大气CO2浓度和温度升高对水稻叶片及群体光合作用的影响

大气ＣＯ２浓度升高对植物光合作用的影响研究多集中在单叶水平,在高ＣＯ２及高温下对植物单叶及群体光合进行比较的研究少有报道,而群体水平的研究则是预测生态系统反应所不可缺少的。采用田间开顶式培养室研究了大气ＣＯ２浓度和温度升高对水稻（ＯｒｙｚａｓａｔｉｖａＬ．）叶片及群体光合作用的影响。发现ＣＯ２浓度和温度对水稻叶片光合作用有协同促进作用,而对群体光合作用的促进则随时间的推移而减弱;单叶光合受到的促进作用大于群体光合;叶面积指数只在营养生长期受到促进,冠层叶片含氮量受ＣＯ２影响降低。群体呼吸（包括茎杆）增加及冠层叶片早衰可能是后期ＣＯ２对群体光合促进作用下降的原因。     

祁连山中部优势灌木物种叶功能性状参数变化特征

植物叶功能性状是连接植物与外界环境的重要桥梁,是探索植物适应环境、进行全球气候变化研究的指标,也是生态水文模型重要的输入参数。通过高时间分辨率样品的采集结合图像处理技术,对祁连山中部优势物种金露梅(Dasiphora fruticosa)和鬼箭锦鸡儿(Caragana jubata)叶功能性状变化特征进行研究,结果表明：叶面积(LA)从生长初期到生长末期发生明显变化,且2020年6—7月初灌木叶面积变化最为明显;比叶面积(SLA)分别在76—157 cm²/g之间和120—217 cm²/g之间,金露梅SLA平均值(123 cm²/g)<鬼箭锦锦鸡儿SLA平均值(176 cm²/g);在生长初期金露梅的叶面积指数(LAI)明显大于鬼箭锦鸡儿,金露梅和鬼箭锦鸡儿的叶面积指数随时间的变化都符合三次函数;生物量与基径和高度的乘积呈显著性相关,最优回归模型均为幂函数;地上生物量与叶功能性状参数在不同阶段的模拟方程均不同,地上总生物量与叶生物量在生长期的最优选择模型分别呈S型函数和幂函数,金露梅的单株地...     

Abundance and stratification of foliage arthropods in a lowland rain forest of Cameroon

Abstract.

• 1 Arthropod densities and apparent leaf damage were compared within the canopy ecotone and the shrub layer of a lowland rain forest in Cameroon, using a branch clipping method.
• 2 Most of the individuals collected consisted of ants (average 44%) and various herbivores (31%). Overall arthropod densities amounted to 17 individuals per sample, which, on average, consisted of 0.85 m² of foliage area. Arthropod densities were lower than on temperate foliage.
• 3 Arthropod densities were about 3 times higher in the canopy than within the shrub layer. In particular, ants and herbivores were significantly more abundant in the canopy than within the shrub layer. Usually, layer effects rather than site effects appeared to cause greater variance in arthropod abundance.
• 4 Arthropod species-richness, as estimated by the number of operational taxonomic units sorted, was higher in canopy samples than in samples obtained from the shrub layer. However, apparent leaf damage was higher within the shrub layer (10.9%) than on the canopy (5.2%).
• 5 Possible factors responsible for the high densities of ants and herbivores on the canopy and for the high leaf damage within the shrub layer are discussed.

     

Light partitioning among species and species replacement in early successional grasslands

Abstract. We studied canopy structure, shoot architecture and light harvesting efficiencies of the species (photon flux captured per unit above‐ground plant mass) in a series of exclosures of different age (up to 4.5 yr) in originally heavily grazed grassland in N Japan.Vegetation height and Leaf Area Index (LAI) increased in the series and Zoysia japonica, the dominant in the beginning, was replaced by the much taller Miscanthus sinensis. We showed how this displacement in dominance can be explained by inherent constraints on the above‐ground architecture of these two species. In all stands light capture of plants increased with their above‐ground biomass but taller species were not necessarily more efficient in light harvesting. Some subordinate species grew disproportionally large leaf areas and persisted in the shady undergrowth. Some other species first grew taller and managed to stay in the better‐lit parts of the canopy, but ultimately failed to match the height growth of their neighbours in this early successional series. Their light harvesting efficiencies declined and this probably led to their exclusion. By contrast, species that maintained their position high in the canopy managed to persist in the vegetation despite their relatively low light harvesting efficiencies. In the tallest stands ‘later successional’ species had higher light harvesting efficiencies for the same plant height than ‘early successional’ species which was mostly the result of the greater area to mass ratio (specific leaf area, SLA) of their leaves. This shows how plant stature, plasticity in above‐ground biomass partitioning, and architectural constraints determine the ability of plants to efficiently capture light, which helps to explain species replacement in this early successional series.     

