Deconvolution of pigment and physiologically related photochemical reflectance index variability at the canopy scale over an entire growing season

The sensitivity of the photochemical reflectance index (PRI) to leaf pigmentation and its impacts on its potential as a proxy for light‐use efficiency (LUE) have recently been shown to be problematic at the leaf scale. Most leaf‐to‐leaf and seasonal variability can be explained by such a confounding effect. This study relies on the analysis of PRI light curves that were generated at the canopy scale under natural conditions to derive a precise deconvolution of pigment‐related and physiologically related variability in the PRI. These sources of variability were explained by measured or estimated physiologically relevant variables, such as soil water content, that can be used as indicators of water availability and canopy chlorophyll content. The PRI mainly reflected the variability in the pigment content of the canopy. However, the corrected PRI, which was obtained by subtracting the pigment‐related seasonal variability from the PRI measurement, was highly correlated with the upscaled LUE measurements. Moreover, the sensitivity of the PRI to the leaf pigment content may mask the PRI versus LUE relationship or result in an artificial relationship that reflects the relationship of chlorophyll versus LUE, depending on the species phenology.     

Constitutive changes in pigment concentrations: implications for estimating isoprene emissions using the photochemical reflectance index

The photochemical reflectance index (PRI), through its relationship with light use efficiency (LUE) and xanthophyll cycle activity, has recently been shown to hold potential for tracking isoprene emissions from vegetation. However, both PRI and isoprene emissions can also be influenced by changes in carotenoid pigment concentrations. Xanthophyll cycle activity and changes in carotenoid concentrations operate over different timescales, but the importance of constitutive changes in pigment concentrations for accurately estimating isoprene emissions using PRI is unknown. To clarify the physiological mechanisms behind the PRI–isoprene relationship, the light environment of potted Salix viminalis (osier willow) trees was modified to induce acclimation in photosynthetic rates, phytopigments, isoprene emissions and PRI. Acclimation resulted in differences in pigment concentrations, isoprene emissions and PRI. Constitutive changes in carotenoid concentration were significantly correlated with both isoprene emissions and PRI, suggesting that the relationship between PRI and isoprene emissions is significantly influenced by constitutive pigment changes. Consequently knowledge regarding how isoprene emissions are affected by both longer term changes in total carotenoid concentrations and shorter term dynamic adjustments of LUE is required to facilitate interpretation of PRI for monitoring isoprene emissions.     

毛乌素沙地油蒿叶性状对光能利用效率动态的影响

目前对于荒漠灌木光能利用效率（LUE）的季节变异及其调控因素,尤其是其生物调控因素的认识非常有限,导致了荒漠生态系统生产力模型的不确定性。拟验证假设：长期干旱环境下,典型荒漠灌木油蒿光能利用效率日均值（LUE_day）的动态变化与叶片性状的季节性调整有关。试验采用Li-6400便携式光合仪定期测量了油蒿生长季叶片LUE_day的季节动态及相关叶性状指标,探究叶性状对LUE_day的影响。结果表明：LUE_day的季节波动范围为0.003-0.017 mol/mol,整体变异系数（CV）为38.75%。完全展叶期LUE_day均值相比生长季平均值降低17.37%,相比展叶期和落叶期时降低30%;8个叶性状的季节变异幅度差异较大,其中总叶绿素含量（Chl）、类胡萝卜素含量（Car）和叶氮含量（LNC）均表现出较大的季节变异性（CV ≥ 20%）,叶碳含量（LCC）和叶片相对含水量（LRWC）的变异程度最低（CV<7%）。LRWC与所有叶片化学性状（Chl、Chl a/b、Car、LNC和LCC）均存在显著相关,表明其变化与叶片的养分吸收、光合色素合成以及碳同化的运输过程密切相关;油蒿LUE_day的相对变化与LRWC、Chl a/b和LNC显著正相关,而LRWC和LNC的季节动态受空气温度（T_a）和土壤含水量（VWC）的共同调节,Chl a/b的季节波动主要由浅层土壤含水量（10 cm VWC）控制。以上研究结果强调,在未来预计极端的气候事件（如极端干旱和持续热浪事件）发生更频繁的旱地场景中,时间尺度植物叶性状对于土壤干旱和高温的适应性调整应当被充分考虑到旱地生态系统的通量建模方案中。该结果将为构建叶片尺度的光合生理模型与厘清LUE的生物调控机制提供理论依据。     

