Radial and stand‐level thinning treatments: 15‐year growth response of legacy ponderosa and Jeffrey pine trees

Restoration efforts to improve vigor of large, old trees and decrease risk to high‐intensity wildland fire and drought‐mediated insect mortality often include reductions in stand density. We examined 15‐year growth response of old ponderosa pine (Pinus ponderosa) and Jeffrey pine (Pinus jeffreyi) trees in northeastern California, U.S.A. to two levels of thinning treatments compared to an untreated (control) area. Density reductions involved radial thinning (thinning 9.1 m around individual trees) and stand thinning. Annual tree growth in the stand thinning increased immediately following treatment and was sustained over the 15 years. In contrast, radial thinning did not increase growth, but slowed decline compared to control trees. Available soil moisture was higher in the stand thinning than the control for 5 years post‐treatment and likely extended seasonal tree growth. Our results show that large, old trees can respond to restoration thinning treatments, but that the level of thinning impacts this response. Stand thinning must be sufficiently intensive to improve old tree growth and health, in part due to increasing available soil moisture. Importantly, focusing stand density reductions around the immediate neighborhood of legacy trees was insufficient to elicit a growth response, calling into question treatments attempting to increase vigor of legacy trees while still maintaining closed canopies in dry, coniferous forest types. Although radial thinning did not affect tree growth rates, this treatment may still achieve other resource objectives not studied here, such as protecting wildlife habitat, reducing the risk of severe fire injury, and decreasing susceptibility to bark beetle attacks.     

辽东山区蒙古栎径向生长对林分密度和气候因子的响应

为探究林分密度和气候因子对蒙古栎径向生长的影响,利用树木年代学方法研究了不同林分密度调控(间伐)下次生蒙古栎林径向生长变化,并结合气象数据,分析了蒙古栎生长变化的驱动因子。结果表明: 次生蒙古栎林径向生长受林分密度的影响显著。低密度原始林蒙古栎径向年均增长量为3.12 mm,2个中密度次生蒙古栎林分别为1.55和1.42 mm,高密度次生蒙古栎林为0.96 mm。20%的间伐强度对促进高密度(1900 株·hm^-2以上)栎林径向生长恢复作用有限,而对于中等密度(1600 株·hm^-2)栎林效果显著。该地区蒙古栎径向生长主要对当年1月和2月的降水变化敏感。抚育间伐降低了蒙古栎径向生长对气候因子的敏感性。在未来暖干化的气候情境下,密度调控有利于减缓气候变化对蒙古栎生长的不利影响。     

Managing red pine stand structure to mitigate drought impacts

Reducing forest stand density through silvicultural thinning has demonstrated potential to mitigate drought impacts on growth; however, less has been studied on how changes in stand structure created by different thinning methods influence forest growth responses to drought. This research examined the growth responses to drought of natural-origin red pine in a long-term study contrasting thinning methods. Dendrochronological methods were used to examine growth responses during several drought events among stands where different thinning methods have been applied since 1950. Growth responses to drought were expressed as resistance (maintaining growth during drought), and resilience (regaining pre-drought growth). Results indicate that periodic thinning from above, which resulted in smaller diameters, has the potential to moderate drought-induced growth reductions. Larger tree diameters negatively influenced tree-level resistance and resilience across all treatments; however, the proportion of dominant trees in a stand had contrasting effects on stand-level drought responses. Stands thinned from above exhibited more complex vertical structure and increased stand-level resistance and resilience to drought-induced growth declines because competition is more stratified among smaller diameter trees. Opposite trends were observed in stands thinned from below, where the larger diameters and monolayered structure create greater competition among trees of similar size and crown position. The results of this study highlight the utility in managing for greater structural diversity to mitigate the negative effects of drought in red pine forest ecosystems.     

