Biomass responses to elevated CO<Subscript>2</Subscript>, soil heterogeneity and diversity: an experimental assessment with grassland assemblages

While it is well-established that the spatial distribution of soil nutrients (soil heterogeneity) influences the competitive ability and survival of individual plants, as well as the productivity of plant communities, there is a paucity of data on how soil heterogeneity and global change drivers interact to affect plant performance and ecosystem functioning. To evaluate the effects of elevated CO₂, soil heterogeneity and diversity (species richness and composition) on productivity, patterns of biomass allocation and root foraging precision, we conducted an experiment with grassland assemblages formed by monocultures, two- and three-species mixtures of Lolium perenne, Plantago lanceolata and Holcus lanatus. The experiment lasted for 90 days, and was conducted on microcosms built out of PVC pipe (length 38 cm, internal diameter 10 cm). When nutrients were heterogeneously supplied (in discrete patches), assemblages exhibited precise root foraging patterns, and had higher total, above- and belowground biomass. Greater aboveground biomass was observed under elevated CO₂. Species composition affected the below:aboveground biomass ratio and interacted with nutrient heterogeneity to determine belowground and total biomass. Species richness had no significant effects, and did not interact with either CO₂ or nutrient heterogeneity. Under elevated CO₂ conditions, the two- and three-species mixtures showed a clear trend towards underyielding. Our results show that differences among composition levels were dependent on soil heterogeneity, highlighting its potential role in modulating diversity–productivity relationships. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible to authorized users.     

Soil heterogeneity and community composition jointly influence grassland biomass

Question: Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness? Location: Duke University Phytotron (Durham, North Carolina, USA). Methods: We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations of Lolium perenne, Poa pratensis and Plantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous‐up, and heterogeneous‐down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively). Results: Communities containing Plantago and Lolium responded to nutrient heterogeneity by increasing above‐ and below‐ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition X nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch. Conclusions: Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small‐scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.     

Inorganic Nutrient Supplements Constrain Restoration Potential of Seedlings of the Seagrass,Posidonia australis

Seed represents a potentially ecologically sustainable source of planting units for restoring seagrasses, particularly for seagrasses where transplanting negatively impacts donor beds. However, newly germinated seeds may be nutrient limited as their underdeveloped root systems may constrain capacity to access sediment‐based resources. We conducted a study in land‐based aquaculture tanks to determine whether early growth of newly germinated Posidonia australis seedlings could be enhanced by adding inorganic nutrients to the sediment. Sediments were supplemented with nitrogen and phosphorus in a factorial design (no nutrients, N, P, N + P). Shoot survival, whole shoot biomass, root morphology, root architecture, and nutrient concentration of seedlings were assessed monthly for the first 4 months after germination. More than 90% of seedlings survived during the 4 months of the experiment, irrespective of nutrient treatment. Growth of P. australis seedlings was not enhanced by addition of N or P to the sediment despite nutrient uptake occurring. Seedling growth was found to be more dependent on seed nutrient reserves rather than external nutrient sources for at least the first 4 months after germination. Adding inorganic nutrients to the sediment also significantly reduced the development of the seedling root system in terms of biomass, length, and density of lateral root branches. This study demonstrated that inorganic nutrient supplements constrain root development and therefore capacity for successful anchorage of seagrass seedlings, and pose a significant limitation on seedling establishment when transferred to the field, as well as potentially limiting natural and transplanted seedling establishment in eutrophic sediments.     

