DEVELOPMENT OF FLORAL ORGANS IN THE SITES OF LEAF PRIMORDIA IN PINGUICULA VULGARIS

Plants of Pinguicula vulgaris L. have either clockwise or counterclockwise spiral phyllotaxy. The inception of floral primordia occurs in leaf sites as a normal sequence of development. Only two leaf primordia initiated late in the season develop into floral primordia in the following year. They do not represent a direct modification of the apical meristem nor of the detached meristem. The apical meristem continues to produce leaves in the vegetative phase and flowers in the reproductive phase, and thus the plants show a monopodial growth. Axillary buds are not developed in this perennial species and instead additional buds of adventitious ontogeny appear. Such buds are produced on the older leaves of larger plants, and they are extremely useful in the vegetative propagation of the species.     

兰州市北山不同林地春夏季土壤纤毛虫群落特征

为了研究兰州市北山绿化工程的植被恢复状况,于2016年4月和7月,对北山罗九公路绿化工程区的人工林及荒坡、半荒坡共6个样点进行野外调查采样,分析土壤纤毛虫群落组成及其影响因素。研究结果表明:(1)春季有10纲21目34科44属80种,夏季10纲21目38科54属104种。春夏季的优势类群均为尖毛科(Plagiocampidae),优势种为膨胀肾形虫(Colpoda inflate)和盘状肾形虫(Colpoda patella)。(2)土壤纤毛虫的丰度、物种数和多样性指数夏季高于春季,且人工林样点的土壤纤毛虫的丰度、物种数和多样性指数均高于荒坡;(3)春季土壤有机碳、土壤温度和电导率是影响土壤纤毛虫物种数分布的主要环境因子;夏季土壤有机碳、土壤温度及pH是影响土壤纤毛虫物种数分布的主要因子。总体而言,人工林土壤恢复较荒坡、半荒坡好,而人工林中杨树林和侧柏林的土壤环境质量较优。     

闽西南红壤侵蚀区黑莎草功能性状特征及其与土壤因子的关系

为了解黑莎草(Gahnia tristis)在南方红壤侵蚀区的适应状况,测定了长汀县红壤侵蚀区的黑莎草叶片、根系的功能性状及土壤理化性质,并应用数理统计方法分析了黑莎草叶片与根系功能性状之间的相关性,及其对土壤因子的响应。结果表明,黑莎草叶片表型性状在冬夏季间存在显著差异,叶长、叶宽、叶组织密度和叶绿素表现为夏季显著大于冬季,根系表型性状则更具稳定性,冬季的根系养分含量均高于夏季,养分的分配上叶片养分高于根系养分。叶组织密度与叶绿素含量呈显著正相关,与比叶面积呈显著负相关;根组织密度与比根长和比根面积均呈显著负相关,叶片和根系养分间均呈显著正相关,土壤碳、氮、磷含量是影响黑莎草功能性状主要因子。因此,黑莎草可通过调节功能性状以适应环境变化,可作为地带性植物应用于南方红壤侵蚀区的植被恢复和水土流失治理。     

A Framework for Identifying Plant Species to Be Used as ‘Ecological Engineers’ for Fixing Soil on Unstable Slopes

Major reforestation programs have been initiated on hillsides prone to erosion and landslides in China, but no framework exists to guide managers in the choice of plant species. We developed such a framework based on the suitability of given plant traits for fixing soil on steep slopes in western Yunnan, China. We examined the utility of 55 native and exotic species with regard to the services they provided. We then chose nine species differing in life form. Plant root system architecture, root mechanical and physiological traits were then measured at two adjacent field sites. One site was highly unstable, with severe soil slippage and erosion. The second site had been replanted 8 years previously and appeared to be physically stable. How root traits differed between sites, season, depth in soil and distance from the plant stem were determined. Root system morphology was analysed by considering architectural traits (root angle, depth, diameter and volume) both up- and downslope. Significant differences between all factors were found, depending on species. We estimated the most useful architectural and mechanical traits for physically fixing soil in place. We then combined these results with those concerning root physiological traits, which were used as a proxy for root metabolic activity. Scores were assigned to each species based on traits. No one species possessed a suite of highly desirable traits, therefore mixtures of species should be used on vulnerable slopes. We also propose a conceptual model describing how to position plants on an unstable site, based on root system traits.     

