Postfire energy exchange in arctic tundra: the importance and climatic implications of burn severity

Fires produce land cover changes that have consequences for surface energy balance and temperature. Three eddy covariance towers were setup along a burn severity gradient (i.e. Severely, Moderately, and Unburned tundra) to determine the effect of fire and burn severity on arctic tundra surface energy exchange and temperature for three growing seasons (2008–2010) following the 2007 Anaktuvuk River fire. The three sites were well matched before the fire, experienced similar weather, and had similar energy budget closure, indicating that the measured energy exchange differences between sites were largely attributable to burn severity. Increased burn severity resulted in decreased vegetation and moss cover, organic layer depth, and the rate of postfire vegetation recovery. Albedo and surface greenness steadily recovered with Moderately matching Unburned tundra by the third growing season. Decreased albedo increased net radiation and partly fueled increased latent and ground heat fluxes, soil temperatures, and thaw depth. Decreases in moss cover and the organic layer also influenced the ground thermal regime and increased latent heat fluxes. These changes either offset or decreased the surface warming effect from decreased albedo, resulting in a small surface warming in Severely and a small surface cooling in Moderately relative to Unburned tundra. These results indicate that fires have a significant impact on surface energy balance and highlight the importance of moss and permafrost thaw in regulating arctic surface energy exchange and temperature.     

Heat production and body temperature during cooling and rewarming in overweight and lean men

Objective: To compare overweight and lean subjects with respect to thermogenesis and physiological insulation in response to mild cold and rewarming. Research Methods and Procedures: Ten overweight men (mean BMI, 29.2 ± 2.8 kg/m²) and 10 lean men (mean BMI, 21.1 ± 2.0 kg/m²) were exposed to cold air for 1 hour, followed by 1 hour of rewarming. Body composition was determined by hydrodensitometry and deuterium dilution. Heat production and body temperatures were measured continuously by indirect calorimetry and thermistors, respectively. Muscle activity was recorded using electromyography. Results: In both groups, heat production increased significantly during cooling (lean, p = 0.004; overweight, p = 0.006). The increase was larger in the lean group compared with the overweight group (p = 0.04). During rewarming, heat production returned to baseline in the overweight group and stayed higher compared with baseline in the lean group (p = 0.003). The difference in heat production between rewarming and baseline was larger in the lean (p = 0.01) than in the overweight subjects. Weighted body temperature of both groups decreased during cold exposure (lean, p = 0.002; overweight, p < 0.001) and did not return to baseline during rewarming. Discussion: Overweight subjects showed a blunted mild cold‐induced thermogenesis. The insulative cold response was not different among the groups. The energy‐efficient response of the overweight subjects can have consequences for energy balance in the long term. The results support the concept of a dynamic heat regulation model instead of temperature regulation around a fixed set point.     

Use of Shrubs as Nurse Plants: A New Technique for Reforestation in Mediterranean Mountains

Common techniques currently used for afforestation in the Mediterranean basin consider the pre‐existing vegetation (mainly shrubs) as a source of competition for trees, and consequently it is generally eliminated before planting. Nevertheless, it has been demonstrated that woody plants can facilitate the establishment of understory seedlings in environments that, like the Mediterranean area, are characterized by a pronounced dry season. In this study, we experimentally analyze the usefulness of shrubs as nurse plants for afforestation of two native conifers, Pinus sylvestris L. (Scots pine) and Pinus nigra Arnold (black pine). Two‐year‐old seedlings were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method used in afforestation programs), (2) under individuals of Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Pine survival was remarkably higher when planted under individuals of the shrub S. lavandulifolia (54.8% for Scots pine, 81.9% for black pine) compared with open areas (21.5% for Scots pine, 56.8% for black pine; chi square, p < 0.05). The survival of both pines was also higher when planted on the north side of spiny shrubs, although the survival on the south side was similar to that found in open areas. In addition, pine growth was not inhibited when planted in association with shrubs. This pattern appears to result from the combination of abiotic conditions imposed by the presence of a nurse shrub, which leads to improvement in seedling water status and therefore reduced summer mortality by drought. The results show that the use of shrubs as nurse plants is a technique that offers both economic and ecological advantages, in terms of savings in labor and plant material and reduced and even negligible impact on the pre‐existing vegetation.     

