Evidence that the Montseny mountains are still a good climatic refugium for the southernmost silver fir forest on the Iberian Peninsula

Many European temperate tree species reach their southern distribution limits in the Mediterranean region, and ongoing climate change will further restrict their climatic niche in this area. In this study, we investigated the effects of forest management and climate change on tree growth and the spatial extension of a silver fir forest (Abies alba Mill.) located at the species' southern distribution limit on the Iberian Peninsula (Montseny Mountains Natural Park, Spain). Different growth variables such as tree-ring width (RW), basal area increment (BAI), earlywood width (EwW) and latewood width (LwW) were assessed, and climate-growth relationships were established for the period 1914-2010. Our results revealed that the main growth reductions and releases in the raw tree-ring width series were related to both volcanic activity and intensive logging. Since the establishment of the Natural Park in 1977, RW series have levelled off, and this has translated into an increase in BAI. This positive performance may have also facilitated the spatial expansion of the stand. Low precipitation during spring and summer was found to be the most limiting factor for tree growth during the period 1914-2010. Temperature had only a minor influence on tree growth. LwW was the growth variable most sensitive to climatic conditions. Such sensitivity explained the decreasing LwW trend since 1975. In contrast, EwW mostly depended on the previous year's climatic conditions, and was not climatically limited during the growing season, resulting in an increasing trend over the study period. However, the temporal instability of most of these climate-growth relations indicated that climate change might have been beneficial for tree performance. Past logging events have fostered tree growth in the stand due to the increase in the availability of water, light, and nutrients, potentially alleviating the negative impacts of climate change. Furthermore, it is possible that the increase in the EwW improved water transport in the silver firs, which may also have helped them to endure ongoing climate change. Therefore, it is crucial to assess the role of forest management, as well as the potential acclimation of the tree species when considering the effects of climate change

How to introduce connectance in a frame of an expression for diversity

Systems made of parts that are totally connected do not work, neither ecosys- tems nor artifacts. Relative connectance is inversely related to diversity, and both magnitudes can find a common frame of expression, in which some constant expressing the constraints of any organization might be embodied. If S is Simp- son's index, the expression (1 - S)IS as a measure of diversity offers some advantages or, at least, helps further reasoning. Such expression is the ratio between total interspecific possible interactions and possible intraspecific inter- actions

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

Describimos la estructura de un ecotono bosque subalpino-pastos alpinos en los Pirineos Centrales, que incluye los límites altitudinales del bosque y del árbol y está dominado por Pinus uncinata Ram. Para cada individuo de P. uncinata situado dentro de una parcela rectangular paralela a la pendiente anotamos su localización y medimos diversas variables estructurales o deforma (número y tipo de pies por individuo). Estos individuos fueron clasificados según su tamaño (adultos, jóvenes, vastagos y plántulas) y forma («krummholz» ¿individuos arbustivos policórmicos¿, krummholz con pies verticales). La estructura del ecotono se describió mediante los cambios de tamaño y forma de los individuos de P. uncinata a lo largo del ecotono y en relación con el clima de la zona, que se caracteriza por espesores de nieve máximos en primavera (abril) y fuertes vientos del N-NW-W. La mayoría de los individuos vivos eran krummholz, situándose por encima del límite del bosque y mostrando proximidad espacial con las plántulas. Los individuos grandes, verticales y unicórmicos predominaban en el bosque. Los cambios estructurales o deforma de los individuos eran bruscos a lo largo del ecotono. Los daños de las copas debidos al viento eran evidentes en individuos arbustivos y predominaban en las direcciones de viento más fuertes durante todo el año. La interacción nieve-viento permite explicar en parte la localización de este límite del bosque que puede considerarse un fenómeno local. La situación espacial de las distintas clases de individuos, la interacción espacial entre plántulas y krummholz más los cambios deforma de crecimiento (de arbustiva a arbórea o viceversa) pueden modificar la respuesta de estas poblaciones de P uncinata a cambios climáticos

Dendrocronología en bosques neotropicales secos: métodos, avances y aplicaciones.

Los bosques neotropicales secos (BTSs) se caracterizan por una sequía marcada que permite la formación de anillos anuales de crecimiento en diversas especies de árboles. La aplicación de la dendrocronología en los BTSs requiere identificar, datar y medir los anillos de crecimiento. Esto permite tener una visión retrospectiva del crecimiento de los árboles a una escala temporal acorde con su longevidad. Para tal fin, es necesario tener un buen conocimiento de la anatomía de la madera. Por ejemplo, la mayoría de las especies de angiospermas utilizadas en estudios dendrocronológicos en BTSs presentan anillos de crecimiento delimitados por una banda de parénquima marginal. La información que proporcionan los anillos de crecimiento ha permitido: (i) cuantificar cómo el crecimiento de las especies de árboles de los BTSs responde a las variables climáticas (principalmente a la precipitación) y determinar cómo está relacionado con patrones atmosféricos a gran escala (El Niño-Oscilación del Sur) (dendroclimatología), y (ii) reconstruir y comprender diversos aspectos fundamentales de la historia de vida de las especies arbóreas de los BTSs (dendroecología), permitiendo conocer la edad y las tasas de crecimiento. La dendroecología es una herramienta útil que permitirá a los gestores forestales cuantificar o proyectar ciclos de corta específicos para cada especie arbórea, lo que contribuirá a un manejo sostenible de los BTSs

