Guillermo Gea-Izquierdo

Climate-growth variability in Quercus ilex L. west Iberian open woodlands of different stand density

Abstract• We present the longest tree-ring chronology (141 y) of Quercus ilex L. (holm oak), and discuss the species climate-growth relationships and the influence of stand density on tree sensitivity to climate.• Similarly to Quercus suber L., the most influential climatic variables upon holm oak growth were late spring and early summer precipitation, which enhanced growth, and high temperatures in the previous August and current July, which negatively affected growth.• High density stands responded to similar climatic factors as low density stands, but their response was generally weaker. Holm oak sensitivity to climate has increased in recent decades, which might be related to increasing temperatures in the region. Sensitivity was higher in low density stands. Additionally, the effect of summer stress on growth seems to have increased during the same period, similarly to other species in the Iberian Peninsula, suggesting that trees are more vulnerable to climatic changes.• Stand density could buffer the response to climate by smoothing climatic extremes. Nevertheless, the effect of competition might reverse this positive effect at the individual tree level. Precautions should be taken before providing management guidelines regarding the effect of climate change and stand density on holm oak.Résumé• Nous présentons la plus longue chronologie de cernes (141 ans) de Quercus ilex L. (chêne vert) comme et nous discutons les relations climat-croissance chez cette espèce et l’influence de la densité du peuplement sur la sensibilité des arbres au climat.• De façon similaire à Quercus suber L., les variables climatiques les plus influentes sur la croissance du chêne vert ont été les précipitations de la fin du printemps et du début de l’été, qui ont augmenté la croissance, et les températures élevées d’août de l’année précédente et du mois de juillet de l’année, qui ont affecté négativement la croissance.• Les peuplements de densité élevé ont répondu à des facteurs climatiques similaires que les peuplements de faible densité mais leur réponse a été généralement plus faible. La sensibilité du chêne vert au climat a augmentée au cours des dernières décennies, elle pourrait être liée à l’augmentation des températures dans la région. Cette sensibilité est plus élevée dans les peuplements de faible densité. En outre, le effet du stress estival sur la croissance semble avoir augmenté au cours de la même période, de la même façon que pour d’autres espèces dans la péninsule Ibérique, ce qui suggère que les arbres sont plus vulnérables aux changements climatiques.• La densité du peuplement pourrait amortir la réponse au climat en lissant les extrêmes climatiques. Néanmoins, les effets de la concurrence pourraient inverser cet effet positif au niveau des arbres individuels. Des précautions doivent être prises avant de fournir, pour le chêne vert, des lignes directrices de gestion concernant les effets du changement climatique et de la densité.

Stand Competition Determines How Different Tree Species Will Cope with a Warming Climate

Plant-plant interactions influence how forests cope with climate and contribute to modulate species response to future climate scenarios. We analysed the functional relationships between growth, climate and competition for Pinus sylvestris, Quercus pyrenaica and Quercus faginea to investigate how stand competition modifies forest sensitivity to climate and simulated how annual growth rates of these species with different drought tolerance would change throughout the 21st century. Dendroecological data from stands subjected to thinning were modelled using a novel multiplicative nonlinear approach to overcome biases related to the general assumption of a linear relationship between covariates and to better mimic the biological relationships involved. Growth always decreased exponentially with increasing competition, which explained more growth variability than climate in Q. faginea and P. sylvestris. The effect of precipitation was asymptotic in all cases, while the relationship between growth and temperature reached an optimum after which growth declined with warmer temperatures. Our growth projections indicate that the less drought-tolerant P. sylvestris would be more negatively affected by climate change than the studied sub-Mediterranean oaks. Q. faginea and P. sylvestris mean growth would decrease under all the climate change scenarios assessed. However, P. sylvestris growth would decline regardless of the competition level, whereas this decrease would be offset by reduced competition in Q. faginea. Conversely, Q. pyrenaica growth would remain similar to current rates, except for the warmest scenario. Our models shed light on the nature of the species-specific interaction between climate and competition and yield important implications for management. Assuming that individual growth is directly related to tree performance, trees under low competition would better withstand the warmer conditions predicted under climate change scenarios but in a variable manner depending on the species. Thinning following an exponential rule may be desirable to ensure long-term conservation of high-density Mediterranean woodlands, particularly in drought-limited sites.

