Species distribution models predict temporal but not spatial variation in forest growth

Bioclimate envelope models have been widely used to illustrate the discrepancy between current species distributions and their potential habitat under climate change. However, the realism and correct interpretation of such projections has been the subject of considerable discussion. Here, we investigate whether climate suitability predictions correlate to tree growth, measured in permanent inventory plots and inferred from tree‐ring records. We use the ensemble classifier RandomForest and species occurrence data from ~200,000 inventory plots to build species distribution models for four important European forestry species: Norway spruce, Scots pine, European beech, and pedunculate oak. We then correlate climate‐based habitat suitability with volume measurements from ~50‐year‐old stands, available from ~11,000 inventory plots. Secondly, habitat projections based on annual historical climate are compared with ring width from ~300 tree‐ring chronologies. Our working hypothesis is that habitat suitability projections from species distribution models should to some degree be associated with temporal or spatial variation in these growth records. We find that the habitat projections are uncorrelated with spatial growth records (inventory plot data), but they do predict interannual variation in tree‐ring width, with an average correlation of .22. Correlation coefficients for individual chronologies range from values as high as .82 or as low as ?.31. We conclude that tree responses to projected climate change are highly site‐specific and that local suitability of a species for reforestation is difficult to predict. That said, projected increase or decrease in climatic suitability may be interpreted as an average expectation of increased or reduced growth over larger geographic scales.     

Volcanic explosive eruptions of the Vesuvio decrease tree-ring growth but not photosynthetic rates in the surrounding forests

Volcanic eruptions impact the global and the hemispheric climate, but it is still unknown how and to what degree they force the climate system and in particular the global carbon cycle. In this paper, the relationships between individual eruptions (reconstructed for the past using written records), tree primary productivity (estimated using ring widths), photosynthetic rate and stomatal conductance (assessed by carbon and oxygen isotope data) are investigated, to understand the impact of volcanic eruptions on net primary production. Data from a mixed stand of Fagus sylvatica L. and Acer pseudoplatanus L. located in the area of the Vesuvio volcanic complex (Southern Italy) showed a significant decrease in ring width following each eruption. Isotope analyses indicate a change in climatic conditions after such events. Specifically, the lower oxygen isotope ratio in the tree‐ring cellulose strongly suggests an increase in relative humidity and a decrease in temperature, with the latter resulting in a strong limitation to tree‐ring growth. The carbon isotope ratio was only moderately but not significantly reduced in the years of volcanic eruption, suggesting no major changes in C fixation rates. This work is a case study on the effects of volcanic eruptions resulting in strong climatic changes on the local scale. This is an opportunity to explore the process and causal relationships between climatic changes and the response of the vegetation. Thus, we propose here a realistic model scenario, from which we can extrapolate to global scales and improve our interpretations of results of global studies.     

Tree-ring responses in Araucaria araucana to two major eruptions of Lonquimay Volcano (Chile)

Palaeoclimate proxies have demonstrated links between climate changes and volcanic activity. However, not much is known about the impact of volcanic eruptions on forest productivity. Here we used tree-ring width and annually resolved carbon and oxygen isotopic records from tree rings of Araucaria araucana (Molina) K. Koch, providing a centennial-scale reconstruction of tree ecophysiological processes in forest stands nearby the Lonquimay Volcano (Chile). We observed a mean decrease in tree-ring width following the major eruption of 1988–1990 (with aerosol emission), most probably caused by the modified ecological conditions due to acid rain and ash deposition, while a generally negative relationship between δ¹³C and δ¹⁸O would point to a decline in humidity and precipitation. More negative δ¹³C and lower δ¹⁸O values (positive correlation) following the major eruption of 1887–1890 (without aerosol emission) would suggest high stomatal conductance and moisture availability, though tree-ring width (and probably photosynthetic rate) was unaltered. At least for this sample of trees, in the case of eruption with large tephra emission, the beneficial effect of aerosol light scattering on tree productivity appears to be outweighed by the detrimental effect of eruption-induced toxic deposition. Signals of the two major eruptions of the past 200?years at Lonquimay were present in tree rings of nearby A. araucana. No unique response of tree functions to volcanic eruptions can be expected, but rather (1) the variable volcanic properties and (2) the complex interplay of diffuse light increase (aerosol scattering), air temperature decrease (cloud shading), and toxic deposition impact (volcanic ash), makes any prediction of tree growth and ecophysiological response very challenging.     

