Subfossil bog-pine horizons document climate and ecosystem changes during the Mid-Holocene

Extended dendrochronological investigations were performed on subfossil pine entombed in peat layers of former raised bogs in Lower Saxony (NW Germany). The aim was to study of dynamics in bog development in response to local environmental conditions and regional changes in climate throughout the Holocene. To date, 1702 samples have been collected from 36 locations. Crossdating with the Lower Saxony Bog Oak Chronology (LSBOC) resulted in five absolutely dated pine chronologies covering large parts of the period from 5600 BC to 2200 BC. Radiocarbon dating of eight additional chronologies extends this time-span from 7000 BC to 1500 BC. By combining dendrochronology with information on stratigraphic position as well as stem and root morphology we found that major changes in site hydrology cause changes in growth pattern and population dynamics of subfossil pine whereas storm and fire were of minor importance. The fact that shifts in growth patterns and population dynamics occurred simultaneously in trees from different sites indicates regional climate changes as main drivers of pines forest development in peatland ecosystems.     

Mid‐Holocene pine woodland phases and mire development – significance of dendroecological data from subfossil trees from northwest Germany

Question: Can investigations of subfossil bog‐pine woodlands contribute to the understanding of mire development, especially the influence of climate fluctuations on the fen–bog transition? Location: Lowlands of northwest Germany. Methods: We investigated pine (Pinus sylvestris L.) tree remains (stumps and trunks) buried in peat deposits. Dendrochronology was used to date each sampled tree to calendar years and to reconstruct population dynamics of the pine woodlands. Ecological changes, especially changes in site hydrology during the pine woodland phases were inferred from peat stratigraphic analyses and investigations of stem and root morphology of the tree remains. Results: The subfossil pine woodlands occurred mostly during the transition from fen to raised bog conditions within the mire development. The population dynamics are strikingly wave‐like whereas woodland phases of 100 to 250 years duration are separated by much shorter (10–50 years) phases of high germination and dying‐off rates (GDO phases). Such GDO phases are often synchronous at different sites and are also linked to growth depressions of the independent regional oak master chronology (LSBOC), indicating a climate trigger. Conclusions: The development of raised bogs started about 7000 BC and had a main phase between 5100 and 3600 BC in northwest Germany. The subfossil bog‐pine woodlands document the transitional phase towards the onset of raised bog formation, as characterized by initial dry conditions that were followed by increasing wetness of the sites, whereas this development is at least partly the result of climate variations.     

Evaluating the temperature sensitivity of radial growth patterns from whitebark pine in the western Canadian Cordillera

We present a network of thirteen annual ring-width chronologies from high elevation whitebark pine (Pinus albicaulis Engelm.) sites in the western Canadian Cordillera in order to assess the dendroclimatic potential of this long-lived tree species. The temperature signal within the chronologies is complex and strongly influenced by diverging trends in the summer temperature and ring-width records from across the region. A first differences transformation of the tree-ring and temperature records illustrates a loss of frequency coherence in growth response to summer temperatures following reduced radial growth in the 1950s. Prior to reduced growth, we note a positive association with summer temperatures for both first differenced (r_d = 0.60) and traditional (r = 0.50) records. Following reduced growth, the association at first differences is maintained (r_d = 0.49) whereas there is a change in the lower frequency component of tree growth response to summer temperatures (r = ?0.34). We suggest the cause of this reduced temperature sensitivity is related to the interaction between diurnal temperature and cloud cover patterns, the hydrological regime of snowpack, and site conditions which have been amenable to the initiation of moisture stress during the latter half of the 20th century. Reduced radial growth is coincident with the arrival of white pine blister rust (Cronatium ribicola J.C. Fisch. ex Raben) into the study region which suggests this infestation may be related to the observed reduction in radial growth. Whitebark pine has considerable potential for the field of dendroclimatology. Unfortunately, the decline of the species due to the combined effects of climate change, white pine blister rust, mountain pine beetle (Dendroctonus ponderosae Hopk.), and forest fire exclusion practices indicate this potential may remain unfulfilled.     

