Tree growth acceleration and expansion of alpine forests: The synergistic effect of atmospheric and edaphic change

Transitional responses of tree growth to climate warming at the southernmost margin of high latitudinal permafrost distribution

The marked increase in temperature warming and permafrost degradation has raised apprehensions about the fate of forests of boreal forests in permafrost regions. However, the impact of climate on tree growth is not limited to direct effects but also involves complex interactions with permafrost. The degradation of permafrost poses a threat to forest growth that has received insufficient attention thus far, after analyzing the impact of permafrost degradation and climate on Dahurican larch (Larix gmelinii) growth from six forest sites with two maximum active layer thickness (ALT) classifications (more and less than tree root length) across the southern margin of the permafrost region. We found that accompanying the continued degradation of permafrost, tree growth was inhibited (slope = -0.67, p < 0.05) by the degradation of permafrost and the growth-climate relationship was shifted from positive to negative at maximum ALT less than tree root length sites. However, the growth rate of trees significantly accelerated (slope = 5.46, p < 0.05) at maximum ALT more than tree root length sites. Path analysis indicated that tree growth did not benefit from temperature warming and more stress could be caused by waterlogging due to permafrost degradation at maximum ALT less than tree root length sites, however, enhanced tree growth primarily by reducing the physical spatial constraints and root layer additional water source with permafrost degradation at maximum ALT more than tree root length sites. It also implies that the matchiness between tree root and maximum active layer depth is critical to the effect of permafrost degradation on tree growth. The transitional response to climate warming and the opposite trend of tree growth at two ALT classification sites suggest that future tree growth responds to the different stages of permafrost degradation differently. Our study provides a new insight on permafrost degradation impact on tree growth.

Phenotypic plasticity of growth ring traits in Pinus hartwegii at the ends of its elevational gradient

Introduction

Phenotypic plasticity (PP) could be an important short-term mechanism to modify physiological and morphological traits in response to climate change and global warming, particularly for high-mountain tree species. The objective was to evaluate PP response of growth ring traits to temperature and precipitation in Pinus hartwegii Lindl. populations located at the ends of its elevational gradient on two volcanic mountains in central Mexico (La Malinche and Nevado de Toluca).

Methods

Increment cores collected from 274 P. hartwegii trees were used to estimate their PP through reaction norms (RN), which relate the ring width and density traits with climate variables (temperature and precipitation). We estimated the trees' sensitivity (significant RN) to climatic variables, as well as the relative proportion of RN with positive and negative slope. We also estimated the relationship between the PP of ring width and density traits using correlation and Principal Component (PC) analyses.

Results

Over 70% of all trees showed significant RN to growing season and winter temperatures for at least one growth ring trait, with a similar proportion of significant RN at both ends of the gradient on both mountains. Ring width traits had mostly negative RN, while ring density traits tended to have positive RN. Frequency of negative RN decreased from lower to higher elevation for most traits. Average PP was higher at the lower end of the gradient, especially on LM, both for ring width and ring density traits, although high intrapopulation variation in PP was found on both mountains.

Discussion

Results indicate that P. hartwegii presents spatially differentiated plastic responses in width and density components of radial growth. PP was particularly strong at the lower elevation, which has higher temperature and water stress conditions, putting these populations at risk from the continuing global warming driven by climate change.

Impacts of human pressure and climate on biodiversity-multifunctionality relationships on the Qinghai-Tibetan Plateau

Many studies have investigated the effects of environmental context on biodiversity or multifunctionality in alpine regions, but it is uncertain how human pressure and climate may affect their relationships. Here, we combined the comparative map profile method with multivariate datasets to assess the spatial pattern of ecosystem multifunctionality and further identify the effects of human pressure and climate on the spatial distribution of biodiversity-multifunctionality relationships in alpine ecosystems of the Qinghai-Tibetan Plateau (QTP). Our results indicate that at least 93% of the areas in the study region show a positive correlation between biodiversity and ecosystem multifunctionality across the QTP. Biodiversity-multifunctionality relationships with increasing human pressure show a decreasing trend in the forest, alpine meadow, and alpine steppe ecosystems, while an opposite pattern was found in the alpine desert steppe ecosystem. More importantly, aridity significantly strengthened the synergistic relationship between biodiversity and ecosystem multifunctionality in forest and alpine meadow ecosystems. Taken together, our results provide insights into the importance of protecting and maintaining biodiversity and ecosystem multifunctionality in response to climate change and human pressure in the alpine region.