Bioclimatic model of tree radial growth: application to the French Mediterranean Aleppo pine forests

The potential effects of global changes on forests are of increasing concern. Dendrochronology, which deals with long-term records of tree growth under natural environmental conditions, can be used to evaluate the impact of climatic change on forest productivity. However, assessment of climatic change impacts must be supported by accurate and reliable models of the relationships between climate and tree growth. In this study, a bioclimatic model is used to explore the relationships between tree radial growth and bioclimatic variables closely related to the biological functioning of a tree. This model is at an intermediate level of complexity between purely empirical and process-based models. The method is illustrated with data for 21 Aleppo pine (Pinus halepensis Mill.) stands grown under a Mediterranean climate in south-east France. The results show that Aleppo pine growth is mainly controlled by soil water availability during the growing season. The bioclimatic variable which best expresses the observed inter-annual tree growth variations is the actual evapotranspiration (AET). Four parameters were adjusted to simulate dendrochronological data: the soil water capacity, the wilting point, the minimum temperature for photosynthesis, and the end of the growing season. The bioclimatic model gives better results than the standard response function and provides better insight into the functional processes involved in tree growth. The convincing results obtained by the bioclimatic model as well as the limited numbers of parameters it requires demonstrate the feasibility of using it to explore future climatic change impacts on Aleppo pine forests.     

Linking drought legacy effects across scales: From leaves to tree rings to ecosystems

Severe drought can cause lagged effects on tree physiology that negatively impact forest functioning for years. These “drought legacy effects” have been widely documented in tree‐ring records and could have important implications for our understanding of broader scale forest carbon cycling. However, legacy effects in tree‐ring increments may be decoupled from ecosystem fluxes due to (a) postdrought alterations in carbon allocation patterns; (b) temporal asynchrony between radial growth and carbon uptake; and (c) dendrochronological sampling biases. In order to link legacy effects from tree rings to whole forests, we leveraged a rich dataset from a Midwestern US forest that was severely impacted by a drought in 2012. At this site, we compiled tree‐ring records, leaf‐level gas exchange, eddy flux measurements, dendrometer band data, and satellite remote sensing estimates of greenness and leaf area before, during, and after the 2012 drought. After accounting for the relative abundance of tree species in the stand, we estimate that legacy effects led to ~10% reductions in tree‐ring width increments in the year following the severe drought. Despite this stand‐scale reduction in radial growth, we found that leaf‐level photosynthesis, gross primary productivity (GPP), and vegetation greenness were not suppressed in the year following the 2012 drought. Neither temporal asynchrony between radial growth and carbon uptake nor sampling biases could explain our observations of legacy effects in tree rings but not in GPP. Instead, elevated leaf‐level photosynthesis co‐occurred with reduced leaf area in early 2013, indicating that resources may have been allocated away from radial growth in conjunction with postdrought upregulation of photosynthesis and repair of canopy damage. Collectively, our results indicate that tree‐ring legacy effects were not observed in other canopy processes, and that postdrought canopy allocation could be an important mechanism that decouples tree‐ring signals from GPP.     