Spatio-temporal growth variability of three Pinus species of Northeast Himalaya with relation to climate

This research aimed to evaluate spatio-temporal growth variability of three Pinus species viz. Pinus kesiya (Khasi pine), Pinus merkusii (Merkus pine) and Pinus wallichiana (Blue pine) along with the existence of species differentiation among the taxa in northeast India. Several statistical analyses were used, namely Pearson correlation and multivariate approaches involving UPGMA Cluster Analysis; ordination methods by Principal Component Analysis (PCA) and Non-metric multidimensional scaling (NMDS) on tree-ring width chronologies from 13 sites. The tree growth-climate relationships were assessed with both correlation and bootstrap response function using regional climate datasets of each sampling site prepared by averaging the nearest grid points of 0.5 × 0.5° of CRU TS-2.1 climate dataset. Pronounced species differentiation in the growth pattern among the three Pinus taxa was inferred. The observed spatio-temporal variability revealed inter-species tree growth variations were not uniform suggesting no common factor influenced the radial tree growth in this region, which may be related to anthropogenic impact or non-climatic factors. The tree growth-climate relationship showed that climatic factors limiting the radial growth of Pine are mostly similar for intra-species but diverse in inter-species. This study is extremely relevant in terms of species and site selection for the long-term climate reconstruction and forest management in the Northeast Himalaya.     

Elevation-specific tree-ring chronologies of Norway spruce and Silver fir in Southern Germany

Tree-ring (TR) chronologies are important instruments for the dating and provenance analyses of historical wood, as well as for climate reconstructions. However, radial growth patterns differ between tree species and growing environments. Therefore chronologies are more or less specific for a certain tree species, region and elevation. Chronologies that are restricted to more confined regions could extend the possibilities for dating, dendroprovenancing and regional climate reconstructions.In Southern Germany, the transport of wood by raft – for the supply of towns and cities with timber – has been documented since early mediaeval times. Consequently, not only local timber from the lowlands was used for construction purposes, but also alpine wood originating from sites up to the timberline. Since pronounced altitudinal gradients cause distinct climate differences, elevation-specific chronologies have the potential to improve dating precision in this region.In this contribution, a model is presented and applied in order to separate elevation-specific provenances of Norway spruce (Picea abies L. Karst) and Silver fir (Abies alba Mill.) in Southern Germany. The model is derived from more than 2100 living TR series originating from sites between 200 and 1710 m above see level. Absolute (mean, maximum) ring-width values and the variation of ring-widths (mean sensitivity) show distinct correlations with their altitudinal provenance, reflecting improvements in growth conditions as well as the increase of their yearly variation from high over intermediate to low elevation sites. Mean ring widths and mean sensitivity values were used as independent parameters in an exponential regression model which exhibits a coefficient of determination (r²) of 77% for spruce and 74% for fir. The prediction accuracy of the elevation amounts ±300 m for spruce and ±200 m for fir within the 95% confidence interval.The model was used to estimate the elevation origin of around 5000 historical spruce and 800 historical fir series from buildings located in Southern Germany. The historical TR series covering the AD 990–1800 period were allocated by the model to elevations between 120 and 2090 m. In a second step the individual TR series were combined to elevation-specific chronologies representing low, intermediate, and high altitudinal belts. The chronologies show distinct differences among the altitudinal belts in terms of signatures and pointer years, especially for spruce. Elevation-specific chronologies are assumed to amplify the dating possibilities of ancient timber, to provide valuable evidences for the origin of historical wood, and to offer more specific proxy data for regional climate reconstructions.     

Precipitation reconstruction for the southern Altay Mountains (China) from tree rings of Siberian spruce,reveals recent wetting trend

