Renzo Motta

Growth trends and dynamics in sub‐alpine forest stands in the Varaita Valley (Piedmont, Italy) and their relationships with human activities and global change

Abstract. A study of the forest lines, tree lines and the structures of the sub-alpine forest was performed in Vallone Vallanta and in Aleve forest in the Varaita Valley (Cottian Alps, Piedmont, Italy). Forest- and tree lines were analysed over 1728 ha while forest structures were studied on six 3000-m2 plots located at the tree line (2), at the forest line (2) and inside the sub-alpine forest (2). Dendro-ecological analysis of living plants and stumps showed that Larix decidua was more abundant in the past than today and that Pinus cembra has expanded, both upwards and within sub-alpine forests. Age structure analysis revealed that the current sub-alpine forest stands were established 200–220 yr ago, probably following a clearcut. At the forest lines the tree density decreases, and some trees are more than 500 yr old, whereas at the tree lines most of the trees (almost exclusively Pinus cembra) are younger than 100 yr. Growth dynamics were investigated both by observing Basal Area Increment (BAI) in the old and dominant trees, and by comparing the BAIs of classes of trees with a given cambial age range in different time periods. The results showed that the growth rates of mature Pinus cembra and Larix decidua had increased. These increments are more substantial for Pinus than for Larix. The growth rate of young trees (< 100 yr) of both species has decreased over recent decades. This could be due to competition caused by increased tree densities that have resulted from a decrease in grazing.

Impact of wild ungulates on forest regeneration and tree composition of mountain forests in the Western Italian Alps

Abstract The impact of ungulates on forest regeneration was observed in six localities of the Italian Western Alps recently colonized by deer and having different densities of ungulates. The selectivity of bark stripping, browsing and fraying damage was analysed. The incidence of damage on forest regeneration varies between 79.8% (class A, height 10–150 cm) in the most severely damaged area to 10.2% in the least affected area. The incidence of damage in each species varies considerably and the tree species most sensitive to browsing damage is the silver fir ( Abies alba ). Browsing and bark stripping are always highly selective, whilst a difference between red and roe deer was found in relation to fraying damage. The lethality (dead trees/damaged trees) of damage resulting from fraying is important also with low densities, but increases slowly at high densities; the lethality of damage resulting from browsing is nil with low densities of ungulates, whereas it increases rapidly with good correlation when ungulate densities are high.

Protective forests and silvicultural stability. An example of planning in the Aosta Valley.

Abstract Protection is of vital importance to human populations and activities in the European Alps. In the short to medium term, failure to manage alpine protective forests leads to intolerable risks for people who live and make a living in alpine valleys. The most important features of a protective forest are its stability properties, that is, its ability to carry out its protective function reliably and continuously and, if this is achieved, its ability to maintain its structure and vitality in the face of internal and external influences. Since maintaining and improving stability properties is costly and labor intensive, the objective of interventions should be an acceptable—rather than an ideal—degree of stability in order to ensure the functions required of the protective forest over a 20–50 year period. Such interventions are collectively referred to as minimal tending. A case study of the Ban de Ville forest in Courmayeur (Aosta Valley, Italy) illustrates aspects of silvicultural planning. Only one third of the Ban de Ville forest was found to be acceptably stable. The main causes of instability were unsuitable species composition, simplified vertical structure and cover, presence of Ips typographus and Heterobasidion annosum, and presence of high densities of wild ungulates. Measures to improve stability properties aimed to increase the presence of larch among a homogenous stand of Norway spruce and gradually establish a multilayered, small group structure.

Spatial structure along an altitudinal gradient in the Italian central Alps suggests competition and facilitation among coniferous species

Abstract Questions: What is the structure of the anthropogenic upper forest-grassland ecotone and are there differences in the spatial relationships between the tree species involved? Location: Valfurva Valley, Italian central Alps. Methods: We conducted a spatial distribution and structure analysis in three 1-ha permanent plots along an altitudinal gradient, from the treeline to the sub-alpine forest. We reconstructed the age structure from cores from each individual with diameter > 4 cm at 50 cm height. Results: All tree species and age classes examined had a clumped structure. The cluster tendency was more evident at the treeline where the environmental conditions are more severe. In the sub-alpine forest there was a repulsion between Pinus cembra and Pinus mugo but at the treeline P. cembra was frequently found downslope from P. mugo. Conclusions: Although human influence has been the main driving force in shaping the present forest structure, in the last few decades natural dynamics have become the predominant force acting on forest structure and processes, showing a higher magnitude as altitude increases. Our results emphasize the existence of facilitating and interfering mechanisms between different species. P. cembra seems to be favoured compared to the other tree species.

