J. Julio Camarero

Pace and Pattern of Recent Treeline Dynamics: Response of Ecotones to Climatic Variability in the Spanish Pyrenees

Treeline ecotones are regarded as sensitive monitors of the recent climatic warming. However, it has been suggested that their sensitivity depends more on changes in tree density than on treeline position. We study these processes and the effect of climate, mainly air temperature, on tree recruitment and recent treeline dynamics. We selected three relatively undisturbed sites in the Spanish Pyrenees, dominated by Pinusuncinata, and analyzed their recent dynamics at local spatial (0.3–0.5 ha) and short temporal scales (100–300 years). We wanted to establish whether higher temperature was the only climatic factor causing an upward shift of the studied alpine treelines. The data we report show that treelines were ascending until a period of high interannual variability in mean temperature started (1950–95). During the late twentieth century, treeline fluctuation was less sensitive to climate than was the change in tree density within the ecotone. Tree recruitment and treeline position responded to contrasting climatic signals; tree recruitment was favored by high March temperatures whereas treeline position ascended in response to warm springs. We found a negative relationship between mean treeline-advance rate and March temperature variability. According to our findings, if the interannual variability of March temperature increases, the probability of successful treeline ascent will decrease.

To die or not to die: early warnings of tree dieback in response to a severe drought

This study was supported by projects CGL2011-26654 (Spanish Ministry of Economy and Competitiveness), 1032S/2013 and 387/2011 (Organismo Autonomo Parques Nacionales, Spanish Ministry of Agriculture and Environment, Spain). We thank ARAID for supporting J.J.C. and the AEET for providing climatic data. We also thank M. Maestro and E. Lahoz for performing the chemical analyses and R. Hernandez, A.Q. Alla and E. Gonzalez de Andres for their help in the field. The authors declare no conflict of interest.

TREE-RING GROWTH AND STRUCTURE OF PINUS UNCINATA AND PINUS SYLVESTRIS IN THE CENTRAL SPANISH PYRENEES

We compared tree-ring structure and formation for Pinus uncinata Ram. and Pinus sylvestris L. in the Central Spanish Pyrenees. Ring width and the number of tracheids in the latewood and in the enti...

Spatial pattern of subalpine forest-alpine grassland ecotones in the Spanish Central Pyrenees

We describe the spatial structure of two contrasting subalpine Pinus uncinata forest-alpine grassland ecotones located in the Central Pyrenees (Ordesa and Tessosites) as a preliminary step to infer the processes that produced their spatial patterns. All trees were mapped and measured within 4200 m 2 rectangular plots parallel to the maximum slope and encompassing timberline and treeline. The spatial description of the ecotones was accomplished using several methodologies. Point pattern analysis (Ripleys K) was first used to quantify the spatial pattern of trees using each stem x-y coordinates. Then, surface pattern analyses (Moran and Mantel spatial correlograms) were used to quantify the spatial pattern of tree characteristics across the ecotone (size, growth-form, estimated age). In the Ordesa site, krummholz individuals showed significant and positive spatial interaction with seedlings. In this site, P. uncinata individuals evolved from shrubby to vertical growth-forms abruptly, producing a steep spatial gradient. In the Tessosite, regeneration was concentrated near the treeline and the spatial gradient was gradual. Both ecotones formed45 m long zones of influence along the slope based on different variables. Wind and snow avalanches seem to be the main controlling factors of the spatial pattern of trees in the sites Ordesa and Tesso ´, respectively. Our results point out potential different responses of treeline populations to environmental changes according to the spatial pattern. # 2000 Elsevier Science B.V. All rights reserved.

Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir Abies pinsapo

The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO2 have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ13C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO2 concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ13C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high- (H) and a low-elevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive micro-environment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in low-elevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming.

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

AbstractTreeline 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 a...

