Pointer years in tree-ring width and earlywood-vessel area time series of Quercus robur—Relation with climate factors near its northern distribution limit

For a long time, radial growth of pedunculate oak (Quercus robur) in relation to environmental factors has been studied in Central Europe. However, there is insufficient information on oak growth in the Baltic region. Climate–growth interactions have been mostly investigated by correlation/response analysis between ring width and climatic factors. Other wood anatomical proxies, and also pointer year analysis, which focuses on weather extremes, can be sources of additional information. Wood samples were taken from 40 sites across Latvia. Tree-ring width (TRW) and mean area of earlywood vessels (EVA) were measured. To assess differences in wood formation among sites, a PCA was performed based on pointer year series of TRW and EVA per site. The relation with climate was assessed by Pearson's correlation. Patterns of wood formation differed along west/east gradient, so that two regions (western and eastern) of Latvia were distinguished. Pointer year values were higher for TRW than for EVA and higher in the eastern than in the western region. However, the pointer year values were moderate, suggesting that there was no strict limiting factor for their formation. Pointer years of EVA were strongly related with temperature during the dormant period in both regions; June precipitation had a significant effect in the eastern region. Pointer years of TRW were correlated with spring and summer temperature in the western region and with February temperature in the eastern region; precipitation in autumn showed a negative effect. These differences can be explained by a gradient from maritime to continental climate, leading to shifts in the growth limiting factors. Extremely cold winters resulted in most of the negative pointer years of TRW and EVA.     

Influence of climate on tree-ring and earlywood vessel formation in Quercus robur in Latvia

We studied the effects of climatic factors on tree-ring width and vessel lumen area (VLA) in earlywood of English oak (Quercus robur L.) in Latvia. Cores were obtained from healthy canopy oaks in 40 stands located across Latvia. Tree-ring widths and VLA were measured. Principal component analysis was used to arrange the sites along gradients of response of tree-ring width and earlywood to environmental factors. Significant relationships of tree-ring width and mean VLA with climatic factors (mean monthly temperature and precipitation sum) were determined by correlation analysis. Relationships between tree-ring, early- and latewood widths were tested in three sampled stands. The patterns of response of VLA and tree-ring width to environmental factors differed in relation to a west–east gradient of increasing continentality. Three regions of Latvia (western, central and eastern) were distinguished along this gradient. Responses to climate differed between tree-ring width and mean VLA. Occurrence of significant correlations between climatic factors and the proxies differed between regions, likely due to regional differences in temperature and precipitation. Tree-ring width correlated with climatic factors (most commonly with March, May and June temperature and August precipitation of the current growing season and July–August temperatures of the previous growing season); VLA was more strongly related to climatic factors, particularly with temperature in winter and spring months. The proportion of significant correlation coefficients with climatic factors differed between the regions. Among sites, significant correlation of tree-ring width with temperature in spring and summer was more frequent in the western region, while correlation with winter temperature of the previous growing season and precipitation in August was more frequent in the eastern region. For VLA, the frequency of significant correlation coefficients with temperature in winter and spring was higher in the eastern region.     

Different responses of Korean pine (Pinus koraiensis) and Mongolia oak (Quercus mongolica) growth to recent climate warming in northeast China

Different tree species growing in the same area may have different, or even contrasting growth responses to climate change. Korean pine (Pinus koraiensis) and Mongolia oak (Quercus mongolica) are two crucial tree species in temperate forest ecosystems. Six tree-ring chronologies for Korean pine and Mongolia oak were developed by using the zero-signal method to explore their growth response to the recent climate warming in northeast China. Results showed that Mongolia oak radial growth was mainly limited by precipitation in the growing season, while Korean pine growth depended on temperature condition, especially monthly minimum temperature. With the latitude decrease, the relationships between Korean pine growth and monthly precipitation changed from negative to positive correlation, while the positive correlation with monthly temperature gradually weakened. In the contrary, Mongolia oak growth at the three sampling sites was significantly and positively correlated with precipitation in the growing season, while it was negatively correlated with temperature and this relationship decreased with the latitude decrease. The radial growth of Korean pine at different sites showed a clearly discrepant responses to the recent warming since 1980. Korean pine growth in the north site increased with the temperature increase, decreased in the midwest site, and almost unchanged in the southeast site. Conversely, Mongolia oak growth was less affected by the recent climate warming. Our finding suggested that tree species trait and sites are both key factors that affect the response of tree growth to climate change. In addition, the suitable distribution area of Korean pine may be moved northward with the continued global warming in the future, but Mongolia oak may not shift in the same way.     

