Is N‐resorption efficiency related to secondary compounds and leaf longevity in coexisting plant species of the arid Patagonian Monte,Argentina?

Leaf longevity and nutrient resorption efficiency are important strategies to conserve plant nutrients. Theory suggests a negative relationship between them and also proposes that high concentration of phenolics in long‐lived leaves may reduce nutrient resorption. In order to provide new evidence on these relationships, we explored whether N‐resorption efficiency is related to leaf longevity, secondary compounds and other leaf traits in coexisting plant species of different life forms in the arid Patagonian Monte, Argentina. We assessed N‐resorption efficiency, green leaf traits (leaf mass per area (LMA), leaf longevity and lignin, total soluble phenolics and N concentrations) and N concentration in senescent leaves of 12 species of different life forms (evergreen shrubs, deciduous shrubs and perennial grasses) with contrasting leaf traits. We found that leaf longevity was positively correlated to LMA and lignin, and negatively correlated to N concentration in green leaves. N concentrations both in green and senescent leaves were positively related. N‐resorption efficiency was not associated with the concentration of secondary compounds (total soluble phenolics and lignin) but it was negatively related to LMA and leaf longevity and positively related to N concentration in green leaves. Furthermore, leaf traits overlapped among life forms highlighting that life forms are not a good indicator of the functional properties (at least in relation to nutrient conservation) of species. In conclusion, our findings indicated that differences in N‐resorption efficiency among coexisting species were more related to N concentration in green leaves, leaf lifespan and LMA than to the presence of secondary compounds at least those assessed in our study (soluble phenolics and lignin). Accordingly, N‐resorption efficiency seems to be modulated, at least in part, by the productivity–persistence trade‐off.     

Changes in traits of shrub canopies across an aridity gradient in northern Patagonia, Argentina

Low and highly variable precipitation pulses exert a strong selective pressure on plant traits and this might provide axes of ecological differentiation among plant species in arid ecosystems. We asked whether aridity contributes to maintain high diversity of species and morphotypes in shrub canopies. We selected eleven study sites evenly distributed across a 400-km transect in northern Patagonia, Argentina. Precipitation is low and highly variable within and between years but almost homogeneous across the transect (125–150 mm). Mean annual temperature varied, however, ranging from 8 °C (west) to 13.5 °C (east) creating a west–east gradient of aridity (aridity index from 3.7 to 7.3, respectively). Sheep grazing commenced in the early 1900s at a similar intensity across the transect. We recorded the richness and cover of shrubs by species and by morphotypes (drought deciduous tall shrubs, evergreen tall shrubs, medium shrubs, and dwarf shrubs), and further calculated the species and morphotype Shannon diversity index at each site. We assessed the presence of spiny leaves, leaf pubescence, thorny stems, and photosynthetic stems in shrub species of all morphotypes and collected green leaves of the dominant shrub species (more than 80% of the total shrub cover) to assess the leaf area, leaf mass per unit area, N-, lignin- and soluble phenolic-concentrations per species at each site. Richness and diversity of shrub species and morphotypes were positively associated with aridity. The richness and diversity of shrub species with pubescent leaves and thorny stems, and nitrogen concentration in green leaves of dominant shrubs increased with increasing aridity. We conclude that our findings on increased diversification in life history traits, species and morhotypes in shrub canopies with increasing aridity support the hypothesis that variability in aridity provides axes of ecological differentiation among shrub species facilitating their coexistence.     

Plant phenology,leaf traits and leaf litterfall of contrasting life forms in the arid Patagonian Monte,Argentina

Question: Do coexisting plant life forms differ in overall phenology, leaf traits and patterns of leaf litterfall? Location: Patagonian Monte, Chubut Province, Argentina. Methods: We assessed phenology, traits of green and senesced leaves and the pattern of leaf litterfall in 12 species of coexisting life forms (perennial grasses, deciduous shrubs, evergreen shrubs). Results: We did not identify differences in phenology, leaf traits and patterns of leaf litterfall among life forms but these attributes contrasted among species. Independent of the life form, the maintenance of green leaves or vegetative growth during the dry season was mostly associated with leaves with high leaf mass per area (LMA) and high concentration of secondary compounds. Low LMA species produced low litterfall mass with low concentration of secondary compounds, and high N concentration. High LMA species produced the largest mass of leaf litterfall. Accordingly, species were distributed along two main dimensions of ecological variation, the dimension secondary compounds in leaves ‐ length and timing of the vegetative growth period (SC ‐ VGP) and the dimension leaf mass per area ‐ leaf litterfall mass (LMA ‐ LLM). Conclusions: Phenology, leaf traits and leaf litterfall varied among species and overlapped among life forms. The two dimensions of ecological variation among species (SC ‐ VGP, LMA ‐ LLM) represent distinct combinations of plant traits or strategies related to resource acquisition and drought tolerance which are reflected in the patterns of leaf litterfall.     

