JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to International Journal of Plant Sciences.A detailed understanding of how the climate models the hydraulic system of trees is still lacking, in spite of the need to understand tree response to the ongoing process of climatic change. A systematic correlation of tracheid anatomical features with climate has not been implemented in Mediterranean ecosystems, where climatic change is expected to be particularly intense. We measured ring width and cell number-in addition to intraring position, lumen size, and wall thickness of tracheids-in 10 Juniperus thurifera individuals from north-central Spain. We used this information to perform an exploratory analysis of how these parameters correlated with climatic variables in 1965-2004. Cell number and ring width shared a relatively similar climatic signal, whereas the anatomical variables provided differentiated and diverse signals about climatic conditions during their formation. Earlywood and latewood tracheids differed in controlling factors, with earlywood tracheid size and wall thickness being positively related to rainfall during early summer and latewood tracheid size being positively related to August temperature. Tracheid anatomical variables improved our understanding of climate effects on tree growth and wood formation under harsh environmental conditions, as those experienced in continental Mediterranean climates, where limiting factors show multiple shifts across the year.

Summary1. Plant growth in Mediterranean high mountains is limited by the double climatic stress of low winter temperatures and summer drought. Elevational shifts in response to climate change may be complex for species whose distribution is constrained by several climatic factors. 2. We used herb-chronology, that is, the analysis of annual rings in the secondary root xylem of perennial forbs, to evaluate life-long factors constraining secondary growth and xylem hydraulic anatomy along an elevational gradient from the upper to the lower distribution limits of the alpine forb Silene ciliata at its southernmost distribution range. 3. Generalized additive models (GAM) showed that annual ring width (RW) in S. ciliata was greatest at intermediate elevations and smallest at the upper and lower limits of its elevational range. In contrast, maximal vessel area (MVA) was greatest at lower elevations. RW responded to climatic conditions in early spring and late summer, suggesting the presence of a bimodal xylogenesis. Structural equation modelling (SEM) indicated a positive influence of MVA on RW in the same year; improved hydraulic efficiency seemed to promote higher secondary growth. 4. The observed greatest secondary growth (RW) and maximal vessel area (MVA) at intermediate and intermediate-low elevations, respectively, contrasts with previous evidence of an improvement in plant reproduction and recruitment with increasing elevation for S. ciliata. However, our results are in agreement with other indicators suggesting that best conditions occur at intermediate elevations, such as better seed quality or larger genome size. 5. This study reinforces the evidence that the response of high-mountain plants to climatic change under simultaneous temperature and drought stress is complex and that models that simply assume an increase in elevation as a response to higher temperatures may fail to predict future responses to climate change.

Increasing fire recurrence is a major problem threatening Mediterranean-type ecosystems. Moreover, this pattern is predicted to increase owing to global change. Although a reduction in the density and growth of post-fire regeneration is usually observed in recurrently burnt areas, the potential effects on reproductive ability have seldom been explored. The aim of the present study is to investigate whether structural changes induced by fire recurrence may constrain reproduction ability of Pinus halepensis forests. We conducted the current study in Catalonia (NE Spain) in 12 study sites, consisting of two adjacent areas differing in the number of fire events suffered throughout the last 16 years (one v. two fires). Twice-burnt areas showed a lower density of pines, lower pine height and a lower reproductive ability, namely (i) a 3-year delay in the onset of pine reproduction; (ii) a reduction of 52% in the number of reproductive pines; and (iii) a 36% lower mean cone crop per tree. The lower mean cone production per tree coupled with a lower density resulted in an ~80% lower canopy seed bank in twice-burnt areas. These results suggest that the occurrence of a third fire event in twice-burnt areas would severely constrain natural regeneration.

In 1994, six large fires occurred along a climate gradient in Eastern Spain. Ten-year-old trees were selected in the plots established for each location affected by fire. Each selected tree was measured for height and trunk diameter, the number of female strobili and cones, and grouped into four types. A geographical gradient was determined to study growth by latitude and variations in reproductive characteristics. Height and diameter values decreased southwards with the exception of the lowest-density tree location, where trunk diameters were similar to those in northern locations. Strobilus and cone production were higher in lower tree density locations owing to intraspecific competition and higher availability of water, nutrients and light. Serotiny levels were highest in southern locations, although lower percentages were found in the southern locations with low tree density. The number of reproductive trees was higher with lower tree density. The reproductive phase was reached in the lowest tree density site (also low site quality) and nearly reached in the most northern locations (high tree density as well as high site quality). Results showed a geographical gradient based on growth and reproduction dynamics, where serotiny levels are higher in southern locations. Density was shown to be a significant factor influencing overall pine stand development.