Climate response of cork growth in the Mediterranean oak (Quercus suber L.) woodlands of southwestern Portugal

In the Mediterranean climate regions, drought events are expected to affect the growth of forests ecosystems by changing trees growth rates and eventually inducing shifts in their growth patterns. Cork oak (Quercus suber L.) is a strictly western Mediterranean tree species periodically harvested for its bark, the cork. So far, cork oak has received limited attention for dendroclimatological studies due to its typical faint and erratic tree wood rings. Moreover, its distinct cork rings chronologies have been completely neglected. In this study we introduce an approach using cork ring chronologies dated back 9–10 years for climate response. Despite enhancing interannual variability and increasing statistical response to short-term climatic variability, still poorly understood, this study will possibly allow infer long-term climate response. We analyzed the cork ring chronologies of 55 cork samples collected in mature (under exploitation) trees in three distinct locations in southwestern Portugal. Cork growth recorded a high climate signal, with highly significant and coherent responses to the yearly climate-related sources of variation. We successfully assessed trends of cork growth via correlation analysis including selected climate variables among mean monthly temperature, monthly precipitation and, on an annual basis, eight precipitation indices. The high mean sensitivities and inter-series correlations found for cork ring chronologies combined with the significant variance explained by climate variables suggest that climate is likely one dominant signal that affects cork growth, but local environmental stresses can decisively affect this (climate) signal. Assuming cork growth as a proxy for cork oak growth, it seems conceivable that despite the trees being highly resistant to drought stress, cork oak woodlands in southwestern Portugal would have to face lesser growth in a global warming scenario.     

The influence of winter temperatures on the annual radial growth of six northern range margin tree species

This study explores the influence of temperature on the growth of six northern range margin (NRM) tree species in the Hudson River Valley (HRV). The HRV has excellent geographic and floristic qualities to study the influence of climate change on forested ecosystems. Indices of radial growth for three populations per species are developed and correlated against average minimum and maximum monthly temperatures from 1897 to 1994. Only positive correlations to temperature are considered for this analysis. Principal component analysis (PCA) is performed on chronologies over the entire HRV and at four subregions. PCA reveals a strong common signal among populations at subregional and regional scales. January temperatures most limit growth at the ecosystem level, supporting the hypothesis that winter temperatures may control vegetational ecotones. Surprisingly, growth of the oak–hickory ecosystem is most limited by January temperatures only in the southern half of the study region. Chestnut and white oak are the primary species driving the geographic pattern. As winter xylem embolism is a constant factor for ring-porous species, snow cover and its interaction on fine root mortality may be the leading factors of the pattern of temperature sensitivity. Species-specific differences in temperature sensitivity are apparent. Atlantic white-cedar (AWC) and pitch pine are more sensitive to the entire winter season (December–March) while oak and hickory are most sensitive to January temperatures. AWC is most sensitive species to temperature. Chestnut and white oak in the HRV are more sensitive to winter temperature than red oak. Pignut hickory has the most unique response with significant relations to late growing season temperatures. Interestingly, AWC and pitch pine are sensitive to winter temperatures at their NRM while oak and hickory are not. Our results suggest that temperature limitations of growth may be species and phylogenetically specific. They also indicate that the influence of temperature on radial growth at species and ecosystem levels may operate differently at varying geographic scales. If these results apply broadly to other temperate regions, winter temperatures may play an important role in the terrestrial carbon cycle.     