Recent climate hiatus revealed dual control by temperature and drought on the stem growth of Mediterranean Quercus ilex

A better understanding of stem growth phenology and its climate drivers would improve projections of the impact of climate change on forest productivity. Under a Mediterranean climate, tree growth is primarily limited by soil water availability during summer, but cold temperatures in winter also prevent tree growth in evergreen forests. In the widespread Mediterranean evergreen tree species Quercus ilex, the duration of stem growth has been shown to predict annual stem increment, and to be limited by winter temperatures on the one hand, and by the summer drought onset on the other hand. We tested how these climatic controls of Q. ilex growth varied with recent climate change by correlating a 40‐year tree ring record and a 30‐year annual diameter inventory against winter temperature, spring precipitation, and simulated growth duration. Our results showed that growth duration was the best predictor of annual tree growth. We predicted that recent climate changes have resulted in earlier growth onset (?10 days) due to winter warming and earlier growth cessation (?26 days) due to earlier drought onset. These climatic trends partly offset one another, as we observed no significant trend of change in tree growth between 1968 and 2008. A moving‐window correlation analysis revealed that in the past, Q. ilex growth was only correlated with water availability, but that since the 2000s, growth suddenly became correlated with winter temperature in addition to spring drought. This change in the climate–growth correlations matches the start of the recent atmospheric warming pause also known as the ‘climate hiatus’. The duration of growth of Q. ilex is thus shortened because winter warming has stopped compensating for increasing drought in the last decade. Decoupled trends in precipitation and temperature, a neglected aspect of climate change, might reduce forest productivity through phenological constraints and have more consequences than climate warming alone.     

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against ¹³C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced ¹³C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.     

Continental‐scale tree‐ring‐based projection of Douglas‐fir growth: Testing the limits of space‐for‐time substitution

A central challenge in global change research is the projection of the future behavior of a system based upon past observations. Tree‐ring data have been used increasingly over the last decade to project tree growth and forest ecosystem vulnerability under future climate conditions. But how can the response of tree growth to past climate variation predict the future, when the future does not look like the past? Space‐for‐time substitution (SFTS) is one way to overcome the problem of extrapolation: the response at a given location in a warmer future is assumed to follow the response at a warmer location today. Here we evaluated an SFTS approach to projecting future growth of Douglas‐fir (Pseudotsuga menziesii), a species that occupies an exceptionally large environmental space in North America. We fit a hierarchical mixed‐effects model to capture ring‐width variability in response to spatial and temporal variation in climate. We found opposing gradients for productivity and climate sensitivity with highest growth rates and weakest response to interannual climate variation in the mesic coastal part of Douglas‐fir's range; narrower rings and stronger climate sensitivity occurred across the semi‐arid interior. Ring‐width response to spatial versus temporal temperature variation was opposite in sign, suggesting that spatial variation in productivity, caused by local adaptation and other slow processes, cannot be used to anticipate changes in productivity caused by rapid climate change. We thus substituted only climate sensitivities when projecting future tree growth. Growth declines were projected across much of Douglas‐fir's distribution, with largest relative decreases in the semiarid U.S. Interior West and smallest in the mesic Pacific Northwest. We further highlight the strengths of mixed‐effects modeling for reviving a conceptual cornerstone of dendroecology, Cook's 1987 aggregate growth model, and the great potential to use tree‐ring networks and results as a calibration target for next‐generation vegetation models.     

Tree‐ring analysis and modeling approaches yield contrary response of circumboreal forest productivity to climate change

Circumboreal forest ecosystems are exposed to a larger magnitude of warming in comparison with the global average, as a result of warming‐induced environmental changes. However, it is not clear how tree growth in these ecosystems responds to these changes. In this study, we investigated the sensitivity of forest productivity to climate change using ring width indices (RWI) from a tree‐ring width dataset accessed from the International Tree‐Ring Data Bank and gridded climate datasets from the Climate Research Unit. A negative relationship of RWI with summer temperature and recent reductions in RWI were typically observed in continental dry regions, such as inner Alaska and Canada, southern Europe, and the southern part of eastern Siberia. We then developed a multiple regression model with regional meteorological parameters to predict RWI, and then applied to these models to predict how tree growth will respond to twenty‐first‐century climate change (RCP8.5 scenario). The projections showed a spatial variation and future continuous reduction in tree growth in those continental dry regions. The spatial variation, however, could not be reproduced by a dynamic global vegetation model (DGVM). The DGVM projected a generally positive trend in future tree growth all over the circumboreal region. These results indicate that DGVMs may overestimate future wood net primary productivity (NPP) in continental dry regions such as these; this seems to be common feature of current DGVMs. DGVMs should be able to express the negative effect of warming on tree growth, so that they simulate the observed recent reduction in tree growth in continental dry regions.     