We developed six tree-ring width chronologies of Siberian spruce (Picea obovata) from the low elevation forest of the southern Altay Mountains in northern Xinjiang, China. Although the six chronologies come from different sampling sites, significant correlations existed among the chronologies (r ≥ 0.477), and the first principal component (PC1) accounted for 72.2% of total variance over their common period 1825–2010. Correlation response analysis revealed that radial growth of Siberian spruce is mainly limited by a 12-month precipitation starting from July of the previous year to June of the current year. We therefore developed a July–June precipitation reconstruction spanning 1825–2009, which explained 65.5% of the instrumental variance for the period 1962–2009. The information of our precipitation reconstruction suggested that dry conditions existed for the periods 1829–1838, 1852–1855, 1876–1888, 1898–1911, 1919–1923, 1932–1936, 1943–1955, 1963–1968, 1973–1984 and 2007–2009, and wet conditions for the periods AD 1825–1828, 1839–1851, 1856–1875, 1889–1897, 1912–1918, 1924–1931, 1937–1942, 1956–1962, 1969–1972 and 1985–2006. Spatial climate correlation analyses with gridded land surface data revealed that our precipitation reconstruction contains a strong precipitation signal for the Altay Mountain ranges. Our reconstruction agreed with the moisture-sensitive tree ring width series of Siberian larch from the Altay Mountains of Mongolia on a decadal timescale. In addition, in contrast to a drying trend in north central China, a clear wetting trend has occurred in the southern Altay Mountains since 1980s.     

Detecting changes in climate forcing on the fire regime of a North American mixed-pine forest: A case study of Seney National Wildlife Refuge,Upper Michigan

The study of forests dominated by red pine (Pinus resinosa Ait.), one of the few fire-resistant tree species of eastern North America, provides an opportunity to reconstruct long-term fire histories and examine the temporal dynamics of climate forcing upon forest fire regimes. We used a 300-year long spatially explicit dendrochronological reconstruction of the fire regime for Seney National Wildlife Refuge (SNWR, 38,531 ha), eastern Upper Michigan to: (1) identify fire size thresholds with strong vs. weak climate controls, (2) evaluate effect of landform type (outwash channel vs. sand ridges) in modifying climate–fire associations, and (3) check for the presence of temporal changes in the climate control of large fire events over the time period 1700–1983. We used a summer drought sensitive red pine chronology (ITRDB code can037) as a proxy of past fire-related climate variability. Results indicated that fires >60 ha in sand-ridge-dominated portions of SNWR and >100 ha in outwash channels were likely climatically driven events. Climate–fire associations varied over time with significant climate–fire linkages observed for the periods 1700–1800 (pre-EuroAmerican), 1800–1900 (EuroAmerican settlement) and 1900–1983 (modern era). Although an increase in fire activity at the turn of 20th century is commonly associated with human sources of ignitions, our results suggest that such an increase was also likely a climatically driven episode.     

Cenozoic long-term terrestrial climatic evolution in Germany tracked by δ18O of rodent tooth phosphate

Even though the Cenozoic has been recognized as a period of important climate change, long-term climatic changes that took place in the continental domain are still questioned. For an area, southern Germany, for which other long-term palaeoclimatic records exist, analysis of oxygen isotope composition of small mammal teeth has been carried out for localities ranging in age from the late Eocene (c.36 Ma) to the middle Miocene (c.10 Ma). Comparison of this long-term continental δ¹⁸O record with the marine record reveals comparable trends. The major Cenozoic climatic events are thus recognized in the continental oxygen isotope record.Through comparison with other quantitative palaeoclimatic reconstructions available for the studied area, temperatures in southern Germany are broadly estimated to have fluctuated between 12 °C and 25 °C from late Eocene to early late Miocene. According to the different available chronological frameworks, either a ～ 2 °C (considering the classical biostratigraphy) or a ～ 6 °C (considering a revised biostratigraphy) decrease in mean air temperature is estimated for the Eocene/Oligocene boundary.     

Age-related drought sensitivity of Atlas cedar (Cedrus atlantica) in the Moroccan Middle Atlas forests

Age-related tree responses to climate change are still poorly understood at the individual tree level. In this paper, we seek to disentangle the relative contribution of tree age to growth decline and growth–climate relationships in Atlas cedar (Cedrus atlantica Manetti) trees at the Middle Atlas Mountains, northern Morocco. Dendrochronological methods were applied to quantify growth–climate relationships using tree-ring width indices (TRWi) calculated for cedars of two contrasting age groups (old trees, age ≥150 years; young trees, age <150 years). TRWi–climate relationships were assessed at the site and tree levels by using response functions and linear mixed-effects models, respectively. Growth of the studied Atlas cedars was negatively affected by recurrent droughts and by the steep temperature rise since the 1970s. Response functions and mixed-effects models indicated that the decline in tree growth was mainly explained by diminishing precipitation. The negative association between cedar growth and temperature was stronger in old than in young trees. Vulnerability to temperature-induced drought stress in old cedar trees may lead to an impending growth decline. We argue that the age dependence of growth sensitivity to drought must be quantified and considered at the individual tree level when predicting the future dynamics and persistence of cedar forests in the Moroccan Middle Atlas.     

