No abstract

-In Mediterranean climates trees may go through two periods of dormancy, resulting in special anatomical features such as false rings and other intra-annual density fluctuations (IADFs). In this paper, ring growth and the presence of IADFs were studied in Pinus pinea L. growing in the coastal and inland regions of Alentejo (southern Portugal). In order to identify the triggering factors associated with the IADFs, a new classification was proposed for the IADFs in P. pinea: Type E (latewoodlike cells within earlywood); Type E+ (transition cells between earlywood and latewood); Type L (earlywoodlike cells within latewood) and Type L+ (earlywoodlike cells between latewood and earlywood of the next tree ring). Response function analyses showed that radial growth of P. pinea was strongly correlated with precipitation in southern Portugal. The climatic response of P. pinea was higher in the inland area where the summer drought is more severe, the winter temperatures are lower and the soils have low water-holding capacity, in comparison with the coastal area. IADFs were frequent in P. pinea and most of the IADFs were observed in latewood. The presence of IADFs was correlated with fluctuations in climate parameters during the growing season. The IADF type E+ was linked to precipitation events early in summer. The IADF type L and L+ were associated with above-average precipitation in early autumn.false tree-ring / Mediterranean climate / Pinus pinea / radial growth / wood anatomy Résumé -Signification climatique de la largeur des cernes et des fluctuations intra annuelles de la densité chez Pinus pinea dans une région méditerranéenne sèche du Portugal. Dans les climats méditerranéens, les arbres peuvent traverser deux périodes de dormance, ce qui a pour consé-quence des caractéristiques anatomiques particulières telles que des faux cernes et des fluctuations intra annuelles de densité (IADFs). Dans cet article, la croissance des cernes et la présence de IADFs ont été étudiées chez Pinus pinea L. poussant dans les régions côtières et intérieures de l'Alentejo (sud-ouest du Portugal). Dans le but d'identifier les facteurs déclenchants associés à l'IADFs, une nouvelle classification a été proposée pour l'IADFs chez Pinus pinea : Type E (cellules ressemblant à du bois final dans le bois initial) ; Type E+ (cellules de transition entre bois initial et bois final) ; Type L (cellules ressemblant à du bois initial dans du bois final) et Type L+ (cellules ressemblant à du bois initial entre bois final et bois initial du prochain cerne). Les analyses des fonctions de réponse ont montré que la croissance radiale de Pinus pinea était fortement corrélée avec les précipita-tions dans le sud-ouest du Portugal. La réponse climatique de Pinus pinea a été plus forte dans la zone intérieure où la sécheresse d'été est plus sévère, les températures hivernales plus basses et où les sols ont une plus faible capacité de rétention de l'eau, comparativement aux zones côtières. IADFs a été fréquent chez Pinus pinea et la majorité d'IADFs ...

Dendrochronology generally assumes that climate-growth relationships are age independent once the biological growth trend has been removed. However, tree physiology, namely, photosynthetic capacity and hydraulic conductivity changes with age. We tested whether the radial-growth response to climate and the intra-annual density fluctuations (IADFs) of Pinus pinaster Ait. varied with age. Trees were sampled in Pinhal de Leiria (Portugal), and were divided in two age classes: young (\65 years old) and old ([115 years old). Earlywood and tree-ring width of young P. pinaster trees were more sensitive to climate influence while the response of latewood width to climate was stronger in old trees. Young trees start the growing season earlier, thus a time window delay occurs between young and old trees during which wood cells of young trees integrate environmental signals. Young trees usually have a longer growing season and respond faster to climate conditions, thus young P. pinaster trees presented a higher frequency of IADFs compared with old trees. Most of the IADFs were located in latewood and were positively correlated to autumn precipitation. The radial-growth response of P. pinaster to climate and the IADFs frequency were age dependent. The use of trees with different age to create a tree-ring chronology for climate studies can increase the resolution of climatic signals. Age-dependent responses to climate can also give important clues to predict how young and old trees react to climate change.

Intra-annual density fluctuations (IADFs) in tree rings are generally considered structural anomalies caused by deviations from the “normal course” of xylogenesis during the growing season. This definition is based on the bias that, under “normal conditions”, cambial activity stops once a year. Each tree ring can thus be dated to one calendar year, which is one of the principles of dendrochronology. The formation of IADFs can be triggered directly by environmental changes, especially in precipitation and temperature, that affect cambial activity and cell differentiation. It can also be the result of limited photosynthesis, due to defoliation induced by biotic or abiotic constraints.Often indicated with alternative terms, IADFs were first described in the 1930s, and recently reported for many trees and shrubs from different ecosystems throughout the world, particularly for Mediterranean species. Different types of IADFs have been detected; their formation and structural properties depend on many factors including tree genotype, age, size, rooting depth, habitat, soil, climate, photosynthetic activity, and allocation strategies. Whether IADFs affect the adaptive capability of plants remains, however, unclear.We provide an overview of the main anatomical features of IADFs and their occurrence in tree rings from various environments and climatic regimes. We propose a simplified way of classifying them and discuss the hypotheses about their functional role and the factors triggering their formation. To understand the ecological role of IADFs better, we recommend a multidisciplinary approach, involving wood anatomy, dendroecology, and stable isotopes, which has already been applied for Mediterranean species. We conclude by considering that IADFs appear to be the “rule” rather than “anomalies” in some ecosystems where they help plants cope with fluctuating environmental conditions. Moreover, their anatomical structure represents a valuable proxy of past climatic conditions at a sub-seasonal resolution and may be relevant to adapt hydraulic functioning of living trees to changing climatic conditions.

