Miguel Ángel Saz

Plasticity in Dendroclimatic Response across the Distribution Range of Aleppo Pine (Pinus halepensis)

We investigated the variability of the climate-growth relationship of Aleppo pine across its distribution range in the Mediterranean Basin. We constructed a network of tree-ring index chronologies from 63 sites across the region. Correlation function analysis identified the relationships of tree-ring index to climate factors for each site. We also estimated the dominant climatic gradients of the region using principal component analysis of monthly, seasonal, and annual mean temperature and total precipitation from 1,068 climatic gridpoints. Variation in ring width index was primarily related to precipitation and secondarily to temperature. However, we found that the dendroclimatic relationship depended on the position of the site along the climatic gradient. In the southern part of the distribution range, where temperature was generally higher and precipitation lower than the regional average, reduced growth was also associated with warm and dry conditions. In the northern part, where the average temperature was lower and the precipitation more abundant than the regional average, reduced growth was associated with cool conditions. Thus, our study highlights the substantial plasticity of Aleppo pine in response to different climatic conditions. These results do not resolve the source of response variability as being due to either genetic variation in provenance, to phenotypic plasticity, or a combination of factors. However, as current growth responses to inter-annual climate variability vary spatially across existing climate gradients, future climate-growth relationships will also likely be determined by differential adaptation and/or acclimation responses to spatial climatic variation. The contribution of local adaptation and/or phenotypic plasticity across populations to the persistence of species under global warming could be decisive for prediction of climate change impacts across populations. In this sense, a more complex forest dynamics modeling approach that includes the contribution of genetic variation and phenotypic plasticity can improve the reliability of the ecological inferences derived from the climate-growth relationships.

Cell‐level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (gm ), which is the most limiting factor for area-based photosynthesis (AN ) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar AN to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (Sm /S and Sc /S), may offset the restrictions imposed by high LMA in gm and AN in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in AN that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in gm , the most limiting factor for carbon assimilation in these species. Species with higher AN showed increased Sc /S, which implies increased gm without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach AN values comparable to congeneric deciduous species despite their higher LMA.

Missing Rings in Pinus halepensis - The Missing Link to Relate the Tree-Ring Record to Extreme Climatic Events.

Climate predictions for the Mediterranean Basin include increased temperatures, decreased precipitation, and increased frequency of extreme climatic events (ECE). These conditions are associated with decreased tree growth and increased vulnerability to pests and diseases. The anatomy of tree rings responds to these environmental conditions. Quantitatively, the width of a tree ring is largely determined by the rate and duration of cell division by the vascular cambium. In the Mediterranean climate, this division may occur throughout almost the entire year. Alternatively, cell division may cease during relatively cool and dry winters, only to resume in the same calendar year with milder temperatures and increased availability of water. Under particularly adverse conditions, no xylem may be produced in parts of the stem, resulting in a missing ring (MR). A dendrochronological network of Pinus halepensis was used to determine the relationship of MR to ECE. The network consisted of 113 sites, 1,509 trees, 2,593 cores, and 225,428 tree rings throughout the distribution range of the species. A total of 4,150 MR were identified. Binomial logistic regression analysis determined that MR frequency increased with increased cambial age. Spatial analysis indicated that the geographic areas of south-eastern Spain and northern Algeria contained the greatest frequency of MR. Dendroclimatic regression analysis indicated a non-linear relationship of MR to total monthly precipitation and mean temperature. MR are strongly associated with the combination of monthly mean temperature from previous October till current February and total precipitation from previous September till current May. They are likely to occur with total precipitation lower than 50 mm and temperatures higher than 5°C. This conclusion is global and can be applied to every site across the distribution area. Rather than simply being a complication for dendrochronology, MR formation is a fundamental response of trees to adverse environmental conditions. The demonstrated relationship of MR formation to ECE across this dendrochronological network in the Mediterranean basin shows the potential of MR analysis to reconstruct the history of past climatic extremes and to predict future forest dynamics in a changing climate.

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.

