Pubescence and leaf spectral characteristics in a desert shrub,Encelia farinosa

Summary The effects of leaf hairs (pubescence) on leaf spectral characteristics were measured for the drought-deciduous desert shrub Encelia farinosa. Leaf absorptance to solar radiation is diminished by the presence of pubescence. The pubescence appears to be reflective only after the hairs have dried out. There are seasonal changes in leaf absorptance; leaves produced at the beginning of a growing season have high absorptances, whereas leaves produced during the growing season are more pubescent and have lower absorptances. The decrease in leaf absorptance is the result of an increase in pubescence density and thickness. Between 400 and 700 nm (visible wavelengths), pubescence serves as a blanket reflector. However, over the entire solar spectrum (400–3000 nm), the pubescence preferentially reflects near infrared radiation (700–3000 nm) over photosynthetically useful solar radiation (400–700 nm). Leaf absorptance to solar radiation (400–3000 nm) varies between 46 and 16%, depending on pubescence; whereas leaf absorptance to photosynthetically useful radiation (400–700 nm) may vary from 81 to 29%.C.I.W.-D.P.B. Publication No. 612     

Intraspecific variation of drought adaptation in brittlebush: leaf pubescence and timing of leaf loss vary with rainfall

Reflective leaf pubescence of the desert shrub Encelia farinosa (brittlebrush) reduces leaf temperature and plant water loss, and is considered adaptive in xeric environments. Yet, little is known about intraspecific variation in this trait. Among three populations in the northern range of E. farinosa, which span a very broad precipitation gradient, both leaf absorptance variation and differences in the timing of drought-induced leaf loss were broadly associated with climatic variability. Where mean annual rainfall was greatest, drought-induced leaf loss was earliest, but these plants also had higher population-level mean leaf absorptance values. Higher absorptance increases the relative dependence on latent heat transfer (transpirational cooling), but it also provides greater instantaneous carbon assimilation. Plants at the driest site reached lower leaf absorptance values and maintained leaves longer into the drought period. Lower leaf absorptance reduces water consumption, and extended leaf longevity may buffer against the unpredictability of growing conditions experienced in the driest site. These observations are consistent with a trade-off scenario in which plants from wetter regions might trade off water conservation for higher instantaneous carbon gain, whereas plants from drier regions reduce water consumption and extend leaf longevity to maintain photosynthetic activity in the face of unpredictable growing conditions. Received: 2 April 1997 / Accepted: 11 August 1997     

Effects of elevated atmospheric CO(2) and temperature on leaf optical properties in Acer saccharum

Elevated partial pressures of atmospheric carbon dioxide, similar to numerous causes of plant stress, may alter leaf pigmentation and structure and thus would be expected to alter leaf optical properties. Hypotheses that elevated CO(2) pressure and air temperature would alter leaf optical properties were tested for sugar maple (Acer saccharum) in the middle of its fourth growing season under treatment. The saplings had been growing since 1994 in open-top chambers and partial shade at Oak Ridge, Tennessee under the following treatments: (1) ambient CO(2) pressure and air temperature (control); (2) CO(2) pressure approximately 30 Pa above ambient; (3) air temperatures 3 degrees C above ambient; and (4) elevated CO(2) and air temperature. Under elevated CO(2) or temperature, spectral reflectance, transmittance and absorptance in the visible spectrum (400-720 nm) tended to change in patterns that generally are associated with chlorosis, with maximum differences from the control near 700 nm. However, these changes were not significant at P=0.05. Although reflectance, transmittance and absorptance at 700 nm correlated strongly with leaf chlorophyll concentration, variability in chlorophyll concentration was greater within than among treatments. The lack of treatment effects on pigmentation explained the non-significant change in optical properties in the visible spectrum. Optical properties in the near-infrared (721-850 nm) were similarly unresponsive to treatment with the exception of an increased absorptance throughout the 739-850 nm range in leaves that developed under elevated air temperature alone. This response might have resulted from effects of air temperature on leaf internal structure.     