Dendroclimatology generally assumes that climate-growth relationships are age and size independent. However, there is evidence that climate response can be unstable across different age/size classes. In addition, the occurrence of some anatomical features, such as intraannual density fluctuations (IADFs), is age dependent. The present study investigates whether the climate-growth responses and the occurrence of IADFs in an even-aged stand of Pinus pinaster Ait., growing under Mediterranean climate, are also size-dependent. We randomly selected 60 P. pinaster trees falling within two stem diameter classes: small (\27 cm) and large ([35 cm). Tree rings were crossdated, measured and IADFs identified according to their position within the ring. The residual chronologies of both size classes were strongly correlated, suggesting a common signal. In fact, similar growth-climate relationships were observed in large and small trees. The frequency of IADFs was higher in large than in small trees, suggesting that IADFs were more likely to occur in wider rings of fast-growing trees. In both size classes, most of the IADFs were found in latewood. Latewood IADFs were triggered by the combination of dry June, wet September, and warm December, whereas IADFs located at the end of earlywood were triggered by previous winter precipitation and favorable conditions before summer (high precipitation for large trees and lower temperature for small trees). Our results suggest that IADFs can be a mechanism used at the individual level for adaptation to drought in P. pinaster. The climatic signal of IADFs between earlywood and latewood was mediated by stem size suggesting that future tree-ring studies should include trees stratified by size to better estimate the sensitivity of IADFs to climate. Keywords Tree size Á IADFs Á False rings Á Mediterranean climate Á Ecological wood anatomy Communicated by S. Leavitt.

a b s t r a c tHigh resolution measurements of stem diameter variation can provide valuable information on the growth process as well as the tree water status. This study investigates the hourly variations in stem radial increment of maritime pine growing on the west coast of Portugal, under a Mediterranean climate with a pronounced summer drought. We tested the hypotheses that (1) stem radius variation in maritime pine has a daily and seasonal pattern that reflects the availability of water and (2) that once the internal water storage is depleted the tree enters a quiescent state. During 2010, stem radius variations were monitored in four similar trees with automatic dendrometers. Tree water deficit was extracted from the stem radial variation. The stem cycle approach was used to divide the daily cycles into contraction, recovery and increment phases. The seasonal cycle was divided into five periods: winter dormancy, spring growth, pre-summer contraction, summer quiescence and autumn re-hydration. Amplitude and duration of each phase were calculated for each cycle and correlated with precipitation, maximum and minimum temperature. Continuous positive radial increment started in spring and reached its maximum by the end of June. A shrinking period was observed in summer, with amplitudes of contraction and recovery 10 times higher than in the other periods. The inability of the trees to recover from the water lost due to transpiration was responsible for stem shrinking and quiescence observed during summer. In autumn, a period of re-hydration and rapid expansion was observed after precipitation, restoring the tree to a physiologically active state. Daily variations in stem radius of maritime pine were mainly determined by the course of transpiration and thus dependent on temperature and tree water status.

& Context The knowledge on cambial activity in water-limited environments, such as the Mediterranean, is still fragmentary. Dendrochronological studies have determined that spring precipitation plays an important part in determining tree-ring width and the properties of tracheids. However, the complex relation between cambial phenology and climate is still far from understood. & Aims We studied the influence of climate, especially water stress, on maritime pine wood formation with the aim of determining the influence of drought on cambial activity. & Methods A plantation of maritime pine (Pinus pinaster)was selected in the west coast of Portugal, to monitor cambial activity and wood formation using anatomical observations and band dendrometers. The trees were monitored weekly over 2 years (2010 and 2011). & Results Xylem differentiation started earlier when warmer late winter temperatures were observed. Water stress triggered an earlier stop of wood formation and also the formation of tracheids with smaller lumen area. In both years a bimodal pattern of stem radial increment was registered by band dendrometers with two periods of increment: one in spring and another in autumn. The xylem anatomy study suggests that the autumnal increment period corresponded mostly to stem rehydration, since the differentiation of new xylem cells by the cambium was not observed. & Conclusion Maritime pine cambial activity appears to be under a double climatic control: temperature influences cambial onset and water availability growth cessation.

Intra-annual density fluctuations (IADFs) are anatomical features formed in response to changes in the environmental conditions within the growing season. These anatomical features are commonly observed in Mediterranean pines, being more frequent in younger and wider tree rings. However, the process behind IADF formation is still unknown. Weekly monitoring of cambial activity and wood formation would fill this void. Although studies describing cambial activity and wood formation have become frequent, this knowledge is still fragmentary in the Mediterranean region. Here we present data from the monitoring of cambial activity and wood formation in two diameter classes of maritime pine (Pinus pinaster Ait.), over two years, in order to test: (i) whether the differences in stem diameter in an even-aged stand were due to timings and/or rates of xylogenesis; (ii) if IADFs were more common in large trees; and (iii) if their formation is triggered by cambial resumption after the summer drought. Larger trees showed higher rates of cell production and longer growing seasons, due to an earlier start and later end of xylogenesis. When a drier winter occurs, larger trees were more affected, probably limiting xylogenesis in the summer months. In both diameter classes a latewood IADF was formed in 2012 in response to late-September precipitation, confirming that the timing of the precipitation event after the summer drought is crucial in determining the resumption of cambial activity and whether or not an IADF is formed. It was the first time that the formation of a latewood IADF was monitored at a weekly time scale in maritime pine. The capacity of maritime pine to adjust cambial activity to the current environmental conditions represents a valuable strategy under the future climate change conditions.