内蒙古温带荒漠草原生长季地表反射率特征及数值模拟

基于内蒙古苏尼特左旗温带荒漠草原生态系统野外观测站2008年生长季(5月1日-10月15日)的气象和生物要素观测资料,分析了地表反射率日、季动态,并构建了温带荒漠草原地表反射率模型.结果表明：研究区地表反射率日变化主要受太阳高度角影响,呈早晚高、中午低的U形曲线特征;生长季地表反射率在0.20～0.34,平均为0.25,以5月较高、6月下降、7-9月相对稳定、10月增大;研究区地表反射率的季节动态与冠层叶片的物候变化有关,同时受降水过程的影响;土壤含水量和叶面积指数是影响该区地表反射率的关键因子;反映土壤含水量和叶面积指数共同作用的地表反射率模型的模拟值与野外实测值具有较好的一致性.     

黄土高原半干旱草地地表能量通量及闭合率

利用兰州大学半干旱气候与环境观测站(简称SACOL站)2008年的湍流、辐射、土壤温度和通量梯度观测资料,分析了地表能量通量的日变化、季节变化及能量分配特征,讨论了典型黄土高原沟壑区土壤热量储存对地表能量闭合率的影响.结果表明:黄土高原半干旱草地全年获得的净辐射约为2.269×103 MJ/m2,感热、潜热和土壤热通量年总量分别为1.210×103 MJ/m2、1.117×103 MJ/m2和0.069×103 MJ/m2;能量平衡各分量季节变化明显,日变化呈单峰型.从各能量分量占净辐射的比例来看,黄土高原半干旱草地净辐射主要以感热形式加热大气.草原生长期的能量闭合率为86.8％,非生长期的能量闭合率为76.5％.与未考虑0-5cm深度的土壤热量储存相比,草原生长期能量闭合率提高了11.3％,非生长期能量闭合率提高了12.0％.     

Short-term variability in physical forcing in temperate reservoirs: effects on phytoplankton dynamics and sedimentary fluxes

1. The effects of wind events on phytoplankton dynamics were investigated in two temperate reservoirs. 2. Meteorological forcing, change in physical and chemical structure of the water column and biological responses of phytoplankton communities were followed for 3 weeks in three seasons. 3. Depending on the season, the phytoplankton response differed in response to nutrient and light conditions, and to the intensity of stratification and mixing. 4. We demonstrated that, on a time scale of a few days, wind events can modify phytoplankton dynamics, in terms of size structure and exported biomass. An increase of mixing favoured the largest size class and disadvantaged the smallest size class, while an increase in stratification had the opposite effects. The short‐term change in size structure was reflected in the sedimentary fluxes but with a time lag.     

Radiation budget changes with dry forest clearing in temperate Argentina

Land cover changes may affect climate and the energy balance of the Earth through their influence on the greenhouse gas composition of the atmosphere (biogeochemical effects) but also through shifts in the physical properties of the land surface (biophysical effects). We explored how the radiation budget changes following the replacement of temperate dry forests by crops in central semiarid Argentina and quantified the biophysical radiative forcing of this transformation. For this purpose, we computed the albedo and surface temperature for a 7‐year period (2003–2009) from MODIS imagery at 70 paired sites occupied by native forests and crops and calculated the radiation budget at the tropopause and surface levels using a columnar radiation model parameterized with satellite data. Mean annual black‐sky albedo and diurnal surface temperature were 50% and 2.5 °C higher in croplands than in dry forests. These contrasts increased the outgoing shortwave energy flux at the top of the atmosphere in croplands by a quarter (58.4 vs. 45.9 W m^?2) which, together with a slight increase in the outgoing longwave flux, yielded a net cooling of ?14 W m^?2. This biophysical cooling effect would be equivalent to a reduction in atmospheric CO₂ of 22 Mg C ha^?1, which involves approximately a quarter to a half of the typical carbon emissions that accompany deforestation in these ecosystems. We showed that the replacement of dry forests by crops in central Argentina has strong biophysical effects on the energy budget which could counterbalance the biogeochemical effects of deforestation. Underestimating or ignoring these biophysical consequences of land‐use changes on climate will certainly curtail the effectiveness of many warming mitigation actions, particularly in semiarid regions where high radiation load and smaller active carbon pools would increase the relative importance of biophysical forcing.     