Seedling growth declines in warmed tropical forest soils

The response of plants to a warming climate could have a large feedback on further climatic change. This feedback is especially important for tropical forests, where the global peak in plant productivity and biodiversity occurs. Here we test the response of tropical forest tree seedling growth, photosynthesis and herbivory to 3 years of in situ full-soil profile warming. We studied six species, three of which are known nitrogen-fixers and we hypothesized that the warming response of growth will be mediated by nutrient availability to plants. Across species, growth was significantly lower in warmed soil compared to soil at ambient temperature, and the same pattern was observed for light-saturated photosynthesis, pointing toward a growth decline associated with decreased C fixation. Within species, the relative growth decline was significant for two species, Inga laurina and Tachigali versicolor, both of which are N-fixers. Together our results suggest a growth decline may have resulted from a negative effect of warming on N-fixation, rather than via changes in nutrient mineralization from soil organic matter, which was unchanged for N and increased for P during the dry-to-wet season transition. Overall, our study demonstrates that belowground warming causes species-specific declines in the growth and photosynthesis of seedlings, with a suggestion—requiring further investigation—that this growth decline is larger in N-fixing species.     

Spatiotemporal variation in the relationship between boreal forest productivity proxies and climate data

The impacts of climate change on high-latitude forest ecosystems are still uncertain. Divergent forest productivity trends have recently been reported both at the local and regional level challenging the projections of boreal tree growth dynamics. The present study investigated (i) the responses of different forest productivity proxies to monthly climate (temperature and precipitation) through space and time; and (ii) the local coherency between these proxies through time at four high-latitude boreal Scots pine sites (coastal and inland) in Norway. Forest productivity proxies consisted of two proxies representing stem growth dynamics (radial and height growth) and one proxy representing canopy dynamics (cumulative May-to-September Normalized Difference Vegetation Index (NDVI)). Between-proxy and climate-proxy correlations were computed over the 1982–2011 period and over two 15-yr sub-periods. Over the entire period, radial growth significantly correlated with current year July temperature, and height growth and cumulative NDVI significantly correlated with previous and current growing season temperatures. Significant climate responses were quite similar across sites, despite some higher sensitivity to non-growing season climate at inland sites. Significant climate-proxy correlations identified over the entire period were temporarily unstable. Local coherency between proxies was generally insignificant. The spatiotemporal instability in climate-proxy correlations observed for all proxies underlines evolving responses to climate and challenges the modelling of forest productivity. The general lack of local coherency between proxies at our four study sites suggests that forest productivity estimations based on a single proxy should be considered with great caution. The combined use of different forest growth metrics may help circumvent uncertainties in capturing responses of forest productivity to climate variability and improve estimations of carbon sequestration by forest ecosystems.     