Growth ring response in Paratecoma peroba (Record) Kuhlm. of seasonal semideciduous forest in Southeast Brazil

Seasonal climate can influence the radial growth of woody species, resulting in physical indications of regional environmental events that are identifiable through dendrochronological analysis. The objective of this work was to investigate the dendroclimatological potential of the Paratecoma peroba occurring in the last remnant of seasonal semideciduous forest in Rio de Janeiro, Brazil, and to characterize the anatomical structure of its growth rings. This important species has a large ecological role in the forest as a late succession but is classified as “Endangered" because it is extensively and illegally exploited due to the use potential of wood. Samples were collected and analyzed using dendrochronological methods and wood anatomy. P. peroba has distinct annual growth rings with diffuse to semi-porous porosity, marginal parenchyma, radial flattening, and greater thickening of the fibre wall in the latewood. It may sometimes present weakly distended rays and a higher frequency of vessels in the initial wood. The analyzed individuals are 30–77 years of age and exhibit a radial increase of 2–4 mm year^-1. Our results indicate that precipitation and temperature influence the growth of P. peroba in this forest and that temperature is the climatic factor with the most influence on the growth ring of the species. Given the importance of the species and the study area, knowing its growth rates and the factors that influence it is possible to offer better criteria for managing species and its reforestation for conservation.     

Calibrating the radial-growth averaging method for detecting releases in old-growth forests of coastal British Columbia,Canada

Dendroecological methods that use growth releases to reconstruct the history of canopy disturbances are most useful when calibrated for specific species in specific forest types. In this study, we calibrate the radial-growth averaging method to detect growth releases of western redcedar (Thuja plicata Donn ex D. Don), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Pacific silver fir (Abies amabilis Dougl. ex J. Forbes) following fine-scale canopy disturbances in old-growth forests of coastal British Columbia, Canada. Our goal is to establish a version of the method that will capture the full range of growth increases that occur for the study species following natural, fine-scale canopy gaps, yet will account for the effects of climatic variability on radial growth and growth increases. We used traditional dendroclimatological techniques and the radial-growth averaging method to examine how climatic and regional-scale factors contributed to radial growth and growth increases. In addition, we did a sensitivity analysis, using both ring widths and basal area increments, to explore how varying the values of three parameters of the radial-growth averaging method (threshold, moving average, and window) influenced the proportion of trees that showed a release pattern following the formation of canopy gaps of known timing of origin. Although radial growth of western redcedar and western hemlock was significantly associated with climate, percent-growth change derived from residual chronologies rarely exceeded 25%, which defined our minimum threshold for a release. For the sensitivity analysis, two general trends were common to all three species: (1) as threshold increased, the proportion of trees that showed a release pattern decreased, particularly for western redcedar and (2) a higher proportion of trees showed a release pattern using a 10-year versus a 5-year window, particularly for thresholds <100%. The greatest proportion of trees showed a release pattern using a 25% threshold, 5-year moving average, and 10-year window for both ring widths and basal area increments. Overall, a higher proportion of trees showed a release pattern using basal area increments as opposed to ring widths. Therefore, basal area increments are better suited to assess releases in these old-growth stands that have large inter- and intra-species variability in tree size. By establishing these empirically-based criteria, we have achieved the first step towards quantifying attributes of growth releases of trees in the study stands, allowing future studies to capture the variability of past disturbance events and predict changes in forest structure and composition over time.     

Divergent growth responses of healthy and declining spruce trees to climatic stress: A case study from the Western Carpathians

Long-lived tree species optimize resource allocation processes, and there are likely trade-offs between the growth and response to environmental stressful conditions. The study aims to investigate the growth of healthy and declining Norway spruce trees using dendrochronological analysis. We have selected a natural high mountain spruce population and established 27 sampling plots in Poľana Mts. We measured 270 trees, from which we visually evaluated 133 individuals. Moreover, 51 trees were sampled for increment cores. Our study reveals thatspecific extreme combination of winter, spring and summer temperatures and precipitation sums within the particular year 1974 induced long-term growth divergence of neighboring spruce trees of comparable age and size and differentiated between healthy and declining (highly defoliated) trees. This period accounts for a decrease in size of relativized increments by 30.2% in declining trees compared to visually healthy individuals. Trees with a low ability to recover from climatic stress compensate the response to environmental conditions by lower growth rates. In high mountain temperate forests, the long-term growth decline of spruce trees last about 40 years. The single-tree selection silvicultural system should be preferred in high mountain conditions, where frequent small-scale, low-intensity disturbances drive stand dynamics. Trees showing low growth resilience to revealed combination of weather conditions should be preferentially removed from stands , especially within the transformation of even-aged spruce monocultures using selective cuttings.     