Comparative study of dark respiration in plants inhabiting arctic (Wrangel Island) and temperate climate zones

Comparative analysis of dark respiration was carried out for 18 plant species inhabiting arctic zone (Wrangel Island, lat. 71° N) and temperate zone (Leningrad oblast, lat. 59° N). For 15 pairs of species examined, the Stocker’s rule was proved valid; i.e., respiration rates of identical species were equal at average temperatures of their natural habitats. The concept of respiratory features in boreal and mountain plants is described in its historic development. The possible causes for controversial data are explained. It is concluded that gas exchange measurements in natural plant habitats are the only valid means for characterizing plant respiration. Only such measurements should provide the basis for the discussion of global climate changes.     

A comparison of the water balance characteristics of temperate and tropical fly pupae

Abstract Water balance characteristics of temperate zone fly pupae are compared with the characteristics of flies inhabiting the tropics. The flies, all of which were reared without diapause, had very similar equilibrium weights that were quite high (a_v 0.90-0.92), thus implying a limited capacity to absorb water from a subsaturated atmosphere. Likewise, the critical transition temperatures (CTT) were nearly the same for all the flies. Net transpiration rates at 20^oC are a function of size, but the rate is less size dependent as temperature increases. When water loss is examined across a broad temperature range, as described by activation energies, it is apparent that the tropical flies lose water at a greater rate than their temperate zone counterparts. Activation energy may be a good parameter to use in evaluating habitat preference and suitability for a species because it describes water loss as a function of temperature, and thus is likely to be a good indicator of the insect's response to the fluctuating temperatures that occur naturally.     

亚热带毛竹林生态系统能量通量及平衡分析

利用开路涡度相关系统和常规气象观测仪器,对亚热带(浙江省)毛竹林生态系统2011年的净辐射、显热通量、潜热通量、土壤热通量以及气温、地温、降雨量等气象要素进行了连续观测,定量分析了毛竹林生态系统能量通量的变化和各能量分量的分配特征,并计算了能量闭合度以及波文比。结果表明:毛竹林全年净辐射为2628.00 MJ/m2,显热通量为576.80 MJ/m2,潜热通量为1666.77 MJ/m2,土壤热通量为-7.52 MJ/m2,土壤为热源,各能量分量季节变化明显,日变化基本呈单峰型曲线变化。显热通量占净辐射的22.0%,潜热通量占63.4%,毛竹林生态系统潜热通量为能量散失的主要形式。波文比逐月变化规律不明显,波动较大,在0.07—1.77之间变化,能量平衡比率法得出毛竹林年能量闭合度为0.85,月平均闭合度为0.84,能量闭合度高于线性回归法计算结果,但仍有15%的能量不闭合。     

不同降水梯度下草地生态系统地表能量交换

通过对不同降水梯度下的蒙古中部针茅草原(KBU)、内蒙古羊草草原(NM)、海北高寒灌丛草甸(HB)和当雄高寒草甸草原(DX)4个草地生态系统的能量通量连续4-5 a的测定,分析了影响青藏高原和蒙古高原草地生态系统生长季中地表能量交换的主要因素。研究表明:相对于KBU、NM和DX,HB高寒灌丛草甸NDVI(0.58)和土壤含水量(28.3%)最大,因而地表短波反射率(αk)最低(0.12),从而获得了最大的净辐射(Rn)。KBU、NM和DX 3个草地生态系统生长季中αk随着植被的生长而降低,在生长季末期,随着植被的凋落而增加;HB的αk季节变化趋势与其它生态系统相反。从蒙古高原(KBU和NM)到青藏高原(HB和DX),随着降水量的增加,波文比(β)逐渐减小(2.25-0.53),即生态系统与大气的能量交换从显热(H)占主导转变为潜热(LE)占主导。植被状况对草地生态系统与大气之间能量交换的季节动态有重要的调控作用,在NDVI较低的时候,4个生态系统H/Rn都大于LE/Rn,LE/Rn随着NDVI的增加而增加,而H/Rn呈现出与LE/Rn相反的季节变化趋势。     

