Intra-annual Density Fluctuations Research Articles

Impact of intra-annual wood density fluctuation on tree hydraulic function: insights from a continuous monitoring approach.

Climate change significantly impacts global forests, leading to tree decline and dieback. To cope with climate change, trees develop several functional traits, such as intra-annual density fluctuations (IADFs) in tree rings. The formation of these traits facilitates trees to optimize resource allocation, allowing them to withstand periods of stress and eventually recover when the conditions become favourable again. This study focuses on a Pinus pinaster Aiton forest in a warm, drought-prone Mediterranean area, comparing two growing seasons with different weather patterns. The innovative continuous monitoring approach used in this study combines high-resolution monitoring of sap flow (SF), analysis of xylogenesis and quantitative wood anatomy. Our results revealed the high plasticity of P. pinaster in water use and wood formation, shedding light on the link between IADFs and tree conductance. Indeed, the capacity to form large cells in autumn (as IADFs) improves the total xylem hydraulic conductivity of this species. For the first time, a continuous SF measurement system captured the dynamics of bimodal SF during the 2022 growing season in conjunction with the bimodal growth pattern observed through xylogenesis monitoring. These results highlight the intricate interplay between environmental conditions, water use, wood formation and tree physiology, providing valuable insights into the acclimation mechanisms employed by P. pinaster to cope with weather fluctuations.

Analyses of intra-annual density fluctuation signals in Himalayan cedar trees from Himachal Pradesh, western Himalaya, India, and its relationship with apple production

Intra-annual density fluctuation (IADF) refers to anatomical changes in the tree ring caused by a sudden change in wood density triggered by a combination of climate variations and various biotic and abiotic influences. To reveal the occurrence of IADFs, we analyze the growth rings of Himalayan cedar (Cedrus deodara) growing over the Kullu region, Himachal Pradesh, western Himalaya. Using 30 increment cores, we precisely dated and developed a 214-year-long tree-ring chronology extending back to AD 1808. The tree–growth–climate relationship using ring-width chronology and observed climate data revealed that cool and moist condition provides favorable condition for Himalayan cedar tree growth. Delving deeper into wood anatomy of growth rings, we revealed the frequent occurrences of IADFs in both earlywood (IADFe) and latewood (IADFl). The formation of IADFs in earlywood (IADFe) is related to the reduced precipitation from April to July, causing moisture stress in the soil and surrounding climate. However, wetter conditions in the late growing season, mainly August–October, activated the formation of IADFs in latewood (IADFl). The study revealed several IADF years in earlywood and latewood, such as 1901, 1902, 1903, 1914, 1915, 1919, 1920, 1923, 1925, 1943, 1958, 1959 and 1937, 1955, 1956, 1988, respectively. These IADF years corresponded with unusual climatic fluctuations that severely affected apple production, the major cash crop in the region. The analyses demonstrated that the IADF chronology of Himalayan cedar would be a valuable proxy to understand abrupt and unusual climatic fluctuations from a long-term perspective for the data-scarce western Himalayan region.

Latewood intra-annual density fluctuations indicate wet summer conditions and enhanced canopy activity in a Mediterranean ring-porous oak

Summary The climatic significance and ecosystem implications of latewood intra-annual density fluctuations (IADFs) are still not fully understood in Mediterranean ring-porous oak species. To solve this issue, we investigated: (i) the climate drivers of radial growth and latewood IADFs in the Mediterranean oak Quercus faginea, and (ii) whether they were correlated to increased canopy cover and greenness as recorded by the Normalized Difference Vegetation Index (NDVI). IADFs were formed in the mid to late latewood and they were characterized by rows of parenchyma cells. Such IADFs were uncommon and only present in 30–50% of sampled trees during 1999 and 2014, both years characterized by high precipitation in late July and early August. This relationship between IADFs formation and summer wet conditions differed from the conditions that enhance Q. faginea annual growth, wet winter-spring conditions, and low summer temperatures. Furthermore, IADFs formation showed a positive relationship with August NDVI values, indicating a correspondence of canopy greenness with secondary growth reactivation. We conclude that latewood IADFs in Q. faginea are a robust proxy of rare late-summer wet conditions and enhanced canopy activity as reflected by increased August NDVI values. Further monitoring of xylogenesis in other Mediterranean hardwoods along climatic gradients is suggested to mechanistically explain IADF production in seasonally dry biomes.