Resource allocation in the submerged plant Vallisneria natans related to sediment type, rather than water-column nutrients

1. Phenotypic plasticity in resource allocation by Vallisneria natans was investigated in a greenhouse experiment, using three types of sediment [sandy loam, clay, and a 50 : 50 (by volume) mixture of the two sediments] and two levels of water‐column nutrient. The clay was collected from a highly eutrophic lake in Jiangsu Province, China, and the N and P concentrations applied in nutrient media were at the upper limits observed in most lakes of China. 2. Growth and biomass allocation were significantly affected by sediment type, rather than water‐column nutrients. Plant growth in clay and the mixture were similar, and 2.4–3.4 times higher than that in sandy loam. Compared with the plants grown in clay or the mixed sediments, the plants grown in sandy loam allocated relatively more biomass to root (11–17% versus 7–8% of total biomass), and relatively less to leaf (76–82% versus 86–87% of total biomass). Plastic variations in root area were induced by sediment type alone (P < 0.05), whereas the impacts of sediment type and water‐column nutrients on leaf area were insignificant (P > 0.05). 3. Plant N and P concentrations were significantly affected by both sediment type and water‐column nutrients. Increased nutrient availability in the water column enhanced plant N concentration by 3.5–20.2%, and plant P concentration by 19.1–25.8%. 4. Biomass accumulation and plant nutrient concentration in plants grown in different sediment types and water‐column nutrients indicate that sediment type had more significant impacts on growth and N and P concentrations of V. natans than did water‐column nutrients. Changes in phenotype are a functional response to nutrient availability in sediment, rather than to water‐column nutrients.     

Plant morphology and root hydraulics are altered by nutrient deficiency in Pistacia lentiscus (L.)

The plants in arid and semiarid areas are often limited by water and nutrients. Morpho-functional adjustments to improve nutrient capture may have important implications on plant water balance, and on plant capacity to withstand drought. Several studies have shown that N and P deficiencies may decrease plant hydraulic conductance. Surprisingly, studies on the implications of nutrient limitations on water use in xerophytes are scarce. We have evaluated the effects of strong reductions in nitrogen and phosphorus availability on morphological traits and hydraulic conductance in seedlings of a common Mediterranean shrub, Pistacia lentiscus L.. Nitrogen deficiency resulted in a decrease in aboveground biomass accumulation, but it did not affect belowground biomass accumulation or root morphology. Phosphorus-deficient plants showed a decrease in leaf area, but no changes in aboveground biomass. Root length, root surface area, and specific root length were higher in phosphorus-deficient plants than in control plants. Nitrogen and phosphorus deficiency reduced both root hydraulic conductance and root hydraulic conductance scaled by total root surface area. On the other hand, nutrient limitations did not significantly affect root conductance per unit of foliar surface area. Thus, adaptation to low nutrient availability did not affect seedling capacity for maintaining water supply to leaves. The implications for drought resistance and survival during seedling establishment in semi-arid environments are discussed.     

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N₂-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.     

Root morphological and physiological plasticity of perennial grass species and the exploitation of spatial and temporal heterogeneous nutrient patches

Root morphological and physiological characteristics of four perennial grass species were investigated in response to spatial and temporal heterogeneous nutrient patches. Two species from nutrient-rich habitats (i.e. Holcus lanatus and Lolium perenne) and two species from nutrient-poor habitats (i.e. Festuca rubra and Anthoxanthum odoratum) were included in the study. Patches were created by injecting equal amounts of nutrient solution into the soil either on one location (i.e. spatial heterogeneity) or on several, alternating locations (i.e. temporal heterogeneity) within the pot. The consequences of changes in root morphology and the implications for the exploitation of the nutrient patches by individual plants were quantified by the amount of ¹⁵N captured from the enriched patches. The effects of nutrient heterogeneity on the acquisition of nutrients by species were determined by comparing the total nitrogen and phosphorus acquisition of the species in the two heterogeneous habitats with the total nitrogen and phosphorus acquisition in a homogeneous treatment. In this homogeneous treatment the same amount of nutrient solution was supplied homogeneously over the soil surface. The experiment lasted for 27 days and comprised one harvest. In response to the spatial enrichment treatment, all species produced significantly more root biomass within the enriched patch. The magnitude of the response was similar for species from nutrient-rich and nutrient-poor habitats. In contrast to this response of root biomass, root morphology, including specific root length, branching frequency and mean lateral root length was not affected by the treatments. In response to the temporal enrichment treatment, all species were able to increase the nitrogen uptake rate per unit of root biomass. The species from nutrient-poor habitats had, on average, higher uptake rates per unit root biomass than the species from nutrient-rich habitats, but the magnitude of the response did not differ between the species. These results question the general validity of the assumptions that root foraging characteristics differ among species from nutrient-rich and nutrient-poor habitats. As a result of these root responses, all species captured an equal amount of ¹⁵N from the spatial and temporal enriched nutrient patches and all species acquired significantly more nitrogen in the heterogeneous treatments than in homogeneous treatment. Hence, the ability to exploit local and temporal nutrient heterogeneity does not appear to differ between species from nutrient-rich and nutrient-poor habitats, but is achieved by these species in different ways. The ecological implications of these differences are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同配比施肥对马尾松幼苗生长特征的影响

