Differences in tree and shrub growth responses to climate change in a boreal forest in China

Global climate change has led to rising temperatures and drought in boreal forests in Northeast China. In some areas, shrubs and trees coexist in high altitude and high latitude areas, and their differences with global warming may lead to significant changes in vegetation composition and distribution. Therefore, we compared the relationships between climate and growth for the most widely distributed dwarf shrub (Pinus pumila) and the two dominant tree species (Larix gmelinii and Pinus sylvestris var. mongolica) in boreal forests in the Daxing’an Mountains, China. A total of 340 tree-ring cores from 172 trees and 64 discs from shrubs were collected from four sites and compared the responses of shrub and tree growth to climate patterns using dendrochronological methods. The shrub and two tree species responded differently to interannual climate variance. The negative effect of growing season temperature was greater on growth of L.gmelinii and P. sylvestrisvar.mongolica than on P. pumila, and the promoting effect of winter and spring precipitation was greatest on P. pumila. Compared with the two tree species, P. pumila had a higher temperature threshold and grew over a shorter growing season. Our findings suggested that L. gmelinii and P. sylvestrisvar.mongolica are more susceptible to global warming than the shrubs that coexist with them. However, P.pumila should be studied from an individual perspective in the future due to the dwarf morphology of shrubs and their complex microenvironment.     

Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy

Background and Aims

Dendroclimatology is playing an important role in understanding past climatic changes on the Tibetan Plateau. Forests, however, are mainly confined to the eastern Tibetan Plateau. On the central Tibetan Plateau, in contrast, shrubs and dwarf shrubs need to be studied instead of trees as a source of climate information. The objectives of this study were to check the dendrochronological potential of the dwarf shrub Wilson juniper (Juniperus pingii var. wilsonii) growing from 4740 to 4780 m a.s.l. and to identify the climatic factors controlling its radial growth.

Methods

Forty-three discs from 33 stems of Wilson juniper were sampled near the north-eastern shore of the Nam Co (Heavenly Lake). Cross-dating was performed along two directions of each stem, avoiding the compression-wood side as far as possible. A ring-width chronology was developed after a negative exponential function or a straight line of any slope had been fit to the raw measurements. Then, correlations were calculated between the standard ring-width chronology and monthly climate data recorded by a weather station around 100 km away.

Key Results

Our study has shown high dendrochronological potential of Wilson juniper, based on its longevity (one individual was 324 years old), well-defined growth rings, reliable cross-dating between individuals and distinct climatic signals reflected by the ring-width variability. Unlike dwarf shrubs in the circum-arctic tundra ecosystem which positively responded to above-average temperature in the growing season, moisture turned out to be growth limiting for Wilson juniper, particularly the loss of moisture caused by high maximum temperatures in May–June.

Conclusions

Because of the wide distribution of shrub and dwarf shrub species on the central Tibetan Plateau, an exciting prospect was opened up to extend the presently existing tree-ring networks far up into one of the largest tundra regions of the world.     

Arboreal and prostrate conifers coexisting in Mediterranean high mountains differ in their climatic responses

In contrast to most high elevation areas, plant growth at Mediterranean mountains is exposed to a summer drought period, which represents an additional climatic constraint to low temperatures. Although arboreal and shrubby conifers coexist at high altitudes, most dendroecological studies have focused on climatic responses of tree species, whereas those of shrubby species remain mostly unexplored. We built tree-ring width chronologies for two conifer species, a shrub (Juniperus sabina) and a tree (Pinus sylvestris), coexisting at three high-altitude localities of the Iberian System mountains, eastern Spain. We analyzed their climate–growth relationships for the period 1950–2009 using correlation analyses and multiple regressions. Coexisting species responded to year-to-year climatic variability in different ways. Radial growth in junipers and pines responded positively to April and May temperatures, respectively. Summer drought constrained growth in both cases, although its impact was stronger on junipers than on pines. Our findings suggest that junipers respond earlier than pines to spring temperatures due to their prostrate morphology which may enhance a fast warming of their cambial meristems after snowmelt. The higher dependence of J. sabina on summer rainfall as compared with co-occurring pines confirms that drought stress negatively impacts secondary growth in Mediterranean mountains. This sensitivity to water availability may be caused by the juniper shallow root systems, which mainly use superficial soil water. The climatic signal registered in J. sabina allows studying the response of other similar shrubby woody species growing in Mediterranean alpine areas to the ongoing climate warming, which could also reduce water availability.     