Leaf hairs: Effects on physiological activity and adaptive value to a desert shrub

Summary The effects of leaf hairs on photosynthesis, transpiration, and leaf energy balance were measured on the desert shrub Encelia farinosa in order to determine the adaptive significance of the hairs. The pubescence reduces leaf absorptance resulting in a reduced heat load, and as a consequence lower leaf temperatures and lower transpiration rates. In its native habitat where air temperatures often exceed 40° C, the optimum temperature for photosynthesis in E. farinosa occurs at 25° C, and at leaf temperatures above 35° C net photosynthesis declines precipitously. An advantage of leaf pubescence is that it allows a leaf temperature much lower than air temperature. As a result, leaf temperatures are near the temperature optimum for photosynthesis and high, potentially lethal leaf temperatures are avoided. However, there is a disadvantage associated with leaf pubescence. By reflecting quanta that might otherwise be used in photosynthesis, the presence of leaf hairs reduces the rate of photosynthesis. A tradeoff model was used to assess the overall advantage of possessing leaf hairs. In terms of the carbon gaining capacity of the leaf, the model predicted that for different environmental conditions different levels of leaf pubescence were optimal. In other words, under aird conditions and/or high air temperatures, leaves of E. farinosa would have a higher rate of photosynthesis by being pubescent than by not being pubescent. The predictions from this model agreed closely with observed patterns of leaf pubescence in the field.C.I.W.-D.P.B. Publication No. 613     

Population- and family-level variation of brittlebush (Encelia farinosa, Asteraceae) pubescence: its relation to drought and implications for selection in variable environments

Because leaf pubescence of the desert shrub Encelia farinosa increases in response to drought and influences photosynthesis and transpiration, we hypothesized that differences in water availability across the range of this species may result in genetic differentiation for pubescence and associated productivity traits. We examined maternal family variation of pubescence-moderated light absorption (absorptance) in three populations of E. farinosa. Absorptance was always greatest for plants from the high-rainfall environment and lowest for those from the driest site, but the rate of absorptance change in response to drought was similar among all populations. Similar patterns were found when we compared families within populations-all genotypes had similar initial leaf absorptances, differentiated very early in seasonal growth, then had concordant changes in absorptance thereafter. However, family-level variance was greatest for plants from the driest site, a region with highly heterogeneous precipitation patterns, whereas low variance was found for plants from the wettest, least heterogeneous site. The concordance of leaf absorptance changes, within and among populations, may be due to integration with other drought-related traits; however, the differences in absorptance values within and among populations suggest that variation of leaf pubescence results from selection associated with geographical and local patterns of water availability.     

INFLUENCE OF IRRADIATION,SOIL WATER POTENTIAL,AND LEAF TEMPERATURE ON LEAF MORPHOLOGY OF A DESERT BROADLEAF,ENCELIA FARINOSA GRAY (COMPOSITAE)

Laboratory experiments were performed to evaluate observed seasonal changes in leaf morphology of the desert perennial shrub, Encelia farinosa Gray. Plants were grown under low or high conditions of photosynthetically active irradiation, soil water potential (Ψ^soil), and leaf temperature (8 different experimental regimes). The relative growth rate, leaf water vapor conductance, leaf water potential, and leaf length were all greater for the high Ψ^soil regimes, the largest leaves occurring at low irradiation. High irradiation during growth led to thicker leaves with a higher internal to external leaf area ratio (A^mes/A); low Ψ^soil tended to increase A^mes/A somewhat. High irradiation also led to decreased absorptance to solar irradiation caused by increased pubescence. High leaf temperature during development resulted in slightly smaller, thicker leaves with higher A^mes/A. Thus, irradiation appeared to have its major influence on leaf thickness, A^mes/A, and absorptance, with a secondary effect on leaf length; Ψ^soil affected primarily leaf length, growth rate, and water status, and secondarily A^mes/A. Results are discussed with regard to recent ecophysiological studies on the observed seasonal changes in leaf morphology of E. farinosa.     