A 1700-year Athrotaxis selaginoides tree-ring width chronology from southeastern Australia

Few Southern Hemisphere tree-ring chronologies exceed 1000 years in length. We present a ca. 1700 years of indexed values for the long-lived conifer Athrotaxis selaginoides at Cradle Mt in southeastern Australia and compare it with the only other published millennial-plus length tree-ring chronology for Australia: the nearby Mt Read Lagarostrobos franklinii. We use simple correlation function and pointer year analyses to compare the climate responses of the two species (temperature, precipitation and growing degree days). Both chronologies show accelerated growth at their modern ends, but this growth acceleration is not synchronous, beginning approximately a quarter of a century earlier at the Cradle Mt site. This discrepancy may highlight the relevance of chronology composition and/or physiological differences in the species. Although the seasonality of the climatic responses of the two species is similar, that of A. selaginoides is generally weaker than that of L. franklinii. Somewhat paradoxically, the only pointer years in common between the chronologies are 1898 and 1908 CE. The periods from 600 to 900 CE and ∼1200–1450 CE are conspicuous for their absence of positive pointer years while no negative pointer years occur for either site from ∼1200–1350 CE. It is possible that differing patterns of pointer years can be partially explained by a peak in establishment from ∼1150–1850 CE at the Mt Read L. franklinii site compared to continuous establishment at Cradle Mt. Although statistically significant and time-stable climate responses for the A. selaginoides chronology are too weak to base a single-chronology climate reconstruction on, the long chronology will likely make an important contribution to future multi-proxy temperature reconstructions for southeastern Australia.     

Age-dependent tree-ring growth responses of Schrenk spruce (Picea schrenkiana) to climate—A case study in the Tianshan Mountain,China

For both its climatic and ecological importance, Schrenk spruce (Picea schrenkiana) is a crucial tree species living at mid-altitude on the western area of the Tianshan Mountains. It plays a key role on understanding climatic change in the Tianshan Mountains in the past 500 years. However, whether the relationship between tree growth and limiting climate factors is stable over time is still not well-known. In this study, standard and residual chronologies of four 100-year age classes (AC1 < 110a, 110a < AC2 < 210a, 210a < AC3 < 310a and AC4 > 310a) were established for detecting divergence in climate–growth relationships as well as comparing low-frequency and high-frequency variations. The results show that climate can account for a high amount of variance in tree-ring width and higher climate sensitivity was detected in younger trees. Younger trees (<210a) exhibit significantly negative growth responses to mean monthly air temperature of previous June and positive relationship with total monthly precipitation of current April and May, while mean monthly air temperature of current March may inhibit growth of older trees (>210a). Tree-ring chronology statistics and response function reveal that the age-growth patterns are non-monotonic. Our results together with previous studies demonstrate that the age effects on tree-ring growth–climate response is attributed to a combination of genetic characteristics and site microclimate, which suggests that it is necessary to consider both age-dependent and species-specific climate responses when using tree-ring measurements as a proxy for valid climate reconstructions.     

Maternal smoking during pregnancy and the risk of childhood brain tumors: Results from a Swedish cohort study

BackgroundTobacco metabolites and carcinogens can be found in placental and umbilical cord tissues of fetuses exposed to maternal smoking. However, studies regarding maternal smoking during pregnancy and childhood brain tumor (CBT) have shown inconsistent results.MethodsAll children born in Sweden between 1983 and 2010 and with information about maternal smoking during pregnancy, obtained from the Swedish Medical Birth Register, were included in this population based cohort study (n = 2,577,305). CBT cases were identified from the National Cancer Register. Cox regression models were used to estimate the effect of maternal smoking during pregnancy on the risk of CBTs.ResultsWe identified 1039 cases of CBT in the cohort. Overall, there was little or no effect of maternal smoking during pregnancy on the risk of CBTs. However, in analyses stratified by age at diagnosis and child’s sex, positive associations were found among 5–9 years old children. In this age interval, maternal smoking during pregnancy was associated with an increased risk of all CBTs combined only among male children (RR = 1.50, 95% CI 0.96–2.34), while for astrocytoma there was a positive association in both male (RR = 2.00, 95% CI 1.02–3.91) and female children (RR = 1.80, 95% CI 0.85–3.82).ConclusionResults from this large Swedish cohort study suggest that even though maternal smoking during pregnancy has a limited overall effect on CBTs, it may increase the risk of astrocytomas.     