Topoclimatic modulation of growth and production of intra-annual density fluctuations in Juniperus thurifera

Forest growth is driven by climate variability at continental to regional scales, but other factors play major roles at local scales (0.1–1 ha). Topography impacts on tree growth responses to climate stressors, including drought, by modifying radiation, evapotranspiration rates and the access to soil moisture. However, there is a lack of studies investigating how topographical factors (elevation, aspect, slope) affect climate-growth relationships considering both continuous (tree-ring width) and discrete wood-anatomical features (e.g., intra-annual density fluctuations–IADFs). Here, we investigated how topography modulated the influences of climate and drought on semi-arid Juniperus thurifera forests from north-eastern Spain. We compared two stands located in valley bottoms and two stands located in steep slopes. Radial growth was measured using dendrochronology which also allowed quantifying latewood IADFs. A proxy of topographic influence (incident radiation) was calculated. In addition, the Vaganov-Shashkin (VS) model was used to infer the main climatic constraints of growth at intra-annual scales. We found that junipers growing in valley bottoms were taller and produced more IADFs than junipers growing in steep slopes, but responded less to precipitation variability. This was confirmed by the VS model which showed that low soil moisture in June limited growth, particularly in steep-slope sites and during dry periods. Wet-cool late-summer conditions induced the formation of IADFs. The topoclimatic modulation of growth was explained by the interaction between incident radiation and tree age. Furthermore, climate-growth associations are changing as temperatures rise. Drought impacts on growth are strengthening, but less responsive junipers growing in valley bottoms may be buffered against intensified aridification.

Intra-annual density fluctuations in tree rings are proxies of air temperature across Europe

Intra-Annual Density Fluctuations (IADFs) are an important wood functional trait that determine trees’ ability to adapt to climatic changes. Here, we use a large tree-ring database of 11 species from 89 sites across eight European countries, covering a climatic gradient from the Mediterranean to northern Europe, to analyze how climate variations drive IADF formation. We found that IADF occurrence increases nonlinearly with ring width in both gymnosperms and angiosperms and decreases with altitude and age. Recently recorded higher mean annual temperatures facilitate the formation of IADFs in almost all the studied species. Precipitation plays a significant role in inducing IADFs in species that exhibit drought tolerance capability, and a growth pattern known as bimodal growth. Our findings suggest that species with bimodal growth patterns growing in western and southern Europe will form IADFs more frequently, as an adaptation to increasing temperatures and droughts.

Influence of monsoon anomalies on intra-annual density fluctuations of Chinese pine in the Loess Plateau

In the past few decades, the East Asian summer monsoon (EASM) has experienced an unprecedented weakening, exacerbating drought in northern China, especially in the monsoon margin area. Improving our understanding of monsoon variability will benefit agricultural production, ecological construction, and disaster management. Tree-ring is widely used as proxy data for extending the monsoon history. However, in the East Asian monsoon margin, the tree-ring width were mostly formed before the rainy season, thus may have limited ability to indicate the monsoon variability. Intra-annual density fluctuations (IADFs) can provide higher resolution information on tree growth as well as evidence of short-term climate events. Here, we used Chinese pine (Pinus tabuliformis Carr.) samples from the eastern edge of the Chinese Loess Plateau (CLP), where the climate is deeply affected by monsoon, to investigate the response of tree growth and IADFs frequency to climate variation. We show that tree-ring width and IADFs record significantly different climatic signals. The former was mainly affected by moisture conditions at the end of the previous growing season and the current spring. While the latter was common in years when severe droughts occurred in June and July, especially in June. This period coincides with the onset of the EASM, so we further analyzed the relationship between IADFs frequency and the rainy season. Both correlation analysis and the GAM model suggest that the frequent occurrence of IADFs may be related to the late start of the monsoon rainy season, meaning that we have found a new indicator in tree-ring records that can capture monsoon anomalies. Our results provide further insight into drought variation in the eastern CLP, which also implicates the Asian summer monsoon dynamic.