Spectral changes with leaf aging in Amazon caatinga

 Significant gaps exist in the knowledge of tropical leaf spectra and the manner in which spectra change as leaves age in their natural environment. Leaf aging effects may be particularly important in tropical vegetation growing on nutrient poor soils, such as Amazon caatinga, a white sand community common in the Amazon Basin. Spectral changes observed in six caatinga dominants include decreased reflectance and transmittance and increased absorptance for epiphyll-coated older leaves. Near-infrared (NIR) changes were most significant. More detailed spectral and physical changes were studied in one dominant, Aldina heterophylla. Over 16 months, Aldina study plants produced one or two leaf flushes. During leaf expansion, leaf water content and Specific Leaf Area decreased rapidly. Over the first 6 months spectral changes occurred across the spectrum, resulting in decreased transmittance and increased absorptance in the visible and NIR and decreased visible and increased NIR reflectance. In contrast, significant spectral changes were restricted to the NIR over the last 9 months, which showed a 10% absorptance increase associated primarily with increasing epiphylls and necrosis. At the canopy scale, increased NIR absorptance provides a mechanism for producing seasonally varying forest albedo and changing NIR to red ratios, independent of changes in other canopy attributes. In the Amazon caatinga studied, all canopy dominants were subject to epiphyllic growth providing a mechanism for distinguishing these forest types spectrally from more diverse terra-firme forest or forest types with more rapid leaf turnover, such as second growth. These changes are observable using remote sensing and could be used to map caatinga and monitor interannual or seasonal variability in phenology. If these results can be extended to other communities with long-lived foliage, they may offer a means for mapping vegetation on the basis of leaf longevity. Received: 18 November 1996/Accepted: 24 December 1997     

Morphological changes along an altitude gradient and their consequences for an andean giant rosette plant

Summary Selected morphological features were measured in five populations of the giant rosette plant Espeletia schultzii occurring along an elevation gradient from 2600 to 4200 m in the Venezuelan Andes. Pith volume per amount of leaf area increases with elevation resulting in significantly larger water storage capacity at higher elevations. Thickness of leaf pubescence and, therefore, leaf boundary layer resistance, also increases with elevation resulting in both potentially higher leaf temperatures relative to air temperature and higher leaf to air vapor pressure gradients. The net effect on transpiration rate would depend on ratios of stomatal to boundary layer resistance and leaf energy balance. At higher elevations the central rosette leaves are more vertically oriented and the leaf bases show a pronounced curvature as the intersection with the main axis is approached. This gives these rosettes a distinctly paraboloid appearance and probably enhances capture and retention of incident long and shortwave radiation by the apical bud and expanding leaves. Features which result in enhanced water storage capacity and higher plant temperatures relative to air temperature without greatly increasing water loss are adaptive in high altitude paramo habitats where water availability and growth are limited by year round low temperatures (mean 2–3° C).     

Antagonistic effects of growing season and autumn temperatures on the timing of leaf coloration in winter deciduous trees

Autumn phenology remains a relatively neglected aspect in climate change research, which hinders an accurate assessment of the global carbon cycle and its sensitivity to climate change. Leaf coloration, a key indicator of the growing season end, is thought to be triggered mainly by high or low temperature and drought. However, how the control of leaf coloration is split between temperature and drought is not known for many species. Moreover, whether growing season and autumn temperatures interact in influencing the timing of leaf coloration is not clear. Here, we revealed major climate drivers of leaf coloration dates and their interactions using 154 phenological datasets for four winter deciduous tree species at 89 stations, and the corresponding daily mean/minimum air temperature and precipitation data across China's temperate zone from 1981 to 2012. Results show that temperature is more decisive than drought in causing leaf coloration, and the growing season mean temperature plays a more important role than the autumn mean minimum temperature. Higher growing season temperature and lower autumn minimum temperature would induce earlier leaf coloration date. Moreover, the mean temperature over the growing season correlates positively with the autumn minimum temperature. This implies that growing season mean temperature may offset the requirement of autumn minimum temperature in triggering leaf coloration. Our findings deepen the understanding of leaf coloration mechanisms in winter deciduous trees and suggest that leaf life‐span control depended on growing season mean temperature and autumn low temperature control and their interaction are major environmental cues. In the context of climate change, whether leaf coloration date advances or is delayed may depend on intensity of the offset effect of growing season temperature on autumn low temperature.     