Water Resources and Precipitation Trends in Aragon

This work analyses the trend of annual precipitation in Aragon during the second half of the tentieth century, the spatial differences that this trend might entail in the Autonomous Community, both on an annual and a seasonal scale and the impact on water resources and their management. The regional series (1950–2000) presents drier and more humid phases and a slight negative trend which is not significant from the sixties onwards. The Zaragoza, Huesca and Teruel observatories follow a similar evolution. As regards to space, no significant trends can be observed in the annual totals of any Aragonese county; but instead on a seasonal scale there are some differences: winter shows a precipitation decreasing gradient from south-easat to north-west, opposite to the one produced in summer. Autumn has a clear north–south decreasing gradient, whereas spring does not offer clearly defined patterns. To be able to face the possible adverse effects coming from the rainfall decrease, efficient water policy actions are in need of a high degree of planning and rational water management.

Summer drought reconstruction in northeastern Spain inferred from a tree ring latewood network since 1734

Drought recurrence in the Mediterranean is regarded as a fundamental factor for socio-economic development and the resilience of natural systems in context of global change. However, knowledge of past droughts has been hampered by the absence of high-resolution proxies. We present a drought reconstruction for the northeast of the Iberian Peninsula based on a new dendrochronology network considering the Standardized Evapotranspiration Precipitation Index (SPEI). A total of 774 latewood width series from 387 trees of P. sylvestris and P. uncinata were combined in an inter-regional chronology. The new chronology, calibrated against gridded climate data, reveals a robust relationship with the SPEI representing drought conditions of July and August. We developed a summer drought reconstruction for the period 1734-2013 representative for the northeastern and central Iberian Peninsula. We identified 16 extremely dry and 17 extremely wet summers, and four decadal scale dry and wet periods, including 2003-2013 as the driest episode of the reconstruction.

Oaks Under Mediterranean-Type Climates: Functional Response to Summer Aridity

Mediterranean-type climates are characterized by warm or hot summers, mild or cold winters and, especially, by the existence of a summer drought period driven by the low or even nule precipitation during this season. Mediterranean-type climates are represented in different areas of the world, both in the Northern and the Southern Hemisphere. Specifically, regarding the existence of Quercus under these climatic conditions, two main geographical areas should be considered, namely the Mediterranean Basin in the Palearctic and California (USA) and Baja California (Mexico) in the Nearctic. Despite the relatively low geographical extension of the areas occupied by oaks under this type of climate, it has deserved its own phytoclimatical entity since the first geobotanical synthesis at a global scale. Although evergreen and sclerophyllous oak species are widely assumed as a prototype of mediterranean oaks, both palaeoecological evidences and present biogeographical analysis confirm the co-existence of this oak type with winter-deciduous species of the same genus. In this chapter, the different advantages and disadvantages of both phenological patterns (evergreeness and winter-deciduousness) are presented. Moreover, the strategies for saving water through the overall leaf size reduction, the stomatal control of water losses or some xeromorphic traits for a further reduction of transpiration are also shown. Finally, the development of a high resistance to drought-induced cavitation, as a way for coping with low water potential during dry periods, is discussed.

Influence of different factors on relative air humidity in Zaragoza, Spain

In this study, the spatial patterns of relative air humidity and its relation to urban, geographical and meteorological factors in the city of Zaragoza (Spain) is discussed. We created a relative humidity database by means of 32 urban transects. Data were taken on different days and with different weather types. This data set was used to map the mean spatial distribution of urban dry island (UDI). Using stepwise multiple regression analysis and Landsat ETM+ images the relationships between mean UDI and the main geographic-urban factors: topography, land cover and surface reflectivity, have been analyzed. Different spatial patterns of UDI were determined using Principal Component Analysis (Varimax rotation). The three components extracted accounted for 91% of the total variance. PC1 accounted for the most general patterns (similar to mean UDI); PC2 showed a shift of dry areas to the SE and PC3 a shift to NW. Using data on wind direction in Zaragoza, we have found that the displacement of dry areas to the SE (PC 2) was greater during NW winds while the shift to the NW (PC 3) was produced mainly by SE winds.

SPREAD (Spanish PREcipitation At Daily scale)

The dataset is freely available on the web repository of the Spanish National Research Council (CSIC). There are 6 files in NetCDF format: 3 with precipitation estimations (pcp_) and 3 with standard errors (err_) of each estimation. Each of 3 files represent peninsular Spain (pen_), Balearic islands (bal_) and Canary islands (can_).