查看更多>>摘要：A better understanding of tree growth-climate sensitivity across regions and species may reduce uncertainties in simulating forest carbon budgets. We studied the spatial heterogeneity of growth-climate sensitivity of Pinus yunnanensis and Pinus kesiya across temperature and moisture gradients in the complex mountain regions of Southwest China. We developed tree ring-width chronologies from 22 sites for P. yunnanensis and 20 sites for P. kesiya, totaling 903 trees (1684 cores). We applied generalized additive mixed models (GAMMs) on basal area increment (BAI) series to remove the age and size effects on tree growth, and correlated BAI residual chronol-ogies with climate variables. Radial growth of P. yunnanensis and P. kesiya at most sites (> 75%) were positively correlated with precipitation and one-month Standardized Precipitation-Evapotranspiration Index (SPEI1) dur -ing the early growing season. In contrast, high precipitation and SPEI1 during the late growing season reduced tree growth of both pine species. The growth-climate sensitivity of both pine species varied along the temper-ature and moisture gradients, i.e., moisture sensitivity of tree growth decreased along a dry-to-humid moisture gradients for P. kesiya during early growing season, whereas temperature sensitivity of P. yunnanensis shifted from positive (beneficial) to negative (limitative) along cold-to-warm gradients. Our results indicate that the growth-climate sensitivity of both pine species varies with site-specific environmental conditions. These findings contribute to improve our understanding about the spatial patterns of tree growth-climate responses of these economically important conifer species in Southwest China.
查看更多>>摘要：To assess the impacts of afforestation on land surface temperature (LST) in China and provide a basis for future afforestation and warming mitigation policies, adjacent forest and open land are compared using ten years of satellite data. Most regions in China exhibit a diurnal asymmetry in the magnitude and sign of the effects of afforestation, with daytime cooling and nighttime warming. Afforestation has annual daily cooling effects in most regions of China because the magnitude of annual nighttime warming is generally weaker than that of the annual daytime cooling. We attributed the warming and cooling effects of afforestation to the changes of evapotranspiration (ET) and albedo. The annual average albedo of forest is smaller than that of open land, and the albedo differences between forest and open land vary spatially, with larger differences in places with snow cover. In the annual average, ET over forest is greater than that over open land, with the largest values in the southern of Yangtze River. The daytime differences in LST between forest and open land are driven by the cooling effects of ET and warming effects of albedo, and the cooling effects of ET dominate. The nighttime warming effects of afforestation is related with the release of the energy stored in the soil during daytime. Evergreen broadleaf forest and deciduous broadleaf forest are recommended as the choice of afforestation in south and in north of the Yangtze River respectively to decrease the LST if the afforestation program in China is to be expanded.
查看更多>>摘要：Due to the complex topography and localized climate, active layer thickening and permafrost warming varied distinctly in different regions on the Qinghai-Tibet Plateau (QTP). Based on the borehole-temperature data at 93 sites from 2012 to 2018, we analyzed the temporal and spatial characteristics of active layer thickness, permafrost temperature, and relevant climatic factors in 8 typical geomorphological units on the QTP. The active layer thickened at 86 sites and thinned at 7 sites. The permafrost warmed at 89 sites and cooled at 4 sites. The median values of the annual increase rate of active layer thickness were from 0.04 to 0.13 m/a for the monitored regions. The highest rate reached 0.46 m/a, indicating severe permafrost degradation in local areas. The mean annual soil temperatures at a 6-m depth generally increased faster for cold permafrost, and the active layer thickened more significantly in warm permafrost sites. Among these regions, Kekexili Mountains showed a lower increase rate of active layer thickness, and the temperature rise of permafrost in the Fenghuoshan Mountains was more significant. The temporal change of snow cover duration was closely related to the active layer thickness variation in the northern permafrost regions on the QTP (Kunlunshan Mountains and Chumaerhe High Plain). In contrast, the temporal variation of freezing index was the dominant factor in the southern regions (Wuli Basin, Tongtianhe Basin, and Tanggula Mountains). No linear correlation between the temporal variations of climatical factors and active layer thickness variation was found for the regions in the middle of QTP (Kekexili Mountains, Beiluhe Basin, and Fenghuoshan Mountains ). The comprehensive effects of freezing index and snow cover duration result in the different relationships between air temperature variation and permafrost change in different regions on the QTP. These findings are beneficial for understanding the relationship between climate change and permafrost evolution.
Looze, A. C. M.Raleigh, M. S.Miller, B. W.Friedman, J. M....