为获得马尾松幼苗最佳施肥配方,该文以1年生马尾松幼苗为试验材料,采用L₁₆（4³）正交设计,并通过测定幼苗苗高、地径、生物量、叶绿素含量、叶片N、P、K含量,探讨不同N、P、K配比施肥对马尾松幼苗生长特征影响。结果表明：（1）不同配比施肥处理间马尾松幼苗苗高、地径、生物量、质量指数、叶绿素和养分含量存在显著差异,其中,处理12生物量、质量指数、叶绿素a和总叶绿素含量、隶属值最高。（2）施N对幼苗生长及生理指标均有极显著影响;施K对苗高、地径、地上生物量、总生物量有显著影响,对叶绿素和针叶养分有极显著影响;施P对叶绿素a、叶绿素b、针叶N和P含量有极显著影响,对苗高、地下生物量、总叶绿素含量有显著影响。（3）施N对苗高、地径、地上生物量、总生物量、质量指数、叶绿素a含量、总叶绿素含量和针叶N含量的影响最大,K次之,P最小。各因素对地下生物量和针叶P含量的影响均表现为N>P>K。（4）N3水平利于幼苗苗高地径的生长及生物量的积累,N4水平利于叶绿素a和总叶绿素含量及针叶N、P含量的积累,P4水平利于生物量、叶绿素含量和养分P含量的积累...     

不同生境对苦竹鞭根形态结构及其异速生长的影响北大核心CSCD

鞭根作为竹子吸收养分和水分的主要器官,其形态结构性状与鞭根对养分斑块的敏感性及养分获取能力紧密相关。该研究选取相邻连续的苦竹(Pleioblastus amarus)纯林和苦竹-杉木(Cunninghamia lanceolata)混交林2种林分类型,将其分为苦竹林中心区、苦竹林界面区、混交林界面区和混交林中心区4种生境,测定4种林区生境的苦竹鞭根形态结构性状指标及生物量,比较其间的连续性变化规律,以明确竹子异质性环境下的生态适应策略。结果表明:(1)不同生境下,纯林界面区的苦竹拥有更高的鞭根节点数、根尖数以及更小的根直径;纯林界面区和混交林界面区的苦竹鞭根比根长、比根面积均显著高于纯林中心区,但两个界面区的苦竹鞭根根直径则表现相反。(2)从苦竹纯林中心区至混交林中心区方向,苦竹鞭根生物量呈逐渐降低的趋势,但苦竹林界面区和混交林界面区间差异不显著。(3)生境对苦竹主要鞭根形态结构性状异速增长速率无明显影响,但显著提高了苦竹林界面区鞭根主要形态结构性状的差异性位移量;不同生境下苦竹鞭根形态结构存在显著差异,苦竹纯林界面区的鞭根形态结构可塑性较强,拥有更高的鞭根活性以及更活跃的生理功能。研究发现,生境对苦竹主要鞭根形态结构性状有显著影响,但对其异速增长速率无明显影响;鞭根直径是苦竹获取资源的重要影响因子,异质生境下苦竹趋向于采取增加鞭根面积和降低鞭根直径的策略以最大化地获取资源。     