Tree-ring growth varies with climate and stand density in a red pine plantation forest in the Great Lakes region of North America

Red pine (Pinus resinosa Ait.) was widely planted across the Great Lakes region of North America in the early 20th century to restore tree cover to degraded forest and agricultural lands. In this study, a dendrochronological assessment of radial growth response to climate was conducted in an 82-year-old, previously thinned red pine plantation forest in southern Ontario, Canada. Climate-growth relationships were analyzed at multi-monthly and annual time scales using a 72-year residual growth chronology (1942–2013). Warmer temperatures and periodic drought during the current and previous growing seasons were associated with decreased growth, while higher precipitation during the early part of the current growing season was associated with increased growth. Moving interval correlation analysis of long-term trends indicated that climate-growth relations were temporally unstable due to thinning and variation in climate over the length of the chronology. The correlation between climate and growth was stronger when stand density was relatively high and diminished in the two decades following thinning. These results indicate that growth of red pine plantations near the species’ southern range limit may be much reduced if exposed to a warmer, drier future climate and that periodic thinning can help mitigate the impacts of future climate change on these plantations.     

The carbon costs of mitigating high‐severity wildfire in southwestern ponderosa pine

Forests provide climate change mitigation benefit by sequestering carbon during growth. This benefit can be reversed by both human and natural disturbances. While some disturbances such as hurricanes are beyond the control of humans, extensive research in dry, temperate forests indicates that wildfire severity can be altered as a function of forest fuels and stand structural manipulations. The purpose of this study was to determine if current aboveground forest carbon stocks in fire‐excluded southwestern ponderosa pine forest are higher than prefire exclusion carbon stocks reconstructed from 1876, quantify the carbon costs of thinning treatments to reduce high‐severity wildfire risk, and compare posttreatment (thinning and burning) carbon stocks with reconstructed 1876 carbon stocks. Our findings indicate that prefire exclusion forest carbon stocks ranged from 27.9 to 36.6 Mg C ha^?1 and that the current fire‐excluded forest structure contained on average 2.3 times as much live tree carbon. Posttreatment carbon stocks ranged from 37.9 to 50.6 Mg C ha^?1 as a function of thinning intensity. Previous work found that these thinning and burning treatments substantially increased the 6.1 m wind speed necessary for fire to move from the forest floor to the canopy (torching index) and the wind speed necessary for sustained crown fire (crowning index), thereby reducing potential fire severity. Given the projected drying and increase in fire prevalence in this region as a function of changing climatic conditions, the higher carbon stock in the fire‐excluded forest is unlikely to be sustainable. Treatments to reduce high‐severity wildfire risk require trade‐offs between carbon stock size and carbon stock stability.     

Long-Term Effects of Prescribed Fire and Thinning on Residual Tree Growth in Mixed-Oak Forests of Southern Ohio

Long-term (10 years) growth responses of residual trees to prescribed fire and thinning were evaluated using standard dendrochronological protocols to understand the broader effects of the treatments on mixed-oak forest ecosystems in southern Ohio. Analysis of 696 increment cores (348 trees ≥ 25 cm DBH; five species) from 80 0.1 ha permanent plots distributed evenly across four treatments (control, thin, thin + burn, burn) indicated substantial increase in tree basal area increment (BAI) following the treatment. Post-treatment mean BAI of trees from the three active treatments ranged from 20.52 to 23.55 cm² y^?1 compared with pre-treatment values of 16.86–17.07 cm² y^?1. BAI rates (averaging 15.13 and 16.33 cm² y^?1, respectively, for pre- and post-treatments) in the control plots did not change much over time. Mechanical treatments were more effective than prescribed fire at enhancing BAI of trees. However, basal area growth depended to some degree on the severity of prescribed fire. Analysis of percent BAI change revealed an interesting temporal trend with moderate to major growth releases during the first 5-year post-treatment period, and a slight attenuation thereafter, suggesting the need for periodic application of treatments to sustain growth over a longer timescale. Growth responses varied greatly among species, with yellow-poplar and hickories exhibiting the highest and lowest post-treatment BAI rates of 31.11 and 15.71 cm² y^?1, respectively. Given their variable growth responses, integrating residual trees into current monitoring programs may help in elucidating the consequences of prescribed fire and thinning on forest dynamics and development.     