光化学植被指数估算植物光能利用率的研究进展

 应用遥感技术可以建立光化学植被指数（Photochemical reflectance index, PRI）和光能利用效率（Light use efficiency, LUE）的关系,LUE可用来估算净初级生产力（Net primary productivity, NPP）。因而,用PRI估算植物的LUE,为估算NPP提供了新的方法,弥补了以往以经验模式通过温度和水分对最大LUE的胁迫来获取实际LUE的不足,进而可提高NPP的估算精度。研究表明：PRI和LUE在叶片、冠层和景观尺度上都有着很好的相关性,但是随着尺度的变化,很多因素会对这一关系产生干扰,如水分、氮元素含量、叶面积指数和太阳高度角等,从而削弱了PRI和LUE的关系。该文对建立PRI和LUE的关系过程中的影响因素进行了分析,并指出今后这一研究领域中可能改进的方面,主要包括526 nm 和545 nm 处的反射率对531 nm 处的反射率的作用机制、PRI随LUE的饱和现象、PRI和LUE关系的时间效应以及利用PRI估算LUE的尺度效应。     

基于通量和光谱观测的中亚热带人工针叶林光能利用效率的反演

利用光谱反射率测量的光化学植被指数（PRI）估算植被光合作用的光能利用效率（LUE）,能够更好地为生态系统总初级生产力的估算及尺度扩展提供重要的技术支撑.本研究以中国通量网（ChinaFLUX）千烟洲通量观测站为研究区域,2013年9月和12月在通量塔上测量了中亚热带人工针叶林的植被反射光谱,并获取了通量塔上同步观测的气象数据和涡度相关通量数据,对两者进行回归分析.结果表明： PRI-LUE相关关系（R²=0.20,P<0.001）优于NDVI LUE.在整个观测期内,土壤水分含量（SWC）与PRI组合的二元回归模型能够提高LUE的估算精度（日间观测R²=0.29,P<0.001;正午观测R²=0.30,P<0.01）,而在秋季,饱和水汽压差（VPD）与PRI组合的二元回归模型能较好地估算正午LUE（R²=0.448, P<0.001）,表明环境因子SWC和VPD是影响PRI-LUE关系的重要因素,不同季节的二元回归模型所选择的最佳环境变量有所不同.     

Physiology of the seasonal relationship between the photochemical reflectance index and photosynthetic light use efficiency

The photochemical reflectance index (PRI) is regarded as a promising proxy to track the dynamics of photosynthetic light use efficiency (LUE) via remote sensing. The implementation of this approach requires the relationship between PRI and LUE to scale not only in space but also in time. The short-term relationship between PRI and LUE is well known and is based on the regulative process of non-photochemical quenching (NPQ), but at the seasonal timescale the mechanisms behind the relationship remain unclear. We examined to what extent sustained forms of NPQ, photoinhibition of reaction centres, seasonal changes in leaf pigment concentrations, or adjustments in the capacity of alternative energy sinks affect the seasonal relationship between PRI and LUE during the year in needles of boreal Scots pine. PRI and NPQ were highly correlated during most of the year but decoupled in early spring when the foliage was deeply downregulated. This phenomenon was attributed to differences in the physiological mechanisms controlling the seasonal dynamics of PRI and NPQ. Seasonal adjustments in the pool size of the xanthophyll cycle pigments, on a chlorophyll basis, controlled the dynamics of PRI, whereas the xanthophyll de-epoxidation status and other xanthophyll-independent mechanisms controlled the dynamics of NPQ at the seasonal timescale. We conclude that the PRI leads to an underestimation of NPQ, and consequently overestimation of LUE, under conditions of severe stress in overwintering Scots pine, and most likely also in species experiencing severe drought. This severe stress-induced decoupling may challenge the implementation of the PRI approach.     