光照和氮磷供应比对木荷生长及化学计量特征的影响

光照和养分限制是影响林下植物生长和更新的关键影响因素,以亚热带主要常绿树种木荷（Schima superba）实生幼苗为试验对象,研究了不同光照（全光照、遮阴即45%全光照）和N、P供应比例（5,15,45）对幼苗生长和化学计量特征的影响。结果表明：（1）遮阴不仅严重抑制了木荷各器官和单株生物量积累,更加剧了P限制。尽管N、P添加对木荷生长没有显著促进作用,但N、P供应比例为5时的性状组合更有利于木荷后期生长,但高N、P供应比例可能导致P限制。（2）遮阴下叶N、P含量显著增加,但叶C/N和C/P比显著降低;不同光照处理组中各器官及总N含量均随N、P供应比例增大而显著增加,而C/N比逐渐降低;P的分配格局发生改变,全光照组各器官P含量为茎 > 叶 > 根,遮阴组各器官P含量为根 > 茎 > 叶。（3）随N、P供应比例增加或光照强度降低,木荷均趋向降低根冠比和根质比、增加叶质比或茎质比。（4）木荷生物量与各器官N、P含量、叶质比呈极显著负相关,而与C/N和C/P比及根冠比、茎质比、根质比呈极显著正相关。光强和N、P比例变化均显著影响了木荷幼苗的养分利用特征,因而木荷作为伴生树种优化林分环境对其早期生长具有重要意义。     

Mycorrhizal Networks Interacting with Litter Improves Nutrients and Growth for One Plant through the Vary of N/P Ratio under Karst Soil

Arbuscular mycorrhizae (AM) fungi affect nutrient uptake for host plants, while it is unclear how AM fungi interacting with soil litter affect plant growth and nutrient utilization through mycorrhizal networks in karst soil of deficient nutrients beyond the rhizosphere. An experiment was conducted in a microcosm composed of a planting compartment for Cinnamomum camphora seedlings with or without Glomus mosseae fungus (M⁺ vs. M⁻ ) and an adjacent litter compartment containing or not containing additional litter material of Arthraxon hispidus (L⁺ vs. L⁻ ), where the compartments are connected either by nylon mesh of 20 μm or 0.45 μm which either allow available mycorrhizal networks within the litter compartment or prevent mycelium entering into the litter compartment (N⁺ vs. N⁻ ). Plant biomass and nutrients were measured. The results showed that the addition of litter changed the symbiotic process in mycorrhizal colonization, spore, and hyphal density, which when in association with the host plant then affected the biomass, and accumulations of N (nitrogen) and P (phosphorus) in the individual plant as well as root, stem, and leaf respectively. AM fungi increased N and P accumulations and N/P ratio in individual plants and plant tissues. A decrease of the N/P ratio of the individual plant was observed when AM fungus interacted significantly with litter through mycorrhizal networks in the litter compartment. The results indicate that the C. camphora seedlings benefited from litter in nutrient utilization of N and P through the vary of N/P ratio when accessing mycorrhizal networks. These findings suggest that mycorrhizal networks interacting with litter improve growth and nutrients of N and P for plants through the vary of N/P ratio in order to alleviate nutrient limitation under karst soil.     

