Paola Nola

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.

Old World megadroughts and pluvials during the Common Era

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability. Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

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.

Significant Mean and Extreme Climate Sensitivity of Norway Spruce and Silver Fir at Mid-Elevation Mesic Sites in the Alps

Climate forcing is the major abiotic driver for forest ecosystem functioning and thus significantly affects the role of forests within the global carbon cycle and related ecosystem services. Annual radial increments of trees are probably the most valuable source of information to link tree growth and climate at long-term time scales, and have been used in a wide variety of investigations worldwide. However, especially in mountainous areas, tree-ring studies have focused on extreme environments where the climate sensitivity is perhaps greatest but are necessarily a biased representation of the forests within a region. We used tree-ring analyses to study two of the most important tree species growing in the Alps: Norway spruce (Picea abies) and silver fir (Abies alba). We developed tree-ring chronologies from 13 mesic mid-elevation sites (203 trees) and then compared them to monthly temperature and precipitation data for the period 1846–1995. Correlation functions, principal component analysis and fuzzy C-means clustering were applied to 1) assess the climate/growth relationships and their stationarity and consistency over time, and 2) extract common modes of variability in the species responses to mean and extreme climate variability. Our results highlight a clear, time-stable, and species-specific response to mean climate conditions. However, during the previous-years growing season, which shows the strongest correlations, the primary difference between species is in their response to extreme events, not mean conditions. Mesic sites at mid-altitude are commonly underrepresented in tree-ring research; we showed that strong climatic controls of growth may exist even in those areas. Extreme climatic events may play a key role in defining the species-specific responses on climatic sensitivity and, with a global change perspective, specific divergent responses are likely to occur even where current conditions are less limited.

Diachronic analysis of individual-tree mortality in a Norway spruce stand in the eastern Italian Alps

Abstract• Understanding tree mortality processes across time requires long term studies. Spatiotemporal patterns of mortality in a 200 years-old mono-layered Norway spruce stand were evaluated to determine what factors affected individual-tree mortality.• We performed an analysis on two surveys (1993 and 2005) in a 1-ha permanent plot in the Paneveggio forest (Eastern Italian Alps). Tree diameter and age distribution between surveys were compared. We examined spatial patterns of living and dead trees before 1993, in 1993 and in 2005 using univariate and bivariate Ripley’s K(d) function, and a kernel estimator of local crowding. A logistic model was used to assess the effects of diameter, age, recent growth and competitive pressure on tree mortality.• Spatial pattern analysis indicated mortality was associated to tree neighbourhood (neighbour effect at 2–5 m). An increment of regularization of tree spatial pattern occurred due to density-dependent mortality. Logistic regression showed tree diameter and recent growth were determinant on mortality risk during the monitoring period.• Even if the stand is relatively aged, mortality dynamics are those typical of stem exclusion stage. Mortality was related to competitive dynamics, and small suppressed trees with slow growth rate had higher probability to die.Résumé• Comprendre les processus de la mortalité des arbres à travers le temps exige des études à long terme. Les modèles spatio-temporels de la mortalité dans un peuplement d’épicéa âgé de 200 ans monostrate ont été évalués afin de déterminer quels sont les facteurs qui ont un effet sur la mortalité des arbres individuels.• Nous avons pratiqué une analyse sur deux enquêtes (1993 et 2005) dans une parcelle permanente de 1 ha dans la forêt de Paneveggio (Alpes orientales italiennes). Le diamètre des arbres et la distribution des âges ont été comparés entre les enquêtes. Nous avons examiné les modèles spatiaux des arbres vivants et des arbres morts avant 1993, en 1993 et en 2005 en utilisant des fonctions de Ripley K(d) univariées et bivariées, Un modèle logistique a été utilisé pour évaluer les effets du diamètre, de l’âge, de la croissance récente et de la pression concurrentielle sur la mortalité des arbres.• L’analyse du modèle spatial indique que la mortalité a été associée aux arbres voisins (effet du voisin distant de 2 à 5 m). Un incrément de régularisation du modèle spatial de l’arbre s’est produit en raison de la dépendance de la mortalité de la densité. La régression logistique a montré que le diamètre de l’arbre et la croissance récente ont été déterminant pour le risque de mortalité au cours de la période étudiée.• Même si le peuplement est relativement âgé, les dynamiques de la mortalité sont typiques de l’étape d’exclusion des tiges. La mortalité était liée à la dynamique concurrentielle, et de petits arbres dominés avec un faible taux de croissance avaient une probabilité supérieure de mourir.