Is productivity of mesic savannas light limited or water limited? Results of a simulation study

A soil–plant–atmosphere model was used to estimate gross primary productivity (GPP) and evapotranspiration (ET) of a tropical savanna in Australia. This paper describes model modifications required to simulate the substantial C4 grass understory together with C3 trees. The model was further improved to include a seasonal distribution of leaf area and foliar nitrogen through 10 canopy layers. Model outputs were compared with a 5‐year eddy covariance dataset. Adding the C4 photosynthesis component improved the model efficiency and root‐mean‐squared error (RMSE) for total ecosystem GPP by better emulating annual peaks and troughs in GPP across wet and dry seasons. The C4 photosynthesis component had minimal impact on modelled values of ET. Outputs of GPP from the modified model agreed well with measured values, explaining between 79% and 90% of the variance and having a low RMSE (0.003–0.281 g C m^?2 day^?1). Approximately, 40% of total annual GPP was contributed by C4 grasses. Total (trees and grasses) wet season GPP was approximately 75–80% of total annual GPP. Light‐use efficiency (LUE) was largest for the wet season and smallest in the dry season and C4 LUE was larger than that of the trees. A sensitivity analysis of GPP revealed that daily GPP was most sensitive to changes in leaf area index (LAI) and foliar nitrogen (N_f) and relatively insensitive to changes in maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max) and minimum leaf water potential (ψ_min). The modified model was also able to represent daily and seasonal patterns in ET, (explaining 68–81% of variance) with a low RMSE (0.038–0.19 mm day^?1). Current values of N_f, LAI and other parameters appear to be colimiting for maximizing GPP. By manipulating LAI and soil moisture content inputs, we show that modelled GPP is limited by light interception rather than water availability at this site.     

Leaf attributes and tree growth in a tropical dry forest

Questions: How are leaf attributes and relative growth rate (RGR) of the dominant tree species of tropical deciduous forest (TDF) affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional attributes with each other? Can leaf attributes singly or in combination predict the growth rate of tree species of TDF? Location: Sonebhadra district of Uttar Pradesh, India. Methods: Eight leaf attributes, specific leaf area (SLA); leaf carbon concentration (LCC); leaf nitrogen concentration (LNC); leaf phosphorus concentration (LPC); chlorophyll concentration (Chl), mass‐based stomatal conductance (Gs_mass); mass based photosynthetic rate (A_mass); intrinsic water use efficiency (WUEi); and relative growth rate (RGR), of six dominant tree species of a dry tropical forest on four sites were analysed for species, site and season effects over a 2‐year period. Step‐wise multiple regression was performed for predicting RGR from mean values of SMC and leaf attributes. Path analysis was used to determine which leaf attributes influence RGR directly and which indirectly. Results: Species differed significantly in terms of all leaf attributes and RGR. The response of species varied across sites and seasons. The attributes were positively interrelated, except for WUEi, which was negatively related to all other attributes. The positive correlation was strongest between Gs_mass and A_mass and the negative correlation was strongest between Gs_mass and WUEi. Differences in RGR due to site were not significant when soil moisture was controlled, but differences due to season remained significant. The attributes showed plasticity across moisture gradients, which differed among attributes and species. Gs_mass was the most plastic attribute. Among the six species, Terminalia tomentosa exhibited the greatest plasticity in six functional attributes. In the step‐wise multiple regression, A_mass, SLA and Chl among leaf attributes and SMC among environmental factors influenced the RGR of tree species. Path analysis indicated the importance of SLA, LNC, Chl and A_mass in determining RGR. Conclusion: A _mass, SMC, SLA and Chl in combination can be used to predict RGR but could explain only three‐quarters of the variability in RGR, indicating that other traits/factors, not studied here, are also important in modulating growth of tropical trees. RGR of tree species in the dry tropical environment is determined by soil moisture, whereas the response of mature trees of different species is modulated by alterations in key functional attributes such as SLA, LNC and Chl.     

Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function

In this study the variations in surface reflectance properties and pigment concentrations of Antarctic moss over species, sites, microtopography and with water content were investigated. It was found that species had significantly different surface reflectance properties, particularly in the region of the red edge (approximately 700 nm), but this did not correlate strongly with pigment concentrations. Surface reflectance of moss also varied in the visible region and in the characteristics of the red edge over different sites. Reflectance parameters, such as the photochemical reflectance index (PRI) and cold hard band were useful discriminators of site, microtopographic position and water content. The PRI was correlated both with the concentrations of active xanthophyll‐cycle pigments and the photosynthetic light use efficiency, F_v/F_m, measured using chlorophyll fluorescence. Water content of moss strongly influenced the amplitude and position of the red‐edge as well as the PRI, and may be responsible for observed differences in reflectance properties for different species and sites. All moss showed sustained high levels of photoprotective xanthophyll pigments, especially at exposed sites, indicating moss is experiencing continual high levels of photochemical stress.     

Diurnal and seasonal variations in light-use efficiency in an alpine meadow ecosystem: causes and implications for remote sensing

Aims Light-use efficiency (LUE) is an important tool for scaling up local CO₂ flux (F CO2) tower observations to regional and global carbon dynamics. Using a data set including F CO2 and environmental factors obtained from an alpine meadow on the Tibetan Plateau, we examined both diurnal and seasonal changes in LUE and the environmental factors controlling these changes. Our objectives were to (i) characterize the diurnal and daily variability of LUE in an alpine meadow, (ii) clarify the causes of this variability, and (iii) explore the possibility of applying the LUE approach to this alpine meadow by examining the relationship between daily LUE and hourly LUE at satellite visiting times.Methods First, we obtained the LUE—the ratio of the gross primary production (GPP) to the absorbed photosynthetically active radiation (APAR)—from the flux tower and meteorological observations. We then characterized the patterns of diurnal and seasonal changes in LUE, explored the environmental controls on LUE using univariate regression analyses and evaluated the effects of diffuse radiation on LUE by assigning weights through a linear programming method to beam photosynthetically active radiation (PAR) and diffuse PAR, which were separated from meteorological observations using an existing method. Finally, we examined the relationships between noontime hourly LUE and daily LUE and those between adjusted noontime hourly and daily LUE because satellites visit the site only once or twice a day, near noon.Important findings The results showed that (i) the LUE of the alpine meadow generally followed the diurnal and seasonal patterns of solar radiation but fluctuated with changes in cloud cover. (ii) The fraction of diffuse light played a dominant role in LUE variation. Daily minimum temperature and vapor pressure deficit also affected LUE variation. (iii) The adjusted APAR, defined as the weighted linear sum of diffuse APAR and beam APAR, was linearly correlated with GPP on different temporal scales. (iv) Midday adjusted LUE was closely related to daily adjusted LUE, regardless of the cloud cover. The results indicated the importance of considering radiation direction when developing LUE-based GPP-estimating models.     

Divergent long‐term trends and interannual variation in ecosystem resource use efficiencies of a southern boreal old black spruce forest 1999–2017