Abiotic constraints on the establishment of Quercus seedlings in grassland

High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first‐year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland‐forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long‐term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late‐season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present‐day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue.     

Climate change and extension of the Ginkgo biloba L. growing season in Japan

To understand the effects of climate change on the growing season of plants in Japan, we conducted trend analysis of phenological phases and examined the relationship between phenology and air temperatures. We used phenological data for Ginkgo biloba L., collected from 1953 to 2000. We defined the beginning and the end of the growing season (BGS and EGS) as the dates of budding and leaf fall, respectively. Changes in the air temperature in the 45 days before the date of BGS affected annual variation in BGS. The annual variation in air temperature over the 85 days before EGS affected the date of EGS. The average annual air temperature in Japan has increased by 1.3°C over the last four decades (1961–2000), and this increase has caused changes in ginkgo phenology. In the last five decades (1953–2000), BGS has occurred approximately 4 days earlier than previously, and EGS has occurred about 8 days later. Consequently, since 1953 the length of the growing season (LGS) has been extended by 12 days. Since around 1970, LGS and air temperatures have shown increasing trends. Although many researchers have stated that phenological events are not affected by the air temperature in the fall, we found high correlations not only between budding dates and air temperatures in spring but also between leaf‐fall dates and air temperatures in autumn. If the mean annual air temperature increases by 1°C, LGS could be extended by 10 days. We also examined the spatial distribution of the rate of LGS extension, but we did not find an obvious relationship between LGS extension and latitude.     

Evidence for facultative deciduousness in Colophospermum mopane in semi‐arid African savannas

Leaf phenology dictates the time available for carbon assimilation, transpiration and nutrient uptake in plants. Understanding the environmental cues that control phenology is therefore vital for predicting climate‐related changes to plant and ecosystem function. In contrast to temperate systems, and to a lesser degree, tropical forest systems, the cues initiating leaf drop in tropical savannas are poorly studied. We investigated the cues for leaf fall in a tropical monodominant arid savanna species, Colophospermum mopane, using an irrigation experiment. We tracked soil moisture, solar radiation, air temperature, leaf water status, leaf health and leaf carbon balance through the dry season in both irrigated and control plants. Water was the primary cue driving leaf loss of C. mopane rather than temperature or light. Trees watered throughout the dry season retained their canopies. These leaves remained functional and continued photosynthesis throughout the dry season. Leaf carbon acquisition rates did not decline with leaf age but were affected by soil moisture availability and temperature. Leaf loss did not occur when leaf carbon gain was zero, or when a particular leaf carbon threshold was reached. Colophospermum mopane is facultatively deciduous as water availability determines leaf drop in this widespread arid savanna species. Obligate deciduosity is not the only successful strategy in climates with a long dry season.     

The effect of a dry spring on seasonal carbon allocation and vegetation dynamics in a poplar bioenergy plantation

In this study the seasonal variation in carbon, water and energy fluxes as well as in net primary productivity (NPP) of different tree components is presented for a 2‐year‐old poplar (Populus spp.) plantation. A thorough ecophysiological study was performed at ecosystem scale, at tree and at leaf level, in this high‐density bioenergy plantation. Seasonal variation in NPP and fluxes was analysed in relation to meteorological parameters at the field site. The growing season length in terms of carbon uptake was controlled by leaf area development until the maximum leaf area index (LAI_max) was reached. Afterwards, a shift to belowground carbon allocation was observed. A dry period in spring caused a reduced leaf area production as well as a decrease in net ecosystem exchange and gross primary production (GPP) due to stomatal closure. Water use efficiency and fine root growth increased in response to limiting soil water availability in the root zone. When soil water availability was not limiting, GPP was controlled by a decrease in solar radiation and air temperature. The results of this study indicate that the productivity of recently established bioenergy plantations with fast‐growing trees is very sensitive to drought. The interaction between soil water availability and factors controlling ecosystem GPP is crucial in assessing the CO₂ mitigation potential under future climate conditions.     