Divergent Tree Growth and the Response to Climate Warming and Humidification in the Tianshan Mountains, China

In recent decades, the global climate has changed significantly. The climate in Northwest China became warm-wet, especially in the Tianshan Mountains. In order to explore the response of tree growth to recent climate change, the two dominant trees species, Picea schrenkiana Fisch. et Mey. and Larix sibirica Ledeb., were studied with the dendrochronological method in the western Tianshan Mountains (WT) and the eastern Tianshan Mountains (ET). Our results showed that: (1) The tree growth of four sample sites in the WT significantly increased in recent decades, while the trees in the three sample sites in the ET significantly decreased. (2) In the WT, except for the Manas site, the tree-ring chronologies of the other three sites were significantly positively correlated with the mean annual minimum temperature. Tree-ring chronologies in the WT, except for Bangfanggou site, were significantly positively correlated with annual precipitation. In the ET, only the tree chronology of L. sibirica in the Balikun site was significantly negatively correlated with the annual temperatures, including the mean minimum, mean and mean maximum temperature. (3) The proportion of trees with a significant upward growth trend at each site decreased from west to east, and the proportion of trees with a significant downward growth trend at each site increased from west to east along the whole Tianshan Mountains. (4) The correlation of tree-ring chronologies with the annual temperature and annual precipitation was not stable during the study period. Warm-humidification promoted the growth of trees in the WT but inhibited tree growth in the ET, which may be exacerbated drought stress in the ET where the increase in precipitation was not enough to offset the increased evapotranspiration potential caused by warming.

Where should China practice forestry in a warming world?

As a nature-based and cost-effective solution, forestation plays a crucial role in combating global warming, biodiversity collapse, environmental degradation, and global well-being. Although China is acknowledged as a global leader of forestation and has achieved considerable overall success in environmental improvements through mega-forestation programs, many negative effects have also emerged at local scales due to the planting of maladapted tree species. To better help achieve carbon neutrality and the new vision of an ecological civilization, China has committed to further increase forestation. However, where forestation lands and such efforts should really be located is not so well understood yet and agreed upon, especially in the face of rapid climate change. Based on an ensemble-learning machine, we predicted the spatial habitats (ecological niche) of the forest, grassland, shrubland, and desert under present and future climate conditions based on the natural climax vegetation distribution across China. We show that the potential forestation lands are mainly located in eastern China, which is east of the Hu Line (also known as the Heihe-Tengchong Line). Under future climate change, forests will shift substantially in the latitudinal, longitudinal, and elevational distribution. Potential forestation lands will increase by 33.1 million hectares through the 2070s, mainly due to the conversions of shrub and grassland to forests along the Hu Line. Our prediction map also indicates that grassland rehabilitation is the universal optimal vegetation restoration strategy in areas west of the Hu Line. This analysis is consistent with much of the observed evidence of forestation failures and recent climate-change-induced forest range shifts. Our results provide an overview and further show the importance of adaptive science-based forestation planning and forest management.