Leaf litterfall patterns of perennial plant species in the arid Patagonian Monte,Argentina

The objective of this study was to investigate the variation in leaf litterfall patterns of desert plant species in relation to the intra- and interannual variation of precipitation. We collected the leaf litterfall of 12 representative species of the dominant life forms in the arid Patagonian Monte (evergreen shrubs, deciduous shrubs, and perennial grasses) at monthly intervals during three consecutive years. All shrub species showed a marked seasonality in the pattern of leaf litterfall, but the date of the peak of leaf litterfall differed among them. The peak of leaf litterfall in three deciduous and three evergreen shrubs occurred in summer months while in one deciduous shrub and in two other evergreen shrubs the peak of leaf litterfall was in autumn and winter, respectively. In contrast, the leaf litterfall of perennial grasses occurred through the year without a seasonal pattern. In most shrub species, increasing annual precipitation was related to increasing leaf litterfall and the peak of leaf litterfall was positively related to precipitation events occurred some months before, during winter. Moreover, the magnitude of responses in terms of variation in leaf litterfall in relation to interannual variation of precipitation was not the same for all species. Evergreen shrubs showed lower responses than deciduous species. These differences in leaf litterfall patterns were consistent with differences in leaf traits. In conclusion, we found new evidence of species-specific responses of leaf litterfall patterns to precipitation, suggesting that other factors than precipitation may control leaf litterfall in desert plants.     

藏东南色季拉山林线过渡带7种灌木植物的叶氮回收潜力

在湿润的青藏高原东南部, 为什么常绿灌木广泛占据高海拔的林线过渡带及以上的高山带, 而落叶灌木只能零星分布?未来气候变暖对该区不同功能群物种的影响是否相同?通过测定西藏东南部色季拉山林线过渡带7种灌木凋落叶的氮含量, 比较了极端高海拔地区灌木不同表达单位的叶氮回收潜力在不同功能群间的差异, 以及不同海拔、不同坡向间的差异, 试图从养分限制的角度为解答上述科学问题提供基础数据。研究结果表明: 1)从基于单位质量叶氮含量(N_mass)的叶氮回收潜力来看, 常绿灌木裂毛雪山杜鹃(薄毛海绵杜鹃) (Rhododendron aganniphum var. schizopeplum)显著高于其他6种落叶灌木, 但由于受比叶重的影响, 基于单位面积叶氮含量(N_area)的叶氮回收潜力则表现为落叶灌木总体较高; 2)落叶灌木山生柳(Salix oritrepha)和拉萨小檗(Berberis hemsleyana)的叶氮回收潜力在不同海拔或不同坡向间均无显著差异, 但裂毛雪山杜鹃基于N_mass的叶氮回收潜力在高海拔地段明显偏高。在极端高海拔的林线过渡带, 通过降低凋落叶中的氮含量(增加叶氮回收潜力)以达到高效的养分利用可能是常绿灌木裂毛雪山杜鹃适应高寒胁迫环境的重要策略。与落叶灌木相比, 常绿灌木裂毛雪山杜鹃叶氮回收潜力对未来气候变暖可能更敏感。     

Leaf functional traits of Neotropical savanna trees in relation to seasonal water deficit

The seasonal savannas (cerrados) of Central Brazil are characterized by a large diversity of evergreen and deciduous trees, which do not show a clear differentiation in terms of active rooting depth. Irrespective of the depth of the root system, expansion of new foliage in deciduous species occurs at the end of the dry season. In this study, we examined a suite of leaf traits related to C assimilation, water and nutrients (N, P) in five deciduous and six evergreen trees that were among the dominant families of cerrado vegetation. Maximum CO₂ assimilation on a mass basis (A_mass) was significantly correlated with leaf N and P, and specific leaf area (SLA; leaf area per unit of leaf mass). The highest leaf concentrations of both nutrients were measured in the newly mature leaves of deciduous species at the end of the dry period. The differences in terms of leaf N and P between evergreen and deciduous species decreased during the wet season. Deciduous species also invested less in the production of non-photosynthetic leaf tissues and produced leaves with higher SLA and maintained higher water use efficiency. Thus, deciduous species compensated for their shorter leaf payback period by maintaining higher potential payback capacity (higher values of A_mass) and lower leaf construction costs (higher SLA). Their short leafless period and the capacity to flush by the end of the dry season may also contribute to offset the longer payback period of evergreen species, although it may involve the higher cost of maintaining a deep-root system or a tight control of plant water balance in the shallow-rooted ones.     

