Pinus Uncinata Ram Research Articles

Weak genetic differentiation but strong climate-induced selective pressure toward the rear edge of mountain pine in north-eastern Spain

Local differentiation at distribution limits may influence species' adaptive capacity to environmental changes. However, drivers, such gene flow and local selection, are still poorly understood. We focus on the role played by range limits in mountain forests to test the hypothesis that relict tree populations are subjected to genetic differentiation and local adaptation. Two alpine treelines of mountain pine (Pinus uncinata Ram. ex DC) were investigated in the Spanish Pyrenees. Further, an isolated relict population forming the species' southernmost distribution limit in north-eastern Spain was also investigated. Using genotyping by sequencing, a genetic matrix conformed by single nucleotide polymorphisms (SNPs) was obtained. This matrix was used to perform genotype-environment and genotype-phenotype associations, as well as to model risk of non-adaptedness. Increasing climate seasonality appears as an essential element in the interpretation of SNPs subjected to selective pressures. Genetic differentiations were overall weak. The differences in leaf mass area and radial growth rate, as well as the identification of several SNPs subjected to selective pressures, exceeded neutral predictions of differentiation among populations. Despite genetic drift might prevail in the isolated population, the Fst values (0.060 and 0.066) showed a moderate genetic drift and Nm values (3.939 and 3.555) indicate the presence of gene flow between the relict population and both treelines. Nonetheless, the SNPs subjected to selection pressures provide evidences of possible selection in treeline ecotones. Persistence in range boundaries seems to involve several selective pressures in species' traits, which were significantly related to enhanced drought seasonality at the limit of P. uncinata distribution range. We conclude that gene flow is unlikely to constrain adaptation in the P. uncinata rear edge, although this species shows vulnerability to future climate change scenarios involving warmer and drier conditions.

Minimum Leaf Conductance (g min) Is Higher in the Treeline of Pinus uncinata Ram. in the Pyrenees: Michaelis' Hypothesis Revisited.

The search for a universal explanation of the altitudinal limit determined by the alpine treeline has given rise to different hypotheses. In this study, we revisited Michaelis’ hypothesis which proposed that an inadequate “ripening” of the cuticle caused a greater transpiration rate during winter in the treeline. However, few studies with different explanations have investigated the role of passive mechanisms of needles for protecting against water loss during winter in conifers at the treeline. To shed light on this, the cuticular transpiration barrier was studied in the transition from subalpine Pinus uncinata forests to alpine tundra at the upper limit of the species in the Pyrenees. This upper limit of P. uncinata was selected here as an example of the ecotones formed by conifers in the temperate mountains of the northern hemisphere. Our study showed that minimum leaf conductance in needles from upper limit specimens was higher than those measured in specimens living in the lower levels of the sub-alpine forest and also displayed lower cuticle thickness values, which should reinforce the seminal hypothesis by Michaelis. Our study showed clear evidence that supports the inadequate development of needle cuticles as one of the factors that lead to increased transpirational water losses during winter and, consequently, a higher risk of suffering frost drought.

Elevation modulates the phenotypic responses to light of four co-occurring Pyrenean forest tree species

Elevation markedly modified the species-specific phenotypic responses of trees to light. The effects from elevation mostly occurred at organ level (SLA) for the montane species, but at whole-plant level (allocation traits) for the subalpine ones. The subalpine species allocate more carbon to roots under closed canopies at low elevation. Contrary to expectations, most species displayed smaller specific leaf area at high elevation. The most drought-intolerant species was more severely affected by drought under shade than in open microsites. Understanding the phenotypic responses of mountain tree species to different levels of local irradiance can be of critical importance for elucidating their capacity to relocate above their current distributional limit in response to environmental changes. To evaluate whether the response of different co-occurring forest tree species to local irradiance varies when they are growing at various elevations or beyond their current distributional limit. Seedlings of four tree species (Betula pendula Roth., Pinus sylvestris L., Abies alba Mill., and Pinus uncinata Ram. ex DC.) were planted under different irradiance levels (forest understory vs natural gaps) and at various elevation (montane–subalpine ecotone and subalpine belt). After four growing seasons, 48 plants per species were excavated to assess allocational (biomass distribution) and morphological (specific leaf area) traits. Midday leaf water potential was recorded during a period of intense drought. The subalpine species (A. alba and P. uncinata) increased their allocation to the root system at low elevation and under dense canopy. We observed constant or higher SLA in all species when they develop in the subalpine belt. B. pendula was affected more severely by drought at low elevations and under shade than in open microsites. We found marked species-specific phenotypic variability of tree seedlings to increasing irradiance, with these responses modulated by the elevation at which the trees were growing.