A hydroclimatic regionalization of central Mongolia as inferred from tree rings

In arid and semi-arid regions of the world, such as Mongolia, the future of water resources under a warming climate is of particular concern. The influence of increasing temperatures on precipitation is difficult to predict because precipitation trends in coming decades could have a high degree of spatial variability. In this study, we applied a rotated principal component analysis (RPCA) to a network of 20 tree-ring chronologies across central Mongolia from 1790 to 1994 to evaluate spatial hydroclimatic variability and to place recent variability in the context of the past several centuries. The RPCA results indicate that the network consists of four tree-growth anomaly regions, which were found to be relatively stable through time and space. Correlation analyses reveal spatial linkages between the tree-growth anomalies and instrumental data, where annual streamflow variability was strongly associated with tree-growth anomalies from their respective regions from 1959 to 1994 (r = 0.52–0.64, p < 0.05). This study highlights the extent of spatial variability in hydroclimate across central Mongolia and emphasizes the value of using tree-ring networks in locations with limited instrumental records.     

Survival of black alder (Alnus glutinosa L.), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) seedlings in a reclaimed oil shale mining area

Early survival and growth of black alder, silver birch and Scots pine were investigated on reclaimed extremely stony and heterogeneous calcareous (pH 8) opencast oil shale mining areas (OOSMAs). Biomass allocation, production, leaf and root adaptations, and mineral nutrition in relation to tree species and soil heterogeneity were analysed. The adaptive strategies of tree species in first-year plantations on OOSMA were different. Scots pine allocated 1.5–2 times more biomass into leaves and fine roots than deciduous trees. The lower leaf/fine root biomass ratio was in proportion to the better survival (%) of seedlings, decreasing in the following order: black alder (93%) ≥ Scots pine (83%) > silver birch (64%). Deciduous trees improved mineral nutrition more by fine-root morphological adaptations than Scots pine; e.g. the mean specific root length (SRL, m g^?1) of short roots increased in the following order: Scots pine (62) < black alder (172) < silver birch (314). The effect of soil heterogeneity on growth and adaptations was minor. All studied species suffered from P and N, and deciduous species also from K deficiency. In the first year after planting, black alder was best adapted to the harsh conditions of the post-mining substrate. The approaches of this study can be used for other regions where wastelands require reclamation.     

Responses to climate change in radial growth of Picea schrenkiana along elevations of the eastern Tianshan Mountains,northwest China

Tree growth is largely driven by climate conditions in arid and alpine areas. A strong change in climate from warm-dry to warm-wet has already been observed in northwest China. However, little is known about the impacts of regional climate variability on the radial growth of trees along elevations of the eastern Tianshan Mountains. Consequently, we developed three tree-ring width chronologies of Schrenk spruce (Picea schrenkiana Fisch. et Mey.) ranging in elevation from 2159 to 2552 m above sea level (a.s.l.), which play an important role in the forestry ecosystem, agriculture, and local economy of Central Asia. In our study, the correlation analyses of growth-drought using the monthly standardized precipitation-evapotranspiration index (SPEI) at different temporal scales demonstrated that drought in growing season was the main factor limiting tree growth, regardless of elevation. The relationships between radial growth of Schrenk spruce and main climate factors were relatively stable by moving correlation function, and the trend of STD chronologies and basal area increment (BAI) also showed a synchronous decline across the three elevations in recent decades. And meanwhile, slight differences in responses to climate change in radial growth along elevations were examined. The drought stress increased as elevations decreased. Radial growth at the higher elevation depended on moisture availability due to high temperature, as indicated by the significant negative correlation with mean temperature in the late growing season of the previous year (August-September, p < 0.001). However, radial growth at the lower elevation were restricted by drought stress due to less precipitation and higher temperatures, as demonstrated by the significant negative correlation with mean temperature but positive with total precipitation in the early growing season of the current year (April-May, p < 0.05). In addition, the decline of radial growth (BAI) at the higher elevation (3.710 cm² yr⁻¹/decade, p < 0.001) was faster than that of the middle elevation (2.344 cm² yr⁻¹/decade, p < 0.001) and the lower elevation (3.005 cm² yr⁻¹/decade, p < 0.001) since 2000, indicating that the trees at higher elevation of a relatively humid environment were more susceptible to the effects of climate change due to their poor adaptability to water deficit. Therefore, the forest ecosystems would be suppressed as a result of increasing drought stress in the future, especially in the high-elevation forests of arid and semi-arid areas.     