SPRING GROWTH OF SHOOTS AND ROOTS IN SHRUBS OF AN ALASKAN MUSKEG

Early spring shoot and fine-root development of four evergreen and three deciduous shrub species were analyzed in a subarctic muskeg at Fairbanks, Alaska. The overwintered foliage of the evergreen shrubs regreened earlier than new leaves developed on the deciduous species. Likewise, the evergreen shrubs produced new fine roots earlier than the deciduous species. The total nonstructural carbohydrate (TNC) concentration did not decline in the evergreen shrub Ledum palustre during the spring development. This contrasted with the deciduous shrub Betula glandulosa, where a significant TNC reduction in stem tissue coincided with bud break and fine-root growth flush.     

Predicting vegetation type through physiological and environmental interactions with leaf traits: evergreen and deciduous forests in an earth system modeling framework

Earth system models are incorporating plant trait diversity into their land components to better predict vegetation dynamics in a changing climate. However, extant plant trait distributions will not allow extrapolations to novel community assemblages in future climates, which will require a mechanistic understanding of the trade‐offs that determine trait diversity. In this study, we show how physiological trade‐offs involving leaf mass per unit area (LMA), leaf lifespan, leaf nitrogen, and leaf respiration may explain the distribution patterns of evergreen and deciduous trees in the temperate and boreal zones based on (1) an evolutionary analysis of a simple mathematical model and (2) simulation experiments of an individual‐based dynamic vegetation model (i.e., LM3‐PPA). The evolutionary analysis shows that these leaf traits set up a trade‐off between carbon‐ and nitrogen‐use efficiency at the scale of individual trees and therefore determine competitively dominant leaf strategies. As soil nitrogen availability increases, the dominant leaf strategy switches from one that is high in nitrogen‐use efficiency to one that is high in carbon‐use efficiency or, equivalently, from high‐LMA/long‐lived leaves (i.e., evergreen) to low‐LMA/short‐lived leaves (i.e., deciduous). In a region of intermediate soil nitrogen availability, the dominant leaf strategy may be either deciduous or evergreen depending on the initial conditions of plant trait abundance (i.e., founder controlled) due to feedbacks of leaf traits on soil nitrogen mineralization through litter quality. Simulated successional patterns by LM3‐PPA from the leaf physiological trade‐offs are consistent with observed successional dynamics of evergreen and deciduous forests at three sites spanning the temperate to boreal zones.     

Responses of alpine shrubs to simulated environmental change during three years in the mid-latitude mountain, northern Japan

Effects of artificial warming on phenology, individual leaf traits, vegetative growth, and reproduction of five alpine species (two deciduous and three evergreen shrubs) were investigated during three years in the mid-latitude alpine, northern Japan. Eleven open-top chambers (OTCs) were set up on a fellfield (1680 m a. s. l.) in the Taisetsu Mountains by which air temperature at plant height was increased by ca 2°C. Vaccinium uliginosum (deciduous shrub) showed earlier leaf emergence in every season and earlier flowering only in the first season in the OTCs. By contrast, acceleration of leaf emergence in the OTCs was not clear for other species, i.e. Arctous alpinus (deciduous shrub). Ledum palustre. V. vitis-idaea , and Empetrum nigrum (evergreen shrub). Both deciduous species showed longer leaf life-span in the OTCs every season. All evergreen species had higher leaf survival rates in the OTCs. indicating extension of leaf life-span. Leaf nitrogen concentration and leaf mass per unit leaf area (mg cm ⁻²) generally tended to decrease in the OTCs. Relationships between the individual leaf traits and cumulative air temperature during the leaf developing period were not clear. Total leaf production during the three seasons increased in the OTCs in A. alpinus. L. palustre. V. vitis-idaea , and E. nigrum. All evergreen shrubs showed larger shoot growth in the OTCs but both deciduous shrubs did not show significant changes. In contrast to the vegetative growth, deciduous shrubs produced more flowers in the OTCs. Fruit production was not influenced by the OTCs for all species. The extension of photosynthetic period in the OTCs may contribute to the larger vegetative growth or flower production.     