Modeling acclimation of photosynthesis to temperature in evergreen conifer forests.

• In this study, we used a canopy photosynthesis model which describes changes in photosynthetic capacity with slow temperature-dependent acclimations. • A flux-partitioning algorithm was applied to fit the photosynthesis model to net ecosystem exchange data for 12 evergreen coniferous forests from northern temperate and boreal regions. • The model accounted for much of the variation in photosynthetic production, with modeling efficiencies (mean > 67%) similar to those of more complex models. The parameter describing the rate of acclimation was larger at the northern sites, leading to a slower acclimation of photosynthesis to temperature. The response of the rates of photosynthesis to air temperature in spring was delayed up to several days at the coldest sites. Overall photosynthesis acclimation processes were slower at colder, northern locations than at warmer, more southern, and more maritime sites. • Consequently, slow changes in photosynthetic capacity were essential to explaining variations of photosynthesis for colder boreal forests (i.e. where acclimation of photosynthesis to temperature was slower), whereas the importance of these processes was minor in warmer conifer evergreen forests.

How do trees affect spatio-temporal heterogeneity of nutrient cycling in mediterranean annual grasslands?

Abstract• In this study we analyzed heterogeneity in nutrient cycling induced by trees in Mediterranean annual grasslands, comparing years of higher and lower than average precipitation and analyzing the effects of two different solar radiation scenarios.• Organic matter and consequently upper soil N, K, Ca and Mg were significantly greater in those locations receiving the highest levels of solar radiation, and as expected from many other studies in the literature, there was an increase in all macronutrients (except P) as well as pH below the canopy.• Contrary to what was expected, plant nutrient concentrations did not directly reflect those found in the soil, with the exception of K. The studied grassland responded to increased nutrient availability by enhancing growth and changing botanical composition rather than by increasing plant nutrient concentrations. Hence, the total amount of accumulated nutrients in the ecosystem was larger below the tree than outside it, although this is mainly a consequence of plant growth enhancement. The levels of Ca, Mg, and Na in plants decreased during the driest year, and the N content was mostly determined by the composition of the grass.• Temporal nutrient variability, particularly within-years, explained most of the variability in plant nutrient concentration, while spatial variability induced by trees was determined to be of secondary importance. These results are significant for ecosystem nutrient modelling.Résumé• Dans cette étude, nous avons analysé l’hétérogénéité du cycle des nutriments induite par les arbres dans les prairies méditerranéennes annuelles, en comparant les années à précipitations supérieures et inférieures à la moyenne et en analysant les effets de deux scénarios de rayonnements solaires différents.• La matière organique et par conséquent N, K, Ca et Mg des horizons supérieurs du sol étaient significativement plus élevés dans les stations recevant le plus de rayonnement solaire. Comme prévu à partir de nombreuses autres études dans la littérature, une augmentation de tous les macronutriments (sauf P) et aussi du pH a été observée sous la canopée des arbres.• Contrairement à ce qui était prévu, les concentrations de nutriments dans les végétaux n’ont pas reflété directement celles trouvées dans le sol, à l’exception de K. Les prairies étudiées ont répondu à un accroissement de la disponibilité en éléments nutritifs par une augmentation de la croissance et un changement de la composition botanique plutôt que par une augmentation des concentrations de nutriments dans les plantes. Par conséquent, les quantités totales de nutriments accumulées dans l’écosystème étaient plus importantes sous les arbres qu’en plein découvert, bien que cela soit principalement une conséquence de l’amélioration de la croissance des plantes. Les niveaux de Ca, Mg, et Na dans les plantes ont diminué au cours de l’année la plus sèche et la teneur en nutriments azotés était largement déterminée par la composition de l’herbe.• La variabilité temporelle intra-annuelle des éléments nutritifs expliquait la plus forte part de la variabilité de la concentration en nutriments des plantes annuelles, tandis que la variabilité spatiale induite par les arbres était d’importance secondaire. Ces résultats sont importants pour la modélisation des éléments nutritifs dans les écosystèmes.

