Moon, MinkyuRichardson, Andrew D.O'Keefe, JohnFriedl, Mark A....
10页
查看更多>>摘要：Incomplete understanding of the processes controlling senescence limits our ability to forecast how the timing of leaf senescence will change in coming decades. In this study, we use a hierarchical Bayesian model (HBM) in association with a 27+ year record of field observations for 12 temperate deciduous tree species collected at Harvard Forest in central Massachusetts to examine how variability in bioclimatic controls affects the timing of leaf senescence. To test how general and extensible our results are over a broader biogeographic range, we used a multi-year record of land surface phenology derived from remote sensing encompassing all forested lands in New England. Results from the HBM showed that while air temperature is an important factor that influences the timing of leaf senescence, photoperiod uniformly exerts the strongest control across all 12 species. Species exhibiting the strongest dependence on photoperiod, particularly Acer species, showed low inter-annual variation and no long-term trends in the timing of leaf senescence. In contrast, species with greater dependence on air temperature, particularly Quercus species, showed statistically significant trends toward later senescence dates in response to long-term warming. Results from analyses conducted at regional scale across all of New England using data derived from remote sensing corroborated results obtained at Harvard Forest. Specifically, relative to ecoregions dominated by Quercus species, the timing of leaf senescence in ecoregions dominated by Acer species exhibited lower interannual variability and lower correlation with year-to-year variation in pre-senescence period mean air temperatures. These results suggest that forecasting how the timing of leaf senescence in temperate forests will change in the future requires species-specific understanding of how bioclimatic forcing controls the timing of leaf senescence.
查看更多>>摘要：When applied to plant canopies, classical radiation theory for a turbid medium yields relatively simple expressions for average fluxes of absorbed radiation within an arbitrary volume of vegetation. However, due to the effects of shading and leaf angle, these averaging volumes usually contain a continuous distribution of leaf-level radiative fluxes ranging from full sun to full shade. These distributions are obscured within turbid media models and are thus usually not considered directly unless a computationally expensive leaf-resolving model is used. Consideration of the full probability distribution of absorbed radiative fluxes can yield valuable information about interactions between plant structure and function, not only for radiative fluxes but also for fluxes of radiation-dependent biophysical processes such as photosynthesis. This work presents the theoretical derivation of probability distributions of absorbed direct, diffuse, scattered, and total radiative fluxes for homogeneous canopies with varying structure. The theory is verified against predictions of a three-dimensional leaf-resolving model, and used to explore the impacts of canopy structure on the distribution of absorbed radiation.
Kannenberg, Steven A.Cabon, AntoineBabst, FlurinBelmecheri, Soumaya...
11页
查看更多>>摘要：The ability of forests to withstand, and recover from, acute drought stress is a critical uncertainty regarding the impacts of climate change on the terrestrial carbon (C) cycle, but it is unclear how drought responses scale from individual trees to whole forests. Here, we assembled a dataset of tree-ring chronologies co-located within the footprint of eddy covariance towers across North America and Europe, with the aim of quantifying the sensitivity of tree radial growth versus gross primary productivity (GPP) during and following drought. We found that drought induced a large decoupling across C cycle processes, whereby GPP was relatively resistant to water stress despite large reductions in tree-ring widths. This decoupling also occurred in the year following drought (i.e., a "drought legacy effect'), and was similar in magnitude in response to both summer and winter droughts. By modeling whole-forest C turnover time, we show that a radial growth-GPP decoupling has important ramifications for the forest C cycle, especially if the C not used to support radial growth is instead allocated towards pools with short residence times. Our results demonstrate that quantifications of drought impacts that rely solely on C uptake are missing this fundamental pathway through which drought alters the forest C cycle and the resulting feedbacks to the climate system.
Gomez-Aparicio, LorenaSerrano, Maria S.Romero, Maria AngelesHomet, Pablo...