The rise and fall of the black locust (Robinia pseudoacacia L.) in the “Siro Negri” Forest Reserve (Lombardy, Italy): lessons learned and future uncertainties

Abstract• The alluvial forests of the Ticino valley have been greatly reduced in size and are now represented by only a small number of fragmented remnants. To study the natural development of the black locust, an invasive species, on relatively undisturbed lowland forests, two permanent plots were established in 2005 in the “Siro Negri” Forest Reserve.• The black locust became established almost exclusively between 1940 and 1960. The observed dynamic of the black locust in the Reserve was very similar to what has been observed in its native North American range: following the initial colonization, the black locust firmly established itself in the dominant and intermediate layers but did not regenerate. In addition to the absence of a significant younger population, the decline of the black locust is evident in an elevated mortality rate and higher proportion of black locust biomass in the total coarse woody debris (CWD)• Our results support the hypothesis that the best strategy to control the spread of black locust is to avoid disturbances that favour black locust colonization, and to wait for natural suppression of the species by other trees. Due to the lack of past reference conditions and the future uncertainties, ongoing monitoring will be needed to fully understand the dynamics of forest ecosystem change in the Reserve.Résumé• Les forêts alluviales de la vallée du Tessin ont été fortement réduites en taille et ne sont maintenant représentées que par un petit nombre de vestiges fragmentaires. Pour étudier le développement naturel du robinier, une espèce envahissante, dans des forêts de plaine relativement intactes, deux placettes permanentes ont été établies en 2005 dans la Réserve Forestière « Siro Negri ».• Le robinier s’est établi presque exclusivement entre 1940 et 1960. La dynamique d’installation observée de Robinia pseudoacacia L. dans la réserve est très similaire à ce qui a été observé dans son habitat naturel en Amérique du Nord : après la première colonisation, le robinier s’est fermement établi dans les strates dominantes et intermédiaires, mais il ne s’est pas régénéré. En plus de l’absence d’une population plus jeune, l’évidence du déclin du robinier est révélée par une élévation du taux de mortalité et une plus grande proportion de la biomasse du robinier dans le total des débris ligneux grossiers (CWD).• Nos résultats appuient l’hypothèse que la meilleure stratégie pour contrôler la propagation du robinier est d’éviter les perturbations qui favorisent sa propagation, et d’attendre la suppression physique de l’espèce par d’autres arbres. En raison de l’absence de références sur les conditions passées, et les incertitudes futures, la surveillance continue sera nécessaire pour comprendre la dynamique de l’écosystème forestier dans la réserve.

Old-growth forests and silviculture in the Italian Alps: The case-study of the strict reserve of Paneveggio (TN)

ABSTRACT All forests in the Italian Alps have been affected by humans in some way, either through direct periodic destruction of the forest or by more subtle forms of management and habitat manipulation. For this reason it is very difficult to find stands with “old-growth properties”. However, in the last century there has been a noticeable reduction in many human activities in the mountains and, as a result, many forest stands have developed naturally over the past few decades, even if their composition and structure still reflect past human activity. This makes it particularly important to study the unmanaged parts of previously managed forests: the past history, present structures and long-term monitoring of these forest stands are fundamental steps for increasing our knowledge of the natural forest stand dynamics. Knowledge of this sort is an important reference tool for the present and future development of near-natural silviculture aimed at producing commodities as well as maintaining environmental values. In this paper, a case study in the forest of Paneveggio is presented. In this forest, a strict forest reserve of about 100 ha was established, and a long-term forest dynamics study was set up in four 1-ha plots. The main results of the study of the stand histories of these plots are presented, and their current naturalness status is discussed.

Evidences of drought stress as a predisposing factor to Scots pine decline in Valle d’Aosta (Italy)

Scots pine (Pinus sylvestris L.) forests of many inner Alpine valleys have recently displayed a quick loss of vitality. A decline disease has been suggested as the cause, with drought as the main predisposing factor and the additional contribution of biotic agents inciting tree dieback. This study is focused on Valle d’Aosta, a dry, inner-Alpine region in NW Italy. We inferred vitality changes between years 2000 and 2007 by computing reductions in enhanced vegetation index (EVI). Image differencing was carried out on pre-processed Moderate Resolution Imaging Spectroradiometer (MODIS) imagery taken in late springtime and validated against ancillary ground truth. We: (1) tested whether EVI reductions in Scots pine forests were significantly higher than those of a control species and of a wetter region for the same species, (2) analyzed decline incidence as a function of site and topographic variables, and (3) assessed the relative influence of site and stand structure on decline probability by means of path analysis. Mean EVI in the study area increased due to an early onset of the 2007 growing season. Nevertheless, the incidence of decline was 6.3% and significantly greater for Scots pine than the control species and site. Low-elevation, northerly exposed sites exhibited the highest incidence of decline. Path analysis suggested that the most important determinants of decline probability were slope, solar radiation, and stand sparseness.