Comparison of Organic and Chemical Soil Amendments Used in the Reforestation of a Harsh Sierra Nevada Site

Abstract A comparison of a composted organic amendment, a controlled‐release fertilizer, and induced mycorrhizal inoculation as affecting the establishment and nutrition of bareroot Jeffrey pine (Pinus jeffreyi Grev. & Balf.) was conducted on a Sierra Nevada surface mine. The soil amendments were applied at outplanting to the backfill of augered planting holes, with a low rate of 8 g and a high rate of 16 g per seedling for the fertilizer, Gromax 21‐6‐2 + Minors, whereas a single rate of 2.0 L was used for organic matter. Colonization by Pisolithus tinctorius (Pers.) Coker & Couch was induced by coating the root systems with basidiospores suspended in a gel carrier. The organic amendment especially, but also mycorrhizal inoculation, caused substantial seedling mortality, whereas survival was unaffected by controlled‐release fertilization. Gromax applied at the high rate produced a 74% increase in shoot volume after three growing seasons, whereas the organic amendment reduced volume by 28%. Growth was unaffected by mycorrhizal treatment. The growth response to the 16‐g Gromax application probably reflected enhanced N, P, and K nutrition and decreased concentrations of potentially toxic metallic elements, including Mn and Al among others, as revealed through foliar analysis. Because they were accompanied by growth reduction, nutritional responses to the organic amendment, which involved both macronutrients and trace elements, were of little consequence. Impaired water relations may account for the poor response to this amendment. Likewise, nutritional responses to mycorrhizal inoculation produced no discernible benefit in terms of seedling performance. An inoculation procedure that failed to induce substantially greater P. tinctorius colonization in inoculated than uninoculated seedlings, and that may have also impaired water relations, likely explains this result. Overall, these findings indicate that further research is needed before either the organic amendment or the mycorrhizal inoculation procedure used here can be used in forest restoration efforts on dry sites.     

The economic value of chaff and straw in arid and temperate zones

Chaff and straw is extensively used as a source of fuel, building material and fodder in arid zones, but this by-product of the cereal harvest is generally under-represented in the archaeobotanical record from north-west Europe. While this is partially a result of differential preservation, the evidence from a number of North African sites suggests that dissimilarities between the two regions in the availability of firewood, grazing and building material play an important role too. Differences in the way wheat and barley were used may represent an additional factor. Three types of use are distinguished: casual ‘use’, intentional local use and use as a commercial commodity, and these can be linked to scales of production and organization in the agricultural system.     

宁南退耕还草区近地面湍流通量观测初报

根据 2 0 0 2～ 2 0 0 3年小气候考察资料 ,分别分析了四季晴天条件下宁南退耕还草区净辐射、感热、潜热和土壤热通量比例和变化特征。结果表明 ,近地层各通量有较规则的日变化。白天 ,感热通量占净辐射的 5 0 %以上 ,一般达 80 %～ 10 0 %。只有夏季全天和秋季白天的潜热通量为正 ,在春季的早晨和秋季的傍晚潜热通量为负。冬春季节土壤的热量收支大于夏秋季节。     

A comparative kinetic analysis of nitrate and ammonium influx in two early-successional tree species of temperate and boreal forest ecosystems