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub‐dominated and evergreen/graminoid‐dominated community‐level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant‐community‐specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra‐specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10‐day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.     

湖北星斗山地形变化对不同生活型植物叶功能性状的影响

探究地形变化对不同生活型植物叶功能性状的影响有助于深入理解森林群落物种组成的维持特征。该研究以湖北星斗山常绿落叶阔叶混交林为研究对象, 测量了50个样地中224种木本植物的叶面积、叶厚度、叶干质量、叶干物质含量和比叶面积, 运用单因素方差分析揭示了乔木、灌木和木质藤本的叶功能性状变异特征, 并采用偏曼特尔检验分别从群落水平和物种水平分析了地形变化对不同生活型木本植物叶功能性状的影响。研究发现: 不同生活型植物叶性状变异系数分布范围为23.42%-110.45%; 不同生活型之间的植物叶功能性状差异明显。群落水平上, 海拔与乔木叶干质量、灌木叶面积和木质藤本叶厚度显著正相关, 坡度仅对灌木和木质藤本比叶面积具有显著影响, 坡向与灌木叶厚度、叶干质量和比叶面积显著正相关。物种水平上, 海拔比坡度和坡向对植物叶功能性状影响更为显著, 且不同物种对地形变化的敏感度不一致; 在控制空间结构影响后, 地形因子对植物叶功能性状的影响降低。该研究结果表明, 不同生活型植物的叶功能性状对地形变化的响应格局不同, 这可能是星斗山常绿落叶阔叶混交林植物多样性的主要维持机制。     

Relationship between plant nitrogen conservation strategies and the dynamics of soil nitrogen in the arid Patagonian Monte, Argentina

During three consecutive years with contrasting precipitation, we analysed the relationship between strategies of N conservation in the dominant plant functional groups (perennial grasses and evergreen shrubs) of the Patagonian Monte and the main components of N cycling in soil. We hypothesised that the different patterns of N conservation in perennial grasses and evergreen shrubs would have direct consequences for soil-N, inorganic-N release and microbial-N flush in soil. In autumn and late spring of 1999, 2000, and 2001, we assessed N and C concentration in green and senesced leaves, N-resorption efficiency and C/N ratio in senesced leaves of three dominant species of each plant functional group. In the soil associated with species of each plant functional group, we determined N and C concentration, potential-N mineralisation, and the associated microbial-N flush. Slow-growing evergreen shrubs exhibited low N-concentration in green leaves, high N-concentration in senesced leaves and low N-resorption from senescing leaves. In contrast, fast-growing perennial grasses showed high N-concentration in green leaves, low N-concentration in senesced leaves, and high N-resorption from senescing leaves. In evergreen shrubs, the maintenance of long-lasting green leaves with low N-concentration was the most important mechanism of N conservation. In contrast, perennial grasses conserved N through high N-resorption from senescing leaves. Soil-N concentration, potential N-mineralisation, and microbial-N flush in the soil were higher underneath evergreen shrubs than beneath perennial grasses. Observed differences, however, were lower than expected considering the quality of the organic matter supplied by each plant fuctional group to the soil. A possible reason for this relatively weak trend may be the capacity of evergreen shrubs to slow down N cycling through low leaf turnover and the presence of secondary compounds in leaves. Alternatively or simultaneously, the weak relationship between plant and soil N could result from shrubs being able to colonise N-poor soils while grasses may preferably occupy fertile microsites previously influenced by the decomposition pathway of evergreen shrubs. Differences between evergreen shrubs and perennial grasses in the mechanisms of plant N-conservation and in components of N cycling in the underlying soil were consistent over the three years of the study with differing precipitation. Inter-annual differences in N concentration in green leaves and in the microbial-N flush in soil indicate that during the wettest year fast-growing perennial grasses would outcompete slow-growing evergreen shrubs and microorganisms for N uptake.     