Small-scale spatial heterogeneity in the vertical distribution of soil nutrients has limited effects on the growth and development of Prosopis glandulosa seedlings

We examined the above- and below-ground responses of seedlings of the woody shrub Prosopis glandulosa to the spatial heterogeneity of soil nutrients within the root zone. We performed a microcosm experiment where seedlings were grown with different combinations of nutrients [nitrogen (N), phosphorus (P), and both combined (NP)] and under different levels of nutrient heterogeneity (nutrients supplied as patches located in the bottom and/or upper portion of rooting zone versus homogeneous distribution). Seedling morphology and biomass did not show a strong response to changes in nutrient ion or spatial heterogeneity. Height, number of leaves, and specific leaf area did not vary significantly between treatments. The number of leaves, foliar biomass, stem biomass and biomass allocation to stems of seedlings showed more responsiveness to the addition of N and NP than to the addition of P. The spatial heterogeneity of nutrients affected the diameter, root biomass and leaf N content. Seedlings had higher diameter and root biomass when the nutrients were homogeneously distributed as compared to their placement as patches in the bottom of the microcosms. Their leaf N concentration increased in those treatments where the nutrient patch was located in the lower half as compared to the upper half of the microcosms. Root foraging responses to nutrient patches varied with their location. Significant root proliferation was observed when patches of N, P and NP were located in the upper portion of the rooting zone; when they were located in the lower portion such a response was observed only for P. Despite our findings that Prosopis seedlings have a low overall responsiveness to small-scale vertical differences in soil nutrient heterogeneity, our results suggest that these differences may modify the growth dynamics of the secondary roots of this ecologically important dryland species during the early stages of its development.     

Root characteristics of representative Mediterranean plant species and their erosion-reducing potential during concentrated runoff

Gully erosion is an important soil degradation process in Mediterranean environments. Revegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass on reducing water erosion rates, whereas the role of the below-ground biomass is often neglected. In a Mediterranean context, the above-ground biomass can temporally disappear because of fire or overgrazing and when concentrated flow erosion occurs, roots can play an important role in controlling soil erosion rates. Unfortunately, information on root characteristics of Mediterranean plants, growing on semi-natural lands, and their effects on the topsoil resistance to concentrated flow erosion is lacking. Therefore, typical Mediterranean grass, herb, reed, shrub and tree root systems of plants growing in habitats that are prone to concentrated flow erosion (i.e. in ephemeral channels, abandoned fields and steep badland slopes) are examined and their erosion-reducing potential was evaluated. Root density (RD), root length density (RLD) and root diameters are measured for 26 typical Mediterranean plant species. RD values and root diameter distribution within the upper 0.10–0.90 m of the soil profile are then transformed into relative soil detachment rates using an empirical relationship in order to predict the erosion-reducing effect of root systems during concentrated runoff. Comparing the erosion-reducing potential of different plant species allows ranking them according to their effectiveness in preventing or reducing soil erosion rates by concentrated flow. RD in the 0.10 m thick topsoil ranges between 0.13 kg m⁻³ for Bromus rubens (L.) and 19.77 kg m⁻³ for Lygeum spartum (L.), whereas RLD ranges between 0.01 km m⁻³ for Nerium oleander (L.) and 120.43 km m⁻³ for Avenula bromoides ((Gouan) H. Scholz.) Relative soil detachment rates, compared to bare soils, range between 0.3 × 10^-12 and 0.7 for the 0.10 m thick topsoil. The results show that grasses such as Helictotrichon filifolium ((Lag.) Henrard), Piptatherum miliaceum ((L.) Coss.), Juncus acutus (L.), Avenula bromoides ((Gouan) H. Scholz), Lygeum spartum (L.) and Brachypodium retusum ((Pers.) Beauv.) have the highest potential to reduce soil erosion rates by concentrated flow in the 0–0.1 m topsoil. But also shrubs such as Anthyllis cytisoides (L.) and Tamarix canariensis (Willd.), having high root densities in the topsoil, can reduce erosion rates drastically. Among the species growing in channels, Juncus acutus (L.) has the highest erosion reducing potential, whereas Phragmites australis (Cav.) is the least effective. On abandoned fields, Avenula bromoides ((Gouan) H. Scholz) and Plantago albicans (L.) are the most effective species in reducing concentrated flow erosion rates, while Thymelaea hirsuta (L. (Endl.)) and Bromus rubens (L.) perform the worst. On steep badland slopes, Helictotrichon filifolium ((Lag.) Henrard) and Anthyllis cytisoides (L.) perform the best in the analysis of erosion reducing potential, while Ononis tridentata (L.) is the least effective species. These findings have implications for ecological restoration and management of erosion-prone slopes.     