Trends in climatically driven extreme growth reductions of Picea abies and Pinus sylvestris in Central Europe

Extreme tree growth reductions represent events of abrupt forest productivity decline and carbon sequestration reduction. An increase in their magnitude can represent an early warning signal of impending tree mortality. Yet the long-term trends in extreme growth reductions remain largely unknown. We analyzed the trends in the proportion of trees exhibiting extreme growth reductions in two Central-European conifer species-Pinus sylvestris (PISY) and Picea abies (PCAB)-between 1901 and 2018. We used a novel approach for extreme growth reduction quantification by relating their size to their mean recurrence interval. Twenty-eight sites throughout Czechia and Slovakia with 1120 ring width series representing high- and low-elevation forests were inspected for extreme growth reductions with recurrence intervals of 15 and 50 years along with their link to climatic drivers. Our results show the greatest growth reductions at low-elevation PCAB sites, indicating high vulnerability of PCAB to drought. The proportions of trees exhibiting extreme growth reductions increased over time at low-elevation PCAB, decreased recently following an abrupt increase in the 1970-1980s at high-elevation PCAB, and showed nonsignificant trends in high- and low-elevation PISY. Climatic drivers of extreme growth reductions, however, shifted over time for all site categories as the proportion of low-temperature-induced extreme growth reductions declined since the 1990s, whereas events caused by drought consistently increased in frequency during the same period. We observed higher growth volatility at the lower range of distribution compared with the upper range margin of PISY and PCAB. This will undoubtedly considerably impact tree growth and vitality as temperatures and incidence of drought in Central Europe are expected to further increase with ongoing climate change.

Climate-induced reversal of tree growth patterns at a tropical treeline

Globally, cold-limited trees and forests are expected to experience growth acceleration as a direct response to warming temperatures. However, thresholds of temperature limitation may vary substantially with local environmental conditions, leading to heterogeneous responses in tree ecophysiology. We used dendroecological and isotopic methods to quantify shifting tree growth and resource use over the past 143 years across topographic aspects in a high-elevation forest of central Mexico. Trees on south-facing slopes (SFS) grew faster than those on north-facing slopes (NFS) until the mid-20th century, when this pattern reversed notably with marked growth rate declines on SFS and increases on NFS. Stable isotopes of carbon, oxygen, and carbon-to-nitrogen ratios suggest that this reversal is linked to interactions between CO₂ stimulation of photosynthesis and water or nitrogen limitation. Our findings highlight the importance of incorporating landscape processes and habitat heterogeneity in predictions of tree growth responses to global environmental change.

The what, how, why, and when of dendrochemistry: (paleo)environmental information from the chemical analysis of tree rings

The chemical analysis of tree rings has attracted the interest of researchers in the past five decades in view of the possibility of exploiting this biological indicator as a widely available, high-resolution environmental archive. Information regarding the surrounding environment can be derived either by directly measuring environmental variables (nutrient availability, presence of pollutants, etc.) or by exploiting proxies (e.g. paleoclimatic and paleoenvironmental reconstructions). This review systematically covers the topic and provides a critical view on the reliability of dendrochemical information. First, we introduce the determinable chemical species, such as major elements, trace metals, isotopic ratios, and organic compounds, together with a brief description of their uptake mechanisms and functions in trees. Subsequently, we present the possibilities offered by analytical techniques in the field of tree ring analysis, focusing on direct methods and recent developments. The latter strongly improved the details of the accessible information, enabling the investigation of complex phenomena associated with plant life and encouraging the direct analysis of new analytes, particularly minor organic compounds. With regard to their applications, dendrochemical proxies have been used to trace several processes, such as environmental contamination, paleoclimate reconstruction, global environmental changes, tree physiology, extreme events, ecological trends, and dendroprovenance. Several case studies are discussed for each proposed application, with special emphasis on the reliability of tracing each process. Starting from the reviewed literature data, the second part of the paper is devoted to the critical assessment of the reliability of tree ring proxies. We provide an overview of the current knowledge, discuss the limitations of the inferences that may be drawn from the dendrochemical data, and provide recommendations for the best practices to be used for their validation. Finally, we present the future perspectives related to the advancements in analytical instrumentation and further extension of application fields.