Quantifying climate–growth relationships at the stand level in a mature mixed‐species conifer forest

A range of environmental factors regulate tree growth; however, climate is generally thought to most strongly influence year‐to‐year variability in growth. Numerous dendrochronological (tree‐ring) studies have identified climate factors that influence year‐to‐year variability in growth for given tree species and location. However, traditional dendrochronology methods have limitations that prevent them from adequately assessing stand‐level (as opposed to species‐level) growth. We argue that stand‐level growth analyses provide a more meaningful assessment of forest response to climate fluctuations, as well as the management options that may be employed to sustain forest productivity. Working in a mature, mixed‐species stand at the Howland Research Forest of central Maine, USA, we used two alternatives to traditional dendrochronological analyses by (1) selecting trees for coring using a stratified (by size and species), random sampling method that ensures a representative sample of the stand, and (2) converting ring widths to biomass increments, which once summed, produced a representation of stand‐level growth, while maintaining species identities or canopy position if needed. We then tested the relative influence of seasonal climate variables on year‐to‐year variability in the biomass increment using generalized least squares regression, while accounting for temporal autocorrelation. Our results indicate that stand‐level growth responded most strongly to previous summer and current spring climate variables, resulting from a combination of individualistic climate responses occurring at the species‐ and canopy‐position level. Our climate models were better fit to stand‐level biomass increment than to species‐level or canopy‐position summaries. The relative growth responses (i.e., percent change) predicted from the most influential climate variables indicate stand‐level growth varies less from to year‐to‐year than species‐level or canopy‐position growth responses. By assessing stand‐level growth response to climate, we provide an alternative perspective on climate–growth relationships of forests, improving our understanding of forest growth dynamics under a fluctuating climate.     

Tree ring responses to climate variability of xerophytic thickets from South Soalara,Madagascar

Xerophytic thickets occur along the southwestern part of Madagascar. Although providing a wide variety of resources and services to the local population, this particular vegetation is subjected to deforestation. This study focuses on linking dendroclimatology and dendroecology by examining the spatial and temporal variability of the ecological growth conditions. Information from tree rings was retrieved, on one hand, to identify the problem of the limiting effects of past climate on growth and, on the other hand, to show how local environment takes part in the growth pattern of South Soalara species, in the southwestern part of Madagascar. Methods and principles of dendrochronology were applied on nine species belonging to seven botanical families. A total of 42 stem discs from 3 to 5 trees per species were collected at 30 cm height. All discs exhibited visible tree rings, but anatomical distinctness varied between species. This study highlighted the annual formation of tree rings through successful crossdating techniques. Then, from the nine constructed chronologies, species were grouped into three clusters. Analysis between precipitation and radial growth showed that the response to climate occurred mainly in rainy season. Large-scale climatic drivers such as sea surface temperature (SST¹) of ENSO² regions revealed a teleconnection with tree growth in cluster 1. Mean radial increments were computed from the measured tree ring width and varied from 0.66 to 1.98 mm year⁻¹, showing that those species are slow-growing. All species were recorded as having a certain dendrochronological potential, which was ranked as useful for Rhizogum madagascariense and Terminalia gracilipes, poor for Gyrocarpus americanus and problematic for the other species. It is recommended to increase the number of wood samples and to provide more knowledge on the characteristics of the species in order to improve the quality of the chronology and the climatic signal on tree rings.     

Seasonal prevalence of arthropods after line thinning of overstocked Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis> D. Don) plantations in central Japan

We investigated the seasonal changes in the abundance of coleopteran and hymenopteran functional groups to evaluate the ecological suitability of line thinning in overstocked Japanese cedar plantations in central Japan. In line thinning, 3- to 5-m-wide sections of the stand are harvested parallel to the mountain slope and perpendicular to the forest roads. The thinning intensity varies from 25 to 35% of the total number of trees. We compared the seasonal abundance between the thinned stand and the unthinned (control) stand in two plantation forests: a low-elevation Sugi site (4 years since thinning) and a high-elevation Kuchiotani site (6 years since thinning). Most coleopteran and hymenopteran functional groups were consistently more abundant in the thinned stand than in the unthinned stand. The differences in arthropod abundance between the thinned and unthinned stands were mainly caused by marked differences in the abundance of seasonally strongly changing functional groups during their peak seasons. The predators and parasitoids of insects as well as the plant and pollen feeders (plant-dependent functional groups) responded to seasonal changes in the vegetation (food and microhabitats) and microclimate. The ants were less affected by line thinning. Our results indicate that line thinning is an ecologically suitable silvicultural treatment for the improvement of biodiversity conditions in the overstocked plantation forests.     