氮磷添加对树木生长和森林生产力影响的研究进展

人为活动所导致的氮、磷输入和大气氮、磷沉降使生态系统中的氮、磷可利用性大幅提高, 对陆地生态系统的碳循环过程产生了显著影响。树木生长和森林生产力在全球碳循环中发挥着重要作用, 它决定着陆地碳固存的大小和方向。目前, 在全球范围内开展了很多氮、磷添加调控树木生长和森林生产力的野外控制实验, 但是研究结果并不一致, 受到多种生物、环境和实验处理条件等因素的影响。该文从野外氮添加和磷添加实验的文献数量、实验数量及其全球空间分布三个方面概述了氮、磷添加对树木生长和森林生产力影响的研究现状, 并总结了氮、磷添加实验中树木生长和森林生产力的评估方法, 包括相对生长速率和绝对增长量。基于相关的研究结果, 阐述了氮、磷添加影响树木生长和森林生产力的调控因素及其潜在影响机制, 包括气候、树木径级与林龄、植物功能性状(共生菌根类型、树木固氮属性和保守性与获得性性状)、植物和微生物相互作用关系、区域养分沉降速率和实验处理条件等。最后, 基于当前的研究进行了系统总结, 并指出今后需要加强的几个方面的研究, 以期为后续研究提供参考: 树木生长响应氮、磷添加的生理学机制, 树木各部分生长对氮、磷添加响应的权衡与分配, 植物功能性状在调节与预测树木生长响应氮、磷添加中的作用, 树木之间的竞争关系如何调控氮、磷添加对树木生长的影响, 以及开展长期的和联网的氮、磷添加对树木生长和森林生产力影响的野外控制实验。     

A review on the effects of nitrogen and phosphorus addition on tree growth and productivity in forest ecosystems

Nitrogen (N) and phosphorus (P) inputs induced by anthropogenic activities and atmospheric N and P deposition have largely increased the availability of soil N and P in terrestrial ecosystems, which have considerably affected terrestrial carbon cycling processes. Tree growth and productivity in forest ecosystems play an important role in global carbon cycling, and determine the magnitude and direction of terrestrial carbon sequestration. Currently, a large number of field manipulation experiments have been conducted to investigate the effects of N and/or P addition on tree growth and forest productivity, but the results from these studies were inconsistent. Such inconsistent results might be affected by multiple factors, including biological, environmental and experimental variables. Here, we reviewed the present research status of the effects of N and P addition on tree growth and forest productivity in forest ecosystems based on three aspects, including the number of publications and experiments with field N and P addition, and the global distributions of these experiments. Then, we summarized the methods for assessing tree growth and forest productivity at ecosystem level in forest ecosystems, including relative growth rate and absolute increment. According to the related results, we reviewed the regulating factors that affect tree growth and productivity, and the potential mechanisms for such factors, including climate, tree size and stand age, plant functional traits (including type of tree-associated mycorrhizal fungi, N-fixation property of trees, and conservative and acquisitive functional traits), plant-microbe interaction, ambient nutrient (i.e., N and P) deposition rate, and experimental variables. Finally, we summarized the current studies, and pointed out five aspects that are urgently needed to provide further insights in future studies, including the physiological mechanism of how tree growth responds to N and P addition, the tradeoff and allocation among growth of various parts of tree under N and P addition, the role of plant functional traits in regulating and predicting the responses of tree growth to N and P addition, how the competition among trees regulates the responses of tree growth to N and P addition, and conducting long-term and coordinated distributed field experiments investigating the effects of N and P addition on tree growth and forest productivity at the global scale.     

Disentangling the Legacies of Climate and Management on Tree Growth

Ecosystems - Legacies of past climate conditions and historical management govern forest productivity and tree growth. Understanding how these processes interact and the timescales over which they...     

Geographic variation in growth response of Douglas‐fir to interannual climate variability and projected climate change