Long-term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

High-elevation forests are experiencing high rates of warming, in combination with CO₂ rise and (sometimes) drying trends. In these montane systems, the effects of environmental changes on tree growth are also modified by elevation itself, thus complicating our ability to predict effects of future climate change. Tree-ring analysis along an elevation gradient allows quantifying effects of gradual and annual environmental changes. Here, we study long-term physiological (ratio of internal to ambient CO₂ , i.e., C_i /C_a and intrinsic water-use efficiency, iWUE) and growth responses (tree-ring width) of Himalayan fir (Abies spectabilis) trees in response to warming, drying, and CO₂ rise. Our study was conducted along elevational gradients in a dry and a wet region in the central Himalaya. We combined dendrochronology and stable carbon isotopes (δ¹³ C) to quantify long-term trends in C_i /C_a ratio and iWUE (δ¹³ C-derived), growth (mixed-effects models), and evaluate climate sensitivity (correlations). We found that iWUE increased over time at all elevations, with stronger increase in the dry region. Climate-growth relations showed growth-limiting effects of spring moisture (dry region) and summer temperature (wet region), and negative effects of temperature (dry region). We found negative growth trends at lower elevations (dry and wet regions), suggesting that continental-scale warming and regional drying reduced tree growth. This interpretation is supported by δ¹³ C-derived long-term physiological responses, which are consistent with responses to reduced moisture and increased vapor pressure deficit. At high elevations (wet region), we found positive growth trends, suggesting that warming has favored tree growth in regions where temperature most strongly limits growth. At lower elevations (dry and wet regions), the positive effects of CO₂ rise did not mitigate the negative effects of warming and drying on tree growth. Our results raise concerns on the productivity of Himalayan fir forests at low and middle (<3,300 m) elevations as climate change progresses.

Potential Elevation Shift of the European Beech Stands (Fagus sylvatica L.) in Serbia

According to climate projection models, the global temperature is expected to rise by at least 1.5°C by the end of this century. According to some studies the expected rise in Serbia is even higher. Global warming may result in creating new areas for forest growth. Although creating new forests would be a positive outcome in some areas, global warming can cause negative impacts in other areas, and this can lead to forest loss and the shift of geographical ranges, or even extinction, of plant species. The European beech is the dominant forest tree species in Serbia, featuring high ecological importance and economic value. In mixed or pure stands, beech forests cover approximately 660,400 ha, accounting for 29.3% of the total Serbian forest area. In the present study, the effects of climate change on the distribution of the European beech stands in Serbia, with an emphasis on their elevation shifts, were examined using species distribution models (SDMs). Data for the present tree cover in Serbia, climate projections, and environmental data were used for model building. The models were first tested against present inventory data. In these tests, the models were found to provide accurate projections, as shown by their true skills statistics (TSS) values ranging from 0.652 to 0.736 and area under the curve (AUC) values ranging from 0.868 to 0.937. The potential distribution patterns predicted by the models indicate that the European beech elevational distribution in Serbia would decrease, exhibiting a significant upward shift in elevation during the first part of this century. Current beech stand locations could be changed, and other areas at higher elevations may be more suitable for beech growth. After 2071, European beech stands at elevations below 500 m would be even smaller. This change is caused by temperature rise and occurrence of climate extremes. However, on the highest elevations, further upward shift of the species is not expected.

Large apparent growth increases in boreal forests inferred from tree-rings are an artefact of sampling biases

