Process-Based Modeling of Phenology and Radial Growth in Pinus tabuliformis in Response to Climate Factors over a Cold and Semi-Arid Region

(1) Background: Climate change significantly impacts the phenology and dynamics of radial tree growth in alpine dryland forests. However, there remains a scarcity of reliable information on the physiological processes of tree growth and cambial phenology in response to long-term climate change in cold and semi-arid regions. (2) Methods: We employed the process-based Vaganov-Shashkin (VS) model to simulate the phenology and growth patterns of Chinese pine (Pinus tabuliformis) in the eastern Qilian Mountains, northeastern Tibetan Plateau. The model was informed by observed temperature and precipitation data to elucidate the relationships between climate factors and tree growth. (3) Results: The simulated tree-ring index closely aligned with the observed tree-ring chronology, validating the VS model's effectiveness in capturing the climatic influences on radial growth and cambial phenology of P. tabuliformis. The model outputs revealed that the average growing season spanned from mid-April to mid-October and experienced an extension post-1978 due to ongoing warming trends. However, it is important to note that an increase in the duration of the growing season did not necessarily result in a higher level of radial growth. (4) Conclusions: While the duration of the growing season was primarily determined by temperature, the growth rate was predominantly influenced by water conditions during the growing season, making it the most significant factor contributing to ring formation. Our study provides valuable insights into the potential mechanisms underlying tree growth responses to climate change in cold and semi-arid regions.

Ecological and methodological drivers of non-stationarity in tree growth response to climate

Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate-growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate-growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate-growth response for temperate forests.

Stable carbon isotopes of woods during carbonization and their correlation with climatic factors

To explore the applicability of the carbon isotope composition (δ13C) of fossil charcoal for the quantitative reconstruction of paleoclimates, we selected five points in Shaanxi province, from north to south, to collect modern Pinus species and Quercus species to sample covering areas with obvious climatic differences. In order to reveal the relationships between δ13C of charcoal and climate variables on the basis of carbonization experiments, we evaluated the fractionation mechanism of δ13C of charcoal, and compared the differences between δ13C of charcoal in wildfire experiments and indoor experiments regarding genera and species. The results showed significant differences in δ13C between genera but no significant differences among species. Additionally, the δ13C of charcoal was significantly negatively correlated with precipitation and positively correlated with evaporation, which could be determined from δ13C values in the study area to reconstruct ancient precipitation and evaporation in the future.

Plant-Soil-Climate Interaction in Observed and Simulated Tree-Radial Growth Dynamics of Downy Birch in Permafrost

Climate change projections forecast most significant impacts on high-latitude forest ecosystems. Particularly, climate warming in boreal regions should increase fire severity and shorten its return interval. These processes can change the dynamics of boreal forests as younger stands become more dominating with a shift from gymnosperm to angiosperm. However, despite angiosperm's phenological and physiological traits have a high potential for ecophysiological and dendroclimatological studies in Siberia, they have been rarely investigated due to their short-term lifespan in comparison with gymnosperm. Modeling tree growth is a common way to understand tree growth responses to environmental changes since it allows using available experiment or field data to interpret observed climate-growth relationships based on the biological principles. In our study, we applied the process-based Vaganov-Shashkin (VS) model of tree-ring growth via a parameterization approach VS-oscilloscope for the first time to an angiosperm tree species (Betula pubescens Ehrh.) from continuous permafrost terrain to understand its tree-radial growth dynamic. The parameterization of the VS model provided highly significant positive correlations (p < 0.05) between the simulated growth curve and initial tree-ring chronologies for the period 1971-2011 and displayed the average duration of the growing season and intra-seasonal key limiting factors for xylem formation. Modeled result can be valid at the regional scale for remote birch stands, whereas, justification of the local non-climatic input data of the model provided precise site-specific tree growth dynamic and their substantiated responses to driving factors.

