Biomass partitioning,architecture and turnover of six herbaceous species from habitats with different nutrient supply

Three grasses (Holcus lanatus, Anthoxanthum odoratum and Festuca ovina) and three herbs (Rumex obtusifolius, Plantago lanceolata and Hieracium pilosella) were grown in a greenhouse at 3 nutrient levels in order to evaluate plant allocation, architecture and biomass turnover in relation to fertility level of their habitats.Four harvests were done at intervals of 4 weeks. Various plant traits related to biomass partitioning, plant architecture, biomass turnover and performance were determined. Differences in nutrient supply induced a strong functional response in the species shoot:root allocation, but architecture and turnover showed little or no response. Architectural parameters like specific leaf area and specific root length, however, in general decreased during plant development.Species from more nutrient-rich successional stages were characterized by a larger specific leaf area and longer specific shoot height (height/shoot biomass), resulting in a higher RGR and total biomass in all nutrient conditions. There was no evidence that species from nutrient-poor environments had a longer specific root length or any other superior growth characteristic. The only advantage displayed by these species was a lower leaf turnover when expressed as the fraction of dead leaves and a shorter specific shoot height (SSH) which might prevent herbivory and mowing losses.The dead leaf fraction, which is a good indicator for biomass and nutrient loss, appeared to be not only determined by the leaf longevity, but was also found to be directly related to the RGR of the species. This new fact might explain the slow relative growth rates in species from a nutrient-poor habitat and should be considered in future discussions about turnover.     

Survival of three cool-temperate tree species in shade with lime and NPK applications

Knowledge of the site specificity of plastic reactions and shade tolerance of tree species within the complexity of natural forests is limited but critical to understanding forest regeneration. Paper birch (Betula papyrifera), yellow birch (B. alleghaniensis), and eastern hemlock (Tsuga canadensis) were planted in a randomized block design under natural deciduous canopy and within small canopy gaps with a single application of lime and NPK at time of planting in 2003. Saplings were excavated in 2004 and 2008 for analysis of biomass allocation and testing of foliar N. In 2008, mean survival rate differed significantly among species at 0.14 for paper birch, 0.57 for yellow birch, and 0.76 for eastern hemlock, respectively. Yellow birch emerged as a species of intermediate shade tolerance associated with a sharp decline in shoot development after two growing seasons, high leaf allocation of biomass, and foliar plasticity. Foliation and physiological adaptation of leaves appeared N-limited in yellow birch. Biomass allocation analysis revealed strong allometric growth in yellow birch and eastern hemlock across experimental treatments imposed. Increased soil reaction from pH 4.7 to 6.3 following lime applications had no discernable effect on the juvenile growth and survival of the three species during six growing seasons. The experiment underlined the need for experimental testing of species under natural forest canopy. Cyclical cutting of forests with moderate canopy opening is recommended for sustainable management of yellow birch and eastern hemlock.     

Nutrient and water availability alter belowground patterns of biomass allocation, carbon partitioning, and ectomycorrhizal abundance in Betula nigra

In managed settings, seedlings are often fertilized with the objective of enhancing establishment, growth, and survival. However, responses of seedlings to fertilization can increase their susceptibility to abiotic stresses such as drought. Seedlings acclimate to variation in soil resources by reallocating carbon among different physiological processes and compartments, such as above versus belowground growth, secondary metabolism, and support of ectomycorrhizal fungi (EMF). We examined the effects of nutrient and water availability on carbon allocation to above and belowground growth of river birch (Betula nigra), as well as partitioning among root sugars, starch, phenolics, lignin, and EMF abundance. As nutrient availability increased, total plant biomass and total leaf area increased, while percent root biomass decreased. Root sugars, total root phenolics and EMF abundance responded quadratically to nutrient availability, being lowest at intermediate fertility levels. Decreased water availability reduced total leaf area and root phenolics relative to well-watered controls. No interactions between nutrient and water availability treatments were detected, which may have been due to the moderate degree of drought stress imposed in the low water treatment. Our results indicate that nutrient and water availability significantly alter patterns of carbon allocation and partitioning in roots of Betula nigra seedlings. The potential effects of these responses on stress tolerance are discussed.     