Effects of Elevated CO2 and High Temperature on Single Leaf and Canopy Photosynthesis of Rice

The increase of atmospheric CO2 concentration is indisputable. In such condition, photosynthetic response of leaf is relatively well studied, while the comparison of that between single leaf and whole canopy is less emphasized. The stimulation of elevated CO2 on canopy photosynthesis may be different from that on single leaf level. In this study, leaf and canopy photosynthesis of rice ( Oryza sativa L. ) were studied throughout the growing season. High CO2 and temperature had a synergetic stimulation on single leaf photosynthetic rate until grain filling. Photosynthesis of leaf was stimulated by high CO2, although the stimulation was decreased by higher temperature at grain filling stage. On the other hand, the simulation of elevated CO2 on canopy photosynthesis leveled off with time. Stimulation at canopy level disappeared by grain filling stage in beth temperature treatments. Green leaf area index was not significantly affected by CO2 at maturity, but greater in plants grown at higher temperature. Leaf nitrogen content decreased with the increase of CO2 concentration although it was not statistically significant at maturity. Canopy respiration rate increased at flowering stage indicating higher carbon loss. Shading effect caused by leaf development reached maximum at flowering stage. The CO2 stimulation on photosynthesis was greater in single leaf than in canopy. Since enhanced CO2 significantly increased biomass of rice stems and panicles, increase in canopy respiration caused diminishment of CO2 stimulation in canopy net photosynthesis, keaf nitrogen in the canopy level decreased with CO2 concentration and may eventually hasten CO2 stimulation on canopy photosynthesis. Early senescence of canopy leaves in high CO2 is also a possible cause.     

A Study on Selected Physiological Parameters of Plants Grown Under Lithium Supplementation

Exposure of sunflower and maize plants to increasing concentrations of lithium (0?C50?mg Li dm^?3) in a nutrient solution induced changes in biomass, leaf area and photosynthetic pigment accumulation, as well as levels of lipid peroxidation. The highest applied lithium dose (50?mg Li dm^?3) evoked a significant reduction in the shoot biomass for both examined species, as well as necrotic spots and a reduction of the leaf area in sunflower plants. An enrichment of a nutrient solution with 5?C50?mg Li dm^?3 did not significantly affect chlorophylls a and b and the carotenoid content in sunflower plants. However, in maize, a significant decrease in all pigment content under highest used lithium concentration was noted. The levels of lipid peroxidation of the cell membranes in leaves of sunflower plants and the roots of maize increased significantly in the presence of 50?mg Li dm^?3, which suggests disturbances of the membrane integrity and pro-oxidant properties of the excess lithium ions. Nonetheless, in maize, an increase of shoot biomass and leaf area in the presence of 5?mg Li dm^?3 was found. An analysis of the metal content indicated that lithium accumulated significantly in sunflower and maize shoots in a dose-dependent manner, but differences occurred between species. The sunflower plants accumulated considerably greater amounts of this metal than maize. The potassium content in shoots remained unchanged under lithium treatments, except for a significant increase in the potassium levels for sunflower plants grown in the presence of 50?mg Li dm^?3. These results suggest that lithium at 50?mg Li dm^?3 is toxic to both plant species, but the symptoms of toxicity are species-specific. Moreover, the lithium influence on plants is dose-dependent and its ions can exert toxicity at high concentrations (50?mg Li dm^?3) or stimulate growth at low concentrations (5?mg Li dm^?3).     