Time-dependent effects of climate and drought on tree growth in a Neotropical dry forest: short-term tolerance vs. long-term sensitivity

A seasonal period of water deficit characterizes tropical dry forests (TDFs). There, sympatric tree species exhibit a diversity of growth rates, functional traits, and responses to drought, suggesting that each species may possess different strategies to grow under different conditions of water availability. The evaluation of the long-term growth responses to changes in the soil water balance should provide an understanding of how and when coexisting tree species respond to water deficit in TDFs. Furthermore, such differential growth responses may be linked to functional traits related to water storage and conductance. We used dendrochronology and climate data to retrospectively assess how the radial growth of seven coexisting deciduous tree species responded to the seasonal soil water balance in a Bolivian TDF. Linear mixed-effects models were used to quantify the relationships between basal area increment and seasonal water balance. We related these relationships with wood density and sapwood production to assess if they affect the growth responses to climate. The growth of all species responded positively to water balance during the wet season, but such responses differed among species as a function of their wood density. For instance, species with a strong growth response to water availability averaged a low wood density which may facilitate the storage of water in the stem. By contrast, species with very dense wood were those whose growth was less sensitive to water availability. Coexisting tree species thus show differential growth responses to changes in soil water balance during the wet season. Our findings also provide a link between wood density, a trait related to the ability of trees to store water in the stem, and wood formation in response to water availability

Disentangling the formation of contrasting tree line physiognomies combining model selection and Bayesian parameterization for simulation models.

Alpine tree-line ecotones are characterized by marked changes at small spatial scales that may result in a variety of physiognomies. A set of alternative individual-based models was tested with data from four contrasting Pinus uncinata ecotones in the central Spanish Pyrenees to reveal the minimal subset of processes required for tree-line formation. A Bayesian approach combined with Markov chain Monte Carlo methods was employed to obtain the posterior distribution of model parameters, allowing the use of model selection procedures. The main features of real tree lines emerged only in models considering nonlinear responses in individual rates of growth or mortality with respect to the altitudinal gradient. Variation in tree-line physiognomy reflected mainly changes in the relative importance of these nonlinear responses, while other processes, such as dispersal limitation and facilitation, played a secondary role. Different nonlinear responses also determined the presence or absence of krummholz, in agreement with recent findings highlighting a different response of diffuse and abrupt or krummholz tree lines to climate change. The method presented here can be widely applied in individual-based simulation models and will turn model selection and evaluation in this type of models into a more transparent, effective, and efficient exercise

Disparate effects of global-change drivers on mountain conifer forests: warming-induced growth enhancement in young trees vs. CO2 fertilization in old trees from wet sites

Theory predicts that the postindustrial rise in the concentration of CO2 in the atmosphere (ca ) should enhance tree growth either through a direct fertilization effect or indirectly by improving water use efficiency in dry areas. However, this hypothesis has received little support in cold-limited and subalpine forests where positive growth responses to either rising ca or warmer temperatures are still under debate. In this study, we address this issue by analyzing an extensive dendrochronological network of high-elevation Pinus uncinata forests in Spain (28 sites, 544 trees) encompassing the whole biogeographical extent of the species. We determine if the basal area increment (BAI) trends are linked to climate warming and increased ca by focusing on region- and age-dependent responses. The largest improvement in BAI over the past six centuries occurred during the last 150 years affecting young trees and being driven by recent warming. Indeed, most studied regions and age classes presented BAI patterns mainly controlled by temperature trends, while growing-season precipitation was only relevant in the driest sites. Growth enhancement was linked to rising ca in mature (151-300 year-old trees) and old-mature trees (301-450 year-old trees) from the wettest sites only. This finding implies that any potential fertilization effect of elevated ca on forest growth is contingent on tree features that vary with ontogeny and it depends on site conditions (for instance water availability). Furthermore, we found widespread growth decline in drought-prone sites probably indicating that the rise in ca did not compensate for the reduction in water availability. Thus, warming-triggered drought stress may become a more important direct driver of growth than rising ca in similar subalpine forests. We argue that broad approaches in biogeographical and temporal terms are required to adequately evaluate any effect of rising ca on forest growth

Hydrological evidence for a North Atlantic oscillation during the Little Ice Age outside its range observed since 1850.