Root architecture and allocation patterns of eight native tropical species with different successional status used in open-grown mixed plantations in Panama

We investigated biomass allocation and root architecture of eight tropical species with different successional status, as classified from the literature, along a size gradient up to 5 m. We focused on belowground development, which has received less attention than aboveground traits. A discriminant analysis based upon a combination of allocational and architectural traits clearly distinguished functional types and classified species according to successional status at a 100% success rate. For a given plant diameter, the pioneer species presented similar root biomass compared to the non-pioneer ones but higher cumulative root length and a higher number of root apices. A detailed study on the root system of a sub-sample of three species showed that the most late-successional species (Tabebuia rosea) had longer root internodes and a higher proportion of root biomass allocated to the taproot compared to the other two species (Hura crepitans and Luehea seemannii). Most pioneer species showed a higher leaf area ratio due to a higher specific leaf area (SLA). We conclude that the functional differences between pioneer and non-pioneer tree species found in natural forests were maintained in open-grown plantation conditions.     

Effects of ramet clipping and nutrient availability on growth and biomass allocation of yellow nutsedge

A glasshouse experiment was conducted to examine how the interactions of nutrient availability and partial ramet clipping affect growth, reproduction and biomass allocation of Cyperus esculentus, an invasive sedge. The plants sprouting from tubers were grown at low and high nutrient levels, and were subject either to no clipping, one, two or three clippings, with each clipping cutting half of the existing ramets at soil level. Our results show that nutrient availability and clipping frequency tended to independently affect most of growth, reproduction and biomass allocation parameters of Cyperus esculentus examined in the present study. Increased supply of nutrients led to an increase in plant productivity and its associated traits. All of the traits, except for the number of ramets, displayed a decreasing pattern with increasing clipping frequency, indicating that Cyperus esculentus had undercompensatory responses to ramet clipping. It is likely that the patterns of plants response to clipping are species specific, and depend on morphological characters of species. Its susceptibility to ramet clipping can offer opportunities for controlling this invasive species through mechanical methods such as mowing. Clipping had little effects on biomass allocation; however, root weight fraction increased with increasing clipping frequency. While nutrient availability and clipping frequency had no influence on leaf carbon concentration at harvest, both of them increased leaf nitrogen concentration, and hence reduced leaf C/N ratio.     

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.     

施氮及不同根系分隔模式对尾叶桉和降香黄檀幼苗生长及叶片生理特性的影响

豆科与非豆科树种混交作为一种人工林培育可持续发展模式,在保证木材产量和维持氮素平衡中发挥了重要作用。该研究通过大棚盆栽试验,设计3个施氮水平(0、3、6g·株-1)及3种根系分隔方式(不隔、网隔、膜隔),分析了不同氮素水平及根系分隔模式对尾叶桉(Eucalyptus urophylla)与降香黄檀(Dalbergia odorifera)植株幼苗生长、叶片生理特性、根系形态及生物量的影响。结果表明:(1)随施氮量的增加,尾叶桉与降香黄檀的苗高、地径均呈递增趋势;不同根系分隔模式下,尾叶桉苗高、地径均在不隔模式下生长最好,降香黄檀则在膜隔模式下生长最好。(2)与不施氮处理相比,施氮3、6g·株-1水平下尾叶桉与降香黄檀叶片的超氧化物歧化酶(SOD)活性、过氧化物酶(POD)活性及可溶性蛋白含量均显著升高,而丙二醛(MDA)含量则呈降低趋势。(3)尾叶桉与降香黄檀根系的总长度、总表面积、总体积、平均直径、根尖数和比根长均随氮素水平的增加而增加,且各氮素水平间的差异显著;同一氮素水平下,尾叶桉的根系生长总体表现为不隔网隔膜隔,而降香黄檀根系生长则表现为膜隔不隔网隔,且两树种不同分隔模式间差异显著。(4)尾叶桉与降香黄檀各器官生物量及总生物量均随施氮量的增加而增加,并在6g·株-1施氮水平下生物量最大;各器官生物量分配中,尾叶桉各器官生物量所占比例大小依次为:茎(40.59%)叶(32.37%)根(27.04%),降香黄檀各器官生物量所占比例大小依次为:根(47.67%)茎(40.08%)叶(12.25%)。研究表明,尾叶桉与降香黄檀混交一定程度上扩展了根系横向和纵向水平的养分生态位,扩大了根系吸收土壤养分的空间,同时根系互作提高了降香黄檀的固氮能力,对土壤有效氮的产生有较大影响。     