土壤结构与光照水平对秋茄某些生长和生理参数的影响

在香港的３个红树森样地即黄竹湾（沙土）、西径（沙壤土）和米埔（粘壤土）进行了土壤结构对秋茄（Kandelia candel(L.)Durce)生长和生理影响的研究,并在米埔比较了林内和林外秋茄幼苗的生长和生理参数以观察光照水平的效应。在沙土和沙壤土生长的１．５年秋茄幼苗比粘壤土具有较粗的基径的较高的总生物量,说明秋茄幼苗在沙土和沙壤土中比在粘壤土中生长更好。沙土１．５ 茄幼苗的叶片厚度分别为沙壤土和粘壤土的１．７５和２．０５倍,表明沙土中的秋茄幼苗具有旱生结构以维持体内水分。然而,沙土和沙壤土４．５年秋茄幼树的叶片厚度无显著差异,沙土和沙壤土中１．５年秋茄幼苗分配于根系的生物量比例约为５０％,高于粘壤土的值（约４０％）。沙土和沙壤土中１．５年的秋茄比粘壤土具有较低的叶绿素含量、根系活力、硝酸盐还原酶活性、过氧化物酶（ＰＯＸ）活性、超氧化物歧化酶（ＳＯＤ）活性及较高的丙二醛（ＭＤＡ）含量。米埔１．５年秋茄幼苗在红树林外比林内有更好的长势,具有更大的叶面积、特殊叶面积、叶片数量及生物量。林内幼苗具有较高叶绿素含量,较低叶绿素a/b比值,较高硝酸盐还原酶活性和较强的根系活力,林外幼苗的叶片ＰＯＸ和ＳＯＤ活性比林内的值稍高,ＭＤＡ含量比林内显著要高。     

Seasonal changes of fine root density in the Southern Californian chaparral

Summary Fine root extractions from soil cores of a south facing slope in the Southern Californian chaparral were used to study the dynamics of feeder root growth in a summer-dry area. The studies were concentrated on the root systems of Adenostoma fasciculatum, Arctostaphylos glauca, Ceanothus greggii, and Rhus ovata. The total fine root biomass of Adenostoma fasciculatum increased from 0.6 g dm^-3 in early spring to 3.6 g dm^-3 in late summer. Considering the specific soil conditions at this site and earlier gained information on fine root distribution with depth, the value of 3.6 g dm^-3 converts to 1.58 kg m^-2 of ground shaded by the shrub canopy. The observed seasonal biomass increase is mainly due to the accumulation of dead root material in the soil when low soil moisture contents presumably inhibited decomposition processes. The total length of living fine roots also increased during the season, e.g. from 0.8 m dm^-3 to more than 5 m dm^-3 (0.35 km m^-2 to 2.2 km m^-2) in A. fasciculatum. Unusual summer rains in the research year stimulated vigorous fine root growth at a time when the normally low soil moisture would prohibit further fine root growth. The average fine root diameters and total lengths of fine roots beneath one square meter of ground surface allowed an estimate of root area indices (RAI) analogous to the leaf area indices (LAI). The data provide evidence for a significant fine root turnover in the chaparral.     

Canopy structure within a Quercus ilex forested watershed: variations due to location,phenological development,and water availability

Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m² m^–2 at the ridge top, and 5.3 m² m^–2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.     

Leaf litterfall, fine-root production, and decomposition in shrublands with different canopy structure induced by grazing in the Patagonian Monte, Argentina

Selective sheep grazing in the Patagonian Monte induces the reduction of total and perennial grass cover, species replacement within life forms, and the increase in dominance of long-lived evergreen woody plants with slow growth rates and high concentration of secondary compounds in leaves. We hypothesized that these changes in the canopy structure induced by sheep grazing will affect the mass, chemistry and decomposability of leaf litter and fine roots. We selected two sites in the Patagonian Monte, representative of ungrazed and grazed vegetation states. At each site, we assessed canopy structure (total cover and absolute and relative grass and shrub cover), monthly leaf litterfall, and fine-root biomass and production in the upper soil (15 cm). We also estimated the rates of mass, C, soluble phenolics, lignin and N decay in litterbags containing both leaf litter and fine roots of each site under field conditions during two consecutive years. The ungrazed site exhibited higher total plant cover, absolute and relative grass- and shrub-cover than the grazed one. Leaf litterfall was lower at the grazed site than at the ungrazed site. Fine-root production did not vary between sites. Leaf litter and fine root tissues had higher concentration of secondary compounds at the grazed than at the ungrazed site. However, fine roots showed lower mass and C decay than leaf litter, attributable to the predominant secondary compound (lignin and soluble phenolics, respectively). Leaf litter decomposed slower but released more N during decay at the ungrazed than at the grazed site, probably due to its low concentration of secondary compounds. We concluded that changes in canopy structure induced by grazing disturbance such as those explored in our study could reduce leaf litterfall mass and increase the concentration of secondary compounds of both leaf litter and fine roots leading to slow N release to soil during decay.     