Savannah woodland vegetation in the South‐East District of South Australia: the influence of evaporative aerodynamics on the foliage structure of the understorey invaded by introduced annuals

Abstract Evaporative aerodynamics determine the foliage projective cover of the understorey of perennial tussock grasses and associated perennial herbs in the savannah woodland dominated by Eucalyptus camalduknsis on gleyed podsolic soils in the Mediterranean climate of the South‐East District of South Australia. By the mid 1940s, winter‐spring evapotranspiration from the ‘thin’ leaves (with low leaf specific weight) of introduced annual plants was depleting surface soil water and thus reducing the annual growth of the summer‐growing savannah understorey; perennial herbs between the tussock grasses were the first to succumb to this competition. During spring, the percentage of the ground covered by the savannah understorey was increased by 10% in the subhumid zone to 30% in the humid zone as the pre‐European perennial herbs between the tussock grasses were replaced by introduced annuals. Application of phosphatic fertilizer to the understorey increased the growth of introduced annuals, which formed a dense stratum during their winter‐spring growing season, increasing evapotranspiration and leading eventually to the extinction of the native perennial grasses. When the savannah understorey, invaded by introduced annuals in the mid‐1940s, was converted to improved pasture, the percentage of ground covered by the seasonal foliage was increased by 20–30%; 100% coverage of overlapping foliage resulted in the humid zone.     

Selection of plant parts by Depressaria multifidae (Lep., Oecophoridae) on its seasonally-restricted hostplant, Lomatium grayi (Umbelliferae)

ABSTRACT.

• 1 Depressaria multifidae Clarke feeds on a broader variety of Umbelliferae plant parts than other Depressaria species.
• 2 Early instar larvae feed in the sheaths surrounding floral buds and leaves. Later instar larvae feed in the sheaths and floral stems and on flowers and leaves.
• 3 Floral stems bored by larvae had significantly larger basal stem diameters than floral stems that were not bored. Smaller stems usually have umbels with only male flowers, and wither after flowering, too soon for larvae to complete development. In contrast, larger stems often have umbels with some hermaphroditic flowers, which remain green and erect long enough for larvae to complete development. Hence, selection may favour larvae that bore only in relatively large stems.
• 4 In the laboratory, larvae fed sheaths with enclosed floral buds, flowers, or leaves all pupated at the same weight, but larvae fed floral stems pupated at a significantly lower weight. Larval and pupal development time was the same on all plant parts.
• 5 In the field, larvae restricted to a single umbel throughout development pupated at the same weight as those restricted to a single leaf.
• 6 Unlike in other Depressaria species, nitrogen levels only partly correspond to the pattern of use of plant parts in D.multifidae. Nitrogen values varied as follows: floral buds > immature leaves ≥ flowers > floral stalks > sheaths excluding floral buds or leaves.
• 7 The broad variety of plant parts used by D.multifidae may result partly from the problem of feeding on a small, seasonally restricted hostplant; the greater use of sheaths and floral stems than in other Depressaria species may result from selection for safety from parasites or predators.
• 8 The results for D.multifidae indicate that the way in which an insect feeds on a plant species can vary broadly even at a single site.