Range position and climate sensitivity: The structure of among‐population demographic responses to climatic variation

Species’ distributions will respond to climate change based on the relationship between local demographic processes and climate and how this relationship varies based on range position. A rarely tested demographic prediction is that populations at the extremes of a species’ climate envelope (e.g., populations in areas with the highest mean annual temperature) will be most sensitive to local shifts in climate (i.e., warming). We tested this prediction using a dynamic species distribution model linking demographic rates to variation in temperature and precipitation for wood frogs (Lithobates sylvaticus) in North America. Using long‐term monitoring data from 746 populations in 27 study areas, we determined how climatic variation affected population growth rates and how these relationships varied with respect to long‐term climate. Some models supported the predicted pattern, with negative effects of extreme summer temperatures in hotter areas and positive effects on recruitment for summer water availability in drier areas. We also found evidence of interacting temperature and precipitation influencing population size, such as extreme heat having less of a negative effect in wetter areas. Other results were contrary to predictions, such as positive effects of summer water availability in wetter parts of the range and positive responses to winter warming especially in milder areas. In general, we found wood frogs were more sensitive to changes in temperature or temperature interacting with precipitation than to changes in precipitation alone. Our results suggest that sensitivity to changes in climate cannot be predicted simply by knowing locations within the species’ climate envelope. Many climate processes did not affect population growth rates in the predicted direction based on range position. Processes such as species‐interactions, local adaptation, and interactions with the physical landscape likely affect the responses we observed. Our work highlights the need to measure demographic responses to changing climate.     

Relating annual increments of the endangered Blanding's turtle plastron growth to climate

This research is the first published study to report a relationship between climate variables and plastron growth increments of turtles, in this case the endangered Nova Scotia Blanding's turtle (Emydoidea blandingii). We used techniques and software common to the discipline of dendrochronology to successfully cross‐date our growth increment data series, to detrend and average our series of 80 immature Blanding's turtles into one common chronology, and to seek correlations between the chronology and environmental temperature and precipitation variables. Our cross‐dated chronology had a series intercorrelation of 0.441 (above 99% confidence interval), an average mean sensitivity of 0.293, and an average unfiltered autocorrelation of 0.377. Our master chronology represented increments from 1975 to 2007 (33 years), with index values ranging from a low of 0.688 in 2006 to a high of 1.303 in 1977. Univariate climate response function analysis on mean monthly air temperature and precipitation values revealed a positive correlation with the previous year's May temperature and current year's August temperature; a negative correlation with the previous year's October temperature; and no significant correlation with precipitation. These techniques for determining growth increment response to environmental variables should be applicable to other turtle species and merit further exploration.     

Influence of climate and disturbance on the growth of Tsuga canadensis at its southern limit in eastern North America

It has long been hypothesized that trees growing at range limits likely also occur near the limit of their ecological amplitude and thus, should be more sensitive to climate variability than individuals growing nearer the range core. We developed a tree-ring chronology using Tsuga canadensis individuals from three disjunct stands at the species’ southern limit to quantify the influence of climate and disturbance on radial growth patterns. The tree-ring record extended 158 years from 1850 to 2007. Significant negative relationships were found between the STANDARD chronology and monthly mean temperature, monthly maximum temperature, and monthly minimum temperature during the previous and current summer, while significant positive relationships were documented between the STANDARD chronology and monthly minimum temperature for September and October of the current year. Also, significant positive relationships were documented between the STANDARD chronology and monthly total precipitation for September of the previous year and May of the current year. Response function analysis showed that monthly climate variables (r ² = 0.22) and prior growth (r ² = 0.40) explained 62% of the variance in the T. canadensis tree-ring chronology. A time series plot for the T. canadensis chronology showed that actual tree growth agreed relatively well with the predicted growth based on significant climate variables. However, positive departures from the predicted growth were noted. Dendroecological analysis revealed these departures were likely related to disturbance events. Our results indicated that T. canadensis individuals at its southernmost extent are sensitive to regional climate, but not more so than trees nearer the range core. We hypothesize that microenvironmental conditions of T. canadensis stands at its southern limit are similar to conditions within the contiguous distribution of the species, which may explain this pattern.     