土壤氮素异质性分布和马先蒿寄生对长芒棒头草生长发育及根系分布的影响

无论在农田还是自然生态系统中,土壤养分异质性普遍存在。植物具有感知土壤养分异质性的能力,并通过调节根系生物量分配及空间分布以获取更多资源。了解寄生胁迫在不同养分条件下对寄主生长发育及根系空间分布的影响,对解析寄主应对寄生胁迫和养分胁迫的适应策略,进而指导寄生性杂草防控具有重要的指导意义。该文采用分根试验,通过对寄主分根,并控制根室两侧氮供应水平及寄生胁迫程度,考察了氮胁迫及两种寄主依赖程度不同的马先蒿的寄生对寄主长芒棒头草生长发育及根系空间分布的影响。结果表明:(1)土壤氮水平与马先蒿寄生均可显著影响长芒棒头草生物量及根冠比,并且两者之间存在显著交互作用,其中土壤氮水平为主要影响因子。(2)两种马先蒿对长芒棒头草的危害程度不同。在NPK和2NPK 处理时,三色马先蒿的寄生显著降低长芒棒头草生物量(茎叶:37.1%、51.5%; 根系:35.6%、63.6%); 在NPK处理时,大王马先蒿的寄生显著增加长芒棒头草生物量(茎叶:29.9%,根系:61.2%)。(3)长芒棒头草的根系生长和空间分布受氮营养的异质分布和寄生的影响,具有明显的感知养分空间分布及调节根系生长能力。     

遮荫对青桐幼苗生长性状与化学计量特征的影响

营养元素含量及化学计量比可反映植物器官营养元素的分配及互作关系,亦可反映其营养利用效率及生长环境的养分限制状况。以青桐(Firmiana platanifolia)幼苗为材料,分析4种不同光强(全光照,50%、75%和95%遮荫)对青桐幼苗生长,C、N、P、K含量及其化学计量特征的影响。结果表明:遮荫对青桐幼苗的生长性状、生物量、元素含量和积累量及化学计量比均有显著影响(P<0.05)。苗高和比叶面积随遮荫强度增加而升高,而地径和粗壮度随遮荫强度增加而减小。总生物量在75%遮荫下最大,在95%遮荫下最小。在各遮荫处理下,叶片C、N、P、K含量均大于根和茎,且N含量顺序为叶>根>茎,K含量顺序为叶>茎>根。C、P、K积累量随遮荫强度增大而增加,在75%遮荫下达到最大值,而在95%遮荫下达到最小值。青桐幼苗C∶N和C∶P呈现出相同的变化趋势,随遮荫强度变大先增加后减小,均在50%遮荫下达最大值。青桐各器官N∶P远低于14,即青桐幼苗在不同遮荫条件下的生长严重受N限制。这些结果说明,遮荫可调节青桐幼苗在不同光环境中的养分积累与分配,进而影响生物量。因此,青桐苗期培育宜选75%遮荫,有助于苗高、比叶面积、生物量、养分含量和积累量的增加,而过度遮荫可抑制幼苗生长发育。     

黑果枸杞功能性状对氮磷添加的响应及其可塑性

分析养分添加对荒漠植物功能性状的影响,对揭示其响应和适应环境变化的规律至关重要。本研究以黑果枸杞为材料,设置3个氮磷(NP)添加量(低、中、高)和N/P(5∶1、15∶1、45∶1),量化分析了整株、根、茎、叶和果实性状对NP添加的响应。结果表明: 黑果枸杞功能性状差异化响应了NP添加量和比例,随NP添加量的增加,生物量和比叶面积增加,根冠比、叶干物质含量、根组织密度和比根长降低;随N/P的提高,地下生物量、比根长和净光合速率增大。17个功能性状指标的变异系数为7.3%～69.1%,生物量、根冠比和比根长为响应氮磷的敏感性状(可塑性指数PI>0.5),变异性较大(49.4%～69.1%);而叶长宽比、叶厚、叶组织密度、叶茎干物质含量为惰性性状(PI<0.20)。主成分分析(PCA)结果显示,黑果枸杞在多元特征空间的位置随NP添加量横向迁移,趋向于地上、地下生物量更大、根冠比更小的策略;同时叶组织密度与叶厚、比叶面积呈负相关;叶干物质含量与叶厚、比叶面积呈负相关,与叶组织密度呈正相关;生物量与比叶面积呈正相关,与比根长呈负相关。逐步回归分析进一步表明,比根长、比叶面积和叶片净光合速率是影响黑果枸杞生物量的主要功能性状。黑果枸杞通过资源利用策略的转变、根系碳分配的改变以及性状的权衡协变与非一致性响应适应土壤养分环境的波动。     

