Ignacio Barbeito

Factors driving mortality and growth at treeline: a 30-year experiment of 92 000 conifers

Understanding the interplay between environmental factors contributing to treeline formation and how these factors influence different life stages remains a major research challenge. We used an afforestation experiment including 92 000 trees to investigate the spatial and temporal dynamics of tree mortality and growth at treeline in the Swiss Alps. Seedlings of three high-elevation conifer species (Larix decidua, Pinus mugo ssp. uncinata, and Pinus cembra) were systematically planted along an altitudinal gradient at and above the current treeline (2075 to 2230 m above sea level [a.s.l.]) in 1975 and closely monitored during the following 30 years. We used decision-tree models and generalized additive models to identify patterns in mortality and growth along gradients in elevation, snow duration, wind speed, and solar radiation, and to quantify interactions between the different variables. For all three species, snowmelt date was always the most important environmental factor influencing mortality, and elevation was always the most important factor for growth over the entire period studied. Individuals of all species survived at the highest point of the afforestation for more than 30 years, although mortality was greater above 2160 m a.s.l., 50-100 m above the current treeline. Optimal conditions for height growth differed from those for survival in all three species: early snowmelt (ca. day of year 125-140 [where day 1 is 1 January]) yielded lowest mortality rates, but relatively later snowmelt (ca. day 145-150) yielded highest growth rates. Although snowmelt and elevation were important throughout all life stages of the trees, the importance of radiation decreased over time and that of wind speed increased. Our findings provide experimental evidence that tree survival and height growth require different environmental conditions and that even small changes in the duration of snow cover, in addition to changes in temperature, can strongly impact tree survival and growth patterns at treeline. Further, our results show that the relative importance of different environmental variables for tree seedlings changes during the juvenile phase as they grow taller.

Generalized additive models reveal the intrinsic complexity of wood formation dynamics

The intra-annual dynamics of wood formation, which involves the passage of newly produced cells through three successive differentiation phases (division, enlargement, and wall thickening) to reach the final functional mature state, has traditionally been described in conifers as three delayed bell-shaped curves followed by an S-shaped curve. Here the classical view represented by the ‘Gompertz function (GF) approach’ was challenged using two novel approaches based on parametric generalized linear models (GLMs) and ‘data-driven’ generalized additive models (GAMs). These three approaches (GFs, GLMs, and GAMs) were used to describe seasonal changes in cell numbers in each of the xylem differentiation phases and to calculate the timing of cell development in three conifer species [Picea abies (L.), Pinus sylvestris L., and Abies alba Mill.]. GAMs outperformed GFs and GLMs in describing intra-annual wood formation dynamics, showing two left-skewed bell-shaped curves for division and enlargement, and a right-skewed bimodal curve for thickening. Cell residence times progressively decreased through the season for enlargement, whilst increasing late but rapidly for thickening. These patterns match changes in cell anatomical features within a tree ring, which allows the separation of earlywood and latewood into two distinct cell populations. A novel statistical approach is presented which renews our understanding of xylogenesis, a dynamic biological process in which the rate of cell production interplays with cell residence times in each developmental phase to create complex seasonal patterns.

Snow Fungi—Induced Mortality of Pinus cembra at the Alpine Treeline: Evidence from Plantations

Abstract Identifying the factors controlling tree mortality is key to understanding the effects of ongoing global change on treeline movement and species composition. One potentially significant but little studied factor that impacts the formation of treelines is mortality caused by snow fungi. We studied the mortality patterns of Pinus cembra, a typical treeline species of the Central Alps of Switzerland, in two plantations located on opposite slopes of the Dischma valley (Northeast [NE] and Southwest [SW]) above the current treeline. In 1975, 33,000 P. cembra trees were planted at the NE site and 550 trees at the smaller SW site. All trees have been periodically monitored for survival since then. After 30 years, only ca. 5% of all P. cembra trees survived in both plantations. Two species of pathogenic snow fungus, Gremmeniella abietina and Phacidium. infestans, were major mortality agents for saplings. High rates of infection by Gremmeniella corresponded to late snowmelt and high ratios of rainfall to temperature. High rates of infection by Phacidium corresponded to earlier snowmelt and were spatially associated with P. cembra mature trees that had been present in the area before the time of planting. We provide experimental evidence that snow fungi are a primary cause of sapling tree mortality at treeline. Although additional evidence from different geographic regions and more natural treeline distributions is needed, our results suggest that the prevalence of snow fungi may prevent establishment of trees above the current treeline at present, and under future climate scenarios.