Long‐term trends in ecosystem resource use efficiencies (RUEs) and their controlling factors are key pieces of information for understanding how an ecosystem responds to climate change. We used continuous eddy covariance and microclimate data over the period 1999–2017 from a 120‐year‐old black spruce stand in central Saskatchewan, Canada, to assess interannual variability, long‐term trends, and key controlling factors of gross ecosystem production (GEP) and the RUEs of carbon (CUE = net primary production [NPP]/GEP), light (LUE = GEP/absorbed photosynthetic radiation [APAR]), and water (WUE = GEP/evapotranspiration [E]). At this site, annual GEP has shown an increasing trend over the 19 years (p < 0.01), which may be attributed to rising atmospheric CO₂ concentration. Interannual variability in GEP, aside from its increasing trend, was most strongly related to spring temperatures. Associated with the significant increase in annual GEP were relatively small changes in NPP, APAR, and E, so that annual CUE showed a decreasing trend and annual LUE and WUE showed increasing trends over the 19 years. The long‐term trends in the RUEs were related to the increasing CO₂ concentration. Further analysis of detrended RUEs showed that their interannual variation was impacted most strongly by air temperature. Two‐factor linear models combining CO₂ concentration and air temperature performed well (R²~0.60) in simulating annual RUEs. LUE and WUE were positively correlated both annually and seasonally, while LUE and CUE were mostly negatively correlated. Our results showed divergent long‐term trends among CUE, LUE, and WUE and highlighted the need to account for the combined effects of climatic controls and the ‘CO₂ fertilization effect’ on long‐term variations in RUEs. Since most RUE‐based models rely primarily on one resource limitation, the observed patterns of relative change among the three RUEs may have important implications for RUE‐based modeling of C fluxes.     

Responses of CO2, CH4 and N2O fluxes to livestock exclosure in an alpine steppe on the Tibetan Plateau, China

Background and aims

The photochemical reflectance index (PRI) is correlated to photosynthetic efficiency and has been successfully applied at multiple scales for remote estimation of physiological functioning. However, interpretation of the PRI signal can be confounded by many different variables including declines in photochemical pigments. Our study was aimed at investigating PRI in response to salinity stress, and evaluating physiological and pigment responses of two co-occurring shrubs, Baccharis halimifolia and Myrica cerifera in laboratory studies.

Methods

Photosynthesis, water relations, chlorophyll fluorescence, hyperspectral reflectance and leaf pigment contents were measured following salinity treatment.

Results

Physiological measurements indicated that both species exhibit adaptations which protect PSII during periods of stress. Chlorophyll fluorescence parameters were affected in both species, but indicated that other photochemical reactions (e.g. photorespiration) were important for energy dissipation in absence of chlorophyll changes. After many days of reduced photosynthesis, photochemical changes were detectable using PRI indicating chronic stress.

Conclusions

Variations in PRI were not related to changes in pigments but strongly related to tissue chlorides indicating salinity effects on the PRI signal. Thus, PRI is an indicator of salinity stress in these coastal species and may be as an early signal for increasing salt exposure associated with rising sea-level and climate change.     

Light and temperature, but not UV radiation, affect chlorophylls and carotenoids in Norway spruce needles (Picea abies (L.) Karst.)

Concentrations of chlorophyll a/freshweight (Chl a FW) and photosynthetic pigments/chlorophyll a were studied during one growing season in the current year's (CYN) and last year's needles (LYN) from Norway spruce (Picea abies (L.) Karst.) grown under natural or close‐to‐natural climate. Climate regimes differed in photosynthetic active radiation (PAR), temperature (T) and UV‐B radiation. Pigments were not affected by UV‐B but most of the differences between climate regimes, and also seasonal variations within climate regimes, could be related to PAR and T. Generally, two types of response to climate were observed: firstly, pigments reacted primarily to PAR without marked sensitivity to T and exhibited slow response times (> 30 d), and, secondly, pigments were affected by the combined action of PAR and T and responded faster than 20 d. The Chl a FW and chlorophyll b/chloprophyll a ratio exhibited slow‐type response in CYN and fast‐type response in LYN. Higher amplitudes in CYN than in LYN were observed for the latter two parameters, which are known to be associated with levels of pigment–protein complexes. It is suggested that slow response in CYN ensures that the high investments in proteins in these needles occur only in response to longer‐lasting climate episodes.     