Root NO₃ ^? and NH₄ ⁺ influx systems of two early‐successional species of temperate (trembling aspen: Populus tremuloides Michx.) and boreal (lodgepole pine: Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forest ecosystems were characterized. NO₃ ^? and NH₄ ⁺ influxes were biphasic, consisting of saturable high‐affinity (HATS) and constitutive non‐saturable low‐affinity transport systems (LATS) that were evident at low and relatively high N concentrations, respectively. NO₃ ^? influx via HATS was inducible (IHATS); nitrate pre‐treatment resulted in 8–10‐fold increases in the V_max for influx in both species. By contrast, HATS for NH₄ ⁺ were entirely constitutive. In both species, V_max values for NH₄ ⁺ influx were higher than those for NO₃ ^? uptake; the differences were larger in pine (6‐fold) than aspen (1·8‐fold). In aspen, the K_m for NH₄ ⁺ influx by HATS was approximately 3‐fold higher than for IHATS NO₃ ^? influx, while in pine the K_m for IHATS NO₃ ^? influx was approximately 3‐fold higher than for NH₄ ⁺ influx. The aspen IHATS for NO₃ ^? influx appeared to be more efficient than that of pine (V_max values for aspen being approximately 10‐fold higher and K_m values being approximately 13‐fold lower than for pine). By contrast, only small differences in values for the NH₄ ⁺ HATS were evident between the two species. The kinetic parameters observed here probably result from adaptations to the N availabilities in their respective natural habitats; these may contribute to the distribution and niche separation of these species.     

When water interacts with temperature: Ecological and evolutionary implications of thermo‐hydroregulation in terrestrial ectotherms

The regulation of body temperature (thermoregulation) and of water balance (defined here as hydroregulation) are key processes underlying ecological and evolutionary responses to climate fluctuations in wild animal populations. In terrestrial (or semiterrestrial) ectotherms, thermoregulation and hydroregulation closely interact and combined temperature and water constraints should directly influence individual performances. Although comparative physiologists traditionally investigate jointly water and temperature regulation, the ecological and evolutionary implications of these coupled processes have so far mostly been studied independently. Here, we revisit the concept of thermo‐hydroregulation to address the functional integration of body temperature and water balance regulation in terrestrial ectotherms. We demonstrate how thermo‐hydroregulation provides a framework to investigate functional adaptations to joint environmental variation in temperature and water availability, and potential physiological and/or behavioral conflicts between thermoregulation and hydroregulation. We extend the classical cost–benefit model of thermoregulation in ectotherms to highlight the adaptive evolution of optimal thermo‐hydroregulation strategies. Critical gaps in the parameterization of this conceptual optimality model and guidelines for future empirical research are discussed. We show that studies of thermo‐hydroregulation refine our mechanistic understanding of physiological and behavioral plasticity, and of the fundamental niche of the species. This is illustrated with relevant and recent examples of space use and dispersal, resource‐based trade‐offs, and life‐history tactics in insects, amphibians, and nonavian reptiles.     

Fruit fall in tropical and temperate forests: implications for frugivore diversity

There have been few attempts to compare fruit productivity throughout the world, although this is indispensable for understanding the global variations in frugivore diversity. The purposes of this study are (1) to reveal the patterns in fruit fall in tropical and temperate forests, (2) to examine the environmental factors (location, climate, and total litterfall) affecting these patterns, and (3) to assess the effect of fruit fall on frugivore diversity by using bird and primate data. Fruit fall was compared among 53 forests, from around the equator to the cool-temperate zone at 62°N, in Asia, Africa, North and South America, and Australia. Average ± SD of fruit fall (kg/ha/year) was 454 ± 258 in tropical, and 362 ± 352 in temperate forests. Fruit fall was exceptionally high in Australia (812 ± 461). When Australia was excluded, fruit fall significantly decreased with increasing absolute latitude and altitude, and fruit fall in tropical forest was 1.7 times larger than that in temperate forests (265 ± 227). Total litterfall affected fruit fall significantly, explaining 32, 28, and 64% of the variations of fruit fall in the entire data, tropical data, and temperate data, respectively. The fruit fall/litterfall ratio did not differ between temperate and tropical forests but was significantly higher in Australia than in other regions. Among climatic parameters (annual temperature, precipitation, actual evapotranspiration), a positive relationship was found between temperature and fruit fall in the entire dataset and within temperate forests. Fruit fall seemed to explain the temperate/tropical difference in frugivorous primate diversity to some extent, but not for frugivorous bird diversity. This study shows that the difference in fruit fall in tropical and temperate forests is smaller than that in frugivore diversity, and that it could explain at least part of the frugivore diversity.