Coping with Central European climate – xylem adjustment in seven non-native conifer tree species

The introduction of tree species raises the question of whether they find suitable conditions in their secondary range and whether they can successfully adapt to the new conditions in the context of an unstable climate. Stem secondary growth of trees and the changes in xylem cell structure make it possible to trace the adaptation process to the new climate on an inter- and intra-seasonal scale. The objective of this study was to determine whether the seven non-native conifer species in Central Europe differ in climate sensitivity and to what extent late frost and spring-summer drought can influence xylem changes. In our study, we investigated the growth pattern and xylem cell structure modifications of seven introduced conifer species Pseudotsuga menziesii, Pinus strobus, Tsuga canadensis, Abies grandis, Pinus banksiana, Pinus rigida, Pinus nigra, growing under uniform conditions in Poland. To determine species-specific temporal variations in climate sensitivity, we used stationary and non-stationary correlations of ring width with daily climate variables: mean, minimum, and maximum temperatures, precipitation sum, and SPEI, supported by a pointer-year analysis. Changes in xylem structure (frost rings and intra-annual density fluctuations) were analyzed using light and fluorescence microscopy. Warming of the winter-spring period benefits both southern European and North American introduced conifers, as reflected in the extended responsiveness period. However, as the growing season warms and the risk of late frosts increases, the potential for acclimatization of non-native species decreases, especially with respect to their juvenile growth phase. Negative precipitation and evapotranspiration balance at spring-summer period can lead to reduced growth and anomalies in earlywood xylem structure, commonly referred to as intra annual density fluctuations. P. strobus, the species that loses both frost-induced cambium damage and drought-induced xylem anomalies in summer is the fastest growing non-native species, along with P. menziesii. Although the results presented here show the potential and the limitations in the acclimatization of coniferous species, the obvious limitations related to local environmental conditions and the different ages of the trees make us cautious in generalizing the conclusions. Therefore, we call for the creation of a pan-European dendro-network of sites of non-native coniferous species.

Why Intra-Annual Density Fluctuations Should Be Formed at Night? Implications for Climate–Growth Relationships in Seasonally Dry Conifer Forests

Trees grow at night, when the vapor pressure deficit (VPD) is low enough. Therefore, intra-annual density fluctuations (IADFs) should be formed when the VPD drops below a certain threshold. This idea is tested by assessing climate-latewood IADF relationships in six conifer species under Mediterranean climate conditions. Hourly climate and dendrometer data were analyzed for years with elevated IADF production in two species (Pinus halepensis, Juniperus thurifera). Lastly, climate–growth relationships were evaluated in two drought-prone sites to assess the relative role of minimum vs. maximum temperatures as growth drivers. Latewood IADF production was positively related to growth rate. IADFs were more abundant when monthly or 10-day long precipitation was high in the late growing season (August and September). According to dendrometer data, growth mainly occurred in early night (20–2 h) and early morning (6–8 h). This growth window corresponded to rainy periods with VPD below a minimum threshold associated with summer storms. Latewood IADFs are produced in response to these wet late-summer conditions, which could be related to bimodal growth. These associations are in line with correlations showing that high minimum (night) rather than maximum (day) temperatures reduce growth. This last idea should be further checked in drought-prone forests using global tree-ring databases.