Tree rings are thought to be a powerful tool to reconstruct historical growth changes and have been widely used to assess tree responses to global warming. Demographic inferences suggest, however, that typical sampling procedures induce spurious trends in growth reconstructions. Here we use the world’s largest single tree-ring dataset (283,536 trees from 136,621 sites) from Quebec, Canada, to assess to what extent growth reconstructions based on these - and thus any similar - data might be affected by this problem. Indeed, straightforward growth rate reconstructions based on these data suggest a six-fold increase in radial growth of black spruce (Picea mariana) from ~0.5 mm yr⁻¹ in 1800 to ~2.5 mm yr⁻¹ in 1990. While the strong correlation (R² = 0.98) between this increase and that of atmospheric CO₂ could suggest a causal relationship, we here unambiguously demonstrate that this growth trend is an artefact of sampling biases caused by the absence of old, fast-growing trees (cf. “slow-grower survivorship bias”) and of young, slow-growing trees (cf. “big-tree selection bias”) in the dataset. At the moment, we cannot envision how to remedy the issue of incomplete representation of cohorts in existing large-scale tree-ring datasets. Thus, innovation will be needed before such datasets can be used for growth rate reconstructions.

Linking variation in intrinsic water-use efficiency to isohydricity: a comparison at multiple spatiotemporal scales

Species-specific responses of plant intrinsic water-use efficiency (iWUE) to multiple environmental drivers associated with climate change, including soil moisture (θ), vapor pressure deficit (D), and atmospheric CO₂ concentration (c_a ), are poorly understood. We assessed how the iWUE and growth of several species of deciduous trees that span a gradient of isohydric to anisohydric water-use strategies respond to key environmental drivers (θ, D and c_a ). iWUE was calculated for individual tree species using leaf-level gas exchange and tree-ring δ¹³ C in wood measurements, and for the whole forest using the eddy covariance method. The iWUE of the isohydric species was generally more sensitive to environmental change than the anisohydric species was, and increased significantly with rising D during the periods of water stress. At longer timescales, the influence of c_a was pronounced for isohydric tulip poplar but not for others. Trees' physiological responses to changing environmental drivers can be interpreted differently depending on the observational scale. Care should be also taken in interpreting observed or modeled trends in iWUE that do not explicitly account for the influence of D.

Limited evidence for CO2 -related growth enhancement in northern Rocky Mountain lodgepole pine populations across climate gradients

Forests sequester large amounts of carbon annually and are integral in buffering against effects of global change. Increasing atmospheric CO₂ may enhance photosynthesis and/or decrease stomatal conductance (g_s ) thereby enhancing intrinsic water-use efficiency (iWUE), having potential indirect and direct benefits to tree growth. While increasing iWUE has been observed in most trees globally, enhanced growth is not ubiquitous, possibly due to concurrent climatic constraints on growth. To investigate our incomplete understanding of interactions between climate and CO₂ and their impacts on tree physiology and growth, we used an environmental gradient approach. We combined dendrochronology with carbon isotope analysis (δ¹³ C) to assess the covariation of basal area increment (BAI) and iWUE over time in lodgepole pine. Trees were sampled at 18 sites spanning two climatically distinct elevation transects on the lee and windward sides of the Continental Divide, encompassing the majority of lodgepole pine's northern Rocky Mountain elevational range. We analyzed BAI and iWUE from 1950 to 2015, and explored correlations with monthly climate variables. As expected, iWUE increased at all sites. However, concurrent growth trends depended on site climatic water deficit (CWD). Significant growth increases occurred only at the driest sites, where increases in iWUE were strongest, while growth decreases were greatest at sites where CWD has been historically lowest. Late summer drought of the previous year negatively affected growth across sites. These results suggest that increasing iWUE, if strong enough, may indirectly benefit growth at drier sites by effectively extending the growing season via reductions in g_s . Strong growth decreases at high elevation windward sites may reflect increasing water stress as a result of decreasing snowpack, which was not offset by greater iWUE. Our results imply that increasing iWUE driven by decreasing g_s may benefit tree growth in limited scenarios, having implications for future carbon uptake potential of semiarid ecosystems.