Nonstructural carbohydrate allocation following different frequencies of simulated browsing in three semi-arid shrubs

Nonstructural carbohydrate allocation patterns in response to different frequencies of simulated browsing (leaf and twig removal) were studied in the following semi-arid shrubs: Osteospermum sinuatum, a dwarf deciduous shrub, Pteronia pallens, a dwarf evergreen shrub, and Ruschia spinosa, a dwarf leaf-succulent shrub. Simulated browsing at all frequencies resulted in the elevation, or had no effect, on total nonstructural carbohydrate (TNC) concentrations of O. sinuatum plant parts, and resulted in the decrease in TNC concentrations of R. spinosa plant parts. The responses of P. pallens were intermediate with elevations as well as declines in TNC concentrations of plant parts measured in response to various clipping frequencies. At the low frequency of simulated browsing (every 26 weeks) elevations in plant TNC content were measured in the two non-succulent shrubs O. sinuatum and P. pallens. It was concluded that the overcompensation with respect to TNC accumulation observed in the two non-succulent species represents one of the ways in which excess photosynthate is utilized by browsed shrubs with a limited regiowth potential. Simulated browsing was the least detrimental with respect to biomass production to the non-succulent O. sinuatum and P. pallens, and most injurious to the leaf-succulent shrub, R. spinosa. The observed TNC allocation patterns could not adequately explain the variation among species in the production of new growth and it was concluded that some factor(s) other than the carbon resource was limiting regrowth.     

Abiotic factors modulate post-drought growth resilience of Scots pine plantations and rear-edge Scots pine and oak forests

The proportion of planted forests in the Mediterranean Basin is one of the largest in the world. These plantations are dominated by pine species and present a series of characteristics such as low elevation, high competition or small tree size that make them more vulnerable to droughts. However, quantitative assessments of their post-drought growth resilience in accordance with species, site factors and tree characteristics are lacking. In this study we sampled 164 trees at four forest sites located in the drought-prone Sierra Nevada, southeastern Spain. We compared growth responsiveness to drought in rear-edge planted vs. relic natural Scots pine (Pinus sylvestris) and coexisting Pyrenean oak (Quercus pyrenaica) stands. Our objective was to characterize and compare the different growth responses to drought between species and sites and the effect of the main physiographic factors (altitude, aspect, and slope) on these responses since the influence of these factors on post-drought resistance and resilience has received little attention to date. Our results reveal that the planted pine sites with the lowest mean growth rates displayed greater resistance during drought, and that higher altitude was associated with improved resistance and/or resilience for all species and sites. Natural pine and Pyrenean oak stands were better adapted to the dry climatic conditions of the Mediterranean region where the study was undertaken, displaying greater resistance and/or resilience and lower influence of drought on growth in comparison to stands of planted pines. These results suggest that promoting the conservation of high-elevation pine plantations and enhancing the regeneration of natural pine and oak may improve the resistance and resilience of these drought-prone forest ecosystems.     

Limits of pine forest distribution at the treeline: herbivory matters

Herbivores can affect future forest composition by feeding selectivity. At temperature-sensitive treelines, herbivory can exacerbate or constrain climate-driven distributional shifts in tree species. This study analyses the impact of herbivory in a Mediterranean treeline of widespread Pinus sylvestris and P. nigra pinewoods, testing whether herbivory damage reinforces or inhibits the climatic responses of these trees. We used naturally occurring sapling pairs of similar size and age of both species, thereby isolating plant characteristics from environmental effects in herbivore behaviour. Herbivory damage by ungulates proved higher than that caused by insects in saplings of both species. Low plant density and extreme abiotic conditions at the treeline could in part be responsible for the observed low incidence of insect herbivory. Ungulates preferred P. sylvestris over P. nigra, implying heavier browsing damage for a large number of P. sylvestris saplings, suffering reduced internode growth as a consequence. In addition, P. sylvestris could not compensate height-growth reductions due to browsing with higher growth rate than P. nigra. In fact, P. sylvestris showed similar or lower relative height growth with respect to P. nigra. Under a scenario of increasing aridity and maintenance of ungulate populations, the upward migration of P. sylvestris in its southern range could be restricted by higher drought vulnerability than P. nigra, a situation exacerbated by ungulate herbivory. Our results indicate that ungulate herbivory reinforces climatic response of coexisting P. sylvestris and P. nigra at treeline, favouring a potential change in community dominance towards Mediterranean P. nigra.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Developmental instability as an index of adaptation to drought stress in a Mediterranean oak