ANATOMICAL AND PHYSIOLOGICAL ACCLIMATION OF FATSIA JAPONICA LEAVES TO IRRADIANCE

Anatomical and physiological leaf characteristics and biomass production of Fatsia japonica plants were studied. Plants were grown in a growth chamber at 300 μmol m^-2 s^-1 (high light) and 50 μmol m^-2 s^-1 (low light) photosynthetic photon flux density. Plants grown under high light showed a net maximum photosynthetic rate 44% higher than plants grown under low light; the light compensation point and the light saturation point were also higher in high-light plants. Photosynthetic oxygen evolution in isolated chloroplasts was about 40% higher in high-light plants. However, chlorophyll content on a dry weight basis, on a leaf area basis, and per chloroplast was greater in plants grown under low light. Leaf thickness in high-light plants was 13% higher than in low-light plants. The number of chloroplasts was 30% higher in high-light leaves, while chloroplast size was only slightly higher. Chloroplast ultrastructure was also affected by light. Leaf dry weight, leaf area, and biomass production per plant were drastically reduced under low light. Thus, F. japonica is a plant that is able to acclimate to different photosynthetic photon flux density by altering its anatomical and physiological characteristics. However, low-light acclimation of this plant has a considerable limiting effect on biomass production.     

Biomass estimation models and allocation patterns of 14 shrub species in Mountain Luya,Shanxi, China

Aims Shrub species have evolved specific strategies to regulate biomass allocation among various organs or between above- and belowground biomass and shrub biomass model is an important approach to estimate biomass allocation among different shrub species. This study was designed to establish the optimal estimation models for each organ (leaf, stem, and root), aboveground and total biomass of 14 common shrub species in Mountain Luya, Shanxi Province, China. Furthermore, we explored biomass allocation characteristics of these shrub species by using the index of leaf biomass fraction (leaf to total biomass), stem biomass fraction (stem to total biomass), root biomass fraction (root to total biomass), and root to shoot mass ratio (R/S) (belowground to aboveground biomass).
Methods We used plant height, basal diameter, canopy diameter and their combination as variables to establish the optimal biomass estimation models for each shrub species. In addition, we used the ratios of leaf, stem, root to total biomass, and belowground to aboveground biomass to explore the difference of biomass allocation patterns of 14 shrub species.
Important findings Most of biomass estimation models could be well expressed by the exponential and linear functions. Biomass for shorter shrub species with more stems could be better estimated by canopy area; biomass for taller shrub species with less stems could be better estimated by the sum of the square of total base diameter multiply stem height; and biomass for the rest shrub species could be better estimated by canopy volume. The averaged value for these shrub species was 0.61, 0.17, 0.48, and 0.35 for R/S, leaf biomass fraction, stem biomass fraction, and root biomass fraction, respectively. Except for leaf biomass fraction, R/S, stem biomass fraction, and root biomass fraction for shrubs with thorn was significantly greater than that for shrubs without thorn.     

Leaf life span of floating-leaved plants

Photosynthetic capacity of floating-leaved plants is relatively high comparable with terrestrial herbaceous plants, though floating-leaved plants have a much smaller biomass with a leaf area index seldom exceeding 2m²m^-2. Their rather small biomass accumulation is related to higher turnover of leaf biomass or shorter leaf life span. Life span of floating leaves reported in the literature ranged mostly from 13 to 35 days, shorter than that of any other groups of herbaceous macrophytes. Floating-leaved plants are known to show considerably high plasticity in their leaf form. Leaf life span could be prolonged for Nymphoides peltata (Gmel.) O. Kuntze grown in a terrestrial environment and for emergent leaves of Nelumbo nucifera Gaertn. Their short leaf life span seems to be closely related to the fact that old leaves covered by newly formed ones are inevitably compelled to be submerged and lose their function as a photosynthetic apparatus.Abbreviations LAI leaf area index - PFD photosynthetic photon flux density     

贵阳市区灌木林生态系统生物量及碳储量

采用直接收获法和实测数据,以贵州省贵阳市区天然灌木林内木本和草本植物、凋落物及土壤为研究对象,研究了灌木林生态系统的生物量、碳含量及碳储量。结果表明:灌木林植被层生物量为23.16 t/hm2,其中木本植物层生物量为12.46 t/hm2;草本植物层为3.74 t/hm2;凋落物层为6.96 t/hm2,分别占植被层生物量的53.08%、16.15%、30.05%。木本植物25种的碳含量范围为445.91—603.46 g/kg;草本植物6种的碳含量为408.48—523.04 g/kg;凋落物层碳含量为341.01—392.81 g/kg;土壤层碳含量为5.73—26.68 g/kg。生态系统总碳储量为88.34 t/hm2,其中植被层为8.10 t/hm2;凋落物层为2.56 t/hm2;土壤层为77.68 t/hm2,分别占系统总碳储量的9.17%、2.89%、87.94%。灌木林生态系统碳储量的空间分布格局为:土壤层植被层凋落物层。研究结果,可为喀斯特城市估算森林生态系统碳储量和碳平衡提供科学依据。     