Individual and time-varying tree-ring growth to climate sensitivity of Pinus tabuliformis Carr. and Sabina przewalskii Kom. in the eastern Qilian Mountains, China

Individual tree-ring width chronologies and mean chronologies from Pinus tabuliformis Carr. (Chinese pine) and Sabina przewalskii Kom. (Qilian juniper) tree cores were collected and analyzed from two sites in the eastern Qilian Mountains of China. The chronologies were used to analyze individual and time-varying tree-ring growth to climate sensitivity with monthly mean air temperature and total precipitation data for the period 1958–2008. Climate–growth relationships were assessed with correlation functions and their stationarity and consistency over time were measured using moving correlation analysis. Individuals’ growth–climate correlations suggested increased percentages of individuals are correlated with certain variables (e.g., current June temperature at the P. tabuliformis site; previous June, December and current May temperature and May precipitation at the S. przewalskii site). These same climatic variables also correspond to the mean chronology correlations. A decreased percentage of individuals correlated with these climatic variables indicates a reduced sensitivity of the mean chronology. Moving correlation analysis indicated a significant change over time in the sensitivity of trees to climatic variability. Our results suggested: (1) that individual tree analysis might be a worthwhile tool to improve the quality and reliability of the climate signal from tree-ring series for dendroclimatology research; and (2) time-dependent fluctuations of climate growth relationships should be taken into account when assessing the quality and reliability of reconstructed climate signals.     

Growth traits of juvenile American chestnut and red oak as adaptations to disturbance

American chestnut (Castanea dentata) was a dominant species in eastern North America prior to the importation of chestnut blight. In light of recent efforts to restore viable populations of chestnut in eastern forests, an increased understanding of its association with other co‐occurring, disturbance‐adapted oak species is necessary. We evaluated crown architecture and leaf morphology in juvenile chestnut and red oak (Quercus rubra) to assess potential differences in establishment strategies of both species. We also investigated differences in nonstructural carbohydrate reserves and whole tree biomass partitioning between species. Seedlings of both species were planted in forest stands treated either with midstory removal or small patch cuts, simulating potential restoration plantings. After 5–7 years, chestnut's allocation to its root system was lower than red oak's, with chestnut saplings instead diverting resources to branches and foliage. Chestnut had lower leaf area index, greater crown projection area, and higher specific leaf area than red oak, indicating the species may have an advantage in shaded understories. There were only minor differences in nonstructural root carbohydrate reserves, between red oak and American chestnut, indicating that chestnut may respond similarly to oak by resprouting after disturbances topkill young saplings. We suggest that American chestnut has morphological and physiological attributes that allow it to function as an opportunistic and plastic species that can utilize gaps to facilitate its canopy recruitment, yet still persist after occasional surface fire. This knowledge can guide restoration strategies for this iconic species of the eastern temperate forest region.     

Common climatic signals affecting oak tree-ring growth in SE Central Europe

A network of 41 local tree-ring chronologies of oak (Quercus petraea and Quercus robur) in Austria, Hungary, Slovenia, Croatia and Serbia (latitudes 45.00–48.00N, longitudes 13.14–21.63E, altitudes 80–800 m a.s.l.) was constructed and used to establish common climatic signals in oak tree rings in the region. Co-variation of residual chronologies could be resumed in 11 significant principal components (PC), explaining 79 % of common variability. Three of them, PC1, PC2 and PC3, made it possible to identify similarities among the sites. PC1, significantly correlated with all 41 chronologies, indicated a common positive response to precipitation in spring and summer (March and June) and a negative response to temperature in spring and summer (April and June). PC2, significantly correlated with 12 chronologies, indicated a common positive response to precipitation especially in spring (May) and a negative one to high summer temperatures (especially in August) with a pronounced north to south gradient. PC3, significantly correlated with ten chronologies, indicated that a warm previous December and warm current September have a positive effect on tree growth, especially in the south-western part of the study area. The obtained climate–growth relationships will help to understand better the variability of oak growth, to fill palaeoclimatic gaps and to improve dendrochronological research in the region.     