An increase in temperature and water deficits caused by the ongoing climate change might lead to a decline growth rates and threaten the persistence of tree species in drought-prone areas within the Mediterranean Basin. Developmental instability (the error in development caused by stress) may provide an index of the adaptability of woody plants to withstand climatic stressors such as water shortage. This study evaluated the effects of drought stress on growth variables in three stands of a Mediterranean oak (Quercus faginea) exposed to differing climatic conditions (xeric, mesic and cooler) along an altitudinal gradient in northeastern Spain, in two climatically contrasting years (wet and dry years). Two indices of developmental instability, fluctuating and translational asymmetries, which reflect environmental stress, were measured in leaves and current-year shoots, respectively. We also measured branch biomass and fractal complexity of branches as indicators of the species’ performance. After a period of drought the individuals’ at the most xeric site presented lower developmental instability and less branch biomass than did the individuals from the mesic and cooler sites. We interpret that difference as an adaptive response to drought which reflects a trade-off between maintenance of homeostasis and growth when water is scarce. The study demonstrated that developmental instability constitutes a useful index to assess the degree of adaptation to stressful environmental conditions. The assessment of developmental instability in sites and years with contrasting climatic conditions provides a means of quantifying the capacity of plants to develop plastic adaptive responses to climatic stress.     

Contrasting seasonal overlaps between primary and secondary growth are linked to wood anatomy in Mediterranean sub‐shrubs

Whole‐plant approaches allow quantification of the temporal overlap between primary and secondary growth. If the amount of time available to grow is short, there may be a high temporal overlap between shoot growth and wood formation. We hypothesise that such overlap depends on the duration of the growing season and relates to wood anatomy. We evaluated wood anatomy, shoot longitudinal and radial growth rates, fine root production and the concentrations of non‐structural carbohydrates (NSC) in the wood of six sub‐shrub species growing in sites with contrasting climatic conditions (Lepidium subulatum, Linum suffruticosum, Salvia lavandulifolia, Satureja montana, Ononis fruticosa, Echinospartum horridum). Sub‐shrub species living in sites with a short growing season displayed a high overlap between aboveground primary and secondary growth and formed wide vessels, whereas species from the warmest and driest sites presented the reverse characteristics. The highest overlap was linked to a rapid shoot extension and thickening through the enhanced hydraulic conductivity provided by wide vessels. The reductions in NSC concentrations when growth peaked were low or moderate, indicating that sub‐shrubs accumulate NSC in excess, as do trees. The temporal overlap among primary and secondary growth in woody plants may be connected to the duration and rates of shoot and wood growth, which in turn depend on the vessel lumen area.     

When do nurse plants stop nursing? Temporal changes in water stress levels in Austrocedrus chilensis growing within and outside shrubs

Question: Does the proximity of shrubs affect seasonal water stress of young Austrocedrus chilensis trees (a native conifer of the Austral Temperate Forest of South America) in xeric sites? Location: A. chilensis xeric forest in northwest Patagonia, Argentina. Methods: We examined the dependence of predawn twig water potential on tree development (seedling to adult) and proximity to nurse shrubs during spring and summer. We analysed spatial associations of seedlings, saplings and adult trees with nurse shrubs, and also evaluated if trees affected shrub canopy vitality. Results: Water stress in Austrocedrus trees was affected by shrub presence. Small trees (i.e.<0.5 m in height) growing in the open were most stressed, particularly in summer. Small trees growing within a shrub canopy had low water stress and little change between spring and summer. The opposite trend, however, was true for the medium‐height category (i.e. 0.5‐1.5 m in height); trees in this size category were more stressed when growing within the shrub canopy than in the open. Larger Austrocedrus trees (i.e.>2 m in height) were not affected by shrub presence. Austrocedrus trees were spatially associated with shrubs in all height classes; however, the percentage of living shrub canopy decreased with tree height. Conclusions: In xeric areas of northwest Patagonia, the strength and direction of interactions between A. chilensis and shrubs, in terms of tree water stress, are dynamic and modulated by tree size and environmental conditions. Overall, positive effects of shrubs on early developmental stages appear to be more important than subsequent negative interactions, since nursing effects could generate a spatial association of shrubs and Austrocedrus trees that persists through later successional stages. These findings shed light on mechanisms behind successional changes, and have important conservation and management implications.     