Effect of springtime solar ultraviolet-B radiation on growth of Colobanthus quitensis at Palmer Station, Antarctica

We examined the influence of solar ultraviolet‐B radiation (UV‐B; 280–315 nm) on the growth of Colobanthus quitensis plants by placing them under contrasting UV‐B filters at Palmer Station, along the Antarctic Peninsula. The filters reduced diurnal biologically effective UV‐B (UV‐B_BE) either by 83% (‘reduced UV‐B’) or by 12% (‘near‐ambient UV‐B’) over the 63 day experiment (7 November 1998–8 January 1999). Ozone column depletion averaged 17% during the experiment. Relative growth and net assimilation rates of plants exposed to near‐ambient UV‐B were 30 and 20% lower, respectively, than those of plants exposed to reduced UV‐B. The former plants produced 29% less total biomass, as a result of containing 54% less aboveground biomass. These reductions in aboveground biomass were mainly the result of a 45% reduction in shoot biomass, and a 31% reduction in reproductive biomass. Reductions in shoot biomass were owing to an 18% reduction in branch production by main shoots, while reductions in reproductive biomass were the result of a 19% reduction in individual capsule mass. Total plant leaf area was reduced by 19% under near‐ambient UV‐B, although total leaf biomass was unaffected because leaves had a greater specific leaf mass. The reduction in plant leaf area under near‐ambient UV‐B was attributable to: (1) production of 11% fewer leaves per main shoot system and plant, which resulted from an 18% reduction in branch production by main shoots. Leaf production per individual main shoot or branch was not affected; (2) shorter leaf longevity—main shoots contained 14% fewer green leaves at a given time; and (3) smaller individual leaves—leaf elongation rates were 14% slower and mature leaves were 13% shorter.     

树冠位置对香樟叶形态性状和异速生长关系的影响

为深入了解树冠位置对植物叶形态性状的影响,在常绿乔木香樟树冠上下2层和东南西北4个方位开展调查取样,系统分析了不同树冠位置间叶形态性状(叶长、叶宽、叶厚、叶柄长、叶柄直径和叶形指数)及其异速生长关系的差异性。结果表明,叶形态性状在不同树冠方位间均差异显著,但上下2层变化趋势不完全一致。在树冠上层,除叶形指数和叶炳长外,其余4个性状均表现为东侧最大。在树冠下层,除叶形指数外,其余5个性状指标均表现为东侧最小。在同一方位上,叶形态性状在上下2层间也存在一定差异,其中叶形指数多为下层高于上层,而其他形态性状多呈相反趋势。此外,树冠层次和方位的交互作用对叶片长、叶片厚、叶柄长和叶柄直径有显著影响。各层次和各方位叶形态性状间多为异速生长关系(即异速生长指数不等于1),且多无显著差异。在所有树冠层次和树冠方位,叶宽与叶厚、叶宽与叶炳长、叶长与叶厚及叶长与叶柄长之间均呈异速生长关系。可见,树冠位置对香樟叶形态性状的影响较大,但形态性状间的异速生长关系相对稳定,这是香樟叶形态性状表型可塑性和内在关系稳定性的重要体现。     

Variation in leaf nitrogen and phosphorus stoichiometry in the nitrogen-fixing Chinese sea-buckthorn (Hippophae rhamnoides L. subsp. sinensis Rousi) across northern China

