres : la première (1850-1930) durant laquelle il n'y a pas d'établissement d'arbre à cause de la forte pression du pâturage, la deuxième durant laquelle il y a établissement de mélèzes, car la pression du pâturage diminue et la troisième (1960-au...

Recent forest limit changes in south-east Norway: Effects of climate change or regrowth after abandoned utilisation?

“…In this respect, the chosen study area seems to be representative of the valleys and mountain villages, the mountain forests, and the low-alpine mountains of Norway (Puschmann 2005). Similar findings from the Swiss Alps document the importance of land abandonment for recent forest expansion in areas near the forest limits (Motta et al 2006;Gehrig-Fasel et al 2007;Tasser et al 2007;Albert et al 2008;Gellrich et al 2008).…”

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Land-use and climate change effects in forest compositional trajectories in a dry Central-Alpine valley

“…Only recently, the magnitude of grazing impact in forests has been revealed by the surprising abundant emergence of the rare, presumably goat-dispersed plant species Blitum virgatum, after a large forest fire (Moser et al, 2006). Similar to the Valais, such legacies derived from former grazing activities have been detected for forests throughout the whole European Alps (Chauchard et al, 2007;Curt et al 2003, Motta et al 2006. To study the combined effects of changes in land-use and climate on forest ecosystems, we reconstructed compositional changes on more than 300 parcels of forested land with known land-use history.…”

“…Disentangling the effects of climate from land-use change is needed to better understand current forest composition and to evaluate future trajectories of forest dynamics. Whereas climate change slowly impacts nature at all scales continuously, changes of land-use affect forests quicker though at limited scales (Albert et al, 2008;Motta et al, 2006).…”

Relevance of secondary forest successions and their possible management

“…In the Alpine region, the progressive raising of the tree line due to the abandonment of pasture and global warming is produced by direct colonization of trees. A few pine species, larch, spruce, birch, alder, and other broadleaved tree species colonize directly abandoned land and pasture (Motta et al, 2006;Gambarino et al, 2001Gambarino et al, , 2014. Sometimes, the first stages of colonization are characterized by a shrub layer (hazel, green alder, dwarf mountain Pine, Rhododendron, Juniper, etc.).…”