Dendrochronology and dendroclimatology of Ceiba speciosa (A. St.-Hil.) Ravenna (Malvaceae) exposed to urban pollution in Rio de Janeiro city,Brazil

Nowadays, the biological monitoring through the growth rings has received increasing attention from ecologists and toxicologists. Structural analysis of these rings allows the incorporation of a time component in the study of plant responses to environmental variation. This allows also to evaluate long time series from the woody plants. In this paper, we assessed the dendrochronological characteristics of Ceiba speciosa growing in forest environment and under urbanization impact. Stem samples were obtained with Pressler probe into trees growing the campus of the Oswaldo Cruz Foundation, adjacent to one of the main urban thoroughfares of the city of Rio de Janeiro (Avenida Brasil), and at Tinguá Biological Reserve, an important remnant of Atlantic Forest. The samples were processed and analyzed following usual dendrochronological methods, with COFECHA and ARSTAN softwares. A negative exponential curve was used for standardization of the series. The residual chronologies were correlated with precipitation and temperature indexes obtained from NOAA weather database. Growth rings are distinct and annual, marked by bands of marginal parenchyma, thick-walled and radially flattened fibres in latewood and distended rays in earlywood. In both sites, the intercorrelation between the trees was above 0.40. Ages ranged from 11 to 41 years in the urban site and from 27 to 64 years in the forest site. In urban area, mean annual increment and cumulative average growth rates were 6 mm/year and 142.62 mm, respectively. At the forest site, these rates were 4 mm/year and 173.07 mm, respectively. The comparison between cumulative radial increment of the two sites revealed that trees of the urban site had higher increment rates beginning at the start of their development and consequently, they showed similar diameters despite lower ages. Correlation analysis between the chronologies and climatic factors revealed a positive association between growth and hot and rainy periods for both study sites. However, there is an immediate response of urban trees in relation to the rains and, a late response of forest trees to the same factor. The dry and hot climate, typical of urban environments, and the absence of natural water reserves in urban soil, may explain this more immediate response of urban tree growth to rainfall and temperature indexes. Our results revealed that Ceiba speciosa is a plastic and stress-tolerant species that is able to survive and adapt to polluted urban conditions. These features, along with its wide natural distribution and frequent planting for city landscaping, make this species an important biomarker for environmental monitoring studies.     

An assessment of Schinopsis brasiliensis Engler (Anacardiacea) for dendroclimatological applications in the tropical Cerrado and Chaco forests,Bolivia

Given the scarcity of instrumental climatic data in the South American tropics, it is valuable to explore the dendrochronological potential of the numerous tree species growing in the region. In this paper, we assessed for the first time the dendrochronological characteristics of Schinopsis brasiliensis, an arboreal species from the dry-tropical Cerrado and Chaco forests in Bolivia and adjacent countries. Similar to most woody species in the Cerrado and Chaco regions, growth rings of S. brasiliensis are delimited by the presence of thin but continuous lines of marginal parenchyma. Based on 22 samples from 15 trees, we present the first ring-width chronology for this species covering the period 1812–2011 (200 years). Additionally, a 106-year floating chronology from S. brasiliensis was developed using cores from four columns from the church of San Miguel, Santa Cruz, built in the period 1720–1740. Standard dendrochronological statistics indicate an important common signal in the radial growth of S. brasiliensis. The comparison of variations in regional climate and ring widths shows that tree growth is directly related to spring-summer rainfall and inversely related to temperature. Following the winter dry season, rainfall in late spring and early summer increases soil water supply, which activates tree growth. In contrast, above-average temperatures during the same period increase evapotranspiration, intensify the water deficit and reduce radial growth. The dependence of S. brasiliensis growth on water supply is evidence of its dendrochronological potential for reconstructing past precipitation variations in the extensive tropical Cerrado and Chaco forest formations in South America. Using wood from historical buildings opens the possibility of extending the chronologies of S. brasiliensis over the past 400–500 years.     