Process models and model-data fusion in dendroecology

Dendrochronology (i.e. the study of annually dated tree-ring time series) has proved to be a powerful technique to understand tree-growth. This paper intends to show the interest of using ecophysiological modeling not only to understand and predict tree-growth (dendroecology) but also to reconstruct past climates (dendroclimatology). Process models have been used for several decades in dendroclimatology, but it is only recently that methods of model-data fusion have led to significant progress in modeling tree-growth as a function of climate and in reconstructing past climates. These model-data fusion (MDF) methods, mainly based on the Bayesian paradigm, have been shown to be powerful for both model calibration and model inversion. After a rapid survey of tree-growth modeling, we illustrate MDF with examples based on series of Southern France Aleppo pines and Central France oaks. These examples show that if plants experienced CO2 fertilization, this would have a significant effect on tree-growth which in turn would bias the climate reconstructions. This bias could be extended to other environmental non-climatic factors directly or indirectly affecting annual ring formation and not taken into account in classical empirical models, which supports the use of more complex process-based models. Finally, we conclude by showing the interest of the data assimilation methods applied in climatology to produce climate re-analyses.

Disentangling the effect of competition, CO2 and climate on intrinsic water‐use efficiency and tree growth

Climate change scenarios forecast rising temperatures for the Mediterranean region, which could enhance the vulnerability to drought stress in forest ecosystems. The long‐term effects of climate forcing on tree performance can be, however, modulated by other environmental factors, such as competition and rising atmospheric CO₂ concentrations. We assessed the concomitant effect of competition, climate and CO₂ concentrations on the tree‐ring δ¹³C‐derived intrinsic water‐use efficiency (iWUE) and basal area increments (BAI) of species with different drought tolerance: two Mediterranean deciduous species (Quercus faginea Lam. and Quercus pyrenaica Willd.) and one conifer (Pinus sylvestris L.). Additionally, given that competition may be managed to mitigate the effect of increasing drought stress, we further examined the influence of this variable on iWUE and growth using data compiled from the literature, providing the first review on the response of iWUE to competition. Competition had no significant effect on iWUE in any of the three species studied, whereas, as expected, growth rates were significantly higher under low‐competition levels. This was consistent with the literature review, which showed that shifts in iWUE with competition changes are rare; supporting the hypothesis that leaf‐level gas exchange tends to be a homeostatic trait. In the long term, the three species exhibited a significant increasing trend in iWUE due to the combined effect of increased CO₂ concentration, climate and age. Growth, however, was mostly affected by competition and climate and in most cases was not enhanced as a result of the increase in iWUE. Synthesis. Regardless of their functional response to drought, trees respond to reduced competition through the structural shifts such as increased radial growth rather than leaf‐level gas exchange adjustments. CO₂ and climate are, therefore, the main drivers of iWUE variability, rather than competition. Thus, if temperature‐induced drought becomes limiting, reducing competition for resources may not offset the detrimental effect of increasing drought stress on tree physiology and growth decline may occur without a CO₂ fertilization effect.

Evidence of vulnerability segmentation in a deciduous Mediterranean oak (Quercus subpyrenaica E. H. del Villar)

Key messageThe premature leaf withering inQuercus subpyrenaicaduring the especially dry summers of 2011 and 2012 is interpreted as a mechanism for protecting the stem from xylem cavitation.AbstractThe Mediterranean basin has been proposed as an area especially vulnerable to the effect of global climate change, with a greater probability of extreme drought events which can trigger massive forest decline episodes. The impact of extreme droughts on tree mortality in these territories can be magnified by several local factors, such as soil depth, which can predispose some species or individuals to be the damaged. During the summer of 2011 and 2012 many individuals of the deciduous Q. subpyrenaica in the Spanish Pre-Pyrenees showed a premature leaf withering in patches interspersed with non-affected areas. In this study, we have demonstrated that during 2011 and 2012 an extreme drought episode coincided with this phenomenon. The ultimate mechanism explaining the premature withering in Q. subpyrenaica was the hydraulic failure in the vascular system of the leaves in those trees growing in the poorest soil conditions. This mechanism, identified as an example of vulnerability segmentation, preserved the integrity of the stems and buds and allowed the new foliage production during the following spring. As a negative consequence, this drought avoidance strategy shortens the vegetative period, which may threaten the long-term growth and survival of these trees.