An annual-resolved precipitation reconstruction for the last 800 yr in Southern Spain has been performed using stable carbon isotope (δ13C) of Pinus nigra tree rings. The reconstruction exhibits high- to low-frequency variability and distinguishes a Little Ice Age (LIA, AD 13501850) characterized by lower averaged rainfall than both in the transition from the Medieval Climate Anomaly to the LIA and in the 20th century. The driest conditions are recorded during the Maunder solar Minimum (mid 17thearly 18th centuries), in good agreement with the Spanish documentary archive. Similar linkage between solar activity (maximum/minimum) and precipitation (increase/decrease) is observed throughout the entire LIA. Additionally, the relationship between the hydrological pattern in the Iberian Peninsula and Morocco during the LIA suggests different spatial distribution of precipitation in the south-eastern sector of the North Atlantic region such as it is known currently. Whereas in the instrumental record the precipitation evolves similarly in both regions and opposite to the North Atlantic oscillation (NAO) index, the coldest periods of the LIA shows a contrasting pattern with drier conditions in the South of Spain and wetter in Northern Africa. We suggest an extreme negative NAO conditions, accompanied by a southward excursion of the winter rainfall band beyond that observed in the last century, can explain this contrast. The sustained NAO conditions could have been triggered by solar minima and higher volcanic activity during the LIA

Changes in surface solar radiation in Northeastern Spain over the past six centuries recorded by tree-ring δ13C.

Although solar radiation at the surface plays a determinant role in carbon discrimination in tree rings, stable carbon isotope chronologies (δ13C) have often been interpreted as a temperature proxy due to the co-variability of temperature and surface solar radiation. Furthermore, even when surface solar radiation is assumed to be the main driver of 13C discrimination in tree rings, δ13C records have been calibrated against sunshine duration or cloud cover series for which longer observational records exists. In this study, we use different instrumental and satellite data over northeast Spain (southern Europe) to identify the main driver of tree-ring 13C discrimination in this region. Special attention is paid to periods in which the co-variability of those climate variables may have been weaker, such as years after large volcanic eruptions. The analysis identified surface solar radiation as the main driver of tree-ring δ13C changes in this region, although the influence of other climatic factors may not be negligible. Accordingly, we suggest that a reconstruction of SSR over the last 600 years is possible. The relation between multidecadal variations of an independent temperature reconstruction and surface solar radiation in this region shows no clear sign, and warmer (colder) periods may be accompanied by both higher and lower surface solar radiation. However, our reconstructed records of surface solar radiation reveals a sunnier Little Ice Age in agreement with other δ13C tree-ring series used to reconstruct sunshine duration in central and northern Europe

Increasing drought effects on five European pines modulate Δ13C-growth coupling along a Mediterranean altitudinal gradient.

Climate warming increases vulnerability to drought in Mediterranean water-limited forests. However, we still lack knowledge of the long-term physiological responses of coexisting pine species in these forests regarding their ability to cope with warming-induced drought stress. We investigated spatiotemporal patterns of tree performance for five isohydric pines with partially overlapping ecological niches in the eastern Iberian Peninsula along an altitudinal gradient: Pinus halepensis = P. pinaster ≤ P. nigra ≤ P. sylvestris ≤ P. uncinata. Using indexed tree-ring widths (TRWi) we assessed changes in temporal coherence of radial growth (growth synchrony, âC) over the period 1902-2011 across three elevation belts: low ≈ 1100 m; mid = 1615 m; high = 2020 m. We also examined by mixed modelling whether TRWi showed an increased coupling with leaf-level gas exchange (inferred from indexed carbon isotope discrimination, Δ13Ci) by enhanced stomatal regulation in response to an amplified regional drought stress. Increasingly negative annual water balances (decrease in annual precipitation minus evapotranspiration = -4.8 mm year−1; 1970-2011) prompted more synchronous growth of coexisting pines between low- and mid-elevation belts, with âC rising from 0.25 ± 0.04 (1902-1951) to 0.62 ± 0.05 (1962-2011). This effect was coupled with tighter stomatal regulation at mid-elevation as indicated by high correlations between TRWi and Δ13Ci (>0.60 from the mid-1970s onwards) which resembled those found at low-elevation. Simultaneously, TRWi vs. Δ13Ci uncoupling occurred at the high-elevation belt across species. Weaker growth-climate relationships as elevation increased highlighted the major role of the altitude-dependent thermal gradient in growth responsiveness to drought; however, an intensified Δ13Ci response to spring water availability across elevation belts observed from mid-1970s onwards suggested regional shifts in tree physiological activity linked to earlier seasonal drought impacts. Warming-induced drought stress is spreading to higher altitudes in Iberian pinewoods as multispecies growth is linked to progressively tighter stomatal control of water losses reflected in wood Δ13C