Climate Warming Alters Age-Dependent Growth Sensitivity to Temperature in Eurasian Alpine Treelines

Treeline ecotones are considered early-warning monitors of the effects of climate warming on terrestrial ecosystems, but it is still unclear how tree growth at treeline will track the forecasted temperature rise in these cold environments. Here, we address this issue by analysing and projecting growth responses to climate on two different cold-limited alpine treelines: Pinus uncinata Ram. in the Spanish Pyrenees and Larix sibirica Ledeb. in the Russian Polar Urals. We assess radial-growth changes as a function of tree age and long-term climate variability using dendrochronology and a process-based model of tree growth. Climate‒growth relationships were compared considering young (age < 50 years) and old trees (age > 75 years) separately. Warm summer conditions enhanced radial growth, particularly after the 1980s, in the Polar Urals sites, whereas growth was positively related to warm spring and winter conditions in the Pyrenees sites. These associations were stronger in young than in old trees for both tree species and regions. Forecasted warm conditions are expected to enhance growth rates in both regions, while the growing season is forecasted to lengthen in the Pyrenees treelines, mostly in young trees. The observed age-related responses to temperature also depend on the forecasted warming rates. Although the temperature sensitivity is overall increasing for young trees, those responses seem more divergent, or even reversed, throughout the contrasting emission scenarios. The RCP 8.5 emission scenario corresponding to the most pronounced warming and drier conditions (+4.8 °C) could also amplify drought stress in young trees from the Pyrenees treelines. Our modelling approach provides accessible tools to evaluate functional thresholds for tree growth in treeline ecotones under warmer conditions.

A millennium-long perspective on high-elevation pine recruitment in the Spanish central Pyrenees

Long-term fluctuations in forest recruitment, at time scales well beyond the life-span of individual trees, can be related to climate changes. The underlying climatic drivers are, however, often understudied. Here, we present the recruitment history of a high-elevation mountain pine (Pinus uncinata Ram.) forest in the Spanish central Pyrenees throughout the last millennium. A total of 1108 ring-width series translated into a continuous chronology from 924 to 2014 CE, which allowed estimated germination dates of 470 trees to be compared against decadal-scale temperature variability. High recruitment intensity mainly coincided with relatively warm periods in the early 14th, 15th, 19th, and 20th centuries, whereas cold phases during the mid-17th, early 18th, and mid-19th centuries overlapped with generally low recruitment rates. In revealing the importance of prolonged warm conditions for high-elevation pine recruitment in the Pyrenees, this study suggests increased densification and even possible upward migration of tree-line ecotones under predicted global warming.

Bird community response in mountain pine forests of the Pyrenees managed under a shelterwood system

Abstract Understanding the effects of forest management on biodiversity is a vital challenge given the current regime of large-scale socio-ecological drivers affecting forest ecosystems and their multifunctionality. Here we assessed how forest management affects abundances of common breeding birds in mountain pine (Pinus uncinata Ram. ex DC) stands in the Pyrenees. We assessed, at guild level, avian response to changes in stand structure across different management stages in forests managed under a shelterwood system, as well as in unmanaged forests. Bird guilds were based on habitat breadth, nesting habitat, and foraging habitat. Bird abundance was modelled separately for each guild as a function of stand variables known to be good surrogates of stand density (stand density, quadratic mean diameter, shrub cover) and maturity (dominant height, cavities). For this purpose, we used likelihood methods, which provided flexibility in the shape of the expected responses. For most bird guilds, unmanaged forests showed similar bird abundance to managed forests. Total bird abundance was maximum after regeneration cuts, due to the positive response of canopy nesters and canopy foragers. The typical open stand structure after removal cuts negatively impacted forest specialists, cavity nesters and trunk foragers, but the impact was offset by the higher number of generalists, ubiquitous, ground nesters and ground foragers. General stand descriptors such as stand density, quadratic mean diameter and dominant height were the most influential variables, whereas the association of bird abundance with shrub cover and cavities was less influential and guild-specific. We show that a shelterwood system can be a suitable management tool to promote the abundance of most common bird guilds in dense, homogeneous stands, given that some key structural legacies are retained throughout the rotation and stand structure heterogeneity is promoted. By obtaining quantitative relationships between the main structural features affected by harvests and the abundance of birds, we formulate management recommendations that are valid for forests managed not only under shelterwood systems but also under other silvicultural methods.