Dendrochronological analysis of white oak growth patterns across a topographic moisture gradient in southern Ohio

Moisture availability is a key factor that influences white oak (Quercus alba L.) growth and wood production. In unglaciated eastern North America, available soil moisture varies greatly along topographic and edaphic gradients. This study was aimed at determining the effects of soil moisture variability and macroclimate on white oak growth in mixed-oak forests of southern Ohio. Using accurately dated and measured tree rings, we analyzed 119 white oaks growing across an integrated moisture index (IMI), a computer-generated GIS model that simultaneously combines topographic and edaphic features into a moisture index scale. Growth trends varied considerably across the IMI, with trees in mesic sites exhibiting patterns much different from those in either xeric or intermediate sites. BAI growth and biomass increments were higher for trees growing in the intermediate and mesic sites than those from the xeric sites. Correlation and response function analyses, and redundancy analysis revealed significant relations between ring-width indices and climate, with current year May–July PDSI, precipitation and temperature as the most important correlates of white oak growth. Additionally, climatic influences on growth rate were variable across the IMI; trees in xeric sites showed much greater coefficients relative to those from the intermediate and mesic sites. Despite these differences, xeric and intermediate trees exhibited similar growth patterns. The present results provide further evidence of the usefulness of the IMI for identifying and comparing white oak growth patterns across the complex, dissected landscape of southern Ohio.     

Growth and stable isotope signals associated with drought-related mortality in saplings of two coexisting pine species

Drought-induced events of massive tree mortality appear to be increasing worldwide. Species-specific vulnerability to drought mortality may alter patterns of species diversity and affect future forest composition. We have explored the consequences of the extreme drought of 2005, which caused high sapling mortality (approx. 50 %) among 10-year-old saplings of two coexisting pine species in the Mediterranean mountains of Sierra Nevada (Spain): boreo-alpine Pinus sylvestris and Mediterranean P. nigra. Sapling height growth, leaf δ¹³C and δ¹⁸O, and foliar nitrogen concentration in the four most recent leaf cohorts were measured in dead and surviving saplings. The foliar isotopic composition of dead saplings (which reflects time-integrated leaf gas-exchange until mortality) displayed sharp increases in both δ¹³C and δ¹⁸O during the extreme drought of 2005, suggesting an important role of stomatal conductance (g_s) reduction and diffusional limitations to photosynthesis in mortality. While P. nigra showed decreased growth in 2005 compared to the previous wetter year, P. sylvestris maintained similar growth levels in both years. Decreased growth, coupled with a sharper increase in foliar δ¹⁸O during extreme drought in dead saplings, indicate a more conservative water use strategy for P. nigra. The different physiological behavior of the two pine species in response to drought (further supported by data from surviving saplings) may have influenced 2005 mortality rates, which contributed to 2.4-fold greater survival for P. nigra over the lifespan of the saplings. This species-specific vulnerability to extreme drought could lead to changes in dominance and distribution of pine species in Mediterranean mountain forests.     

Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO₂ balance to climatic warming. We tested this hypothesis in a 19‐year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore‐induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.     

Growth and carbon isotopes of Mediterranean trees reveal contrasting responses to increased carbon dioxide and drought

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species (Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO₂ concentrations (C _a) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21 %) between the 1970s and the 2000s. This increase was positively related to C _a for J. thurifera and to higher C _a and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO₂ uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C _a even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.     