Developmental phenotypic plasticity helps bridge stochastic weather events associated with climate change

The slow, inexorable rise in annual average global temperatures and acidification of the oceans are often advanced as consequences of global change. However, many environmental changes, especially those involving weather (as opposed to climate), are often stochastic, variable and extreme, particularly in temperate terrestrial or freshwater habitats. Moreover, few studies of animal and plant phenotypic plasticity employ realistic (i.e. short-term, stochastic) environmental change in their protocols. Here, I posit that the frequently abrupt environmental changes (days, weeks, months) accompanying much longer-term general climate change (e.g. global warming over decades or centuries) require consideration of the true nature of environmental change (as opposed to statistical means) coupled with an expansion of focus to consider developmental phenotypic plasticity. Such plasticity can be in multiple forms - obligatory/facultative, beneficial/deleterious - depending upon the degree and rate of environmental variability at specific points in organismal development. Essentially, adult phenotypic plasticity, as important as it is, will be irrelevant if developing offspring lack sufficient plasticity to create modified phenotypes necessary for survival.

Integrating effects of species composition and soil properties to predict shifts in montane forest carbon-water relations

This research focuses on how species composition and soil properties interact to control carbon fixation and water loss in California’s montane forests. Two conclusions arise: (i) The amount of carbon fixed per unit of water lost via transpiration varies widely among forest stands depending on dominant tree species and their inherent leaf characteristics, and (ii) different parent materials and stages of soil development regulate the effect of climate on water-use efficiency and productivity of trees across altitudinal gradients. Taken together, dynamic biological processes related to species composition and relatively inert physicochemical properties that characterize soil development can be combined to anticipate changes in forest carbon and water balances.

This study was designed to address a major source of uncertainty pertaining to coupled carbon–water cycles in montane forest ecosystems. The Sierra Nevada of California was used as a model system to investigate connections between the physiological performance of trees and landscape patterns of forest carbon and water use. The intrinsic water-use efficiency (iWUE)—an index of CO₂ fixed per unit of potential water lost via transpiration—of nine dominant species was determined in replicated transects along an ∼1,500-m elevation gradient, spanning a broad range of climatic conditions and soils derived from three different parent materials. Stable isotope ratios of carbon and oxygen measured at the leaf level were combined with field-based and remotely sensed metrics of stand productivity, revealing that variation in iWUE depends primarily on leaf traits (∼24% of the variability), followed by stand productivity (∼16% of the variability), climatic regime (∼13% of the variability), and soil development (∼12% of the variability). Significant interactions between species composition and soil properties proved useful to predict changes in forest carbon–water relations. On the basis of observed shifts in tree species composition, ongoing since the 1950s and intensified in recent years, an increase in water loss through transpiration (ranging from 10 to 60% depending on parent material) is now expected in mixed conifer forests throughout the region.

Influence of riverine input on the growth of Glycymeris glycymeris in the Bay of Brest, North-West France

A crossdated, replicated, chronology of 114 years (1901–2014) was developed from internal growth increments in the shells of Glycymeris glycymeris samples collected monthly from the Bay of Brest, France. Bivalve sampling was undertaken between 2014 and 2015 using a dredge. In total 401 live specimens and 243 articulated paired valves from dead specimens were collected, of which 38 individuals were used to build the chronology. Chronology strength, assessed as the Expressed Population Signal, was above 0.7 throughout, falling below the generally accepted threshold of 0.85 before 1975 because of reduced sample depth. Significant positive correlations were identified between the shell growth and the annual averages of rainfall (1975–2008; r = 0.34) and inflow from the river Elorn (1989–2009; r = 0.60). A significant negative correlation was identified between shell growth and the annual average salinity (1998–2014; r = -0.62). Analysis of the monthly averages indicates that these correlations are associated with the winter months (November–February) preceding the G. glycymeris growth season suggesting that winter conditions predispose the benthic environment for later shell growth. Concentration of suspended particulate matter within the river in February is also positively correlated with shell growth, leading to the conclusion that food availability is also important to the growth of G. glycymeris in the Bay of Brest. With the addition of principle components analysis, we were able to determine that inflow from the River Elorn, nitrite levels and salinity were the fundamental drivers of G. glycymeris growth and that these environmental parameters were all linked.