Spatial and temporal variability in the net primary production of grassland in China and its relation to climate factors

As an important component of the terrestrial carbon (C) cycle, variability in net primary productivity (NPP) plays a crucial role in the C input and accumulation in grasslands system. In this study, the spatial and temporal variability of grassland NPP in China during 2001–2010 and its relation to climate factors were analyzed by using a modified model of Carnegie–Ames–Stanford Approach based on the Comprehensive and Sequential Classification System. The results show that monthly grassland NPP increases from January to July. While the seasonal variability of NPP indicates peak productivity in summer. Annual mean grassland NPP follows a significant increasing trend with fluctuation from 2001 to 2010. The spatial pattern of grassland NPP shows increasing gradients from the west to the east and from the north to the south of China. Annual NPP differs significantly among different grassland types, with the highest NPP in the grassland distributed in sub-tropical perhumid evergreen broad leaved forest and tropical-perhumid rain forest. Time-lag correlation analysis at the monthly scale shows that grassland NPP responded more rapidly to changes in temperature than to precipitation. Among the climate factors, grassland NPP shows the strongest correlation at 1-month lag with moisture index K. There is a significant positive correlation between seasonal NPP and K. The seasonal NPP is significantly correlated with >0 °C annual cumulative temperature. The highest and the lowest NPP sensitivity to precipitation, K, and temperature were observed in the grassland distributed in tropical forest and semi-desert. The results indicate a complex mechanism of climate factors that control grassland C sequestration in terrestrial ecosystems.     

Regional analysis of the impacts of climate change on cheatgrass invasion shows potential risk and opportunity

Interactions between climate change and non-native invasive species may combine to increase invasion risk to native ecosystems. Changing climate creates risk as new terrain becomes climatically suitable for invasion. However, climate change may also create opportunities for ecosystem restoration on invaded lands that become climatically unsuitable for invasive species. Here, I develop a bioclimatic envelope model for cheatgrass ( Bromus tectorum ), a non-native invasive grass in the western US, based on its invaded distribution. The bioclimatic envelope model is based on the Mahalanobis distance using the climate variables that best constrain the species' distribution. Of the precipitation and temperature variables measured, the best predictors of cheatgrass are summer, annual, and spring precipitation, followed by winter temperature. I perform a sensitivity analysis on potential cheatgrass distributions using the projections of 10 commonly used atmosphere–ocean general circulation models (AOGCMs) for 2100. The AOGCM projections for precipitation vary considerably, increasing uncertainty in the assessment of invasion risk. Decreased precipitation, particularly in the summer, causes an expansion of suitable land area by up to 45%, elevating invasion risk in parts of Montana, Wyoming, Utah, and Colorado. Conversely, increased precipitation reduces habitat by as much as 70%, decreasing invasion risk. The strong influence of precipitation conditions on this species' distribution suggests that relying on temperature change alone to project future change in plant distributions may be inadequate. A sensitivity analysis provides a framework for identifying key climate variables that may limit invasion, and for assessing invasion risk and restoration opportunities with climate change.     

Relationship between fire,climate oscillations,and drought in British Columbia,Canada, 1920–2000

Climate oscillations such as El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are known to affect temperature and precipitation regimes and fire in different regions of the world. Understanding the relationships between climate oscillations, drought, and area burned in the past is required for anticipating potential impacts of regional climate change and for effective wildfire‐hazard management. These relationships have been investigated for British Columbia (BC), Canada, either as part of national studies with coarse spatial resolution or for single ecosystems. Because of BC's complex terrain and strong climatic gradients, an investigation with higher spatial resolution may allow for a spatially complete but differentiated picture. In this study, we analyzed the annual proportion burned–climate oscillation–drought relationships for the province's 16 Biogeoclimatic Ecosystem Classification (BEC) zones. Analyses are based on a digital, spatially explicit fire database, climate oscillation indices, and monthly precipitation and temperature data with a spatial resolution of 400 m for the period 1920–2000. Results show that (1) fire variability is better related to summer drought than to climate oscillations, and that (2) fire variability is most strongly related to both, climate oscillations and summer drought in southeastern BC. The relationship of area burned and summer drought is strong for lower elevations in western BC as well. The influence of climate oscillations on drought is strongest and most extensive in winter and spring, with higher indices being related to drier conditions. Winter and spring PDO and additive winter and spring PDO+ENSO indices show BC's most extensive significant relationship to fire variability. Western BC is too wet to show a moisture deficit in summer that would increase annual area burned due to teleconnections.     