Fire effects in Pinus uncinata Ram plantations

Aim of study: Understanding fire ecology of main forest species is essential for a sound, scientifically based on managing of wildlands and also to assess likely implications due to changes in fire regime under a global change scenario. Few references can be found about fire ecology of Pinus uncinata Ram. (PU). PU species grows in the Central Pyrenees where large, severe wildland fires did not occur frequently in the past. However, several fires with extreme fire behavior have affected PU stands in last years and they might disturb other PU forest in the future.Area of study: Cabdella fire (February 2012), in Lleida province, is one of the several wildland fires occurred in 2012 (winter season) in the Central Pyrenees. Fire affected a large PU plantation (102 ha) located at 1.800-2,100 meters above the sea.Material and methods: We have analyzed first order fire effects in three fireline intensity thresholds along three years in terms of mortality ratio, scorched height, percentage of scorched crown volume and bark char height.Main results: PU seems to be a very tolerant species to low and medium fire line intensity but fire effects were very significant when fire line intensity was high. In medium fireline intensity sites, probability of mortality ranged from 15 to 30% and the dead trees had the highest values on scorched height and percentage of scorched crown volume.Research highlights: Results from this work supports that prescribed burning might be used to efficiently decrease fuel load and fuel vertical continuity while avoiding considerable PU mortality. It also displayed that when fuel management has been implemented, PU mortality might be limited even under extreme fire behavior.Abbreviations used: PU: Pinus uncinata Ram.

Estado comparativo de las masas de <i>Pinus uncinata Ram.</i> potencialmente protectoras frente a aludes de una zona de Andorra y Cataluña

A comparative study was carried out among stands of Pinus uncinata Ram. located, respectively, in La Massana (Andorra) and in the Catalonian Pyrenees, and give these sites an important role against snow avalanches. To check if there exist some differential attributes between Andorran and Catalonian sites, a dendrometric database was generated from sampling (Andorra) and from the information provided by the Third Forest National Inventory (Catalonian Pyrenees). As for their tendency towards uneven-aged stand structure, both geographical regions are quite similar. In fact, stand regeneration is not associated to other stocking variables, and stand stocking is not closely linked to the altitudinal gradient (which little affects mortality). However, Catalonian stands have shown less stocking and, in general, they offer worse characteristics for their protective role against avalanches compared with the Andorran stands.

A broad-scale analysis of the main factors determining the current structure and understory composition of Catalonian sub-alpine (Pinus uncinata Ram.) forests

Summary A broad-scale analysis of the structure and understory composition of Pyrenean mountain pine (Pinus uncinata Ram.) stands was performed using data from the Spanish National Forest Inventory. Twelve structure-based forest typologies were defined from variables related to tree size, stand density, vertical structure and standing deadwood, using cluster analysis techniques. These typologies were adequately classified (accuracy >75 per cent) by a dichotomous key obtained from classification and regression trees. Multiple regression models were then used to analyse relationships between the main stand structural variables and a set of climatic and physiographic factors. The models showed significant correlations between winter temperature, slope and continentality (among other variables) and the current structure of mountain pine stands. The relationships between the understory composition of mountain pine forests and different environmental and structural overstory factors were found to be driven by an elevation-pH gradient and a stand density-soil stoniness gradient. The results of this study can be directly used for forest planning at different scales and could help forest managers to establish strategies designed to facilitate a given habitat for species of conservation interest.