8页
查看更多>>摘要：Worldwide, Mediterranean-climate type forests are seriously threatened by climate change and Phytophthora cinnamomi, an extremely aggressive soilborne pathogen listed among the 100 worst invasive alien species. This study examines for first time the effects of annual and seasonal changes in temperature and rainfall under different climatic scenarios on P. cinnamomi dynamics in a Mediterranean forest naturally infested. We followed seasonally and during four years (2016-2020) P. cinnamomi abundance in the soil taking advantage of a climate change infrastructure that simulates a 30% rainfall reduction using rainout shelters and an increase in soil temperature using open top chambers. Seasonal abundance of P. cinnamomi was strongly linked to spring precipitation: the large inoculum density during this season suffered a strong decrease during the dry summer that was maintained until the next spring, despite of fall rains. Thus, inoculum density in spring may be considered the best indicator of the potential of P. cinnamomi infection in natural ecosystems. The two climatic treatments had significant effects on P. cinnamomi abundance, but only in some combinations of seasons and years. Rainfall exclusion had a negative effect on the spring abundance of P. cinnamomi mostly in wet years, causing a reduction as large as 30% in comparison with the control treatment. Meanwhile, warming effect varied from slightly negative in winter to positive in spring and early summer, causing increases as large as 31% in comparison with the control treatment. Overall, we provide novel experimental evidence suggesting that a drier climate might limit the abundance and activity of P. cinnamomi in water-limited forests, but that this negative effect might be at least partly counteracted by the positive effects of warming. These results also imply that P. cinnamomi represents an increasing threat under climate change for the conservation of forest ecosystems where water is not a limiting factor.
查看更多>>摘要：Accurate estimation of chlorophyll content is important for diagnosing the physiological and phenological status of vegetation. Establishing the relationship between vegetation indices (VIs) and leaf chlorophyll content using remote sensing is crucial for large-scale earth observation. However, existing VIs for estimating chlorophyll content generally suffer from the saturation effect or depend on specific scenarios, resulting in insufficient estimation accuracy. Based on the physical mechanism of light-vegetation interaction, this study innovatively proposes the absorption triangle and reflectance triangle in the spectral space to construct the chlorophyll sensitive Sentinel-2 Triangular Vegetation Index (STVI). The STVI uses the Sentinel-2 multispectral instrument (MSI) bands to improve the accuracy of chlorophyll content retrieval by enhancing the relationship with the chlorophyll content and mitigating the saturation effect. Simulated data, measured data, and open data sets were used to test the accuracy and stability of the STVI and 11 classical VIs for retrieving the chlorophyll content using different spectral data types, different winter wheat growth stages, and different vegetation coverages. The results showed that the STVI was more sensitive to the chlorophyll content than the classical VIs and provided the best goodness-of-fit in multiple scenarios. The STVI represents a powerful tool for large-extent chlorophyll content retrieval and a novel approach for scientific research in related fields.
查看更多>>摘要：Representing the altitudinal limit of typical vegetation, the alpine timberline is a unique natural landscape that is also a sensitive indicator of the well-being of alpine ecosystems. Schrenk spruce (P. schrenkiana) is a tree species unique to arid Central Asia. In this study, we found that the P. schrenkiana can grow at elevations of up to 3600 m. To better understand how this species survives at such high elevations, we used tree-ring samples to analyze the relationship between climate and the radial growth of P. schrenkiana at the alpine timberline. Our results suggest that the mean minimum temperature during the growing season is the main factor limiting the radial growth of timberline P. schrenkiana. The limiting effect of temperature on radial growth has been significant and stable over the past 60 years. However, the influence of precipitation on tree growth has increased due to rapid warming. Continued warming may further complicate the response of P. schrenkiana radial growth to climate at the timberline in arid Central Asia.
查看更多>>摘要：Accurate measurements of leaf temperature and area are crucial to calculate leaf gas exchange parameters when using portable gas analysers such as the LI-6400 XT portable photosynthesis system (LI-COR Inc., Lincoln, USA). The importance of leaf temperature data is well known for robust measurements, but there are still technical limitations in measuring it with full accuracy. Here we present a tentative solution by incorporating leaf temperature measurements with thermal imaging. An infrared (IR) camera (IRay T3 Pro, Shandong, China) was combined with a LI-6400XT to measure mean leaf temperatures while avoiding leaf thermal heterogeneity and to accurately calculate leaf area while avoiding destructive sampling. The thermal heterogeneity of Helianthus annuus and Zea mays leaves were studied. We then evaluated the accuracies of leaf area and temperature measurements obtained for 9 species and discussed the influences of these measurements on the resulting photosynthesis-related parameters. Our results provide direct visual evidence of the thermal heterogeneity of leaves in an infrared gas analyzer (IRGA) chamber. The temperature differences across the leaf surface were as high as 0.28 degrees C-1.38 degrees C, causing large errors in stomatal conductance (gs, -5.07%--55.02%) and intercellular CO2 concentrations (Ci, -0.41%--6.72%). The IR camera reduced the area measurement error (-0.71-0.29 cm2) and greatly increased the accuracy by -11.12%-17.60% for the net photosynthesis (Pn) and transpiration rate (E) and by -11.24%-17.69% for gs calculations. Our results showed that combining an IR camera with LI-6400XT can improve leaf area measurements, avoid thermal heterogeneity in leaves and thus dramatically improve the accuracy of photosynthesis-related parameter calculations under semi controlled and moderate climate conditions.