A Band Model of Cambium Development: Opportunities and Prospects

More than 60% of tree phytomass is concentrated in stem wood, which is the result of periodic activity of the cambium. Nevertheless, there are few attempts to quantitatively describe cambium dynamics. In this study, we develop a state-of-the-art band model of cambium development, based on the kinetic heterogeneity of the cambial zone and the connectivity of the cell structure. The model describes seasonal cambium development based on an exponential function under climate forcing which can be effectively used to estimate the seasonal cell production for individual trees. It was shown that the model is able to simulate different cell production for fast-, middle- and slow-growing trees under the same climate forcing. Based on actual measurements of cell production for two contrasted trees, the model effectively reconstructed long-term cell production variability (up to 75% of explained variance) of both tree-ring characteristics over the period 1937−2012. The new model significantly simplifies the assessment of seasonal cell production for individual trees of a studied forest stand and allows the entire range of individual absolute variability in the ring formation of any tree in the stand to be quantified, which can lead to a better understanding of the anatomy of xylem formation, a key component of the carbon cycle.

Stable isotopes of tree rings reveal seasonal-to-decadal patterns during the emergence of a megadrought in the Southwestern US

Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ¹³C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ¹⁸O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ¹³C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ¹³C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species' distribution might be useful in the early detection of long-lasting droughts.

Growth-limiting factors and climate response variability in Norway spruce (Picea abies L.) along an elevation and precipitation gradients in Slovenia

Norway spruce (Picea abies L.) is among the most sensitive coniferous species to ongoing climate change. However, previous studies on its growth response to increasing temperatures have yielded contrasting results (from stimulation to suppression), suggesting highly site-specific responses. Here, we present the first study that applies two independent approaches, i.e. the nonlinear, process-based Vaganov-Shashkin (VS) model and linear daily response functions. Data were collected at twelve sites in Slovenia differing in climate regimes and ranging elevation between 170 and 1300 m a.s.l. VS model results revealed that drier Norway spruce sites at lower elevations are mostly moisture limited, while moist high-elevation sites are generally more temperature limited. Daily response functions match well the pattern of growth-limiting factors from the VS model and further explain the effect of climate on radial growth: prevailing growth-limiting factors correspond to the climate variable with higher correlations. Radial growth correlates negatively with rising summer temperature and positively with higher spring precipitation. The opposite response was observed for the wettest site at the highest elevation, which positively reacts to increased summer temperature and will most likely benefit from a warming climate. For all other sites, the future radial growth of Norway spruce largely depends on the balance between spring precipitation and summer temperature.

Ecological and conceptual consequences of Arctic pollution

Although the effect of pollution on forest health and decline received much attention in the 1980s, it has not been considered to explain the 'Divergence Problem' in dendroclimatology; a decoupling of tree growth from rising air temperatures since the 1970s. Here we use physical and biogeochemical measurements of hundreds of living and dead conifers to reconstruct the impact of heavy industrialisation around Norilsk in northern Siberia. Moreover, we develop a forward model with surface irradiance forcing to quantify long-distance effects of anthropogenic emissions on the functioning and productivity of Siberia's taiga. Downwind from the world's most polluted Arctic region, tree mortality rates of up to 100% have destroyed 24,000 km² boreal forest since the 1960s, coincident with dramatic increases in atmospheric sulphur, copper, and nickel concentrations. In addition to regional ecosystem devastation, we demonstrate how 'Arctic Dimming' can explain the circumpolar 'Divergence Problem', and discuss implications on the terrestrial carbon cycle.

Comparing the Cell Dynamics of Tree-Ring Formation Observed in Microcores and as Predicted by the Vaganov-Shashkin Model