Plastic responses of Abies pinsapo xylogenesis to drought and competition

Radial growth and xylogenesis were studied to investigate the influence of climate variability and intraspecific competition on secondary growth in Abies pinsapo Boiss., a relic Mediterranean fir. We monitored the responses to three thinning treatments (unthinned control -C-, 30% -T30- and 60% -T60- of basal area removed) to test the hypothesis that they may improve the adaptation capacity of tree growth to climatic stress. We also assessed whether xylogenesis was differentially affected by tree-to-tree competition. Secondary growth was assessed using manual band dendrometers from 2005 to 2007. In 2006, xylogenesis (phases of tracheid formation) was also investigated by taking microcores and performing histological analyses. Seasonal dynamics of radial increment were modeled using Gompertz functions and correlations with microclimate and radiation were performed. Histological analyses revealed it as fundamental to calibrate the dendrometer estimates of radial increment and to establish the actual onset and end dates of tracheid production. The lower radial-increment rates and number of produced tracheids were observed in the trees subjected to high competition in the unthinned plots. The growing season differed among the plots, and its duration ranged from an average of 78 days in unthinned plots to 115 days in thinned ones (T60). Variations in the beginning of the growing season (13 April to 22 May) and earlywood-latewood transition (early August) were mainly determined by the temperature pattern, while the onset and the end of the growing season were related to both annual precipitation and tree-to-tree competition. The tracheid-formation phases of radial enlargement and cell-wall thickening showed similar patterns in the trees from thinned and unthinned plots subjected to low and high competition, respectively, but the mean number of tracheids in each phase was always higher in the trees from the thinned plots. The reduction of competition through thinning induced a longer growing season and enhanced the radial growth in A. pinsapo.

Enhanced growth of Juniperus thurifera under a warmer climate is explained by a positive carbon gain under cold and drought

Juniperus thurifera L. is an endemic conifer of the western Mediterranean Basin where it is subjected to a severe climatic stress characterized by low winter temperatures and summer drought. Given the trend of increased warming-induced drought stress in this area and the climatic sensitivity of this species, we expect a negative impact of climate change on growth and ecophysiological performance of J. thurifera in the harsh environments where it dominates. To evaluate this, we measured long- and short-term radial growth using dendrochronology, photosynthesis and water-use efficiency in males, females and juveniles in three sites in Central Spain. Climate was monitored and completed with historical records. Mean annual temperature has increased +0.2 °C per decade in the study area, and the main warming trends corresponded to spring (+0.2 °C per decade) and summer (+0.3 °C per decade). Radial growth and maximum photosynthesis peaked in spring and autumn. Positive photosynthetic rates were maintained all year long, albeit at reduced rates in winter and summer. Radial growth was enhanced by wet conditions in the previous autumn and by warm springs and high precipitation in summer of the year of tree-ring formation. Cloud cover during the summer increased growth, while cloudy winters led to impaired carbon gain and reduced growth in the long term. We argue that maintenance of carbon gain under harsh conditions (low winter temperatures and dry summer months) and plastic xylogenesis underlie J. thuriferas ability to profit from changing climatic conditions such as earlier spring onset and erratic summer rainfall. Our results highlight that not only the magnitude but also the sign of the impact of climate change on growth and persistence of Mediterranean trees is species specific.

Placing unprecedented recent fir growth in a European-wide and Holocene-long context

Forest decline played a pivotal role in motivating Europes political focus on sustainability around 35 years ago. Silver fir (Abies alba) exhibited a particularly severe dieback in the mid-1970s, but disentangling biotic from abiotic drivers remained challenging because both spatial and temporal data were lacking. Here, we analyze 14 136 samples from living trees and historical timbers, together with 356 pollen records, to evaluate recent fir growth from a continent-wide and Holocene-long perspective. Land use and climate change influenced forest growth over the past millennium, whereas anthropogenic emissions of acidic sulfates and nitrates became important after about 1850. Pollution control since the 1980s, together with a warmer but not drier climate, has facilitated an unprecedented surge in productivity across Central European fir stands. Restricted fir distribution prior to the Mesolithic and again in the Modern Era, separated by a peak in abundance during the Bronze Age, is indicative of the long-term interplay of changing temperatures, shifts in the hydrological cycle, and human impacts that have shaped forest structure and productivity.

Growth response to climate and drought change along an aridity gradient in the southernmost Pinus nigra relict forests

ContextTree populations at the rear edge of species distribution are sensitive to climate stress and drought. However, growth responses of these tree populations to those stressors may vary along climatic gradients.AimsTo analyze growth responses to climate and drought using dendrochronology in rear-edge Pinus nigra populations located along an aridity gradient.MethodsTree-ring width chronologies were built for the twentieth century and related to monthly climatic variables, a drought index (Standardized Precipitation–Evapotranspiration Index), and two atmospheric circulation patterns (North Atlantic and Western Mediterranean Oscillations).ResultsGrowth was enhanced by wet and cold previous autumns and warm late winters before tree-ring formation. The influence of the previous year conditions on growth increased during the past century. Growth was significantly related to North Atlantic and Western Mediterranean Oscillations in two out of five sites. The strongest responses of growth to the drought index were observed in the most xeric sites.ConclusionDry conditions before tree-ring formation constrain growth in rear-edge P. nigra populations. The comparisons of climate-growth responses along aridity gradients allow characterizing the sensitivity of relict stands to climate warming.