Nitrogen (N) and phosphorus (P) concentrations and N:P ratios in terrestrial plants and their patterns of change along environmental gradients are important traits for plant adaptation to changes. We determined the leaf N and P concentrations of Chinese sea-buckthorn (Hippophae rhamnoides L. subsp. sinensis Rousi), a non-legume species with symbiotic N fixation (SNF), at 37 sites across northern China and explored their geographical patterns in relation to climate and soil factors. (1) The mean leaf N, P, and N:P ratio were 36.5, 2.1 mg g^?1, and 17.6, respectively, higher than the mean values of most shrub species in the region. (2) Leaf N was correlated with soil mineral N in cool areas (mean annual temperature MAT <3 °C) but with temperature in warm areas (MAT >3 °C). The high leaf N and divergent leaf N–soil N relationship suggested the importance of SNF in plant N uptake; SNF increases with temperature and is probably the major N source in warm areas. (3) Leaf P was positively related to mean annual precipitation. Leaf N:P ratio was primarily driven by changes in leaf P. The high leaf P reflected the greater requirements of the N-fixing species for P. Our results represent a major advance in understanding the elemental stoichiometry of non-legume N-fixing plants, indicating high P and N requirements and a shift in N source from SNF to soil as temperature declines. This knowledge will help in assessing the habitat suitability for the species and predicting the species dynamics under environmental changes.     

Variation in leaf longevity of Pistacia lentiscus and its relationship to sex and drought stress inferred from leaf δ13C

1. Leaf formation, loss, retention, longevity and biomass on male branches of the evergreen mediterranean shrub Pistacia lentiscus , L. correlated strongly with water-use efficiency inferred from leaf δ¹³C across a gradient of precipitation on the island of Mallorca, Spain.
2. The correlations suggest that the leaf phenology is under control of drought-induced constraints on the carbon balance.
3. In fruiting female branches, the correlations between the inferred water-use efficiency and number of formed and retained leaves, leaf biomass and leaf longevity were non-significant. Leaf formation was strongly reduced by fruiting and the females compensated the reduced photosynthetic capacity by retaining older leaves for a longer time than male plants.
4. It is suggested that leaf longevity in females is under strong control of resource allocation to fruit formation which is 'overlaid' on the drought-induced carbon stress, which led to the observed longer leaf longevity in females than in males.     

Tundra plant structure and production in relation to the environment

The alpine and polar climatic limit for growth of woody plants is very much dependent on the mean temperatures of the warmest three or four summer months. Tundra plants with perennating buds close to the ground are sheltered by insulating snow cover. Many tundra plants can grow at temperatures 5–10°C below 0°C and also have low optimum temperatures. Total net production of tundra plants may be as high as 900 g/m²/yr as dry weight in moist and eutrophic low alpine shrub tundra and in antarctic moss mats. The variation in tundra plant production is often observed to be greater between different stands (communities) within one locality than between localities, because of very important variation in soil moisture and nutrients between the stands. On a global scale the biomass of vascular plants increases by an order of magnitude from the climatic severe polar desert to semidesert and again from there to moist shrub tundra. The cryptogam biomass increases only 2–10 fold from polar desert to low arctic shrub tundra. To a certain limit unfavourable climatic conditions are worse to above- than to belowground plant parts. Highest root biomass compared to top (up to 20 times higher) is observed in wet monocotyledonous polar and alpine communities. In polar desert root biomass is small again, as compared to tops and also in lower latitudes and altitudes of temperate regions.Presented at he Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Ecophysiological studies of Sonoran Desert plants

Summary The gas exchange and water relations of two Sonoran Desert plants were measured throughout a 12 month period. Seasonal photosynthesis patterns of both species followed the seasonal variation in soil and plant water potential. Acacia greggii, a winter deciduous shrub, appears to be fall active since the day-long mean photosynthesis rate was maximum, i.e., 16 mg CO₂ dm^-2 h^-1, at this time. Cercidium microphyllum, a drought deciduous and chlorophyllous-stemmed tree, also appears to be mainly fall active. For this species the day-long mean photosynthesis rate was not in excess of 14 mg CO₂ dm^-2 h^-1. Both species initiate leaf production in the spring and neither experiences severe plant water stress. Seasonal minima of dawn plant water potential were-44 and-32 bars for the shrub and tree species, respectively. The two species differ slightly in their tolerance of heat and water stress, since foliated plants of Acacia greggii maintain summer gross photosynthesis.All of the aboveground organs for plants of C. microphyllum are capable of exogenous ¹⁴CO₂ assimilation. This species appears to be unique in the magnitude of the photosynthetic production contributed by stems. Seasonal production by stems, leaves and flowers/fruits averaged 72, 24 and 4% of the total carbon gain per tree, respectively. Aboveground gross primary production was over 4.5-fold greater than aboveground net primary production. This difference between these two production estimates is likely due to the very small foliar biomass maintained throughout the year and the energy expenditure required to maintain the metabolically active cells of the chlorophyllous stems.