     

Seasonal changes in carbohydrate reserves in mature northern<Emphasis Type="Italic"> Populus tremuloides</Emphasis> clones

To assess the changes in seasonal carbohydrate status of Populus tremuloides, sugar and starch concentrations were monitored in roots, stem xylem and phloem and branches of ten different clones. Time of root growth was assessed by extraction of roots from in-growth cores collected five times during the season. Overall the results showed that the main period of root growth in these northern clones was shifted from spring to late summer and fall likely due to the microclimatic conditions of the soil. This increase in root growth was associated with a decline in total non-structural carbohydrate content in the roots during this period. This study also found that the carbohydrate reserves in these clones were being stored as close as possible to the organs of annual growth (leaves and roots). At the time of leaf flush, the largest reduction in stored carbohydrates (3% of dry weight) was observed in the branches of the trees, compared to a slight decline in the stem and roots. Starch and sugar reserves in most tissues were very low in early summer. This suggests that reserves that might be used for the regrowth of foliage after insect defoliation or other disturbances, are relatively small compared to the portion that is needed for maintenance and typical growth developments such as leaf flush.     

黄花杓兰的花芽发育

对黄花杓兰（Cypripedium flavum P.F.Hunt et Summerh.）成年植株做了一个生长季的研究,提出了一年芽、二年芽和多年休眠芽的概念。指出由芽形成到植株开花需两年时间,其具体发育路线是：第一年6－7月份,根状茎顶端二年芽基部外侧有两个新的小芽产生,即“一年芽”,至9－10月份发育出7－9片幼叶,然后随气温下降停止生长;第2年4月份复苏,即为“二年芽”,二年芽在本生长季内发育成混合芽,但一般情况下只有一个充分发育,另一个未能充分发育并且一般将来也不再有发育的机会,被称为“多年休眠芽”;第3年5月份充分发育的二年芽长出地面,形成植株,迅速开花、结果,至9月底植株枯萎。本文还讨论了黄花杓兰发育过程与环境的关系。     

不同龄阶梭梭根区土壤水分时空变化特征

土壤水是荒漠植被发育最主要的制约因子。不仅影响植物的生长和发育,还限制着植被的种类、数量和分布。梭梭作为北方荒漠区重要的固沙植物,研究梭梭林地土壤水分动态对其植被生存或恢复以及群落稳定性维持具有重要意义。鉴于少有学者研究过不同龄阶梭梭根区的土壤含水率差异,于2014年2月至2014年11月,采用中子仪法和烘干法对0—400 cm沙层土壤含水率进行了原位观测,分析了不同龄阶梭梭根区土壤水分的时空变化规律。结果表明:(1)梭梭根区土壤水分时间变化可分为4个阶段:2月下旬—3月下旬是土壤水分快速补给期,4月上旬—5月下旬是土壤水分均衡期,6月上旬—10月下旬是土壤水分耗损期,11月上旬—次年2月中旬是土壤水分稳定期;(2)梭梭根区0—50 cm土层,受降雨、融雪水入渗补给和蒸发的影响较大,土壤水分变异系数较大且随深度增加迅速减小,50 cm以下土层变异系数较小且随深度变化微小;(3)不同龄阶梭梭根区剖面平均土壤含水率全年与春、夏、秋季均表现为:枯树成熟梭梭中龄梭梭裸地;(4)随距梭梭树干距离的增大(0—5 m范围内),土壤含水率整体呈减小趋势;降雨前后,梭梭根区浅层(0—10 cm)土壤含水率增量大于裸地土壤含水率增量。     

Homeosis, morphogenetic gradient and the determination of floral identity in the inflorescences ofPhilodendron solimoesense (Araceae)

A comparative developmental study of the inflorescence ofPhilodendron solimoesense was conducted using scanning electron microscopy. The spadix ofP. solimoesense is characterized by unisexual flowers. Staminate flowers are initiated on the upper portion of the spadix while pistillate flowers develop on the lower portion of the spadix. An intermediate zone located between the upper male and lower female portion of the inflorescence consists of sterile male flowers. Within this intermediate zone a row of flowers exhibit polarity with respect to the identity of sexual organs. Stamens are initiated on the flank of the floral meristem facing the upper male zone and carpels are initiated on the portion of the floral meristem facing the lower female zone. The resulting flowers therefore assume a bisexual identity. At the level of the inflorescence, all floral buds are initiated along a series of contact parastichies and the continuity of these parastichies is not disrupted at any level in the male, intermediate, and female zones on the spadix. Results from this study support the presence of a morphogenetic gradient acting at the level of the inflorescence and appears to be independent of the boundaries of floral primordia.     