Is this the end? Dynamics of a relict stand from pervasively deforested ancient Iberian pine forests

Abstract A drier climate together with intense management can be detrimental for species when they are found at their xeric limit. We analyzed the dynamics of Pinus sylvestris in Central Spain in relation to colonization by more drought-tolerant Quercus pyrenaica. The studied forest presents high biodiversity and is one of the last stands relict from a widespread pine-dominated forest in West-Central Iberian Mountains demised by human deforestation. The observed age patterns could suggest a natural regeneration origin of the current stand for both species. However, while oaks regenerated continuously since the 1950s, there was almost no pine regeneration after the 1870s. Therefore, the lack of pine regeneration was previous to recent climatic changes. Pine stands with ongoing oak colonization were likely thinned in the 1920s in opposition to pure pine stands. Mixed and pure stands expressed certain differences in their response to climate. Pines suffered more from high temperatures from spring to fall, which would reflect their lower tolerance to drought than oaks. Cross-wavelet analysis showed that pine exhibited an increase in their sensitivity to drought intensity in the last years. However, the dominant pine canopy established in the nineteenth century does not show symptoms of growth decline in response to climate change. The factors determining the disruption of pine regeneration need to be determined. Management could have played a dominant role constraining stand dynamics, threatening pine sustainability through modifications of the understory vegetation and soil properties.

Response to the interaction of thinning and pruning of pine species in Mediterranean mountains

Pruning allows knot-free timber to be obtained, thereby increasing the value of the highest-value wood products. However, the effect of pruning on growth is under discussion, and knowledge about the tree response to the simultaneous development of thinning and pruning is scarce. The objective of this study was to analyze the effect of the interaction of thinning and pruning on tree and stand level and the annual radial growth of two pine species native to Mediterranean mountains. We used long-term data of three trials installed in pine stands where several combinations of pruning and thinning were developed. Five inventories were carried out for each trial, and the mean dasometric features of the different treatments were compared using linear mixed models including a competition index. In addition, we collected cores from ten trees per plot in order to evaluate the annual response of trees to the thinning and pruning. We analyzed the annual radial growth using a semiparametric approach through a smooth penalized spline including rainfall and temperature covariates. Pruning did not show any effect on growth. However, larger diameter and increased annual radial growth were found in thinned plots, both with and without pruning, as compared to unthinned plots. Also, we found significant effects of climate on annual radial growth. We recommend the application of thinning and pruning in stands of Mediterranean mountains in order to get knot-free timber since growth reduction was not found in thinned stands.

Effects of competition, drought stress and photosynthetic productivity on the radial growth of white spruce in western Canada

Understanding the complex interactions of competition, climate warming-induced drought stress, and photosynthetic productivity on the radial growth of trees is central to linking climate change impacts on tree growth, stand structure and in general, forest productivity. Using a mixed modeling approach, a stand-level photosynthetic production model, climate, stand competition and tree-ring data from mixedwood stands in western Canada, we investigated the radial growth response of white spruce [Picea glauca (Moench.) Voss] to simulated annual photosynthetic production, simulated drought stress, and tree and stand level competition. The long-term (~80-year) radial growth of white spruce was constrained mostly by competition, as measured by total basal area, with minor effects from drought. There was no relation of competition and drought on tree growth but dominant trees increased their growth more strongly to increases in modeled photosynthetic productivity, indicating asymmetric competition. Our results indicate a co-limitation of drought and climatic factors inhibiting photosynthetic productivity for radial growth of white spruce in western Canada. These results illustrate how a modeling approach can separate the complex factors regulating both multi-decadal average radial growth and interannual radial growth variations of white spruce, and contribute to advance our understanding on sustainable management of mixedwood boreal forests in western Canada.