PHOTOSYNTHESIS AND CARBON ALLOCATION IN TIPULARIA DISCOLOR (ORCHIDACEAE), A WINTERGREEN UNDERSTORY HERB

Seasonal patterns of photosynthesis and carbon allocation were determined for Tipularia discolor, a summer-deciduous wintergreen orchid of the southeastern United States, to assess the effects of environmental conditions and leaf age on carbon acquisition and allocation patterns. There was no shift in the optimum temperature for photosynthesis (T_opt) on a seasonal basis and T_opt (≈26 C) was at least 10 C higher than daily maximum air temperature during most of the growing season. Lack of photosynthetic adjustment in Tipularia to seasonal fluctuations in temperature and light suggested that the photosynthetic characteristics of this wintergreen were more similar to those of spring ephemerals than to those of evergreens and summer-active herbs. The decline in photosynthetic capacity during the winter growing season for Tipularia, largely due to leaf age effects, gradually reduced net photosynthetic rates in the field despite more favorable light and temperature conditions. Photosynthesis in the field was primarily limited by environmental conditions in early- and mid-season and by photosynthetic capacity in late-season. A ¹⁴CO₂ labelling experiment demonstrated that patterns of carbon allocation to vegetative structures were affected by the season of photosynthetic carbon fixation, whereas reproductive structures received 21% of the recovered labelled carbon regardless of the period of labelling. Carbon acquired and stored during all periods of the growing season was used to produce new vegetative and reproductive structures.     

Water relations of epiphytic and terrestrially-rooted strangler figs in a Venezuelan palm savanna

Water use patterns of two species of strangler fig, Ficus pertusa and F. trigonata, growing in a Venezuelan palm savanna were contrasted in terms of growth phase (epiphyte and tree) and season (dry and wet). The study was motivated by the question of how C3 hemiepiphytes accommodate the marked change in rooting environment associated with a life history of epiphytic establishment followed by substantial root development in the soil. During the dry season, stomatal opening in epiphytic plants occurred only during the early morning, maximum stomatal conductances were 5 to 10-fold lower, and midday leaf water potentials were 0.5–0.8 MPa higher (less negative) than in conspecific trees. Watering epiphytes of F. pertusa during the dry season led to stomatal conductances comparable to those exhibited by conspecific trees, but midday leaf water potentials were unchanged. During the rainy season, epiphytes had lower stomatal conductances than conspecific trees, but leaf water potentials were similar between the two growth phases. There were no differences in ¹³C between the two growth phases for leaves produced in either season. Substrate water availability differed between growth phases; tree roots extended down to the permanent water table, while roots of epiphytic plants were restricted to material accumulated behind the persistent leaf bases of their host palm tree, Copernicia tectorum. Epiphytic substrate moisture contents were variable during both seasons, indicating both the availability of some moisture during the dry season and the possibility of intermittent depletion during the rainy season. Epiphytic strangler figs appear to rely on a combination of strong stomatal control, maintenance of high leaf water potentials, and perhaps some degree of stem water storage to cope with the fluctuating water regime of the epiphytic environment.     

SEASONAL PATTERN OF NET PHOTOSYNTHESIS OF ARTEMISIA TRIDENTATA

Gas exchange studies were carried out on Artemisia tridentata during the course of a growing season using microclimatically controlled cuvettes and infrared gas analysis. A definite seasonal pattern of net photosynthesis emerged. This pattern was influenced by the interaction of four major factors: plant water potential, leaf temperature, irradiation, and stage of phenological development. In spring and early summer, when plant water stress was minimal, photosynthesis rate was mainly correlated with leaf temperature and irradiation. During mid and late summer, increased plant water stress and phenological changes assumed at least equal importance with temperature and irradiation in limiting net photosynthesis. Indeed, plant water potential, mainly through its influence on stomatal aperture, r_s‘, was probably the single most important factor influencing assimilation rate of this species on a seasonal basis. However, variations in mesophyll resistance to CO₂ flux, r_m‘, in response to temperature, water stress, or phenological changes also were involved. Sagebrush photosynthesis under field conditions was highest in late May and early June, and declined thereafter, minimum rates occurring in August during the driest period. Optimal temperatures for net photosynthesis were higher later in the season, indicating a change in gas exchange capacity more suitable to the warmer temperatures later in the season.     