Stand and coarse woody debris dynamics in subalpine Norway spruce forests withdrawn from regular management

Abstract• We studied structural characteristics, amount and quality of coarse woody debris (CWD), intensity of competition and mortality in two subalpine Norway spruce stands withdrawn from regular management. The stands, that we measured twice (in 1993 and 2005), have similar age and structure, but a different time has elapsed since the last silvicultural treatments (respectively 22 and about 55 y).• The main purposes were to analyze the current stage of development as compared to the old-growth one and to highlight the legacies of past management.• Although relatively old, the first plot (Valbona 1) was at the end of the pole stage. CWD was low in volume and was mainly of man-made origin (stumps). A recent thinning from below has reduced density-dependent competition and delayed the development of old-growth characteristics. The second plot (Valbona 2a) was at the beginning of the transition stage, with density-dependent and allogenic mortality both active at the same time. CWD volume was higher in plot Valbona 2a than in Valbona 1, but neither was comparable yet to the reference old-growth sites from Central Europe, both in quantity and in quality (e.g., decay rate continuity).• The effects of the past management were: (1) reducing the quality and quantity of the CWD, (2) alleviating competition, (3) increasing resistance to minor disturbances and, as a consequence, (4) delaying the development processes.• In mature or overmature subalpine Norway spruce stands withdrawn from regular management many decades are necessary to develop old-growth characteristics and a longer period of time is necessary to reach a true old-growth stage.

The role of larch budmoth ( Zeiraphera diniana Gn.) on forest succession in a larch ( Larix decidua Mill.) and Swiss stone pine ( Pinus cembra L.) stand in the Susa Valley (Piedmont, Italy)

In the Alps, larch (Larix decidua Mill.) is severely affected by larch budmoth (Zeiraphera diniana Guénée) (LBM) attacks. The impact of these outbreaks on the Swiss stone pine (Pinus cembra L.) and on the dynamic processes acting in subalpine forest stands are still not well known. Dendroecological methods were used in this study to reconstruct past LBM outbreaks in Susa Valley, Piedmont, Italy. The analysis was carried out on 62 cores from larch and 101 cores from stone pine. The length and severity of each outbreak was quantified for both species and for each tree by means of the programme OUTBREAK. The frequency of the outbreaks was determined using singular spectral analysis and superposed epoch analysis was used to test the significance of the associations between outbreaks and tree-ring growth. In order to verify if trees belonging to different age classes are differently affected by LBM, the reconstructed outbreaks are then classified taking into account the cambial age of the tree at the moment of the outbreak. From 1760 to 1999, 19 outbreaks were recorded in the larch chronologies, while only three outbreaks in the stone pine chronologies. The larch growth is strongly influenced by LBM and the identified outbreaks are equally distributed in all the three age classes. On the stone pine the sporadic occurrence of the identified events made difficult any interpretation of the eventual effect of LBM. Our results lead us to argue that LBM has not played an important role both in determining the stone pine growth rate and in influencing the present observed succession from the stage dominated by larch, to a stage dominated by stone pine or by a mixed stone pine-larch forest.

Fraying damages in the subalpine forest of Paneveggio (Trento, Italy): a dendroecological approach

Abstract In the forest of Paneveggio, in the last few years the increase in the density of Red deer (Cervus elaphus) and Roe deer (Capreolus capreolus) has caused the appearance of considerable damage to forest regeneration. To gain an historical documentation of the presence of the two species 82 regeneration specimens damaged by fraying were cut down in August-September 1994. These specimens, 70 spruce (Picea abies), seven cembran pine (Pinus cembra) and five larch (Larix decidua), came from three areas of the forest used as rutting areas by Red deer. An analysis of the stems of these trees was made by cutting a cross-section every 20 cm of height between 0 and 140 cm. In these cross-sections all damage from fraying was identified and dated. The fraying caused by Red deer was distinguished from that caused by Roe deer according to the diameter of the tree at the moment of damage, the height from the ground, and the overall extent of the damage. The presence in the last century of the two deer species has been documented.

Host preference and growth patterns of ivy (Hedera helix L.) in a temperate alluvial forest

Recent studies have highlighted the role of lianas in shaping stand dynamics both in tropical and temperate forests. However, English ivy (Hedera helix L.), one of the most widespread lianas in Europe, has received little attention. We conducted a study in the Siro Negri alluvial forest (NW Italy) to determine what factors most affected ivy distribution and investigate its interactions with the trees in the stand. We evaluated the influence of tree size, age, species, and neighborhood crowding on ivy occurrence. In addition, growth ring widths were used to explore the development pattern of climbing stems. Fifty-two percent of trees in our study plots carried ivy, a value comparable to liana incidence found in mature tropical forests. Tree characteristics and their spatial pattern significantly influenced ivy distribution. Preferred hosts were large, isolated trees, while the effect of tree age and species on ivy occurrence was marginal. Growth pattern analysis revealed that radial growth was positively related to the available space on the tree trunk for each ivy stem. We conclude that neighborhood crowding around trees and competition among climbing stems relying on the same trunk may reduce the colonization rate of ivy.