13页
查看更多>>摘要：Trees are bioindicators of global climate change and regional urbanization, but available monitoring tools are ineffective for fine-scale observation of many species. Using six accelerometers mounted on two urban ash trees (Fraxinus americana), we looked at high-frequency tree vibrations, or change in periodicity of tree sway as a proxy for mass changes, to infer seasonal patterns of flowering and foliage (phenophases). We compared accelerometer-estimated phenophases to those derived from digital repeat photography using Green Chromatic Coordinates (GCC) and visual observation of phenophases defined by the USA National Phenology Network (NPN). We also drew comparisons between two commercial accelerometers and assessed how placement height influenced the ability to extract seasonal transition dates. Most notably, tree sway data showed a greenness signal in an urban environment and produced a clear flowering time-series and peak flowering signal (PF), marking the first observations of a flower phenophase using accelerometer data. Estimated start of spring (SOS) from accelerometers and time-lapse GCC were more similar than start of autumn (SOA); accelerometers lagged behind the time-lapse camera dates by three and four days for SOS and 13 and 14 days for SOA for each tree. Estimates for SOS and SOA from accelerometers and time-lapse cameras aligned closely with different NPN phenophases. The two commercial accelerometers produced similar season onset: a difference of 2.4 to 3.8 days for SOS, 2.1 days for SOA, and 0.5 to 2.0 days for PF. Accelerometers placed at the main crown branch point versus higher in the canopy showed a difference of 0.2 to 4.9 days for SOS and-1.5 to 1.7 days for PF. Our results suggest accelerometers present a novel opportunity to objectively monitor reproductive tree biology and fill gaps in phenology observations. Furthermore, widely available accelerometers show promise for scaling up from individual trees to the landscape level to aid forest management and assessing climate change impacts to tree phenology.
查看更多>>摘要：The forest-floor represents an important interface for various carbon dioxide (CO2) fluxes, however, our knowledge of their variability and drivers across a managed boreal forest landscape is limited. Here, we used a three-year (2016-2018) data set of biometric- and chamber-based flux measurements to investigate the net forest-floor CO2 exchange (NEff) and its component fluxes across 50 forest stands spanning different soil types, tree species, and age classes within a 68 km2 boreal catchment in Sweden. We found that the forest-floor acted as a net CO2 source with the 10th-90th percentile (used hereafter for describing reported variations) ranging from 149 to 399 g C m- 2 yr-1. Among the key landscape attributes, stand age strongly affected most NEff component fluxes, whereas tree species and soil type effects were weak and absent, respectively. Specifically, forest-floor net CO2 emissions increased with stand age due to declining understory gross and net primary production, ranging between 77-275 and 49-163 g C m- 2 yr- 1, respectively. Furthermore, we observed higher understory production rates in pine than in spruce stands. Across the 50 stands, the total forest-floor respiration ranged between 340 and 549 g C m- 2 yr-1 and its spatial variation was primarily regulated by its autotrophic components, i.e., understory and tree root respiration, which displayed divergent increasing and decreasing age-related trends, respectively. Furthermore, heterotrophic soil respiration remained within a relatively narrow range (154-290 g C m- 2 yr- 1), possibly owing to compensating gradients in forest-floor properties. We further identified tree biomass as the major driver of the landscape-scale variations of CO2 fluxes, likely attributable to modulating effects on forest-floor resource availability and growing conditions. This implies that tree growth responses to forest management and global change will be particularly important for regulating magnitudes and spatial variations of forest-floor CO2 fluxes in boreal forests.
查看更多>>摘要：As a main tool for climate variability impact studies on agriculture, crop models are widely used for agricultural assessments. However, the accuracy of input data like radiation data fundamentally affects the reliability of the crop model outputs. In previous studies, estimated radiation was commonly used due to the scarcity of directly measured radiation, which introduced errors into the assessment results. In this study, by assuming the measured radiation data as the true values, we used four sources of estimated solar radiation values to analyze the underlying errors in the results based on estimated solar radiation. The results showed that the total estimated radiation during the crop growing season is all higher than the measured radiation, especially for winter wheat (6.6%~14.8% for maize, and 18.7%~34.0% for winter wheat). When using estimated radiation as input data, regardless of the source, the crop model overestimated the simulated crop yields for maize (13.8%~23.6%) and winter wheat (37.3%~65.4%) in China. The amplitude of such overestimation was relatively greater in the south than in the north, and relatively greater for winter wheat than for maize. As compared to using the measured radiation, the negative impact of climate variability on simulated crop yield was exaggerated when using estimated radiation except for reanalysis datasets (1.6%~4% for maize, and 4.2%~6.3% for winter wheat). Our study results provide scientific evidence of the importance of using accurate radiation data in agricultural impact analyses, and the previous related results may need to be re-evaluated.