Land‐use changes as major drivers of mountain pine (Pinus uncinata Ram.) expansion in the Pyrenees

ABSTRACTAim To assess the spatial patterns of forest expansion (encroachment and densification) for mountain pine (Pinus uncinata Ram.) during the last 50 years at a whole mountain range scale by the study of different topographic and socio‐economic potential drivers in the current context of global change.Location The study area includes the whole distributional area of mountain pine in the Catalan Pyrenees (north‐east Spain). This represents more than 80 municipalities, covering a total area of 6018 km2.Methods Forest cover was obtained by image reclassification of more than 200 pairs of aerial photographs taken in 1956 and 2006. Encroachment and densification were determined according to changes in forest cover, and were expressed as binary variables on a 150 × 150 m cell‐size grid. We then used logistic regression to analyse the effects of several topographic and socio‐economic variables on forest expansion.Results In the period analysed, mountain pine increased its surface coverage by 8898 ha (an increase of more than 16%). Mean canopy cover rose from 31.0% in 1956 to 55.6% in 2006. Most of the expansion was found on north‐facing slopes and at low altitudes. Socio‐economic factors arose as major factors in mountain pine expansion, as encroachment rates were higher in municipalities with greater population losses or weaker primary sector development.Main conclusions The spatial patterns of mountain pine expansion showed a good match with the main patterns of land‐use change in the Pyrenees, suggesting that land‐use changes have played a more important role than climate in driving forest dynamics at a landscape scale over the period studied. Further studies on forest expansion at a regional scale should incorporate patterns of land‐use changes to correctly interpret drivers of forest encroachment and densification.

Geography determines genetic relationships between species of mountain pine (Pinus mugo complex) in western Europe

AbstractAim Our aims were to test whether morphological species of mountain pines were genetically supported in the western part of the distribution range of the Pinus mugo species complex (Pinus mugo Turra sensu lato), to resolve genetically homogeneous clusters of populations, to determine historical demographic processes, and to assess the potential hybridization of mountain pines with Scots pine, Pinus sylvestris L.Location Populations were sampled in the Iberian System, the Pyrenees, the French Mont Ventoux, Vosges and Jura mountains, the German Black Forest and throughout the Alps. This corresponded to a range‐wide sampling for mountain pine sensu stricto (Pinus uncinata Ram.) and to a sampling of the western parts of the ranges of dwarf mountain pine (Pinus mugo Turra sensu stricto) and bog pine/peatbog pine [Pinus rotundata Link/Pinus × pseudopumilio (Willk.) Beck].Methods In total, 786 individuals of P. mugo sensu lato from 29 natural populations, and 85 individuals of P. sylvestris from four natural populations were genotyped at three chloroplast microsatellites (cpSSRs). Populations were characterized for standard genetic diversity statistics and signs of demographic expansion. Genetic structure was explored using analysis of molecular variance, differentiation statistics and Bayesian analysis of population structure (BAPS).Results One hundred haplotypes were identified in P. mugo sensu lato. There was a stronger differentiation between geographical regions than between morphologically identified taxa (P. mugo sensu stricto, P. uncinata and P. rotundata/P. ×pseudopumilio). Overall genetic differentiation was weak (GST = 0.070) and displayed a clear phylogeographic structure [NST = 0.263, NST > NST (permuted), P < 0.001]. BAPS identified a Pyrenean and an Alpine gene pool, along with several smaller genetic clusters corresponding to peripheral populations.Main conclusions The core regions of the Pyrenees and Alps were probably recolonized, respectively by P. uncinata and P. uncinata/P. mugo sensu stricto, from multiple glacial refugia that were well connected by pollen flow within the mountain chains. Pinus rotundata/P. × pseudopumilio populations from the Black Forest, Vosges and Jura mountains were probably recolonized from various glacial populations that kept their genetic distinctiveness despite late glacial and early Holocene expansion. Marginal P. uncinata populations from the Iberian System are compatible with elevational shifts and long‐term isolation. The causes of haplotype sharing between P. mugo sensu lato and P. sylvestris require further research.