Testing the effect of restoration‐focused silviculture on oak regeneration and groundlayer plant communities in urban–exurban oak woodlands

Throughout their global range, oak‐dominated ecosystems have undergone state changes in stand structure and composition. Land managers face an especially acute challenge in restoring oak ecosystems and promoting oak regeneration in urban–exurban areas, where high‐intensity silvicultural treatments are often not feasible. To investigate low‐intensity management alternatives which could be widely applied in urban–exurban forests, a large‐scale adaptive management experiment was implemented in Lake County, IL, in 2012. Five canopy manipulation treatments of varying intensity, timing, and spatial aggregation were replicated across three study areas and oak seedlings were under‐planted into treatment units following management. Responses of understory light environment, shrub and groundlayer plant communities, and survival and growth of underplanted oak seedlings were evaluated. Understory light availability, canopy openness, total groundlayer plant cover, and groundlayer species diversity all differed among treatments. However, although understory light availability was significantly increased by canopy manipulation, groundlayer communities and oak seedling survival and growth did not differ among treatments. High overall seedling survival rates suggest current conditions are amenable to oak regeneration, but long‐term monitoring will be needed to assess the potential for seedlings to transition to the sapling and canopy layers. Early results demonstrate that canopy‐focused silvicultural treatments can affect the understory light environment and, to some degree, groundlayer plant communities. However, underplanting of oak seedlings paired with subcanopy thinning may be sufficient to restore an oak seedling layer, and (when necessary or preferred) canopy manipulation could potentially be deferred until later in the restoration timeline to promote oak recruitment.     

Primary and secondary growth phenology of Schizolobium parahyba (Vell.) Blake at different time scales

Schizolobium parahyba is a tropical species that has expanded its distribution. However, little is known about its ecological performance beyond the tropics. Therefore, the objective of this research was to describe for this species, south of its original distribution, intra-annual variations in primary growth phenology (leaf flushing, leaf shedding, flowering and fruiting) and secondary growth (radial increase of the trunk), as well as secondary growth on an interannual scale (radial xylem increase) and to analyze the influences of climatic conditions on the evidenced phenological patterns. For this purpose, on an intra-annual scale, ten individuals had their phenophases observed fortnightly for 23 months and were described and correlated to climatic variables of photoperiod, temperature and precipitation. Inter-annual activity samples were obtained from the growth rings of 16 individuals, which were evaluated according to dendrochronological protocols and correlated to climatic variables of temperature and precipitation. South of its original distribution, S. parahyba trees presented seasonal growth rates in all observed scales and associations with variations in phenological rhythms and climatic conditions. Differences were found in the climatic response of the species in comparison to studies already performed in the tropics. In this study, on an intra-annual scale, this association is manifested by relationships with photoperiod, being positive for leaf flushing and fruiting and negative for leaf shedding. In the secondary growth, the intra-annual scale positive relationship was found with temperature, but on an inter-annual scale a positive influence of winter rainfall was found. The consistence of those patterns in broader spatial scales deserves further investigation.     

Tree-ring growth of silver fir (Abies alba Mill.) in two stands under different silvicultural systems in central Italy

A preliminary analysis was made of growth trends in two silver fir forests in the Apennines in central Italy. The forests are closely located but were managed differently in the past. The aim was to verify the climatic variation over time, the influence of extreme events on radial growth and the role of silvicultural systems in defining growth response. The two forests have distinct structures due to differing past silvicultural management. The Camaldoli Forest, containing one-storeyed, evenaged, pure silver fir stands, was essentially managed in the past for wood production and featured high plantation density, low to moderate thinning, and clear cutting every 80–100 years. The La Verna Forest includes multi-layered, unevenaged stands that are mixed in with beech and other broadleaves. Increment cores were collected from aged dominant and healthy trees in the two forests. Climatic data were taken from the Camaldoli station and covers the period 1885–1994. All the analysed dendroecological parameters displayed similar patterns and indicated suitable site conditions for the silver fir. Silvicultural systems played an important role in defining growth pattern and rate. In the evenaged system there were higher growth rates and a greater number of pointer years, whereas there were lower growth rates and long, homogeneous development stages in the semi-natural conditions of La Verna. A rising growth trend recorded in the last three decades in both series could be explained by the consistent extension of the growing season due to a significant increase of mean spring and autumn temperatures. No correlation was found with precipitation while significant correlations were found between tree ring growth and February, April and August temperatures. Preliminary results indicated that the silvicultural system does not bias, but just enhances the ability of the species to record the effects of disturbing factors.     

