Photosynthetic Phenology Research Articles

Proximal remote sensing of tree physiology at northern treeline: Do late-season changes in the photochemical reflectance index (PRI) respond to climate or photoperiod?

Relatively little is known of how the world's largest vegetation transition zone – the Forest Tundra Ecotone (FTE) – is responding to climate change. Newly available, satellite-derived time-series of the photochemical reflectance index (PRI) across North America and Europe could provide new insights into the physiological response of evergreen trees to climate change by tracking changes in foliar pigment pools that have been linked to photosynthetic phenology. However, before implementing these data for such purpose at these evergreen dominated systems, it is important to increase our understanding of the fine scale mechanisms driving the connection between PRI and environmental conditions. The goal of this study is thus to gain a more mechanistic understanding of which environmental factors drive changes in PRI during late-season phenological transitions at the FTE – including factors that are susceptible to climate change (i.e., air- and soil-temperatures), and those that are not (photoperiod). We hypothesized that late-season phenological changes in foliar pigment pools captured by PRI are largely driven by photoperiod as opposed to less predictable drivers such as air temperature, complicating the utility of PRI time-series for understanding climate change effects on the FTE. Ground-based, time-series of PRI were acquired from individual trees in combination with meteorological variables and photoperiod information at six FTE sites in Alaska. A linear mixed-effects modeling approach was used to determine the significance (α = 0.001) and effect size (i.e., standardized slope b*) of environmental factors on late-seasonal changes in the PRI signal. Our results indicate that photoperiod had the strongest, significant effect on late-season changes in PRI (b* = 0.08, p < 0.001), but environmental variables susceptible to climate change were also significant (i.e., daily mean solar radiation (b* = −0.03, p < 0.001) and daily mean soil temperature (b* = 0.02, p < 0.001)). These results suggest that interpreting PRI time-series of late-season phenological transitions may indeed facilitate our understanding of how northern treeline responds to climate change.

表征亚热带常绿林光合作用季节变化特征的多种植被指数

PDF HTML阅读 XML下载 导出引用 引用提醒 表征亚热带常绿林光合作用季节变化特征的多种植被指数 DOI: 10.5846/stxb201707211305 作者: 作者单位: 中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,中国科学院华南植物园,中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,中国科学院华南植物园,广东省生态气象中心 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划（2017YFC0503803）；中国科学院重点部署项目（KFZD-SW-310-01）；国家自然科学基金（41571192） Study of multiple vegetation indices reveals photosynthetic phenology in a subtropical evergreen forest Author: Affiliation: Institute of Geographic Sciences and Natural Resources Research, CAS,Institute of Geographic Sciences and Natural Resources Research, CAS,,,,,, Fund Project: National Key R&D Plan，Key Program of the Chinese Academy of Sciences，The National Natural Science Foundation of China 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:利用遥感方法可以在区域尺度反演地表植被的光合生理状况和生产力变化，但亚热带常绿林冠层结构季节变化较小，传统的光谱植被指数对植被光合作用难以准确捕捉。利用2014-2015年中国科学院广东省鼎湖山森林生态试验站多角度自动光谱观测系统的光谱反射数据，分别反演传统冠层结构型植被指数（NDVI）、光合生理生化型植被指数（CCI）和叶绿素荧光型植被指数（NDFI685和NDFI760），并利用不同类型植被指数的组合，构建多元线性回归模型。结果表明：亚热带常绿针阔混交林三种类型植被指数均与GPP的动态变化有显著的相关性，其中，NDVI是表征GPP较优的植被指数（R2=0.60，P < 0.01），其次为CCI（R2=0.55，P < 0.01），而NDFI能够作为辅助指数，有效提高NDVI（R2=0.68，P < 0.001）和CCI（R2=0.67，P < 0.001）表征GPP的程度。多个植被指数参与构建的多元回归模型能够有效提高亚热带地区常绿林GPP季节动态变化的拟合精度，提升遥感精确评估亚热带森林生产力的能力。 Abstract:Remote sensing is an effective method to assess terrestrial vegetation photosynthetic physiology and productivity dynamics at a regional scale. The conventional spectral vegetation index such as normalized difference vegetation index does not accurately reveal the photosynthetic phenology of subtropical evergreen forests because canopy structure is relatively stable across seasons. This study calculated the conventional canopy structural vegetation index (normalized difference vegetation index, NDVI), photosynthetic physiological and biochemical vegetation index (chlorophyll/carotenoid index, CCI), and chlorophyll fluorescence vegetation index (normalized difference fluorescence indices, NDFI) respectively, using the spectral reflection data from the automated multi-angular spectro-radiometer at the Dinghu Mountain Forest Ecosystem Research Station in Guangdong, China. We compared and analyzed their differences in tracking gross primary productivity (GPP) as measured by eddy covariance at the canopy level. A multivariate linear regression model was built to improve the fitting accuracy of GPP seasonal dynamics in this subtropical evergreen forest. The results show:for this mixed subtropical evergreen forest, 1) GPP was significantly correlated with all three indices, and the correlation with NDVI was the strongest (R2=0.60, P < 0.01); 2) CCI could not replace NDVI as a better vegetation index to reveal GPP seasonal dynamics (R2=0.55, P < 0.01); 3) NDFI could be used as a secondary index to effectively improve assessment of photosynthetic phenology (R2=0.68,P < 0.001). 参考文献 相似文献 引证文献

Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees

Tree species in the boreal forest cycle between periods of active growth and dormancy alter their photosynthetic processes in response to changing environmental conditions. For deciduous species, these changes are readily visible, while evergreen species have subtler foliar changes during seasonal transitions. In this study, we used remotely sensed optical indices to observe seasonal changes in photosynthetic activity, or photosynthetic phenology, of six boreal tree species. We evaluated the normalized difference vegetation index (NDVI), the photochemical reflectance index (PRI), the chlorophyll/carotenoid index (CCI), and steady-state chlorophyll fluorescence (FS) as a measure of solar-induced fluorescence (SIF), and compared these optical metrics to gas exchange to determine their efficacy in detecting seasonal changes in plant photosynthetic activity. The NDVI and PRI exhibited complementary responses. The NDVI paralleled photosynthetic phenology in deciduous species, but not in evergreens. The PRI closely paralleled photosynthetic activity in evergreens, but less so in deciduous species. The CCI and FS tracked photosynthetic phenology in both deciduous and evergreen species. The seasonal patterns of optical metrics and photosynthetic activity revealed subtle differences across and within functional groups. With the CCI and fluorescence becoming available from satellite sensors, they offer new opportunities for assessing photosynthetic phenology, particularly for evergreen species, which have been difficult to assess with previous methods.

Multiple drivers of seasonal change in PRI: Implications for photosynthesis 2. Stand level

The goal of this study was to explore the relationships between stand-level photochemical reflectance index (PRI) and canopy structure/pigment pools, as well as light use efficiency (LUE) of photosynthetically active vegetation focusing on seasonal or ontogenetic time frames. PRI was originally designed as a means of assessing the xanthophyll cycle and LUE over short (e.g. diurnal) time frames, and few studies have explored the drivers of PRI over longer, seasonal time frames, particularly in crops having different photosynthetic pathways or canopy structures. Consequently, our purpose was to understand and quantify the drivers of PRI responses over seasonal time scales for two crops, maize (C4) and soybean (C3), contrasting in photosynthetic pathway, leaf structure and canopy architecture. In both crops, PRI was very closely related to green LAI (R2>0.90) and stand chlorophyll (Chl) content (R2>0.93). The slopes of the relationships in different phenological stages, vegetative and reproductive, were substantially different (3-fold smaller in the vegetative stage). The main cause of this disparity was the high PRI value of soil/residue background. While PRI was a sensitive indicator of the changes in stand green LAI and stand Chl content over the full growing season, it was not sensitive to LUE; LUE explained below 12% of PRI variation in maize and 19% in soybean. Unlike leaf-level PRI, stand-level PRI was not clearly related to the Car/Chl ratio, presumably because the large changes in canopy structure (affecting stand Chl and green LAI) had a dominant influence on PRI over this time frame. The strong relationship between PRI and stand Chl content as well as between PRI and Chl-related vegetation index over a growing cycle allowed us to subtract the stand Chl content effect from measured PRI to reveal the component of PRI most likely related to periods of stress. However, for accurate subtraction of the Chl effect from long-term PRI records, thoughtful study of uncertainties related to “natural” variation of PRI-stand Chl relationships, and stand Chl content estimation for different varieties of the same species and for different species is required. The findings of a strong link between stand-level PRI and stand green LAI and Chl content and the lack of a clear relationship between PRI and LUE over seasonal and ontogenetic time spans suggest the need for a more careful evaluation of the relationship between PRI and either LUE or photosynthetic activity. In particular, studies that contrast short-term (e.g. diurnal) vs. long-term (e.g. seasonal) pigment, PRI, and photosynthetic responses in contrasting vegetation types are needed to clarify the different mechanisms involved at different temporal and spatial scales. These findings have important implications for attempts to monitor photosynthetic phenology from remote sensing, many of which have relied on PRI as an indicator of photosynthetic activity.

A remotely sensed pigment index reveals photosynthetic phenology in evergreen conifers

In evergreen conifers, where the foliage amount changes little with season, accurate detection of the underlying "photosynthetic phenology" from satellite remote sensing has been difficult, presenting challenges for global models of ecosystem carbon uptake. Here, we report a close correspondence between seasonally changing foliar pigment levels, expressed as chlorophyll/carotenoid ratios, and evergreen photosynthetic activity, leading to a "chlorophyll/carotenoid index" (CCI) that tracks evergreen photosynthesis at multiple spatial scales. When calculated from NASA's Moderate Resolution Imaging Spectroradiometer satellite sensor, the CCI closely follows the seasonal patterns of daily gross primary productivity of evergreen conifer stands measured by eddy covariance. This discovery provides a way of monitoring evergreen photosynthetic activity from optical remote sensing, and indicates an important regulatory role for carotenoid pigments in evergreen photosynthesis. Improved methods of monitoring photosynthesis from space can improve our understanding of the global carbon budget in a warming world of changing vegetation phenology.