New insights into the intra-annual dynamics of tree-ring formation can improve our understanding of tree-growth response to environmental conditions at high-resolution time scales. Obtaining this information requires, however, a weekly monitoring of wood formation, sampling that is extremely time-intensive and scarcely feasible over vast areas. Estimating the timing of cambial and xylem differentiation by modeling thus represents an interesting alternative for obtaining this important information by other means. Temporal dynamics of cambial divisions can be extracted from the daily tree-ring growth rate computed by the Vaganov-Shashkin (VS) simulation model, assuming that cell production is tightly linked to tree-ring growth. Nonetheless, these predictions have yet to be compared with direct observations of wood development, i.e., via microcoring, over a long time span. We tested the performance of the VS model by comparing the observed and predicted timing of wood formation in black spruce [Picea mariana (Mill.)]. We obtained microcores over 15 years at 5 sites along a latitudinal gradient in Quebec (Canada). The measured variables included cell size and the timing of cell production and differentiation. We calibrated the VS model using daily temperature and precipitation recorded by weather stations located on each site. The predicted and observed timing of cambial and enlarging cells were highly correlated (R ² = 0.8); nonetheless, we detected a systematic overestimation in the predicted timing of cambial cells, with predictions delayed by 1-20 days compared with observations. The growth rate of cell diameter was correlated with the predicted growth rate assigned to each cambial cell, confirming that cell diameter developmental dynamics have the potential to be inferred by the tree-ring growth curve of the VS model. Model performances decrease substantially in estimating the end of wood formation. The systematic errors suggest that the actual relationships implemented in the model are unable to explain the phenological events in autumn. The mismatch between the observed and predicted timing of wood formation in black spruce within our study area can be reduced by better adapting the VS model to wet sites, a context for which this model has been rarely used.

Physiological and Growth Responses to Increasing Drought of an Endangered Tree Species in Southwest China

Research Highlights: We compared annually resolved records of tree-ring width and stable isotope of dead and surviving Fokienia hodginsii (Dunn) Henry et Thomas trees. We provide new insights into the relationships and sensitivity of tree growth to past and current climate, and explored the underlying mechanism of drought-induced mortality in F. hodginsii. Background and Objectives: Drought-induced tree decline and mortality are increasing in many regions around the world. Despite the high number of studies that have explored drought-induced decline, species-specific responses to drought still makes it difficult to apply general responses to specific species. The endangered conifer species, Fokienia hodginsii, has experienced multiple drought-induced mortality events in recent years. Our objective was to investigate the historical and current responses to drought of this species. Materials and Methods: We used annually resolved ring-width and δ13C chronologies to investigate tree growth and stand physiological responses to climate change and elevated CO2 concentration (Ca) in both dead and living trees between 1960 and 2015. Leaf intercellular CO2 concentration (Ci), Ci/Ca and intrinsic water-use efficiency (iWUE) were derived from δ13C. Results: δ13C were positively correlated with mean vapor pressure deficit and PDSI from previous October to current May, while ring widths were more sensitive to climatic conditions from previous June to September. Moreover, the relationships between iWUE, basal area increment (BAI), and Ci/Ca changed over time. From 1960s to early 1980s, BAI and iWUE maintained a constant relationship with increasing atmospheric CO2 concentration. After the mid-1980s, we observed a decrease in tree growth, increase in the frequency of missing rings, and an unprecedented increase in sensitivity of 13C and radial growth to drought, likely related to increasingly dry conditions. Conclusions: We show that the recent increase in water stress is likely the main trigger for the unprecedented decline in radial growth and spike in mortality of F. hodginsii, which may have resulted from diminished carbon fixation and water availability. Given that the drought severity and frequency in the region is expected to increase in the future, our results call for effective mitigation strategies to maintain this endangered tree species.

Insights into the BRT (Boosted Regression Trees) Method in the Study of the Climate-Growth Relationship of Masson Pine in Subtropical China

Dendroclimatology and dendroecology have entered mainstream dendrochronology research in subtropical and tropical areas. Our study focused on the use of the chronology series of Masson pine (Pinus massoniana Lamb.), the most widely distributed tree species in the subtropical wet monsoon climate regions in China, to understand the tree growth response to ecological and hydroclimatic variability. The boosted regression trees (BRT) model, a nonlinear machine learning method, was used to explore the complex relationship between tree-ring growth and climate factors on a larger spatial scale. The common pattern of an asymptotic growth response to the climate indicated that the climate-growth relationship may be linear until a certain threshold. Once beyond this threshold, tree growth will be insensitive to some climate factors, after which a nonlinear relationship may occur. Spring and autumn climate factors are important controls of tree growth in most study areas. General circulation model (GCM) projections of future climates suggest that warming climates, especially temperatures in excess of those of the optimum growth threshold (as estimated by BRT), will be particularly threatening to the adaptation of Masson pine.