Aboveground biomass allocation,leaf growth,and photosynthesis patterns in tundra plant forms in arctic Alaska

Summary Tundra plant growth forms can generally be characterized as consisting predominantly of low-growing perennial grasses and sedges, perennial herbaceous forbs, dwarf deciduous shrubs, and dwarf evergreen shrubs. Gross aboveground carbon allocation, leaf growth, and photosynthesis pattern studies were initiated to develop a quantitative understanding of the functional importance of these particular tundra growth forms. Photosynthetic capacities of 13 species were determined under standardized exposure conditions using a¹⁴CO₂ field system and ranged between 5 and 47 mg CO₂·g dry wt^-1·h^-1. These results, in conjunction with detailed leaf growth determinations, support the generalization that species with an evergreen growth form have lower photosynthetic capacities than species with a perennial graminoid, forb, or deciduous shrub growth form. However, these low photosynthetic capacities in evergreen shrubs are associated with relatively extended leaf longevities. Conversely, deciduous shrub forms exhibited high photosynthetic capacities, but were offset by relatively short leaf longevity periods. The perennial grasses, sedges, and forbs showed patterns intermediate to these. As a result, it appears that among tundra species of different growth form, photosynthetic capacity is inversely related to leaf longevity.     

Periodicity of leaf growth and leaf dry mass changes in the evergreen and deciduous species of Southern Assam,India

The study described patterns of leaf dry mass change, leaf mass per area (LMA), relative growth rate and leaf life span (LL) for 14 evergreen and 7 deciduous species of a tropical forest of Southern Assam, India. Leaf expansion in both the groups was, in general, completed before June (i.e. well before the onset of monsoon rains). Although leaf dry mass during leaf initiation phase was significantly higher (P < 0.01) in evergreen species than in deciduous species, at the time of full leaf expansion, average leaf dry mass relative to the peak leaf dry mass, realised by the evergreen species was lower (66 %) than for deciduous species (76 %). Leaf dry mass increase in both groups continued after leaf full expansion. Evergreen species had a longer leaf dry mass steady phase than deciduous species (2–6 vs 2–3 months). Average LMA of mature leaves for evergreen species (77.43 g m^?2) was significantly greater than that of deciduous species (48.43 g m^?2). LL ranged from 165 days in Gmelina arborea (deciduous) to 509 days in Dipterocarpus turbinatus (evergreen). LMA was correlated positively with LL, indicating that evergreen species with higher leaf construction cost retain leaves for longer period to pay back. The average leaf dry mass loss before leaf shedding was greater (P < 0.01) for deciduous species (30.29 %) than for evergreen species (18.31 %). Although the cost of leaf construction in deciduous species was lower than for evergreen species, they replace leaves at a faster rate. Deciduous species perhaps compensate the cost involved in faster leaf replacement through higher reabsorption of dry mass during senescence, which they remobilise to initiate growth in the following spring when soil resources remain limiting.     

Meaningful traits for grouping plant species across arid ecosystems

Grouping species may provide some degree of simplification to understand the ecological function of plants on key ecosystem processes. We asked whether groups of plant species based on morpho-chemical traits associated with plant persistence and stress/disturbance resistance reflect dominant plant growth forms in arid ecosystems. We selected twelve sites across an aridity gradient in northern Patagonia. At each site, we identified modal size plants of each dominant species and assessed specific leaf area (SLA), plant height, seed mass, N and soluble phenol concentration in green and senesced leaves at each plant. Plant species were grouped according with plant growth forms (perennial grasses, evergreen shrubs and deciduous shrubs) and plant morphological and/or chemical traits using cluster analysis. We calculated mean values of each plant trait for each species group and plant growth form. Plant growth forms significantly differed among them in most of the morpho-chemical traits. Evergreen shrubs were tall plants with the highest seed mass and soluble phenols in leaves, deciduous shrubs were also tall plants with high SLA and the highest N in leaves, and perennial grasses were short plants with high SLA and low concentration of N and soluble phenols in leaves. Grouping species by the combination of morpho-chemical traits yielded 4 groups in which species from one growth form prevailed. These species groups differed in soluble phenol concentration in senesced leaves and plant height. These traits were highly correlated. We concluded that (1) plant height is a relevant synthetic variable, (2) growth forms adequately summarize ecological strategies of species in arid ecosystems, and (3) the inclusion of plant morphological and chemical traits related to defenses against environmental stresses and herbivory enhanced the potential of species grouping, particularly within shrubby growth forms.     