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short‐rotation bio‐energy culture

The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E_c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E_c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.     

Relationship between photochemical reflectance index and leaf ecophysiological and biochemical parameters under two different water statuses: towards a rapid and efficient correction method using real‐time measurements

The use of the photochemical reflectance index (PRI) as a promising proxy of light use efficiency (LUE) has been extensively studied, and some issues have been identified, notably the sensitivity of PRI to leaf pigment composition and the variability in PRI response to LUE because of stress. In this study, we introduce a method that enables us to track the short‐term PRI response to LUE changes because of photosynthetically active radiation (PAR) changes. The analysis of these short‐term relationships between PRI and LUE throughout the growing season in two species (Quercus robur L. and Fagus sylvatica L.) under two different soil water statuses showed a clear change in PRI response to LUE, which is related to leaf pigment content. The use of an estimated or approximated PRI0, defined as the PRI of perfectly dark‐adapted leaves, allowed us to separate the PRI variability due to leaf pigment content changes and the physiologically related PRI variability over both daily (PAR‐related) and seasonal (soil water content‐related) scales. The corrected PRI obtained by subtracting PRI0 from the PRI measurements showed a good correlation with the LUE over both of the species, soil water statuses and over the entire growing season.     

刺槐林水分生态研究

 本文以定位观测资料为基础,对晋西北黄土丘陵沟壑区人工刺槐林的水分生态特点进行了研究。结果表明,在年生长期中,刺槐林地水分条件严酷、土壤有效水分少。生长期刺槐的蒸腾强度为0.1140g·g-1·h-1±0.0586(平均值±标准差);蒸腾强度日变化与光照强度日变化的关系最为明显;与气温日变化的关系次之;但与相对湿度日变化的关系则不明显。生长期刺槐的水势为1.759-Mpa±0.457(平均值±标准差)。水势的日变化与气温日变化的关系最为明显,与相对湿度日变化的关系次之;而与光照强度日变化的关系最不明显。刺槐的蒸腾强度季节变化与土壤水分含量季节变化的关系不够显著;水势的季节变化与土壤水分含量季节变化的关系显著;但它们的季节变化与光照强度、气温和相对湿度季节变化的关系均不够显著。刺槐的蒸腾耗水量,林分总蒸散量分别占同期降水量的16.3%和68.0%,     

Effects of elevated CO2 and temperature on leaf characteristics,photosynthesis and carbon storage in aboveground biomass of a boreal bioenergy crop (Phalaris arundinacea L.) under varying water regimes

This work examined the effects of elevated CO₂ and temperature and water regimes, alone and in interaction, on the leaf characteristics [leaf area (LA), specific leaf weight (SLW), leaf nitrogen content (N_L) based on LA], photosynthesis (light‐saturated net carbon fixation rate, P_sat) and carbon storage in aboveground biomass of leaves (C_l) and stem (C_s) for a perennial reed canary grass (Phalaris arundinacea L., Finnish local cultivar). For this purpose, plants were grown under different water regimes (ranging from high to low soil moisture) in climate‐controlled growth chambers under the elevated CO₂ and/or temperature (following a factorial design) over a whole growing season (May–September in 2009). The results showed that the elevated temperature increased the leaf growth, photosynthesis and carbon storage of aboveground biomass the most in the early growing periods, compared with ambient temperature. However, the plant growth declined rapidly thereafter with a lower carbon storage at the end of growing season. This was related to the accelerated phenology regulation and consequent earlier growth senescence. Consequently, the elevation of CO₂ increased the P_sat, LA and SLW during the growing season, with a significant concurrent increase in the carbon storage in aboveground biomass. Low soil moisture decreased the P_sat, leaf stomatal conductance, LA and carbon storage in above ground biomass compared with high and normal soil moisture. This water stress effect was the largest under the elevated temperature. The elevated CO₂ partially mitigated the adverse effects of high temperature and low soil moisture. However, the combination of elevated temperature and CO₂ did not significantly increase the carbon storage in aboveground biomass of the plants.