查看更多>>摘要：We aim to identify the importance of vapour pressure deficit (VPD), soil water content (SWC) and photosynthetic photon flux density (PPFD) as drivers of tree canopy conductance, which is a key source of uncertainty for modelling vegetation responses under climate change. We use sap flow time series of 1858 trees in 122 sites from the SAPFLUXNET global database to obtain whole-tree canopy conductance (G). The driver importance, defined as the percentage of variance (R2) of G explained by the three main hydrometeorological drivers (VPD, SWC and PPFD), was evaluated using linear mixed models. For each driver we assess differences in their importance among biomes, and use multiple linear regression to explain how driver importance varies with climate, soil and vegetation structure. We found that in most areas tree canopy conductance is better explained by VPD than by SWC or PPFD, although results for drylands were inconclusive. Climate, soil and vegetation structure were common controls for the importance of all three hydrometeorological drivers, with wetter climates, fine-textured soils and tall vegetation being associated with stronger influence of the driver on G. Differences across sites in the importance of the hydrometeorological drivers of tree canopy conductance may affect predictions of ecosystem dynamics under future climates, and should be accounted for explicitly in models.
Mortarini, analysis LucaDias-Junior, Cleo Q.Acevedo, OtavioOliveira, Pablo E. S....
11页
查看更多>>摘要：Observations of the vertical structure of the turbulent flow in different stability regimes above and within the Amazon Forest at the Amazon Tall Tower Observatory (ATTO) site have been presented in Part I. Here, the influence of stability on the inception and development of coherent structures is investigated. According to the mixing-layer analogy the coherent vortices that dominate the turbulent flows at the canopy-atmosphere interface are generated by hydrodynamical instabilities triggered by an inflection in the vertical profile of the mean wind speed at or near the canopy top. The coherent motions time, T-peak and separation length scale, lambda, depend on the shear length scale, Ls, close to the canopy top. The present analysis studies the characteristics of L-s, T(peak )and lambda in the five stability regimes defined in Part I. The behaviour of L-s with stability is evaluated and parameterized. L-s increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. Coherent structures and their timescale are detected with an original method based on the auto-correlation functions of 5-min subsets of turbulent quantities. The vertical time scale is larger in neutral conditions and decreases for both increasing and decreasing stability. At the canopy top the separation length scale presents a linear dependence on Ls, whose slope is maximum in neutral conditions and decreases departing from neutrality. An original parameterization of the dependence of this slope on h/L, where h is the canopy height and L is the Obukhov length, is shown. Combining the parameterizations, the dependence of the separation length scale is finally presented. lambda has a maximum for slightly unstable conditions, linearly decreases towards neutral and weakly stable stratifications and then it tends to zero for very intense unstable and stable stratifications.
查看更多>>摘要：Outbreaks of the pine caterpillar (Dendrolimus spp.) have substantial impacts on coniferous forest ecosystem structure, distribution, and productivity, as well as the economy. Furthermore, there is emerging evidence that increasing frequencies of fires and windstorms driven by climate change are contributing to more severe pine caterpillar outbreaks. Therefore, there is a need to identify areas susceptible to pine caterpillar outbreaks to apply prevention measures efficiently and facilitate sustainable forest management. This study constructed an integrated index for assessing the susceptibility of forest ecosystems in Shandong Province, China to pine caterpillar outbreaks based on the indicators of mainly forest structure, climate, topography, landform, and soil conditions. This index suggests the risk and susceptibility of a forest area to a pine caterpillar outbreak. In addition, a geodetector model was used to identify the synergistic or antagonistic effects of environmental factors on risk of pine caterpillar outbreak. The results showed that: (1) mountain and hilly areas are more susceptible to pine caterpillar outbreaks, whereas surrounding areas show a medium susceptibility in central and southeastern part of Shandong Province; (2) there was an increasing susceptibility of the study area to pine caterpillar outbreak in the 1990s, 2000s and 2010s; (3) synergism between certain topography and climate variables enhanced the susceptibility of the study area to pine caterpillar outbreaks, including the interaction between elevation and sunshine hours and between slope and humidity. The integrated susceptibility index was shown to perform well in indicating the risk and susceptibility of forest ecosystems to pine caterpillar outbreaks. The present study can act as a reference for forest management within the identification of areas with high susceptibility to pine caterpillar outbreak.
NSTL
Elsevier