城市工业区园林绿地滞尘效应的研究--以武汉钢铁公司厂区绿地为例

在测定武钢工业区园林植物滞尘能力的基础上,结合绿地实地调查和武钢工业区园林绿地GIS,比较分析了园林绿地不同植物种类和层片类型的滞尘能力,并定量研究了武钢厂区园林绿地的滞尘效应.结果表明,不同园林植物的滞尘能力有较大差异,这主要与不同植物的叶表面特性、树冠结构、枝叶密集程度等差异有关.不同层片类型滞尘能力依次为落叶阔叶灌木＞常绿阔叶灌木＞绿篱＞常绿阔叶乔木＞落叶阔叶乔木＞针叶乔木＞草本.武钢厂区园林绿地的日滞尘量是9.51 t,年总滞尘量为3 089.98 t.其中常绿阔叶乔木层片年滞尘量最大,达991.68 t,落叶阔叶灌木层片最小,只有0.0181 t.由于园林绿地的滞尘量取决于所构成植物的滞尘能力及其叶面积绿量,因此,选择滞尘能力强的植物,并以乔灌草不同生活型植物进行合理配置,是提高园林绿地滞尘效应的有效途径.     

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.     

Responses of leaf structure and photosynthetic properties to intra-canopy light gradients: a common garden test with four broadleaf deciduous angiosperm and seven evergreen conifer tree species

Spectra of leaf traits in northern temperate forest canopies reflect major differences in leaf longevity between evergreen conifers and deciduous broadleaf angiosperms, as well as plastic modifications caused by within-crown shading. We investigated (1) whether long-lived conifer leaves exhibit similar intra-canopy plasticity as short-lived broadleaves, and (2) whether global interspecific relationships between photosynthesis, nitrogen, and leaf structure identified for sun leaves adequately describe leaves differentiated in response to light gradients. We studied structural and photosynthetic properties of intra-tree sun and shade foliage in adult trees of seven conifer and four broadleaf angiosperm species in a common garden in Poland. Shade leaves exhibited lower leaf mass-per-area (LMA) than sun leaves; however, the relative difference was smaller in conifers than in broadleaves. In broadleaves, LMA was correlated with lamina thickness and tissue density, while in conifers, it was correlated with thickness but not density. In broadleaves, but not in conifers, reduction of lamina thickness was correlated with a thinner palisade layer. The more conservative adjustment of conifer leaves could result from a combination of phylogenetic constraints, contrasting leaf anatomies and shoot geometries, but also from functional requirements of long-lived foliage. Mass-based nitrogen concentration (N(mass)) was similar between sun and shade leaves, and was lower in conifers than in deciduous broadleaved species. Given this, the smaller LMA in shade corresponded with a lower area-based N concentration (N(area)). In evergreen conifers, LMA and N(area) were less powerful predictors of area-based photosynthetic rate (A (max(area))) in comparison with deciduous broadleaved angiosperms. Multiple regression for sun and shade leaves showed that, in each group, A (max(mass)) was related to N(mass) but not to LMA, whereas LMA became a significant codeterminant of A (max(mass)) in analysis combining both groups. Thus, a fundamental mass-based relationship between photosynthesis, nitrogen, and leaf structure reported previously also exists in a dataset combining within-crown and across-functional type variation.     

Short-term effects of simulated environmental change on phenology, leaf traits, and shoot growth of alpine plants on a temperate mountain, northern Japan

In order to assess the responses of circumpolar and semicircumpolar plants growing around their southern distribution margins to artificial warming, we set up 11 open-top chambers (OTCs) on a fell-field (1680 m a.s.l.) in the Taisetsu Mountains, northern Japan. The OTCs increased mean air temperature by 1.3°C through the growing season (June–September) and extended the length of the growing season. We examined phenology and leaf traits of plants in the OTCs and control plots during the first season under artificial warming treatment using two deciduous and three evergreen species. Ledum palustre (evergreen shrub), Vaccinium uliginosum , and Arctous alpinus (deciduous shrubs) showed earlier leaf emergence and/or flowering in the OTCs. Deciduous shrubs had longer individual leaf longevity and an extended foliage period in the OTCs than in the control plots. There were no significant differences in specific leaf area and leaf size for many species between the OTCs and the control plots. Vaccinium vitis-idaea (evergreen shrub), L. palustre, A. alpinus , and Empetrum nigrum (evergreen shrub) had lower leaf nitrogen concentration in the OTCs than in the control plots, whereas it was higher in V. uliginosum . Only E. nigrum showed larger annual shoot growth in the OTCs. No clear differences in response to the warming effect were detected between evergreen and deciduous species in the first season. Circumpolar plants growing in temperate alpine regions may be more affected by season length rather than temperature itself.     

Comparative ecophysiological responses to drought of two shrub and four tree species from karst habitats of southwestern China

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F _v/F _m) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri’s weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.