Evaluating the behaviour of vertical structure indices in Scots pine forests

Abstract• Stand structure indices would appear to be good surrogate measures for biodiversity in forest ecosystems.• The vertical structure of Pinus sylvestris L. stands in Central Spain was analysed in order to evaluate their structural diversity. A comparison between two forests with similar ecological conditions but managed under different silvicultural systems was conducted in order to analyse changes in diversity at different stages of stand development. Height diversity was quantified using two non-spatially explicit indices (Shannon’s index and STVI) as well as two spatially explicit indices (Gadow’s differentiation index and the Structure complexity index). A new diversity index was then proposed, based on the sum of square roots of height differences (SQRI).• Correlations between all vertical structure indices were highly significant. All indices showed that height diversity was greater in the forest with the longer regeneration period and where less intensive thinnings were applied throughout the rotation. Diversity was highest in uneven-aged stands and in the period between the regeneration stage and the first thinning. Thinning from below accounted for the decrease in vertical structure complexity throughout the rotation in even-aged stands.• The results show that height distribution along with successional stage data enhance the analysis of vertical diversity since structural complexity is highly related to the silvicultural practices that are carried out at different ages.Résumé• Les indices de structure de peuplements semblent être de bonnes mesures de substitution pour l’estimation de la biodiversité dans les écosystèmes forestiers.• La structure verticale des peuplements de Pinus sylvestris L. dans le centre de l’Espagne a été analysée afin d’évaluer leur diversité structurelle. Une comparaison entre deux forêts, ayant des conditions écologiques similaires mais gérées avec une sylviculture différente, a été réalisée afin d’analyser les changements dans la diversité à différents stades de développement du peuplement. La diversité de hauteur a été quantifiée au moyen de deux indices non-spatialement explicites (indice de Shannon et STVI) ainsi que de deux indices spatialement explicites (l’Indice de différenciation Gadow et l’Index de complexité de structure). Un nouvel indice de la diversité, basé sur la somme des carrés des racines des différences de hauteur (SQRI), a ensuite été proposé.• Les corrélations entre les indices de structure verticale sont très significatives. Tous les indices montrent que la diversité en hauteur est plus grande dans la forêt ayant la plus longue période de régénération et là où les éclaircies les moins intensives ont été appliquées dans l’ensemble de la rotation. La diversité est la plus élevée dans les peuplements inéquiennes et dans la période entre la phase de régénération et la première éclaircie. L’éclaircie par le bas représente la diminution de complexité de la structure verticale dans l’ensemble de la rotation, dans les peuplements équiennes.• Les résultats montrent que la distribution des hauteurs avec les données des stades de succession améliore l’analyse de la diversité verticale puisque la complexité structurelle est fortement liée à la sylviculture qui a été réalisée à des âges différents.

Crown responses to neighbor density and species identity in a young mixed deciduous stand

Key messageDensity was more important in shaping crown structure than neighbor species identity. Both species showed high crown plasticity at alternative levels, which may explain species coexistence in mixed broadleaved forests with functionally similar species.AbstractUnderstanding crown response to local competition is essential to predicting stand development in mixed stands. We analyzed data from an 8-year-old field experimental plantation mixing two species according to a crossed gradient of density and species proportion to quantify the effect of a broad range of local neighborhood conditions on the development of young trees at multiple crown levels. We used Fagus sylvatica and Acer pseudoplatanus, as two model deciduous species. They are considered functionally equivalent at the young stages, but with contrasting architectural patterns. For both species: (1) changes in density explained more of the variation on crown development than species proportion (2) much of the effect of competition was accounted for by variables at the stem level, while branch and leaf development within crowns were not directly altered by competition. Both species were able to modify their crowns at the stem level to compete with intra- and inter-specific neighbors: Acer and Fagus were taller with a highest proportion of Fagus as neighbors; Fagus displayed a lower crown base when the proportion of Fagus decreased, while Acer had a lower crown base when the proportion of Fagus around it increased. Both species showed common shapes in allometric relationships but contrasting responses at alternative crown levels. Acer exhibited broader intra-specific variation in its height–diameter relationship and in its crown length, while Fagus displayed higher individual variation of branch development and leaf area than Acer. This study demonstrates that differences in crown development strategy of each species in response to changes in local neighborhood conditions are an important factor in maintaining species coexistence in broadleaved forests and designing mixtures that persist over time.