We used 179 tree ring chronologies of Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco] from the International Tree‐Ring Data Bank to study radial growth response to historical climate variability. For the coastal variety of Douglas‐fir, we found positive correlations of ring width with summer precipitation and temperature of the preceding winter, indicating that growth of coastal populations was limited by summer dryness and that photosynthesis in winter contributed to growth. For the interior variety, low precipitation and high growing season temperatures limited growth. Based on these relationships, we chose a simple heat moisture index (growing season temperature divided by precipitation of the preceding winter and current growing season) to predict growth response for the interior variety. For 105 tree ring chronologies or 81% of the interior samples, we found significant linear correlations with this heat moisture index, and moving correlation functions showed that the response was stable over time (1901–1980). We proceeded to use those relationships to predict regional growth response under 18 climate change scenarios for the 2020s, 2050s, and 2080s with unexpected results: for comparable changes in heat moisture index, the most southern and outlying populations of Douglas‐fir in Mexico showed the least reduction in productivity. Moderate growth reductions were found in the southern United States, and strongly negative response in the central Rocky Mountains. Growth reductions were further more pronounced for high than for low elevation populations. Based on regional differences in the slope of the growth–climate relationship, we propose that southern populations are better adapted to drought conditions and could therefore contain valuable genotypes for reforestation under climate change. The results support the view that climate change may impact species not just at the trailing edges but throughout their range due to genetic adaptation of populations to local environments.     

Rapid warming accelerates tree growth decline in semi‐arid forests of Inner Asia

Forests around the world are subject to risk of high rates of tree growth decline and increased tree mortality from combinations of climate warming and drought, notably in semi‐arid settings. Here, we assess how climate warming has affected tree growth in one of the world's most extensive zones of semi‐arid forests, in Inner Asia, a region where lack of data limits our understanding of how climate change may impact forests. We show that pervasive tree growth declines since 1994 in Inner Asia have been confined to semi‐arid forests, where growing season water stress has been rising due to warming‐induced increases in atmospheric moisture demand. A causal link between increasing drought and declining growth at semi‐arid sites is corroborated by correlation analyses comparing annual climate data to records of tree‐ring widths. These ring‐width records tend to be substantially more sensitive to drought variability at semi‐arid sites than at semi‐humid sites. Fire occurrence and insect/pathogen attacks have increased in tandem with the most recent (2007–2009) documented episode of tree mortality. If warming in Inner Asia continues, further increases in forest stress and tree mortality could be expected, potentially driving the eventual regional loss of current semi‐arid forests.     

Revisiting projected shifts in the climate envelopes of North American trees using updated general circulation models

Global climate models are constantly being upgraded, but it is often not clear what these changes have on climate change impact projections. We used difference maps to directly compare downscaled projections of temperature and precipitation across North America for two versions (or generations) of three different Atmospheric‐Ocean General Circulation Models (AOGCM)s. We found that AOGCM versions differed in their projections for the end of the current century by up to 4 °C for annual mean temperature and 60% for annual precipitation. To place these changes in an ecological context, we reanalyzed our work on shifts in tree climate envelopes (CEs) using the newer‐generation AOGCM projections. Based on the updated AOGCMs, by the 2071–2100 period, tree CEs shifted up to 2.4 degrees further north or 2.6 degrees further south (depending on the AOGCM) and were about 10% larger in size. Despite considerable differences between versions of a given AOGCM, projections made by the newer version of each AOGCM were in general agreement, suggesting convergence across the three models studied here. Assessing the AOGCM outputs in this way provides insight into the magnitude and importance of change associated with AOGCM upgrades as they continue to evolve through time.     

Stabilizing effects of diversity on aboveground wood production in forest ecosystems: linking patterns and processes

Both theory and evidence suggest that diversity stabilises productivity in herbaceous plant communities through a combination of overyielding, species asynchrony and favourable species interactions. However, whether these same processes also promote stability in forest ecosystems has never been tested. Using tree ring data from permanent forest plots across Europe, we show that aboveground wood production is inherently more stable through time in mixed‐species forests. Faster rates of wood production (i.e. overyielding), decreased year‐to‐year variation in productivity through asynchronous responses of species to climate, and greater temporal stability in the growth rates of individual tree species all contributed strongly to stabilising productivity in mixed stands. Together, these findings reveal the central role of diversity in stabilising productivity in forests, and bring us closer to understanding the processes which enable diverse forests to remain productive under a wide range of environmental conditions.