Five-year population dynamics of plateau pikas (<Emphasis Type="Italic">Ochotona curzoniae</Emphasis>) on the east of Tibetan Plateau

Small mammals mediate trajectories of vegetation change where both their density and the growing season are moderated by temperature and precipitation. On the Tibetan Plateau, the cold and arid climate particularly restricts the plant growing season, but the role of mammals’ density and climate in moderating small mammal populations remains unknown. We conducted a 5-year mark-recapture study of plateau pikas (Ochotona curzoniae) to test the relative importance of density-dependent and climatic factors on survival and reproduction. Plateau pikas had seasonal fluctuations in density and survival. During the warm summer season (May–August), monthly survival was density-independent, ranging from 74.7 to 90.4%, but varied with sex and age, increased with precipitation and NDVI, and decreased with temperature. During the cold season (September–April), monthly survival was around 98%. Density and precipitation had negative effects on reproductive success for the first and second litters of the year, and temperature showed consistently positive effects on reproductive success for both litters of the year. Pika density and climate regulated together the population dynamics of plateau pikas. These data on the relationships between density and climatic factors on survival and reproduction are critical for the management and conservation of plateau pikas on the Tibetan Plateau.     

Effect of ecological factors on intra-annual stem girth increment of holm oak

Key message

The intra-annual stem girth increment of Quercus ilex is mainly driven by water availability and secondly by temperature. Tree size and competition modulate the growth response to climate.

Abstract

Holm oak (Quercus ilex ssp. ballota [Desf.] Samp.) is the most widespread species in the Iberian peninsula, being one of the most representative trees in forests and open woodlands. The analysis of stem girth increment of holm oak may provide valuable information about how Mediterranean ecosystems will respond to the forecasted climate changes. However, due to the variability of the Mediterranean climate, the knowledge of intra-annual patterns of growth is needed for a better understanding of the influence of the climatic variables at this scale. To this end, we used band dendrometers to measure monthly stem girth increments of 96 holm oak trees from 2003 to 2010, located in open woodlands and dense Mediterranean forests in southwestern Spain. We assessed the effects of climate, competition, topography, and initial stem diameter on stem girth increment. The major stem increment periods were in spring and autumn whereas increment rates were very low or even negative in winter and summer. Spring was not every year the season with the higher stem increments, but autumn when spring was very dry. Higher precipitation, soil moisture, and relative humidity had significant positive effects on stem increment, whereas higher temperature, reference evapotranspiration, and solar radiation had significant negative effects. Initial tree diameter and competition from nearby trees partly explained significant differences in stem increment of individual trees. Therefore, the forecasted climatic changes, in which decreased rainfall in spring and increased summer drought are expected in the Mediterranean region, may be a significant threat to the Q. ilex ecosystems.     

Climate–growth relationships of silver fir (Abies alba Mill.) in marginal populations of Central Italy

This study is part of a LIFE+ project on marginal mountain ecosystem conservation. In five mixed European beech-silver fir forests of Central Italy (Tuscany and Marches), classified as priority habitats of the Natura 2000 Network, we analysed the climate–growth relationships of silver fir (Abies alba Mill.) along an altitudinal gradient. The aims of this study were: (i) to identify the main spatial patterns in the frequency domain of both silver fir growth and climate variables in five different sites and (ii) to detect the overall climate sensitivity of the target species through time.Multivariate analysis displayed groups of chronologies with similar growth patterns for each frequency band-pass, discriminating for altitude and geographical location. The spectral density of climate variables at seasonal scale displayed common spatial patterns during late-spring and summer months. In stands where fir grows in optimal conditions, the most significant growth responses to climate were the positive influence of late-spring and summer precipitations of the previous year and the negative effect of summer temperatures of both previous and current year, although decreasing during the last decades. On the other hand, the site at lowest altitude shows a low and not very consistent climate sensitivity as compared to the preferred altitudes. At the highest site (1375 m asl) the positive effect of previous year spring–summer precipitation and summer temperature of both previous and current year disappears.Results suggest that in the studied areas a water-use increase in summer is a possible response of silver fir to the significant reduction of spring precipitation and general temperature increase throughout the 20th century.These findings provide additional information on silver fir responses to climate variability at different altitudes, useful for calibrating silvicultural treatments to apply for conservation of sensitive ecosystems and of tree species in mountain areas.     