Wind and snow damage in the Pyrenees pine forests: effect of stand attributes and location

Wind and snow-induced damage have been analyzed at stand level for three pine forests in the Central-Eastern Pyrenees (Pinus nigra Arn. salzmanii, Pinus sylvestris L. and Pinus uncinata Ram.). Stand-level models have been then developed for the most affected two species, Pinus sylvestris L. and Pinus uncinata Ram., to describe damage severity. The models were based on data from national forest inventory plots. They included variables related to the spatial location and structure of the stands, being validated using a sub-set of the database (25% of the plots randomly selected). Mountain pine forests (Pinus uncinata Ram.) were the most heavily affected by wind and snow disturbances. For both mountain and Scots pine species, topographic exposure and the severity of the local storm regime had an important effect on the degree of damage. Stand’s resistance to wind and snow was found to be dependent on the combined effect of basal area and mean slenderness of the dominant trees. For a given slenderness ratio, damage increased strongly in lower-density stands, particularly in stands with basal areas below 15 m/ha. Stand structure was particularly important to define the resistance of Scots pine stands, which presented a higher vulnerability to wind and snow under higher degree of even-agedness. The models presented in this study provide empirically-based information that can be used to implement silvicultural practices to minimize the risk of those forests to suffer wind and snow-related damages.2

Response of Pinus uncinata Recruitment to Climate Warming and Changes in Grazing Pressure in an Isolated Population of the Iberian System (NE Spain)

ABSTRACTTo infer future changes in the distribution of tree species in response to climatic variability, we need an understanding of the recruitment dynamics and their climatic controls at the species’ distribution limit. We studied the recruitment processes in an isolated population of Pinus uncinata Ram. located at the southwestern limit of the species’ distribution in Europe (Iberian System, NE Spain). We assessed (1) the temporal patterns of pine recruitment, and (2) how climate influenced recruitment. To reconstruct the recent recruitment episodes and to assess the climatic influence on recruitment and radial growth we employed dendrochronological methods. We mapped, measured the size, and estimated the age of all P. uncinata individuals located within a 50 m × 40 m plot. Additional age data were obtained from individuals located in four nearby 20 m × 20 m plots. The main episodes of tree establishment (early 1960s, late 1980s) coincided with low radial growth during a period with reduced grazing pressure. Grazing pressure and tree recruitment were not related at the spatiotemporal scale of this study. High May, August, and September minimum temperatures and high April precipitation were positively associated with recruitment, whereas high maximum April and June temperatures were negatively associated with recruitment. The studied population was in equilibrium with climate until the late 1990s, one of the warmest decades in the 20th century, when recruitment decreased despite the availability of suitable sites for establishment and the presence of reproductive individuals. We suggest that late-summer temperatures might have a non-linear negative threshold effect on recruitment rather than a linear effect. Despite increasing evidence of climate-induced recruitment episodes in isolated cold mountain forests, threshold effects of temperature on recruitment may imply limited range shifts of these populations in response to climate warming.

Structure and spatial variability of Pinus uncinata Ram. growth forms at two contrasting treeline ecotones in the Spanish Pyrenees

We describe the spatial distribution of Pinus uncinata individuals classified according to their growth-form and size at two undisturbed altitudinal treeline ecotones in the Spanish Pyrenees (Ordesa, O; Tessó, T). At each site, a rectangular plot (30 ´140 m) was located with its longest side along the altitudinal gradient, and encompassing treeline and timberline. At both sites, living individuals were bigger and had a higher number of needle cohorts downslope. The distribution of size and growth-form classes at site O followed a neat sequence of increasing size downslope, from abundant shrubby multi-stemmed krummholz individuals with few needle cohorts (1-3) up to bigger uni-stemmed arborescent trees with several needle cohorts (4-12). In contrast, structural changes through the ecotone at site T were gradual, and they did not follow so neatly this sequence of increasing size downslope.