Lodgepole pine forest age class dynamics and susceptibility to mountain pine beetle attack

Because mountain pine beetle attack mature pine stands, an understanding of forest age class dynamics is important to managing forests within the distribution of the beetle. The assumed theoretical negative exponential forest age distribution provides an estimate when ecosystem dynamics are in equilibrium. This study investigates the dynamics of forest age distribution for non-equilibrium ecosystem dynamics, which result primarily from large and irregular stand-replacement fire disturbances that alter the forest age distribution. A model experiment using the SEM-LAND model on a 1 million ha lodgepole pine forest landscape was conducted to estimate how the proportion of susceptible area could be influenced by different fire regimes. The results of the simulation suggest that the temporal dynamics of the area susceptible to mountain pine beetle attack are complex and depend on the fire history of the study area, if the area is experiencing large and irregular stand-replacement fires. The age range of the lodgepole pine forest stands susceptible to mountain pine beetle attack might significantly affect the estimate of the area susceptible to attack.     

Growth response of Sabina tibetica to climate factors along an elevation gradient in south Tibet

We examine the climate significance in tree-ring chronologies retrieved from Sabina tibetica Kom. (Tibetan juniper) at two sites ranging in elevation from 4124 to 4693 m above sea level (a.s.l.) in the Namling region, south Tibet. The study region is under the control of semi-arid plateau temperate climate. The samples were grouped into high- and low-elevation classes and standard ring-width chronologies for both classes were developed. Statistical analysis revealed a decreasing growth rate yet increasing chronology reliability with increasing elevation. Overall, correlation analyses showed that radial growth in S. tibetica at the study sites was controlled by similar climatic factors, regardless of elevation; these factors comprised early winter (November) and early summer (May–June) temperatures as well as annual precipitation (July–June). Slight differences in the correlation between tree growth along the elevation gradient and climate variables were examined. The correlations with early winter temperature varied from significantly positive at the low-elevation site to weakly positive at the high-elevation site, whereas the correlations between radial growth and early summer temperature increased from weakly negative at the low-elevation sites to strongly negative at the high-elevation sites. The abundant precipitation through the year may have masked variations in tree growth on different elevation aspects. Our results will aid future dendroclimatological studies of Namling tree rings in south Tibet and demonstrate the potential of S. tibetica Kom. for improving our understanding of environmental impacts on tree growth.     

Fine-scale distribution of treeline trees and the nurse plant facilitation on the eastern Tibetan Plateau