Growth form and leaf habit drive contrasting effects of Arctic amplification in long-lived woody species

Current global change is inducing heterogeneous warming trends worldwide, with faster rates at higher latitudes in the Northern Hemisphere. Consequently, tundra vegetation is experiencing an increase in growth rate and uneven but expanding distribution. Yet, the drivers of this heterogeneity in woody species responses are still unclear. Here, applying a retrospective approach and focusing on long-term responses, we aim to get insight into growth trends and climate sensitivity of long-lived woody species belonging to different functional types with contrasting growth forms and leaf habits (shrub vs. tree and deciduous vs. evergreen). A total of 530 samples from 7 species (common juniper, dwarf birch, woolly willow, Norway spruce, lodgepole pine, rowan, and downy birch) were collected in 10 sites across Iceland. We modelled growth trends and contrasted yearly ring-width measurements, filtering in high- and low-frequency components, with precipitation, land- and sea-surface temperature records (1967–2018). Shrubs and trees showed divergent growth trends, with shrubs closely tracking the recent warming, whereas trees, especially broadleaved, showed strong fluctuations but no long-term growth trends. Secondary growth, particularly the high-frequency component, was positively correlated with summer temperatures for most of the species. On the contrary, growth responses to sea surface temperature, especially in the low frequency, were highly diverging between growth forms, with a strong positive association for shrubs and a negative for trees. Within comparable vegetation assemblage, long-lived woody species could show contrasting responses to similar climatic conditions. Given the predominant role of oceanic masses in shaping climate patterns in the Arctic and Low Arctic, further investigations are needed to deepen the knowledge on the complex interplay between coastal tundra ecosystems and land-sea surface temperature dynamics.     

高寒山生柳灌丛的群落特征研究

山生柳（Salix oritrepha）是我国的特有种。通过对山生柳灌丛植物群落结构特征的定量分析结果表明：该类型由97种植物组成,隶属30科65属。其中有灌木10种,草本植物87种,山生柳为建群种;地理成分以北温带分布为主;生活型以地面芽植物为主（52.59%）,其次为地下芽植物（27.89%）和高位芽（10.31%）,地上芽和一年生植物仅占9.21%;群落结构比较简单,大体可以分为灌木层和草本层植物。灌木层物种多样性指数偏低,垂直结构各层次间物种多样性大小为草本层>灌木层。     

Sancho,the oldest known Iberian shrub

Cliffs are refuges for old trees and shrubs. In the Mediterranean Basin most dendroclimatic reconstructions have focused on high-elevation forests where tree radial growth is constrained by low temperatures in addition to drought stress. Old shrubs may provide longer ring-width series of hydroclimate proxies in low-elevation, drought-prone Mediterranean ecosystems where old trees are rare. To fill this research gap we investigated the maximum age and climate sensitivity of young, old, and recently dead Phoenician junipers (Juniperus phoenicea L.), growing on calcareous cliffs and nearby plains, in the Guara Natural Park (northeast Spain). The oldest living juniper was ¹⁴C-dated to be 927 years old, and it was named “Sancho” after Don Quixote’s squire. Based on ring counts, the maximum age was 655 years. The difference in age estimates between the ¹⁴C-dates and ring counts was 39 years indicating that ring counts underestimate age. This was due to missing and wedging rings making the cross-dating of old junipers unfeasible. Cool and wet conditions from May to July enhanced radial growth of young junipers. Old shrubs have a high dendroecological potential in Mediterranean sites where their growth is constrained by warm-dry conditions during the growing-season. Further techniques combining dendrochronological and wiggle-match ¹⁴C dating may allow reconstructing long-term hydroclimate in low-elevation Mediterranean areas.     

Drought signal in the tree rings of three conifer species from Northern Pakistan

Anthropogenic and climatic stressors have affected the forests of northern Pakistan in recent decades. Several studies have been conducted to understand forest growth and its relation to the changing climate in this region, but more work needs to be done to understand this complex environment. In this study, we have collected tree core samples of three conifer species (Pinus wallichiana, Picea smithiana, and Abies pindrow) from three different sites in northern Pakistan to understand their radial growth pattern with the goal of finding a relationship between ring-width and climatic parameters (temperature, precipitation, and drought). A 610-year (AD 1406–2015), a 538-year (AD 1478–2015), and a 306-year (AD 1710–2015) long tree-ring width chronology of Pinus wallichiana, Picea smithiana, and Abies pindrow were developed, respectively, using living trees. The ring-width chronologies of these three species showed a strong positive link with the self-calibrated Palmer Drought Severity Index (scPDSI) rather than precipitation or temperature alone, indicating that soil moisture is the primary limiting climatic factor for the growth of these species in the sampling locations. The chronologies of Pinus wallichiana and Picea smithiana exhibited growth suppressions during AD 1570–1610 and the second half of 17th century while their growth was heightened from AD 1540–1560. We have found the lowest growth in Abies pindrow and Picea smithiana from AD 1900–1920, suggesting dry conditions. All three chronologies have exhibited the most rapid increase in growth during the recent decades, suggesting that this region is experiencing climate change with a strong trend towards wetter conditions.     