Humid tropical leguminous tree and pasture grass responsiveness to vesicular-arbuscular mycorrhizal infection

Phosphorus is the major nutrient limiting plant growth in a Costa Rican silvopastoral system located on an acid, high P-retaining, volcanic soil. We investigated plant responsiveness to vesicular-arbuscular mycorrhizal (VAM) inoculation using the leguminous tree species Erythrina berteroana Urban, and the two dominant grass species Paspalum conjugatum Berg and Homolepsis aturensis Chase of this silvopastoral system. We grew grass seedlings in the greenhouse for 15 weeks in a methyl bromide-sterilized study soil to which either mixed-species VAM inoculum (Theobroma cacao feeder roots) or autoclave-sterilized cacao roots (non-inoculated control) were added. E. berteroana was grown from both seedlings and vegetative stakes (40 cm long) for 30 and 19 weeks, respectively. Upon harvest, we measured above and below ground biomass, N and P content, rootshoot ratio, legume nodulation, and VAM infection levels. The total above-ground and root biomass of mycorrhizae-inoculated P. conjugatum seedlings were 2.5 and 2.8 times greater than those of noninoculated seedlings. In contrast, VAM-inoculated seedlings of H. aturensis produced 8.4 and 5.9 times more total above-ground and root mass than noninoculated seedlings. Mycorrhizae-inoculated E. berteroana seedlings produced 10.6 times greater shoot biomass for inoculated versus noninoculated seedlings, while E. berteroana vegetative stakes exhibited a negative growth response to VAM inoculation (an approximately 16% decrease in shoot biomass for VAM-inoculated cuttings). The difference in responsiveness between Erythrina growth forms is hypothesized to reflect the cost-benefit relationship between plant host and fungal symbiont for energy and nutrient reserves.     

Population-level responses to nutrient heterogeneity and density by Abutilon theophrasti (Malvaceae): an experimental neighborhood approach

An experimental approach was used to examine the effects of spatial nutrient heterogeneity and planting density on the sizes of plants within populations of Abutilon theophrasti. Planting locations were generated using random numbers and replicated among populations growing on two different scales of heterogeneity and homogeneous soils. The same quantity of nutrients (dehydrated cow manure) was added to each population, regardless of the spatial nutrient distribution. The higher density was achieved by adding additional planting locations to those present at the lower density. Plant biomass was compared among ten planting locations present in all populations. Plants in seven locations were smaller at the higher density, but the spatial distribution of nutrients affected plant size in only two locations. At the population level, the higher density reduced mean plant biomass and increased both total biomass and the coefficient of variation in biomass, a measure of size inequality. Only when populations on both scales of heterogeneity were together compared with those on homogeneous soils were population-level measurements found to be significantly affected by soil treatment; heterogeneity resulted in decreased total biomass and an increase in the coefficient of variation, apparently due to an increase in the number of small plants in the population. These results, together with the finding that fine root biomass increased in nutrient-enriched patches, suggest that on heterogeneous soils most plants were able to access nutrient patches.     

Effect of phosphorus fertilization on water stress in Douglas fir seedlings during soil drying

A growth chamber experiment was conducted to determine if P fertilization to enhance the P nutrition of otherwise N and P deficient Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings reduces water stress in the seedlings during drought periods. Seedlings were grown in pasteurized mineral soil under well-watered conditions and fertilized periodically with a small amount of nutrient solution containing P at either of three levels: 0, 20, or 50 mg P L^-1. By age 6 mo, leaf nutrient analysis indicated that N and P were deficient in control (0 mg P L^-1) seedlings. The highest level of P fertilization, which doubled leaf P concentration, did not affect plant biomass, suggesting that N deficiency was limiting growth. When these seedlings were subjected to drought, there was no effect of P fertilization on leaf water potential or osmotic potential. Furthermore, P fertilized seedlings had lower stomatal conductance and net photosynthesis rate. These results indicate that enhanced P nutrition, in the presence of N deficiency, does not reduce water stress in Douglas fir seedlings during drought periods.     