Plant species distribution across two contrasting treeline ecotones in the Spanish Pyrenees

We describe the structure of two contrasting (elevation, topography,climate, vegetation, soil) alpine forest–pasture ecotones located in theCentral Pyrenees (sites Ordesa, O, and Tesso, T). We define ecotonestructure as the spatial distribution of trees of different size classes andgrowth-forms and the relationship between these aspects and the spatialdistribution of understory vegetation and substrate. The studied ecotones aredominated by Pinus uncinata Ram. and have been littleaffected by anthropogenic disturbances (logging, grazing) during this century.One rectangular plot (30 × 140 m) was located within eachsite with its longest side parallel to the slope and encompassing treeline andtimberline. The distribution of size and growth-form classes at site O followeda clear sequence of increasing size downslope from shrubby multistemmedkrummholz individuals to bigger arborescent trees. At site O, regeneration wasconcentrated near the krummholz area and over rocky substrates. This suggeststhat krummholz may modify microenvironment conditions and increase seedlingsurvival. At site T, regeneration was abundant above the treeline where thecover of the dominant understory shrub (Rhododendronferrugineum) decreased. In both ecotones the diversity of plants washigher above the treeline than in the forest and decreased going downslopecoinciding with the increase of P. uncinata cover. Thereduction of plant diversity appeared above the current timberline. At site O,the decrease was steep and spatially heterogeneous what may be due in part tothe edaphic heterogeneity. At site T the change was abrupt though smaller. Therelationships between the plant community and tree regeneration should be takeninto account in future ecological studies of treeline pattern.

Why are seed cones of Swiss stone pine (Pinus cembra) not attacked by the specialized pine cone weevil, Pissodes validirostris? A case of host selection vs. host suitability

AbstractThe pine cone weevil, Pissodes validirostris Gyll. (Coleoptera: Curculionidae), attacks seed cones of most Eurasian pine species, except these of Swiss stone pine (Pinus cembra L.). Behavioural responses of adult weevils to cone volatile emissions of Swiss stone pine and to those of a common host, mountain pine (Pinus uncinata Ram.), were compared in an olfactometer. Weevils were significantly attracted by the volatile blend emitted by mountain pine, but Swiss stone pine volatiles elicited an inverse response, with most weevils moving in the opposite direction to the odour source. However, the majority of second instar weevil larvae that were extracted from mountain pine cones and transferred into Swiss stone pine cones were capable of developing to the adult stage. This suggests that Swiss stone pine cones do not contain strong feeding deterrents that could prevent larval development. The possible factors involved in the absence of colonization of Swiss stone pine cones by cone weevils are discussed.

Estructura de un ecotono bosque Subalpino-pastos alpinos (Las Cutas, Ordesa, Pirineos Centrales)

We describe the structure of a subalpine forest-alpine pasture ecotone in the Central Pyrenees, that includes altitudinal timberline and treeline, and it is dominated by Pinus uncinata Ram . We measured for each P. uncinata 21individual located inside a rectangular plot parallel to the slope: the location and several structural and groivth form variables (number and type of stems per individual). Then, P. uncinata individuals were classified according to their size (adults, poles, saplings and seedlings) and growth form (krummholz —shrubby and multistemmed individuals—, krummholz with vertical stems). The ecotone structure was described through the study of groivth form and size changes of P. uncinata individuals across the ecotone and related to the area climate, that is characterized by maximum snow thickness in spring (April) and strong N-NW-W winds. Most of living individuals were krummholz, located above the timberline in the area where seedlings were abundant. Bigger, vertical and unistemmed individuals predominated in the forest. The structure or growth form changes were sharp across the ecotone. Crown injuries due to wind effects were evident in shrubby individuals and predominated in those directions that showed stronger winds during the year. The snow-wind interaction can explain partially the location of the studied timberline that could be considered a local phenomenon. The spatial location of the different classes of individuals, the spatial interaction between seedlings and krummholz individuals, and the changes of growth form (from arborescent to shrubby or vice versa) can modify the response of these P. uncinata populations to climate changes.

Spatial pattern of a subalpine forest-alpine pasture ecotone (Las Cutas, Ordesa, Central Pyrenees)