The Origin of Tree‐Ring Reconstructed Summer Cooling in Northern Europe During the 18th Century Eruption of Laki

AbstractBasaltic fissure eruptions, which are characteristic of Icelandic volcanism, are extremely hazardous due to the large quantities of gases and aerosols they release into the atmosphere. The 1783–1784 CE Laki eruption was one of the most significant high‐latitude eruptions in the last millennium and had substantial environmental and climatic impacts. Contemporary observations recorded the presence of a sulfuric haze over Iceland and Europe, which caused famine from vegetation damage and resulted in a high occurrence of respiratory illnesses and related mortality. Historical records in northern Europe show that the summer of 1783 was anomalously warm, but regional tree‐ring maximum latewood density (MXD) data from that year are low and lead to erroneously colder reconstructed summer temperatures. Here, we measure wood anatomical characteristics of Scots pine (Pinus sylvestris) from Jämtland, Sweden in order to identify the cause of this discrepancy. We show that the presence of intraannual density fluctuations in the majority of 1783 growth rings, a sudden reduction in lumen and cell wall area, and the measurement resolution of traditional X‐ray densitometry led to the observed reduced annual MXD value. Multiple independent lines of evidence suggest these anatomical anomalies were most likely the result of direct acidic damage to trees in Northern Europe and that the normal relationship between summer temperature and MXD can be disrupted by this damage. Our study also demonstrates that quantitative wood anatomy offers a high‐resolution approach to identifying anomalous years and extreme events in the tree‐ring record.

Assessing intra-annual density fluctuations across and along white pine stems

Intra-annual density fluctuations (IADFs) are triggered by environmental cues, but whether they are distributed uniformly throughout the stem is not well documented. The spatial distribution of IADFs could help us understand variations in cambial sensitivity to environmental cues throughout the tree. We investigate how IADF distribution varies radially, longitudinally, and circumferentially within white pine (Pinus strobus L.) stems. We took wood samples at breast height, near branches, and at the top of the trees. We identified IADFs visually and measured their radial position within a ring as well as their circumferential arc in cross-sections. Intra-annual density fluctuations occurred in 22.2% of rings. The radial position of IADFs within a ring was remarkably consistent at roughly 80% of the total annual radial increment across heights, trees, and years of formation. The main factors affecting the likelihood of IADF occurrence were ring width, year of formation, and the interaction between the two. Being near branches or at the top of the tree slightly increased the probability of occurrence. Though the sample size was not large enough to provide conclusive results about the circumferential distribution of IADFs, our data suggest that the circumferential arc of the IADFs might be conserved throughout the stem.

Gönpa Gang—The First Application of Dendrochronological Dating to Study the Traditional Architecture of Upper Mustang (Nepal)

Gönpa Gang is an example of the traditional Buddhist architecture of Upper Mustang. It is also the first monument in Upper Mustang to be studied using the dendrochronological dating method. The gönpa is a two-story building of imposing size, made from simple elements of Tibetan architecture, namely masonry walls, timber posts, and beams. A total of 14 samples were collected from elements on both the ground and the first floor. The limited number of samples results from the cultural and religious character of the object under study. Only the elements consistent with the structure and the space arrangement, interpreted as original features, were examined. Microscopic observation and the analysis of the anatomical features of all 14 samples resulted in the identification of Himalayan pine (blue pine), Pinus wallichiana A.B. Jacks. Intra-annual density fluctuation, false rings, and missing rings were detected. From 14 samples collected in Gönpa Gang, 18 series were worked out. Thus, 15 series from 12 samples were synchronized and used for the development of the mean chronology, UMGG_m, with a total length of 160 rings. The chronology covers the period from 1524 to 1683. Examination of the Gang Gönpa wood resulted in the age determination of 13 elements. The results were compared with architectural stratification by Harrison and historical data from written sources. The timber used in the gönpa comes from the Southern Mustang area. The examined wood demonstrates a correlation with the timber used in the Upper Mustang historical buildings further north.