Divergent phenological response to hydroclimate variability in forested mountain watersheds

Mountain watersheds are primary sources of freshwater, carbon sequestration, and other ecosystem services. There is significant interest in the effects of climate change and variability on these processes over short to long time scales. Much of the impact of hydroclimate variability in forest ecosystems is manifested in vegetation dynamics in space and time. In steep terrain, leaf phenology responds to topoclimate in complex ways, and can produce specific and measurable shifts in landscape forest patterns. The onset of spring is usually delayed at a specific rate with increasing elevation (often called Hopkins' Law; Hopkins, 1918), reflecting the dominant controls of temperature on greenup timing. Contrary with greenup, leaf senescence shows inconsistent trends along elevation gradients. Here, we present mechanisms and an explanation for this variability and its significance for ecosystem patterns and services in response to climate. We use moderate‐resolution imaging spectro‐radiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to derive landscape‐induced phenological patterns over topoclimate gradients in a humid temperate broadleaf forest in southern Appalachians. These phenological patterns are validated with different sets of field observations. Our data demonstrate that divergent behavior of leaf senescence with elevation is closely related to late growing season hydroclimate variability in temperature and water balance patterns. Specifically, a drier late growing season is associated with earlier leaf senescence at low elevation than at middle elevation. The effect of drought stress on vegetation senescence timing also leads to tighter coupling between growing season length and ecosystem water use estimated from observed precipitation and runoff generation. This study indicates increased late growing season drought may be leading to divergent ecosystem response between high and low elevation forests. Landscape‐induced phenological patterns are easily observed over wide areas and may be used as a unique diagnostic for sources of ecosystem vulnerability and sensitivity to hydroclimate change.     

Variation in the phenology of photosynthesis among eastern white pine provenances in response to warming

In higher‐latitude trees, temperature and photoperiod control the beginning and end of the photosynthetically active season. Elevated temperature (ET) has advanced spring warming and delayed autumn cooling while photoperiod remains unchanged. We assessed the effects of warming on the length of the photosynthetically active season of three provenances of Pinus strobus L. seedlings from different latitudes, and evaluated the accuracy of the photochemical reflectance index (PRI) and the chlorophyll/carotenoid index (CCI) for tracking the predicted variation in spring and autumn phenology of photosynthesis among provenances. Seedlings from northern, local and southern P. strobus provenances were planted in a temperature‐free‐air‐controlled enhancement (T‐FACE) experiment and exposed to ET (+1.5/3°C; day/night). Over 18 months, we assessed photosynthetic phenology by measuring chlorophyll fluorescence, gas exchange, leaf spectral reflectance and pigment content. During autumn, all seedlings regardless of provenance followed the same sequence of phenological events with the initial downregulation of photosynthesis, followed by the modulation of non‐photochemical quenching and associated adjustments of zeaxanthin pool sizes. However, the timing of autumn downregulation differed between provenances, with delayed onset in the southern provenance (SP) and earlier onset in the northern relative to the local provenance, indicating that photoperiod at the provenance origin is a dominant factor controlling autumn phenology. Experimental warming further delayed the downregulation of photosynthesis during autumn in the SP. A provenance effect during spring was also observed but was generally not significant. The vegetation indices PRI and CCI were both effective at tracking the seasonal variations of energy partitioning in needles and the differences of carotenoid pigments indicative of the stress status of needles. These results demonstrate that PRI and CCI can be useful tools for monitoring conifer phenology and for the remote monitoring of the length of the photosynthetically active season of conifers in a changing climate.     

Photochemistry, remotely sensed physiological reflectance index and de-epoxidation state of the xanthophyll cycle in Quercus coccifera under intense drought