查看更多>>摘要：Climate extremes will increasingly influence ecosystem productivity of tropical forests, but little is known about their effects on tree stem radial growth, a major component of forest productivity. To analyze the role of climatic drivers and local site differences on tree growth at sub-annual timescales, we studied the timing, magnitude and climatic drivers of stem growth and xylem formation at two tropical forest sites with contrasting soil water availability in southwestern China. We combined high-resolution point dendrometer recordings and microcoring to investigate intra-annual stem radius variation and xylem formation of Toona ciliata over three consecutive years (2018-2020). Stem radial increment of T. ciliata derived from dendrometer data showed distinct seasonal patterns, with growth mainly occurring during March to October. The start day of stem radial growth coincided with the occurrence of the cell enlarging phase, and its end day occurred concomitantly with the cessation of cell enlarging as identified from microcores. Toona ciliata trees at the tropical ravine rainforest site with higher soil water availability showed higher mean annual stem radial increment, higher daily growth rates, and longer duration of xylem productivity than that at the tropical karst forest site. Weekly stem radial growth rates increased with precipitation and air temperature at the wetter ravine rainforest site, while relative humidity had a positive effect at the drier karst forest site. Our findings indicate that drought stress advances and shortens the growing season for T. ciliata. Our study highlights the strong influence of soil water capacity mediating the impact of climate, so response to drought is site-specific, which should be considered in growth models.
Silva, M. R. F.McHugh, I.Neto, A. M. L. PeixotoPauwels, V. R. N....
12页
查看更多>>摘要：The establishment and expansion of commercial plantations for timber production and carbon sequestration raises concerns because of their large water use. Eucalyptus globulus (blue gum) is one of the most planted species globally, as it grows rapidly and is adaptable to a range of climatic conditions. The dearth of experimental observations on water use and growth in blue gum plantations in their early years after establishment makes it difficult to develop management practices. This study quantified the trade-offs between water use and carbon assimilation in a blue gum plantation in the first 4 years after establishment. The study site is located in southwest Victoria, Australia, where energy, water and CO(2 )fluxes were continuously measured above the tree canopy for 4 years after the trees were planted. During the first year after establishment, understory vegetation and ecosystem respiration had a major impact on the net ecosystem exchange (NEE), with the plantation being a net carbon source. Subsequently, the trees started dominating the contributions to NEE, and after approximately 2 years the plantation became a consistent carbon sink. These shifts in NEE were accompanied by smaller increases in annual evapotranspiration rates, which was 70% of the annual precipitation in the first year and 74% in the 3rd year of measurements. As a result, yearly averages of water use efficiency increased from 2.86 gC kg(-1)H(2)O in 2018 to 4.3 gC kg(-1)H(2)O in 2020, following tree development. This shows a remarkable increase in productivity at the expense of a small amount of water.
查看更多>>摘要：Altitudinal treeline is typically considered as being sensitive to climate change, and treelines worldwide have been responding to climate warming by increasing in elevation and stand density. However, little is known how upward shift and tree densification within treelines are interrelated. Studies of these two ecological processes at regional and local scale have yielded heterogeneous results, showing that either some treelines displayed tree densification without significant advance in their positions or inversely some treelines have shifted their positions upward without any noticeable densification. To explore this variation, we here quantified the difference between the rate of altitudinal upward shift and the rate of tree densification within treelines worldwide and identified its global drivers. The mismatch between upslope shift and tree densification has been observed widely within over 63% of the global treelines, which was primarily driven by temperature change, showing that compared with the rate of tree densification, the rate of treeline shift tended to decrease when accompanied by the increasing rate of warming. Thus, our findings revealed that the accelerating climate warming has enlarged the mismatch between the rate of upward shift and the rate of tree densification and thus widened the lagging gap of global treeline shift relative to densification.
查看更多>>摘要：Climate change is expected to pose major challenges to viticulture. The projected increase in temperature in the Mediterranean region due to climate change is likely to influence the timing of grapevine phenological stages. This study developed and validated a phenological model, from budbreak to maturation phenological stages, based on the growing degree days for the Sultana grapevine cultivar on Crete Island, Greece, and used the model to assess potential changes in future phenology timing employing different climate change scenarios. A dataset of unpublished phenological observations from 20 locations spanning four decades was used to validate the phenological model. The root mean squared difference (RMSD) of the calibration-validation procedure was estimated between 5.4 and 11.5 days, depending on the phenological stage. The model outperformed for the flowering and maturity stages. The highest RMSD was found for the shoot development stages. Projections determined an earlier occurrence of the different phenological stages. Near future climate (2020-2060) projections indicate budbreak advancement by 7 to 8 days and maturity by 4 to 5 days on average. For the far future (2060-2100), the respective changes are 11 to 18 and 7 to 9 days earlier. Discussion on the underlying uncertainty sources is provided.
NSTL
Elsevier