Cell-wall fluorescence highlights the phases of xylogenesis

Summary The monitoring of xylogenesis makes it possible to follow tree growth responses to stress factors in real-time, by observing the course of wood cell division and differentiation. Proper microscopy techniques are of key importance to exactly identify the xylem cells during the different phases of differentiation. We aimed to apply epifluorescence microscopy to follow the lignification process during the different phases of xylogenesis in Mediterranean softwood and hardwood. Microcores from trees of Pinus halepensis Mill. and Arbutus unedo L. were collected at a site in southern Italy, during the period June-December. Fluorescence imaging of sections stained with a water solution of safranin and Astra blue clearly highlighted the contrast between lignified and un-lignified tissue. The proposed methodology is useful to quickly and unambiguously detect the different stages of cell differentiation, as well as the progress in the lignification process. Moreover, it proved to be easily applied to demanding wood materials, such as Mediterranean woods and can be helpful to better track stress responses and the development of anomalies during wood formation, such as intra-annual density fluctuations.

Origin of Intra-annual Density Fluctuations in a Semi-arid Area of Northwestern China.

Intra-annual density fluctuation (IADF) is a structural modification of the tree ring in response to fluctuations in the weather. The expected changes in monsoon flow would lead to heterogeneous moisture conditions during the growing season and increase the occurrence of IADF in trees of the arid ecosystems of continental Asia. To reveal the timings and physiological mechanisms behind IADF formation, we monitored cambial activity and wood formation in Chinese pine (Pinus tabuliformis) during 2017–2019 at three sites in semi-arid China. We compared the dynamics of xylem formation under a drought event, testing the hypothesis that drought affects the process of cell enlargement and thus induces the production of IADF. Wood microcores collected weekly from April to October were used for anatomical analyses to estimate the timings of cambial activity, and the phases of enlargement, wall thickening, and lignification of the xylem. The first cells started enlargement from late April to early May. The last latewood cells completed differentiation in mid-September. Trees produced IADF in 2018. During that year, a drought in June limited cell production in the cambium, only 36% of the xylem cells being formed in IADF trees, compared to 68% in normal tree rings. IADF cells enlarged under drought in early July and started wall thickening during the rainfall events of late July. The drought restricted cell enlargement and affected wall thickening, resulting in narrow cells with wide walls. Cambium and cell enlargement recovered from the abundant rainfall, producing a new layer with large earlywood tracheids. IADF is a specific adaptation of trees to cope with water deficit events occurring during xylem formation. Our findings confirmed the hypothesis that the June-July drought induces latewood-like IADFs by limiting the process of cell enlargement in the xylem. Our finding suggests a higher occurrence of IADF in trees of arid and semi-arid climates of continental Asia if the changes to monsoon flows result in more frequent drought events during the earlywood formation in June.

Tree ring anatomy indices of Pinus tabuliformis revealed the shifted dominant climate factor influencing potential hydraulic function in western Qinling Mountains

Trees can adjust xylem anatomical structure related with potential hydraulic functions to cope with climate variability. We therefore need a better understanding of how climate variability constrains wood anatomy and tree radial growth. Pinus tabuliformis dominates natural forests and plantations over the western Qinling Mountains, which is one of the ecologically vulnerable areas in China. Here, we investigated the response of P. tabuliformis tree-ring anatomical structure to climate variability by applying wood anatomy analysis, and evaluated the influences of anatomical traits on potential hydraulic functions and the climate significance of intra-annual density fluctuations (IADFs). We found that with the increasing temperature from spring to summer, the negative effect of temperature on the formation and enlargement of earlywood and transition-wood tracheids was gradually enhanced. However, spring precipitation not only had a direct and positive influence on the formation of earlywood, but also had a delaying impact on the transition-wood cell enlargement. Besides, the smaller earlywood tracheid size of P. tabuliformis could be a substantially characteristic reflecting spring drought. The contribution of lumen diameter on conduit wall reinforcement was dominated in earlywood, while the contribution of cell wall thickness was greater than that of lumen diameter in latewood. The different contributions of anatomical traits on conduit wall reinforcement would further affect the response of potential hydraulic function to climate. IADFs of P. tabuliformis could be a potential indicator to reflect the abnormal summer precipitation events in the western Qinling Mountains. IADFs with strong and weak intensity indicated years with high and low rates of change in mid-summer precipitation, respectively. Future warmer and drier climate in the western Qinling Mountains will likely result in the production of smaller tracheids to ensure hydraulic safety, which means the stronger drought resistant of P. tabuliformis in the future. In this study, we linked the xylem anatomy and potential hydraulics functions with intra-seasonal climate variability in the context of climate warming and drying, and proposed some xylem anatomical indices reflecting potential drought events.