北京东灵山辽东栎林树木生长对气候要素的响应特征

根据北京东灵山辽东栎(Quercus wutaishanica)的年轮宽度资料,分析了该地区树木生长在1951—2010年时段对气候要素的响应特征。相关分析表明,夏季干旱胁迫是限制东灵山辽东栎树木生长的最为重要的气候要素,主要体现在与夏季(7—9月)温度的负相关关系和夏季降雨(7月)的正相关关系,另外春季(5月)温度对树木生长也有一定的限制性影响;年表与生长季节干旱指数普遍呈正相关关系,进一步证实了干旱胁迫对树木生长的限制性作用。滑动相关分析表明,年表与夏季温度负相关关系及与夏季降雨的正相关关系在近期趋于增强,这表明夏季干旱胁迫对树木生长影响作用有不断加强的趋势。辽东栎林是北京东灵山温带落叶阔叶林的优势群落,在暖干化气候不断发展背景下,辽东栎林生长的干旱胁迫效应将更加突出,对北京东灵山地区森林的生产力及固碳能力产生负面影响。     

Future changes in summer MODIS-based enhanced vegetation index for the South-Central United States

Evaluating the response of vegetation to climate change is relevant to improving the management of both human and natural systems. Here, we quantify the response of the MODIS-based enhanced vegetation index (EVI) to temperature, precipitation, and large-scale natural variability across the South-Central U.S. for summer (JJA) from 2000 to 2013. We find statistically significant relationships between climate and EVI that vary across the region and are distinct for each land cover type: the mean coefficient of determination (R²) between EVI and climate is greatest for pasture (0.61 ± 0.13) and lowest for forest (0.55 ± 0.14). Among the climate variables, three-month cumulative precipitation has the strongest influence on summer vegetation, particularly in semi-arid west Texas and eastern New Mexico. Summer monthly maximum temperature plays an important role in the eastern half of Texas and Oklahoma, moderated by the influence of both Atlantic and Pacific teleconnection indices over inter-annual time scales. Based on these relationships, we train, cross-validate, and, where statistically significant relationships exist, combine this multivariate predictive model with projected changes in teleconnection indices and statistically-downscaled temperature and precipitation from 16 CMIP5 global climate models to quantify future changes in EVI. As global mean temperature increases, projected EVI decreases, indicative of stressed and dry vegetation, particularly for grasslands as compared to other land types, and in Oklahoma and western, central and Gulf Coast Texas for mid- and end-of-century. These trends have potentially important implications for agriculture and the regional economy, as well as for ecosystems and endemic species that depend on vegetation.     

Geographical variation in species' population responses to changes in temperature and precipitation

Despite increasing concerns about the vulnerability of species'' populations to climate change, there has been little overall synthesis of how individual population responses to variation in climate differ between taxa, with trophic level or geographically. To address this, we extracted data from 132 long-term (greater than or equal to 20 years) studies of population responses to temperature and precipitation covering 236 animal and plant species across terrestrial and freshwater habitats. Our results identify likely geographical differences in the effects of climate change on populations and communities in line with macroecological theory. Temperature tended to have a greater overall impact on populations than precipitation, although the effects of increased precipitation varied strongly with latitude, being most positive at low latitudes. Population responses to increased temperature were generally positive, but did not vary significantly with latitude. Studies reporting significant climatic trends through time tended to show more negative effects of temperature and more positive effects of precipitation upon populations than other studies, indicating climate change has already impacted many populations. Most studies of climate change impacts on biodiversity have focused on temperature and are from middle to high northern latitudes. Our results suggest their findings may be less applicable to low latitudes.