Seasonal variation of arbuscular mycorrhizal fungi in temperate grasslands along a wide hydrologic gradient

We studied seasonal variation in population attributes of arbuscular mycorrhizal (AM) fungi over 2 years in four sites of temperate grasslands of the Argentinean Flooding Pampas. The sites represent a wide range of soil conditions, hydrologic gradients, and floristic composition. Lotus glaber, a perennial herbaceous legume naturalised in the Flooding Pampas, was dominant at the four plant community sites. Its roots were highly colonised by AM fungi. Temporal variations in spore density, spore type, AM root colonisation, floristic composition and soil chemical characteristics occurred in each site and were different among sites. The duration of flooding had no effect on spore density but depressed AM root colonisation. Eleven different types of spores were recognized and four were identified. Two species dominated at the four sites: Glomus fasciculatum and Glomus intraradices. Spore density was highest in summer (dry season) and lowest in winter (wet season) with intermediate values in autumn and spring. Colonisation of L. glaber roots was highest in summer or spring and lowest in winter or autumn. The relative density of G. fasciculatum and G. intraradices versus Glomus sp. and Acaulospora sp. had distinctive seasonal peaks. These seasonal peaks occurred at all four sites, suggesting differences among AM fungus species with respect to the seasonality of sporulation. Spore density and AM root colonisation when measured at any one time were poorly related to each other. However, spore density was significantly correlated with root colonisation 3 months before, suggesting that high colonisation in one season precedes high sporulation in the next season.     

Morphology and anatomy of shoot, root, and propagation systems in Hoffmannseggia glauca

Abstract: Hoffmannseggia glauca is a perennial weed that has tubers and root-borne buds. Some authors only consider root tubers without mentioning root-borne buds, while others consider that more anatomic studies become necessary to determine the origin of these structures and to interpret their behaviour. The objectives are: to study the growth form of the plant in order to analyze the ontogeny of its propagation organs, and to study its shoot and root anatomical characters that affect water conductivity. Hoffmannseggia glauca was collected in Argentina. Development of its shoot and root systems was observed. Shoots and roots were processed to obtain histological slides. Macerations were prepared to study vessel members. Primary and lateral roots originate buds that develop shoots at the end of the first year. In winter, aerial parts die and only latent buds at soil surface level and subterranean organs remain. In the following spring, they develop innovation shoots. Roots show localized swellings (tuberous roots), due to a pronounced increase of ray thickness and parenchymatous proliferation in the root center. Root vessel members are wider than those of aerial and subterranean shoots. Early development of an extensive root system, presence of root borne buds, anatomic and physiological specialization of innovation shoots, capability of parenchymatous rays to originate buds and tuberous roots, and high water transport efficiency in subterranean organs lead Hoffmannseggia glauca to display higher colonization potential than other species.     

Limiting processes for perennial plant reintroduction to restore dry grasslands

In restored grasslands of southern Europe, perennial plants remain highly underrepresented compared with the reference ecosystems. We tested various treatments to reintroduce common perennial plant species (Brachypodium retusum, Poaceae, and Thymus vulgaris, Lamiaceae), which are usually not or poorly reintroduced via soil and hay transfer. Treatments included microenvironmental manipulations (rock cover and plant interactions) and two grazing intensities. Target perennial species were transplanted in 2002 in the reference grassland ecosystem (intact grassland area used as a control) and in two abandoned fields. Survival was assessed in June 2003 and June 2004. Target species shoot and root biomass were measured in June 2004. Grazing greatly reduced the survival and biomass of both target species and its effects were reinforced by summer drought: plants that did not establish well enough during the autumn and spring did not survive summer. The restored rock cover had a mild positive effect, particularly on B. retusum. There were no negative or positive plant neighbor interactions in the steppe, while there was competition in both abandoned fields. Competition was particularly intense in the abandoned melon field, composed of a dense sward of annual grasses (Bromus sp.). In order to reintroduce perennial species to dry grasslands, the ideal combination of treatments is to exclude or reduce grazing during the first year to allow seedlings to establish and to recreate adequate microenvironmental conditions. Reducing competition from arable weeds may help but is not essential in such dry grasslands.     