A Neighborhood Analysis of the Consequences of Quercus suber Decline for Regeneration Dynamics in Mediterranean Forests

In forests, the vulnerable seedling stage is largely influenced by the canopy, which modifies the surrounding environment. Consequently, any alteration in the characteristics of the canopy, such as those promoted by forest dieback, might impact regeneration dynamics. Our work analyzes the interaction between canopy neighbors and seedlings in Mediterranean forests affected by the decline of their dominant species (Quercus suber). Our objective was to understand how the impacts of neighbor trees and shrubs on recruitment could affect future dynamics of these declining forests. Seeds of the three dominant tree species (Quercus suber, Olea europaea and Quercus canariensis) were sown in six sites during two consecutive years. Using a spatially-explicit, neighborhood approach we developed models that explained the observed spatial variation in seedling emergence, survival, growth and photochemical efficiency as a function of the size, identity, health, abundance and distribution of adult trees and shrubs in the neighborhood. We found strong neighborhood effects for all the performance estimators, particularly seedling emergence and survival. Tree neighbors positively affected emergence, independently of species identity or health. Alternatively, seedling survival was much lower in neighborhoods dominated by defoliated and dead Q. suber trees than in neighborhoods dominated by healthy trees. For the two oak species, these negative effects were consistent over the three years of the experimental seedlings. These results indicate that ongoing changes in species’ relative abundance and canopy trees’ health might alter the successional trajectories of Mediterranean oak-forests through neighbor-specific impacts on seedlings. The recruitment failure of dominant late-successional oaks in the gaps opened after Q. suber death would indirectly favor the establishment of other coexisting woody species, such as drought-tolerant shrubs. This could lead current forests to shift into open systems with lower tree cover. Adult canopy decline would therefore represent an additional factor threatening the recruitment of Quercus forests worldwide.     

Variation in radial growth responses to drought among species, site, and canopy strata

 Radial growth responses to drought were examined in the tree-ring records of six species growing within two locations of differing land-use history and soil moisture characteristics, and in overstory and understory canopy positions in northern Virginia. Tree species experienced differential ring-width reductions during or immediately following four severe drought periods occurring from 1930 to 1965 and were influenced by climatic variables including annual and summer temperatures, annual precipitation, and annual Palmer Drought Severity Index. Relative growth comparisons averaged across species before and after drought years indicated that understory trees on dry-mesic sites grew 11% faster after drought compared to pre-drought rates while mesic site trees in both canopy positions grew approximately 4% slower. Superposed epoch analysis indicated that Liriodendron tulipifera growing on mesic sites experienced greater ring-width reductions associated with drought than co-occurring, more drought-tolerant Quercus alba and Q. velutina. On dry-mesic sites, L. tulipifera also experienced greatly reduced growth as a result of drought but exhibited significant growth increases following individual drought events. Quercus alba was the only species that exhibited a consistent, significant ring-width decrease associated with all droughts on dry-mesic sites. In contrast, Pinus virginiana was least impacted by drought on dry-mesic sites but was much more impacted by drought on mesic sites, indicating a drought×site interaction for this species. Overstory Carya glabra and Q. alba experienced larger growth decreases during drought on dry-mesic versus mesic sites. Understory tree growth reductions did not differ between site types but were often significantly larger than overstory responses of the same species on mesic sites. Following drought, most trees exhibited growth reductions lasting 2–3 years, although several species experienced reductions lasting up to 6 years. The results of this study suggest that tree rings represent an important long-term proxy for leaf-level ecophysiological measurements of growth responses to drought periods. Received: 31 July 1996 / Accepted: 16 April 1997