Carbon sequestration in Mediterranean pine forests

Management objectives and techniques in woodlands dominated by Quercus species have changed dramatically over the last few decades, especially in the case of coppices. The fact that these stands are of little economic importance today is in part compensated by their value, for example, as carbon sinks. The identification of this important role of Quercus woodlands allows us to contemplate the development of specific silviculture and to promote investment in the stands. It is also important, in the context of global change, to analyze the dynamics involved in the transformation of Quercus woodlands from carbon sinks to emitters.

Alternative approaches to assessing the natural regeneration of Scots pine in a Mediterranean forest

Key messageIn modelling regeneration patterns, parametric regression is recommended because it can account for the spatial and temporal correlation present in the data, whereas decision trees allow more complex interactions and can be used to reduce the number of variables.ContextThe establishment of seedlings after regeneration fellings is key to guaranteeing the development and persistence of the forest. Depending on the objective pursued, data available or type of forest, a number of different methods have been employed to assess the relationship between seedling establishment and both environmental and stand factors. Most authors have conducted their analyses using parametric regression or point pattern analysis.AimWe analysed the way in which light, stand conditions, edaphic and topographic variables affect the regeneration of Pinus sylvestris L. in Central Spain. We used different methods to analyse the same data set. The strengths and weaknesses of each method were discussed.MethodsWe used two parametric approaches: generalized linear mixed model regression using a negative binomial followed by the variant explanatory variables reduction prior to regression as well as three nonparametric approaches not commonly employed in forest regeneration: nonmetric multidimensional scaling, regression trees and random forests algorithm.ResultsThe parametric regression identified a larger number of variables associated with the regeneration process and the inclusion of a random effect in the model allowing the consideration of the spatial variability among plots. However, decision trees captured the complex interaction among variables, which typical parametric methods were unable to detect.ConclusionDifferent statistical methods gave similar insights into the underlying ecological process. However, different statistical premises with inference implications can be noticed. This may give misinterpretation of the model depending on the nature of the data. The choice of a given method should be made according to the nature of the data and the achievement of desirable results.

EuMIXFOR empirical forest mensuration and ring width data from pure and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) through Europe

Key messageThis data set provides unique empirical data from triplets of Scots pine (Pinus sylvestrisL.) and European beech (Fagus sylvaticaL.) across Europe. Dendrometric variables are provided for 32 triplets, 96 plots, 7555 treesand 4695 core samples. These data contribute to our understanding of mixed stand dynamics.Dataset access athttp://dx.doi.org/10.5061/dryad.8v04m. Associated metadata available athttps://metadata-afs.nancy.inra.fr/geonetwork/apps/georchestra/?uuid=b3e098ca-e681-4910-9099-0e25d3b4cd52&hl=eng.

Mediterranean Pine Forests: Management Effects on Carbon Stocks

In the Mediterranean area, the role of forest as carbon sinks is particularly significant since usually ecosystem services provided by forests are frequently of greater value than their direct productions. In this chapter, how carbon sequestration changes over time and with different management regimes in Mediterranean pine forests are presented. The information come from a number of sources including: (i) carbon stock estimates under different management plans using a chronosequence trial in Pinus sylvestris forests; (ii) simulations based on the process model 3-PG of the effect of different thinning regimes on Pinus pinaster biomass under a climate change scenario; (iii) a comparison of the effect of different age structures in Pinus pinea forest using the PINEA growth model which includes the biomass allocated in cones and considers the different wood uses; and finally, (iv) a model for estimating coarse woody debris.

New angular competition index and tree crown projection model

Tree architecture plasticity enables individuals to maximize filling of the available space and minimize the competition effect; tree crowns can be distributed more regularly than stem bases. We conducted a study in a temperate mixed forest (Krakow, Poland) to test the idea of a new angular competition index (CI) and its application in modeling crown radii (CR) and crown projections (CPs) of 30 Quercus robur trees. The statistical method used was random forest algorithm. The model allows the prediction of the CP shape on the basis of stem spatial distribution and diameter, measurement of two CR per target tree and local topography; it can be used to compare the inter- and intra-specific architectural plasticity of the canopy trees.