Quercus coccifera L. is a Mediterranean sclerophyllous shrub with a high capacity to resist intense drought stress. Therefore, it could be used in the study of physiological changes suffered by plants at very low water potentials. A remote sensing sensor was used to measure continuously the physiological reflectance index (PRI; defined as the changes in reflectance at 531 nm with respect to those at 570 nm; PRI = [(R531 − R570)/(R531 + R570)] at canopy level and under field conditions in an artificial carpet of seedlings of Q. coccifera during a drought cycle. Correlations between leaf level-measured chlorophyll fluorescence parameters as well as the de-epoxidation state of the xanthophyll cycle [(A + Z)/(V + A + Z)] and canopy level-measured PRI were reasonably good (R ² = 0.57–0.63, P < 0.01), and quite interesting for water stress remote sensing purposes. The instrument’s temporal resolution allowed us to follow the rapid response of PRI to changing photosynthetic active radiation, and to resolve, in response to cloud-induced changes in light intensity, a fast and a slow PRI component. We report the disappearance of the rapid one under conditions of intense drought in response to a sudden increase in light intensity. The underlying photoprotection mechanisms that Q. coccifera shows in response to intense drought stress periods seem to be related to the existence of a low intrathylakoid lumenal pH at the end of the drought cycle. Under intense drought, these mechanisms allow this species to avoid oxidative damage, which was evidenced by the maintenance of an unaltered photosynthetic pigment composition and constant photosystem II efficiency in the mornings. It is concluded that, contrary to early reports, PRI is a sensible, indirect, non-destructive water stress indicator, even in plants experiencing intense drought. Preliminary results of this work were presented at the 3rd International Workshop on Remote Sensing of Vegetation Fluorescence (February 2007, Florence, Italy).     

Soil‐ and plant‐water dynamics in a C3/C4 grassland exposed to a subambient to superambient CO2 gradient

Plants may be more sensitive to carbon dioxide (CO₂) enrichment at subambient concentrations than at superambient concentrations, but field tests are lacking. We measured soil‐water content and determined xylem pressure potentials and δ¹³C values of leaves of abundant species in a C3/C4 grassland exposed during 1997–1999 to a continuous gradient in atmospheric CO₂ spanning subambient through superambient concentrations (200–560 µmol mol^2?1). We predicted that CO₂ enrichment would lessen soil‐water depletion and increase xylem potentials more over subambient concentrations than over superambient concentrations. Because water‐use efficiency of C3 species (net assimilation/leaf conductance; A/g) typically increases as soils dry, we hypothesized that improvements in plant‐water relations at higher CO₂ would lessen positive effects of CO₂ enrichment on A/g. Depletion of soil water to 1.35 m depth was greater at low CO₂ concentrations than at higher CO₂ concentrations during a mid‐season drought in 1998 and during late‐season droughts in 1997 and 1999. During droughts each year, mid‐day xylem potentials of the dominant C4 perennial grass (Bothriochloa ischaemum (L.) Keng) and the dominant C3 perennial forb (Solanum dimidiatum Raf.) became less negative as CO₂ increased from subambient to superambient concentrations. Leaf A/g—derived from leaf δ¹³C values—was insensitive to feedbacks from CO₂ effects on soil water and plant water. Among most C3 species sampled—including annual grasses, perennial grasses and perennial forbs—A/g increased linearly with CO₂ across subambient concentrations. Leaf and air δ¹³C values were too unstable at superambient CO₂ concentrations to reliably determine A/g. Significant changes in soil‐ and plant‐water relations over subambient to superambient concentrations and in leaf A/g over subambient concentrations generally were not greater over low CO₂ than over higher CO₂. The continuous response of these variables to CO₂ suggests that atmospheric change has already improved water relations of grassland species and that periodically water‐limited grasslands will remain sensitive to CO₂ enrichment.     

On the Use of Leaf Spectral Indices to Assess Water Status and Photosynthetic Limitations in Olea europaea L. during Water-Stress and Recovery

Diffusional limitations to photosynthesis, relative water content (RWC), pigment concentrations and their association with reflectance indices were studied in olive (Olea europaea) saplings subjected to water-stress and re-watering. RWC decreased sharply as drought progressed. Following rewatering, RWC gradually increased to pre-stress values. Photosynthesis (A), stomatal conductance (g_s), mesophyll conductance (g_m), total conductance (g_t), photochemical reflectance index (PRI), water index (WI) and relative depth index (RDI) closely followed RWC. In contrast, carotenoid concentration, the carotenoid to chlorophyll ratio, water content reflectance index (WCRI) and structural independent pigment index (SIPI) showed an opposite trend to that of RWC. Photosynthesis scaled linearly with leaf conductance to CO₂; however, A measured under non-photorespiratory conditions (A_1%O2) was approximately two times greater than A measured at 21% [O₂], indicating that photorespiration likely increased in response to drought. A_1%O2 also significantly correlated with leaf conductance parameters. These relationships were apparent in saturation type curves, indicating that under non-photorespiratory conditions, CO₂ conductance was not the major limitations to A. PRI was significant correlated with RWC. PRI was also very sensitive to pigment concentrations and photosynthesis, and significantly tracked all CO₂ conductance parameters. WI, RDI and WCRI were all significantly correlated with RWC, and most notably to leaf transpiration. Overall, PRI correlated more closely with carotenoid concentration than SIPI; whereas WI tracked leaf transpiration more effectively than RDI and WCRI. This study clearly demonstrates that PRI and WI can be used for the fast detection of physiological traits of olive trees subjected to water-stress.     