Tracheid development under a drought event producing intra-annual density fluctuations in the semi-arid China

Drought is the primary limiting factor for tree growth in arid and semi-arid regions, where it can greatly influence xylem anatomical traits and induce multiple responses to wood development. However, the mechanisms driving xylogenesis and intra-annual density fluctuations (IADFs), as well as the seasonal dynamics of wood development in water-limited environments, are still unclear. In this study, we assessed the kinetics of xylem development weekly in Chinese pine (Pinus tabuliformis) and Qinghai spruce (Picea crassifolia), at two altitudes in the Helan Mountains, semi-arid China. Results indicate that summer drought in June inhibited cambial division and reduced xylogenesis, resulting in only 40% of the xylem developing before August. The onset of abundant precipitation in mid-July reactivated radial growth. This second growth extended the growing season by one month and partly compensated for the negative effect of the drought on growth. E-type IADF were characterized by narrow cells with thick walls, formed during July and early August. The duration of cell enlargement explained 57% of variance in tracheid diameter, showing that it played an important role in determining cell size. In contrast, changes in the cell wall thickness were mainly driven by the rate of secondary wall thickening, which explained 65% of the variance. This study reveals the potential mechanisms for xylem development and E-IADF formation under summer drought in semi-arid China, provides evidence for the plasticity and resistance of Chinese pine and Qinghai spruce under varying environmental conditions, and contributes to our understanding of likely future trends in forest productivity under climate changes.

Intra-annual density fluctuations in silver fir are triggered by drought conditions

Key messageIn Abies alba Mill. (silver fir), the frequency of intra-annual density fluctuations (IADFs) increases along the latitudinal transect, from North to South, is higher in pure than in mixed stands, and their formation is linked to spring and/or summer drought conditions.Trees respond to climate, recording information in tree rings and their anatomical features, such as IADFs. The IADFs are regions within the tree ring where density changes in response to tree physiological processes and environmental conditions. The objective of this study was to assess the effect of different forest-stand structures and of climate on IADF formation and frequency in populations of silver fir along a latitudinal gradient in Italy (Trentino, Molise and Calabria regions). In doing so, we aimed to compare the frequency of IADFs in pure or mixed stands and to understand the ability of silver fir to cope with environmental fluctuations. Results showed higher frequency of IADFs in pure than mixed stands (in Trentino and Calabria) and in younger trees (namely in the pure stand in Calabria), because of the higher sensitivity to environmental variability. The formation of IADFs in silver fir emerged as a response to cope with drought. Summer precipitation (both pure and mixed stands in Trentino) and early spring/summer (both pure and mixed stands in Calabria) played a key role in the formation of IADF-type E+. The formation of IADF-type L+, on the other hand, was related to temperature/precipitation in late summer and early autumn (both pure and mixed stands in Molise) and to precipitation in summer (pure stand in Calabria). Our findings support the theory that IADFs are an important structural/functional mechanism for responding to climate fluctuations.