The effect of varying nitrogen and phosphorus availability on nutrient use by Larrea tridentata,a desert evergreen shrub

Summary In a phytotron study of the effects of nitrogen and phosphorus supply ratio on nutrient uptake and use by Larrea tridentata, seedlings responded to increases in N and P availability with increases in leaf size, total biomass, and leaf nutrient concentration, and with decreases in root: shoot ratio. N and P use efficiency decreased with increasing N and P availability, respectively, but increased with increasing availability of the other nutrient, suggesting that Larrea responds both to the absolute and to the relative availability of limiting nutrients. Absolute amounts of N and P resorption, as well as N and P resorption efficiencies did not demonstrate a significant trend with nutrient availability, and there was no evidence of significant interactions between the two nutrients. More studies of the effects of nutrient interactions in the cycling and use of nutrients by different plant species are needed before more general conclusions can be drawn.     

Contrasting species responses to continued nitrogen and phosphorus addition in tropical montane forest tree seedlings

Global changes in nutrient deposition rates are likely to have profound effects on plant communities, particularly in the nutrient‐limited systems of the tropics. We studied the effects of increased nutrient availability on the seedlings of six tree species in montane forests of southern Ecuador in situ. After five years of continued N, P, or N+P addition, naturally grown seedlings of each of the two most common species at each elevation (1000, 2000, and 3000 m asl) were harvested for analyses of leaf morphology, nutrient content, herbivory, and tissue biomass allocation. Most species showed increased foliar N and P concentrations after addition of each respective element. Leaf tissue N:P ratios of >20 in the control plants of all species suggest that P is more growth‐limiting in these forests than N. Leaf morphological responses to nutrient addition were species and nutrient specific, with some species (Hedyosmum purparescens, Graffenrieda emarginata) exhibiting increased specific leaf area (SLA), and others (Graffenrieda harlingii) increased leaf area ratios (LAR). Pouteria torta (1000 m) had lower SLA and LAR after P addition. Increased herbivory was only evident in G. emarginata (after N and N+P addition). Only the species from 3000 m asl modified biomass allocation after nutrient addition. In general, N and N+P addition more strongly affected the species studied at the upper elevations, whereas P addition had a similar range of effects on the species at all elevations. We conclude that the responses of the studied tropical montane forest tree seedlings to chronic N and P addition are highly species‐specific and that successful adaptation to increased nutrient availability will depend on species‐specific morphological and physiological plasticity.     

The influence of light conditions and interspecific competition on the root foraging traits and seedling growth of two tree species

Abstract

Enriched nutrient patches within natural soil represent an important source of nutrients for tree growth. In the present study, pot experiments in a heterogeneous nutrient environment were conducted to investigate the influence of light conditions and interspecific competition on the root foraging traits and seedling growth of Pinus massoniana and Schima superba. The root foraging scale and the whole-seedling biomass of both species were decreased by shading. The result of this treatment was a lower sensitivity to nutrient heterogeneity in plants that underwent the shading treatment than in plants that were exposed to full-light conditions. The above-ground biomass and whole-seedling biomass of S. superba were not affected by competition with P. massoniana. In contrast, the above-ground biomass and whole-seedling biomass of P. massoniana were negatively affected by competition with S. superba. The more rapid rate of root extension and the more efficient resource uptake of S.superba appear to explain this effect. The species-specific patterns of the influence of environmental factors on foraging ability and seedling growth should be given thorough consideration and should be applied to afforestation and to the management of tree plantations.