We describe the spatial pattern of a subalpine forest-alpine pasture ecotone in the Central Pyrenees, that includes altitudinal timberline and treeline, and it is dominated by Pinus uncinata Ram. A rectangular (30 x 140 m) plot was located crossing the ecotone with its longest side parallel to the slope. We measured for each P. uncinata individual inside the plot: location (coordinates x, y), and structure (e. g. height) and growth form variables (number and type —living or dead, vertical or shrubby— of stems per individual). P. uncinata individuals were classified according to their size (adults, poles, saplings and seedlings) and growth form (krummholz —shrubby and multistemmed individuals— and krummholz with vertical stems). We described quantitatively the type of substrate (bare soil, organic matter, gravel and rock) and cover of herbs, shrubs and P. uncinata using transects parallel to the slope. The ecotone structure was described through: (1) point pattern (Ripley’s K) and (2) surface pattern analyses (spatial correlograms of height); (3) the detection and description of boundaries using density, size or growth form variables; (4) the synthesis of variations of presence and diversity of substrates and herbs and shrubs; and (5) the ordination of quadrats (the plot was previously subdivided into 115 6 x 6 m quadrats), according to their spatial position in the ecotone, the type of substrate, the cover of herbs and shrubs and the number, size and growth form of P. uncinata individuals. Most P. uncinata living individuals were krummholz, located above the timberline. Krummholz individuals showed significant and positive spatial interaction with seedlings. Bigger, vertical and unistemmed individuals predominated in the lower area of the ecotone, in the forest. The change of height with increasing elevation was abrupt and masked an underlying pattern of patches of trees with similar height in the forest. The structure variables were more sensitive because they produced a greater number of boundaries. These boundaries were arranged forming a “diagonal” (in the lower and upper areas of the ecotone for big and small individuals, respectively) because of the sequential location of progressively bigger unistemmed individuals descending across the ecotone. The shrubby individuals were associated with rocky substrates, that decreased in the forest, where organic matter predominated. The snow-wind interaction can explain the location of the studied timberline that could be considered a local phenomenon. Krummholz can buffer seedlings against the harsh climatic conditions of this ecotone (strong wind, reduced snowpack, low temperature). The spatial location of the different classes of individuals, the spatial interaction between seedlings and krummholz individuals, and changes of growth form (from shrubby to vertical growth form or vice versa) can cause some inertia in the response of ecotone P. uncinata populations to environmental changes.

Structure and recent recruitment at alpine forest-pasture ecotones in the Spanish central Pyrenees

Treeline ecotones are intensively studied to quantify the response of vegetation to environmental changes. We describe here the size, growth-form, and spatial distribution of trees in two alpine forest-pasture ecotones located in the Central Pyrenees (Ordesa and Tessó), dominated by Pinus uncinata Ram. and little affected by anthropogenic disturbances during the last century. Total variation of tree data was partitioned into spatial, environmental, combined, and unexplained components. The influence of understory plants on P. uncinata recruitment was assessed using cover data and detrended canonical correspondence analysis (DCCA). Climatic effects on recent recruitment were also investigated at Tessó. Most individuals at Ordesa were multistemmed and shrubby (krummholz). Krummholz and seedlings showed significant spatial correlation. The spatial component of variation of tree data was greater at Ordesa (26%) than at Tessó (4%). DCCA revealed a significant effect of elevation on the spatial segregation of the size and growth-form classes across both ecotones. At Tessó, recruitment was concentrated close to the treeline, where the cover of the dominant understory shrub (Rhododendron ferrugineum) decreased. The abundance of young individuals in the upper part of this ecotone might produce a future upward displacement of treeline. During the last 60 years, the important episodes of recruitment at Tessó were favoured by wet and warm springs and summers. Ordesa showed a clear spatial trend of distribution of P. uncinata individuals. Most individuals were krummholz, located at greater elevation. Recruitment was associated with krummholz type vegetation, which could buffer seedlings (nurse effect) from the harsh environment and thereby favour the establishment of seedlings.

Seasonal growth and needle fall in Pinus sylvestris and P. uncinata

In this work, we study monthly development of tree-ring, elongation of branches, needle fall for Pinus sylvestris L., Pinus uncinata Ram. in the Spanish Central Pyrenees during 1993. Tree-ring formation happens from early June to late September, but most tree-ring growth happens from mid June to mid July. P. sylvestris starts its tree-ring development before P. uncinata. P. uncinata begins its growth of shoots earlier than its tree-ring growth. Needle fall reaches its maximum in autumn. P. uncinata shows a delayed maximum rate of needle fall, a longer period of fall of most of its needles compared to P. sylvestris. This delay, the greater needle dry weight, and the smaller average radial diameter of P. uncinata tracheids compared with P. sylvestris, show P. uncinata adaptations to environments with more stress.