Complex Effects of Prescribed Fire on Restoring the Soil Water Content in a High-Elevation Riparian Meadow, Arizona

One of the largest and rarest Bebb willow (Salix bebbiana) communities in the United States occurs at Hart Prairie, Arizona. Low recruitment of the willow over the past several decades has been linked to inadequate soil water content for seed germination and seedling establishment. We tested a hypothesis that a prescribed burn would reduce biomass of and evapotranspiration by herbaceous plants, thereby increasing soil water content. Three treatments (unburned control, early‐growing season burned, late‐growing season burned) were applied in year 2001 to replicated plots in fern‐ and grass‐dominated herbaceous communities. Soil water content (0–30 cm) was measured weekly in plots during the 2001, 2002, and 2003 growing seasons. Both early‐ and late‐season burning reduced herbaceous biomass in the fern‐dominated community in 2002 and 2003 and reduced biomass in the grass‐dominated community in 2002 but not in 2003. Soil water content increased for approximately four weeks in 2001 following the early‐season burn, but the early‐season and late‐season burns reduced soil water content in both communities over much of the 2002 and 2003 growing seasons. Thus, early‐season burning may benefit willow seed germination by increasing soil water content immediately following burning but be detrimental to germination in the second and third growing seasons after burning because of drier soil. Large temporal variation in the effect of prescribed burning on soil water content will complicate the use of fire as a restoration tool to manage soil water available for threatened plants such as Bebb willow and for recharge of groundwater.     

西双版纳热带雨林土壤与叶片生态化学计量特征的干湿度效应

基于地理格局对西双版纳热带雨林的干湿度梯度效应和生态化学计量学的研究思路,结合野外试验监测和室内分析,对西双版纳热带雨林土壤-植物系统元素化学计量特征对海拔和干湿度效应响应进行了研究探讨,结果发现:西双版纳热带雨林土壤和叶片碳氮磷化学计量特征均不同程度的受到海拔和干湿季影响。季雨林与山地雨林的水热梯度受海拔梯度重要影响,随海拔梯度升高,土壤含水率变化显著,且含水率在干湿季均对土壤有机碳(SOC)存在显著影响(P0.01),雨季其对土壤全氮(STN)和土壤全磷(STP)的影响要显著于干季;叶片全磷(TP)随含水率的增大而升高,而叶片全氮(TN)在干季会随含水率的升高而增大,雨季含水率升高到一定程度时会抑制TN含量的增加并出现单峰现象;而土壤C/P与海拔和干季土壤含水率的极显著相关性(P0.01)及干季叶片C/N与叶片含水率的显著相关关系(P0.05)说明,干季水分匮乏条件下,土壤含水率影响土壤P的矿化度和植物对P的吸收利用水平,而且叶片C/N对反馈植物水分含量具有明显指示作用。因此,水热梯度是土壤-叶片系统碳氮磷生态化学计量特征变化的重要驱动因素。此外,全球变化区域响应方面,多雨高温可能会削弱季雨林叶片C的同化能力,且叶N含量降低,但受氮沉降的影响,对C/N的影响尚无法确定;由于P循环对其他元素的耦合作用,雨林土壤-叶片系统的元素循环周期将会被缩短,但干季山地雨林植物生态系统P的限制作用有可能会减弱。