Perspectives of regional paleoclimate modeling

Regional climate modeling bridges the gap between the coarse resolution of current global climate models and the regional‐to‐local scales, where the impacts of climate change are of primary interest. Here, we present a review of the added value of the regional climate modeling approach within the scope of paleoclimate research and discuss the current major challenges and perspectives. Two time periods serve as an example: the Holocene, including the Last Millennium, and the Last Glacial Maximum. Reviewing the existing literature reveals the benefits of regional paleo climate modeling, particularly over areas with complex terrain. However, this depends largely on the variable of interest, as the added value of regional modeling arises from a more realistic representation of physical processes and climate feedbacks compared to global climate models, and this affects different climate variables in various ways. In particular, hydrological processes have been shown to be better represented in regional models, and they can deliver more realistic meteorological data to drive ice sheet and glacier modeling. Thus, regional climate models provide a clear benefit to answer fundamental paleoclimate research questions and may be key to advance a meaningful joint interpretation of climate model and proxy data.

Climate signals in tree-ring δ18 O and δ13 C from southeastern Tibet: insights from observations and forward modelling of intra- to interdecadal variability

Stable isotopes in tree rings are increasingly used as proxies for climatic and ecophysiological changes. However, uncertainties remain about the strength and consistency of their response to environmental variation at different temporal (i.e. seasonal to inter-decadal) scales. We developed 5 yr of intra-seasonal and 62 yr of early- and late-wood δ¹³ C and δ¹⁸ O series of Smith fir (Abies georgei var. smithii) on the southeastern Tibetan Plateau, and used a process-based forward model to examine the relative importance of environmental and physiological controls on the isotopic data. In this temperate high-altitude region, the response, both δ¹⁸ O and δ¹³ C, is primarily to variations in relative humidity, but by different processes. In δ¹⁸ O, the response is via source water δ¹⁸ O but also arises from leaf water ¹⁸ O enrichment. In δ¹³ C, the response is via changes in stomatal conductance but is modified by carry-over effects from prior periods. We conclude that tree-ring δ¹⁸ O may be a more robust climate proxy than δ¹³ C, and δ¹³ C may be more suited to studies of site-related physiological responses to the local environment.

Assessing forest vulnerability to climate warming using a process-based model of tree growth: bad prospects for rear-edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought-prone areas, tree populations located at the driest and southernmost distribution limits (rear-edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear-edges of the continuous distributions of these tree species. We used tree-ring width data from a network of 110 forests in combination with the process-based Vaganov-Shashkin-Lite growth model and climate-growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear-edge. By contrast, growth of high-elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of -10.7% and -16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear-edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear-edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.

Critical minimum temperature limits xylogenesis and maintains treelines on the southeastern Tibetan Plateau

Physiological and ecological mechanisms that define treelines are still debated. It has been suggested that the absence of trees above the treeline is caused by low temperatures that limit growth. Thus, we hypothesized that there is a critical minimum temperature (CT_min) preventing xylogenesis at treeline. We tested this hypothesis by examining weekly xylogenesis across three and four growing seasons in two natural Smith fir (Abies georgei var. smithii) treeline sites on the southeastern Tibetan Plateau. Despite differences in the timing of cell differentiation among years, minimum air temperature was the dominant climatic variable associated with xylem growth; the critical minimum temperature (CT_min) for the onset and end of xylogenesis occurred at 0.7±0.4°C. A process-based modelling chronology of tree-ring formation using this CT_min was consistent with actual tree-ring data. This extremely low CT_min permits Smith fir growing at treeline to complete annual xylem production and maturation and provides both support and a mechanism for treeline formation.