Linking tree growth and intra-annual density fluctuations to climate in suppressed and dominant Pinus sylvestris L. trees in the forest-steppe of Southern Siberia

Climate change affects forest dynamics with potential consequences for essential ecosystem services. The retrospective analysis of secondary growth unveils the effect of climate on forests. However, most tree-ring studies focus on dominant trees, and less is known about the climatic response of their neighbor suppressed trees. We evaluated the influence of tree social status (dominant/suppressed) on climate response in Pinus sylvestris L. trees from two sites with contrasting water availability conditions in the forest-steppe ecotone in southern Siberia. Tree-ring width and intra-annual density fluctuations (IADFs) were used as proxies. Late spring to early summer conditions were the main climate drivers in both tree social status, but the climate response of suppressed trees was stronger and had a longer time window (May-June). IADFs’ occurrence was controlled by temperature and its frequency was modulated by local conditions, being more common at the dry site, with tree status just marginally significant. Our results suggest that under the projected warmer and drier climate, suppressed trees in southern Siberia will be prone to increased water shortage, leading to possible higher mortality of more sensitive suppressed trees, with potential consequences for carbon sequestration in the forest-steppe ecosystems in southern Siberia.

Physiological and anatomical responses to drought stress differ between two larch species and their hybrid

Key MessageHybrid saplings were more reactive to soil water deficit than Japanese and European larch. European larch had hydraulically safer wood and anisohydric behavior, Japanese and hybrid larch showed isohydric strategy.Deciduous larch species could be an alternative to evergreen conifers in reforestation, but little is known about drought sensitivity of their saplings. The effect of an experimental drought on hydraulics and quantitative wood anatomy was tested on saplings of European larch (EL, Larix decidua), Japanese larch (JL, Larix kaempferi) and their hybrid (HL). Across species, biomass, transpiration rate and relative water content were higher in controls than in drought stressed trees, but transpiration efficiency was lower. JL had the highest transpiration efficiency under drought, and EL the lowest, coinciding with slower growth of EL. Wood of EL formed before drought was hydraulically safer as shown by higher wall/lumen ratio and lower pit cavity area. EL neither had a significant increase in transpiration efficiency nor a reduction in transpiration rate under drought, suggesting that the stomata remained open under soil water deficit. HL saplings were the most reactive to water shortage, indicated by intra-annual density fluctuations and a decrease in relative water content of the sapwood. Significant reduction in transpiration by HL suggested a higher stomatal sensitivity, while the same leaf surface area was maintained and radial growth was still similar to its best parent, the JL. The latter showed a significantly lower leaf surface area under drought than controls. EL, with its hydraulically safer wood, followed an anisohydric behavior, while JL and HL revealed an isohydric strategy. Altogether, our results suggest species dependent acclimations to drought stress, whereby HL followed the strategy of JL rather than that of EL.

Climate change impacts on spatial distribution, tree-ring growth, and water use of stone pine (Pinus pinea L.) forests in the Mediterranean region and silvicultural practices to limit those impacts

Stone pine (Pinus pinea L.) has been cultivated since centuries in Mediterranean areas for its products and economic benefits, including edible pine nuts, timber, mushrooms, firewood, and grazing. However, current management objectives of stone pine stands also include recreational use, biodiversity conservation, protection from soil erosion, and CO2 fixation. Stone pine stands are considered to be among the ecosystems most vulnerable to climate change, and the current increase in drought frequency in the Mediterranean Basin has been shown to negatively impact their long-term establishment. Understanding the effects of climate change on the distribution, tree-ring growth and water use of stone pine forests can help assessing the adaptive capacity of the species, and developing management programs aimed at its conservation. This paper reviews the impacts of climate change on stone pine in the Mediterranean region. The high sensitivity of stone pine to climate change has been widely demonstrated in that: (i) climatic models predict the loss of suitable habitats and the shift of its geographical distribution in the next future; (ii) tree-ring analysis showed that winter and spring rainfalls have positive effects on growth, whereas high spring temperature has a negative effect; (iii) the strategy of stone pine to cope with water deficit affects the processes regulating its growth, including wood formation, leading to peculiar tree-ring anatomical features such as intra-annual density fluctuations. The silvicultural interventions and the most effective management strategies for stone pine forests are reviewed and discussed in the context of current climate change in the Mediterranean Basin.

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.