Above-average climate warming occurred during the 20th century in high altitude regions, and alpine treelines are believed to be an early indicator to respond to these warming-related changes. However, empirical investigations on treeline dynamics showed diverse results. The main objectives of this study are: (1) to investigate if treeline position shifted and if tree recruitment changed along with climate warming, and (2) to test if adult trees have “nursing effect” on tree establishment at treelines. We investigated two Balfour spruce (Picea balfouriana Rehd. et Wils.) treelines in Chang Niang (CNT) and Dang Dui (DDT), Dingqing county, Changdu prefecture, eastern Tibet. At each treeline site, three replicate plots with a size 30 m × 50 m were established. The coordinates of each tree within the plots were recorded and the age of each tree was identified by dendrochronological method. The changes in treeline position and tree recruitment were examined from spatially fine-scale distribution of trees and their age structure. The spatial patterns of individual trees were analyzed to infer the neighborhood effects. Results indicate that plots CNT2, CNT3, DDT1 and DDT2 showed stable treeline position during the last century, whereas plots CNT1 and DDT3 showed treeline advancing movement. Tree recruitments in all the six plots were enhanced during the 20th century, with two peaks occurring in the 1890–1910s and the 1950–1990s. Seedlings and saplings showed a general clustered distribution in all the six plots. The diverse pattern of treeline movement and episodic regeneration suggest that the treeline activity is not merely a result of climate change. “Nursing effects” from adult trees may play an important role in shaping the treeline activities on the eastern Tibetan Plateau. Our findings reveal diverse patterns in treeline dynamics at a local scale and highlight the importance of incorporating biotic interactions into species distribution modeling approaches.     

Process-based modeling of tree-ring formation and its relationships with climate on the Tibetan Plateau

Response of climate warming on tree-ring formation has attracted much attention during recent years. However, most studies are based on statistical analysis, lacking understanding of tree-physiological processes, especially in the mountainous regions of the Tibetan Plateau (TP). Herein, we firstly use an updated new version of the tree-ring process-based Vaganov-Shashkin model (VS-oscilloscope) to simulate tree-ring formation and its relationships with climate factors during the past six decades. Our analyses covered 341 sampled trees growing within elevations ranging from 2750 to 4575 m a.s.l. at five sampling sites across the TP. Simulated tree-ring width series are significantly (p < 0.01) correlated with actual tree-ring width chronologies during their common interval periods. Starting dates of tree-ring formation are determined by temperature at all five sampling sites. After the initiation of tree stem cambial activity, soil moisture content has a significant effect on tree radial growth. Ending dates of cambial activity are driven by temperature over the whole study region. Simulated results indicate differences between wide and narrow tree-rings are mostly induced by soil moisture content, especially during the first half of the growing season, when effects from temperature variations are minor. Interestingly, we detected significantly (p < 0.001) increased relative growth rates due to higher soil moisture content after the year 1985 at the five sampling sites. However, the variability of mean relative growth rates due to temperature is negligible before and after that. Based on the successful application of VS-oscilloscope modeling on the high-elevation tree stands on the TP, our study provides a new perspective on tree radial growth process and their varying relationships to climate factors during the past six decades.     

Holocene vegetation and climate evolution of Corpus Christi and Trinity bays: Implications on coastal Texas source-to-sink deposition

The Texas coastline stretches 595 km across almost 4° of latitude and is home to diverse coastal vegetation assemblages, yet only a handful of studies have documented the climate and vegetative change of this region through the Holocene. We provide a detailed palynological record of Holocene climate for coastal Texas, based upon three subaqueous sediment cores from Corpus Christi Bay and Trinity Bay. Cluster analysis and correspondence analysis were used to investigate changes in palynological assemblages through time within each core. Common to both bays are nonarboreal taxa including Asteraceae (mainly Ambrosia and Helianthus), Chenopodium, Poaceae, and arboreal taxa such as Carya, Pinus, and Quercus. Our record shows that the coastal environments of central Texas began a transition from herbaceous (nonarboreal) dominated vegetation to arboreal vegetation as early as 8.4 ka within Corpus Christi Bay, and 3.8 ka within Trinity Bay. We note flooding events at 8.2, 5.4, and 3.6 ka in Corpus Christi Bay, and at 1.7, 1.2, and 0.8 ka in Trinity Bay. These events were caused by storms, sea level changes including flooding of relict river terraces, and changes in sediment delivery to the bays. The pollen record also shows evidence for changes in fluvial discharge to Corpus Christi Bay at 4.1 and 2.2 ka, and at 1.8 ka in Trinity Bay. We also see Zea mays in Trinity Bay, indicating local Native American agriculture. We observe no significant changes during the middle Holocene Climatic Optimum, and subtle but not statistically significant evidence of more variable climate oscillations than other records from more interior sites in Texas available for the late Holocene. This indicates that coastal Texas’ climate has operated semi-independently from central Texas regions, and was primarily driven by a coast-wise gradient of precipitation and evapotranspiration.