Resilience capacity of Araucaria araucana to extreme drought events

Ongoing climate change has induced modification in the frequency and intensity of extreme climatic events, with consequent impact on tree and forest growth resilience. Araucaria araucana is an endangered Patagonian conifer, which provides several ecosystem services to local human societies and plays fundamental ecological roles in natural communities. These woodlands have historically suffered different types of anthropogenic disturbance, such as fire, logging and grazing, nevertheless the species resilience to extreme drought events remains still poorly understood. To fill this gap of knowledge, we applied dendrochronological methods to several A. araucana stands distributed along a steep bioclimatic gradient in order to reconstruct resilience capacity, in term of stem growth resistance and recovery, to three successive extreme spring-early summer droughts which occurred during the 20th century. Results showed an increase in the species recovery along the considered dry spells, whereas no clear trend emerged for resistance, suggesting no cumulative effect of drought upon resilience. Both resistance and recovery presented different values depending on bioclimatic settings, being xeric stands more sensitive to extreme episodes with respect to mesic woodlands, particularly during the more recent drought event when trees growing in drier environments were not able to reach pre-drought stem growth rates. Tree-level characteristics, such as age and growth trends prior to drought, modulated the species resilience, suggesting that future dry spells would possibly induce shifts in population dynamics, and furthermore be detrimental for fast-growing trees. Our analysis highlighted the response of a key Patagonian tree species to extreme drought events, providing bioclimatic-specific useful information for conservation plans of this natural resource.     

How aridity variations affect Prosopis caldenia growth in transitional forests in the semiarid Argentinean Pampas

The semiarid Pampas in central Argentina, occupied by caldén’s (Prosopis caldenia Burkart) woodlands (caldenales), had been affected by several wet and dry periods in the last century. Nowadays, the caldenales cover about 17 million hectares of central Argentina and their current state is critical due to massive deforestation rates. Caldén has an important dendrochronological potential since it produces sensitive tree rings linked to climate oscillations, fire events, competence, insect breakout, among others. The standard dendrochronological analysis can be improved with new advances in serial modeling and multivariate ordination techniques for handling problems related to the comparison of dendrochronological samples. We applied a statistical algorithm, BIOdry, which integrates conventional procedures for modeling patterns between annual diameter increments and drought. P. caldenia dendrochronological data were used accounting for multiple sources of variation from the sample design, and comparing patterns from contrasting climatic portions of the study site. The relation between diameter growth dynamics of caldén with temperature and precipitation fluctuations along the second half of the last century was analyzed. Populations at the Northern limit of its natural distribution area were selected. Monthly temperature and precipitation were evaluated in order to identify relative water surplus and deficit periods and an annual aridity index (AAI) was calculated. The objectives of the present work were: to analyze, describe and model response patterns between the P. caldenia diameter growth increment and the AAIs. A common growth response pattern and differences in tree and population level were also analysed. The algorithm BIOdry showed a good behavior, according to the considered statistical parameters (all the fixed effects were statistically significant). Several and common problems associated with the nature of the tree-ring data in modeling (pseudorreplication, autocorrelation, and nested random effects) were efficiently addressed. P. caldenia diameter growth, at tree level significantly responds to different AAI variation intensities along the studied period. A high frequency growth variability at population level was detected when trees establishment happened. These associations ended when trees become older. Low frequency growth variability at population level was significant throughout the analyzed period, presumably linked to age and management history. These results provide news insights in relation to the climate impact on growth dynamics of caldén in the Argentinean pampas and the value of new statistical tools in order to improve dendrochronological studies.     

Rapid changes in plasticity across generations within an expanding cedar forest

We investigated the inter‐individual variation of phenotypic plasticity and its evolution across three generations within an expanding forest. Plasticity was assessed in situ from dendrochronological data as the response of radial growth to summer rainfall. A linear mixed model was used to account for spatial effects (environment and stand structure), temporal factors (stand dynamics) and the variation with age. Beyond these effects, our results reveal a significant inter‐individual variance of growth and plasticity within each generation. We also show that the mean values and variances of growth and plasticity changed significantly across generations, with different patterns for both traits. The possible environmental and genetic drivers of these changes are discussed. Contrasting with the trade‐off between stress tolerance and plasticity generally observed among populations, we detected a positive covariance at the individual level, which does not support the cost of plasticity hypothesis in this case.