Habitat associations of wild goat in central Iran: implications for conservation

Wild goat (Capra aegagrus) is one of the highly valued but threatened ungulates of Iran. Seasonal habitat use of wild goat was studied from October 2006 to September 2008 in Haftad Gholleh Protected Area, central Iran. We developed averaged logistic regression models based on Akaike Information Criterion weights for each season. The obtained habitat suitability models showed high sensitivity (greater than 88%) for all evaluation datasets. Wild goat habitat use was positively related with steep slopes, areas near rocky substrates and water sources, west-facing slopes and areas far from roads in all seasons. Core zones of protected area covered nearly 70% of suitable habitats for wild goats, predicted by logistic regression models. Northern core zone covered more than 61% of predicted suitable habitats, which should receive more attention in management actions. The southern core zone boundaries should be considered to increase the encompassed suitable habitats. There was a substantial overlap between seasonal suitable habitats with the highest overlap observed between spring and summer and summer and fall.     

Adaptive significance of sprouting ofEuptelea polyandra,a deciduous tree growing on steep slopes with shallow soil

We investigated growth characteristics ofEuptelea polyandra Sieb. et Zucc. (Eupteleaceae), a Japanese endemic deciduous tree species growing on unstable ground such as that of very steep slopes with thin soil.Euptelea polyandra began to sprout at the juvenile stage and had a multiple-stemmed tree form. There was a positive correlation between diameter of the maximumsized stem within a stool (DMS) and the number of stems within the stool. Many stools had suffered from disturbances as shown by the fact that uprooting scars were found on 31.4% and 42.4%, respectively, of the stools of the two populations studied. Sprouting played a significance role in repairing damaged stems and stools, and at least 15.5% and 18.2% of the stools of the two populations, respectively, had apparently avoided death by sprouting. Sprouted stems gradually inclined with the increase in their relative sizes within each stool, and this seemed to facilitate the establishment of younger sprouted stems. The small younger sprouted stems had their own roots. There were dormant buds on stems which originated from axillary buds, and secondary dormant buds occurred by branching. The total number of dormant buds in a stool increased with DMS. It is concluded thatE. polyandra accumulates dormant buds for sprouting in order to respond to disturbances quickly.     

Bacterial blight of kiwifruit

Bacterial blight of kiwifruit (Actinidia deliciosa) is caused by Pseudomonas viridiflava. Symptoms are rot of floral buds and flowers, and spots on leaves. Buds and flowers nearest the ground are most likely to be affected. Isolations made from healthy buds, flowers and leaves suggest that the pathogen is an epiphyte on vines throughout the year. Disease incidence fluctuates between seasons, and varies between orchards. A relationship between disease incidence and rainfall in spring is indicated.     

植物枝叶与根系耦合固土抗蚀的差异性

以4年生4种内蒙古鄂尔多斯地区常见水土保持植物柠条、沙柳、白沙蒿和沙棘为研究对象,选取反映枝叶防风特性和根系特性的13项指标,即单株防风效能、林带防风效能、群落防风效能、直根抗拉强度、代表根本构特征、代表根弹性模量、侧根分支处抗拉强度、累计根表面积、拉拔摩阻特性、根-土界面摩擦系数、累计根长、根-土复合体粘聚力、根-土复合体内摩擦角,采用层次分析法对春季大风和夏季暴雨2种自然条件下4种植物枝叶与根系耦合固土抗蚀的差异性进行评价.结果表明： 4种植物枝叶与根系耦合固土抗蚀指数为：春季大风条件下为沙柳(0.841)>柠条(0.454)>白沙蒿(-0.466)>沙棘(-0.829);夏季暴雨条件下为柠条(0.841)>沙柳(0.474)>白沙蒿(-0.470)>沙棘(-0.844).沙柳可作为研究区重要的防风抗蚀植物种之一;柠条则可作为研究地区重要的水土保持植物种之一.     

Patterns of nitrogen conservation in shrubs and grasses in the Patagonian Monte,Argentina

We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs (Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses (Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.