The larch wood pasture: structure and dynamics of a cultural landscape

As a consequence of land abandonment and marginalization, open areas and traditionally managed wood pastures are disappearing from many alpine valleys. Landscape and stand scale dynamics were analyzed in two neighboring inner valleys of central Italian Alps (Valmalenco, SO) dominated by larch (Larix decidua Mill.) forests and experiencing different historical land use intensities. Land cover mapping obtained from object-oriented analysis of aerial photographs was used to quantify land cover changes between 1961 and 2003. Spatial statistics were used to quantify landscape patterns, and intensive sampling in permanent plots was employed to perform point pattern analysis at the stand scale. Expansion of forested area at the expense of wood pastures was the main land cover transition observed. Differences in land use intensities were responsible for different patterns at the landscape scale: (1) heavier grazing and human pressure created a more diverse and fragmented mosaic of meadows, open woodlands, forests and wood pastures, (2) lower human impact favored a more homogeneous and stable system. A decline in grazing pressure in the late 1970s favored the establishment of larch and spruce in a clustered pattern, typical of the subalpine belt. Subalpine wood pastures, likewise many other semi-natural environments, are in danger of disappearing and lose their productive and cultural characteristics, because they are developing into more closed and dense stands.

Inter-annual and decadal changes in teleconnections drive continental-scale synchronization of tree reproduction

Climate teleconnections drive highly variable and synchronous seed production (masting) over large scales. Disentangling the effect of high-frequency (inter-annual variation) from low-frequency (decadal trends) components of climate oscillations will improve our understanding of masting as an ecosystem process. Using century-long observations on masting (the MASTREE database) and data on the Northern Atlantic Oscillation (NAO), we show that in the last 60 years both high-frequency summer and spring NAO, and low-frequency winter NAO components are highly correlated to continent-wide masting in European beech and Norway spruce. Relationships are weaker (non-stationary) in the early twentieth century. This finding improves our understanding on how climate variation affects large-scale synchronization of tree masting. Moreover, it supports the connection between proximate and ultimate causes of masting: indeed, large-scale features of atmospheric circulation coherently drive cues and resources for masting, as well as its evolutionary drivers, such as pollination efficiency, abundance of seed dispersers, and natural disturbance regimes.Climate oscillations affect weather on different temporal-spatial scales, which poses difficulty in understanding how they influence tree reproduction. Here Ascoli et al. show relationships between low- and high-frequency components of the NAO and masting in two European tree species across multiple decades.

Fire severity, residuals and soil legacies affect regeneration of Scots pine in the Southern Alps

Regeneration of non fire-adapted conifers following crown fires on the European Alps is often delayed or unsuccessful. Fire may limit establishment by eliminating seed trees, altering soil properties, or modifying microsite and soil conditions via disturbance legacies. However, the effect of soil legacies on post-fire establishment has rarely been discussed. We analyzed the abundance of Scots pine regeneration in a 257 ha wildfire in an inner-alpine forest. Our aims were (1) to model fire intensity at the soil surface and topsoil heating along a gradient of increasing fire severities; (2) to assess the differences in soil properties along the fire severity gradient; (3) to model the effect of disturbance and soil legacies on the density of pine seedlings. We reconstructed fire behavior and soil heating with the First Order Fire Effects Model (FOFEM), tested the effect of fire severity on soils by nonparametric distributional tests, and modeled seedling density as a function of site, disturbance and soil legacies by fitting a GLM following a variable selection procedure. Topsoil heating differed markedly between the moderate and high severity fires, reaching temperatures high enough to strongly and permanently alter soil properties only in the latter. High fire severity resulted in decreased soil consistency and wet aggregate stability. Burned soils had lower organic matter and cations than those unburned. Pine seedlings favored low-fertility, eroded, and chemically poor sites. Establishment was facilitated by the presence of coarse woody debris, but hampered by increasing distance from the seed source. These results suggest that in dry, inner-alpine valleys, fire residuals and soil legacies interact in determining the success of Scots pine re-establishment. High severity fire can promote favorable soil conditions, but distance from the seed source and high evaporation rates of bare soils must be mitigated in order to ensure a successful restoration.