黄河口新生湿地碱蓬生物量及氮累积与分配对外源氮输入的响应

2014年4—11月,选择黄河入海口北部滨岸高潮滩的碱蓬湿地为研究对象,基于野外原位氮输入模拟试验,研究了不同氮输入梯度下(N0,无氮输入;N1,低氮输入;N2,中氮输入;N3,高氮输入)碱蓬不同器官生物量以及氮累积与分配特征的差异。结果表明,尽管不同氮输入处理并未改变碱蓬地上生物量的季节变化模式,但在不同程度上均促进了地上生物量的增长(平均增幅为19.71%—62.29%)且整体表现为N3N2N1N0;不同氮输入处理亦延长了碱蓬的生长高峰期,N1、N2和N3处理下地上生物量达到最大值的时间相对于N0处理推迟20 d左右。与地上生物量不同,不同氮输入处理改变了地下生物量的季节变化模式,特别是N2和N3处理均对生长初期的地下生物量产生了明显促进作用,且其初期地下生物量达到较高值的时间相对于N1和N0处理提前20—50d。不同氮输入处理下的枯落物量在产生初期和中期均增幅不大,末期则骤然增加且整体表现为N3N2N1N0。不同氮输入处理下碱蓬各器官的全氮(TN)含量总体上均表现为叶茎根,叶是氮的主要累积器官。尽管不同氮输入处理并未改变碱蓬不同器官的氮累积与分配格局以及地上与地下之间的养分供给关系,但其为适应不同养分条件而调整自身养分供给与分配的特性在N2处理下表现的尤为明显。研究发现,N2处理下碱蓬种子的发育时间相对于N0、N1和N3处理可能会提前约1个月,原因可能与N2氮输入水平可显著影响碱蓬体内的碳分配比以及碱蓬对适量氮养分输入环境的特殊适应对策有关。随着黄河口新生湿地氮养分供给的不断增加,当未来碱蓬湿地氮养分状况达到较高水平(特别是中等水平)时,其生物量、生长节律(特别是种子发育时间)以及不同器官氮累积与分配状况可能将发生明显改变。     

不同混交比例对杉木和大叶榉幼苗功能性状的影响

了解林木功能性状在不同培育模式下的变异和关联,对揭示林木生态适应性及其生态功能具有重要意义。选取了亚热带地区两种常见人工林树种杉木、大叶榉幼苗为研究对象,设置4种不同栽培模式的盆栽试验：单一杉木（4C）,单一大叶榉（4Z）和杉木、大榉树3种混栽模式（1C3Z、2C2Z、3C1Z）,研究不同混交比例对其叶、茎、根功能性状的影响。结果表明：（1）杉木总叶面积、叶干物质含量、净光合速率、蒸腾速率和气孔导度在混栽模式下显著减小,而比叶面积显著增大;根长和比根长在不同处理间无显著差异;叶、茎、根生物量和单株总生物量在混栽模式下显著低于4C处理,不同混栽模式之间差异不显著。（2）大叶榉单叶面积在3C1Z处理下最高,总叶面积随大叶榉在树种组成中所占比例的降低而逐渐增大,比叶面积在不同处理间无显著差异,叶干物质含量、净光合速率、蒸腾速率和气孔导度均在2C2Z处理下最大,而瞬时水分利用效率在2C2Z处理下最小;根长在3C1Z处理下显著增大,比根长在不同处理间无显著差异;叶、茎、根生物量和单株总生物量随大叶榉在树种组成中所占比例的降低而逐渐增大。综合来看,杉木和大叶榉混合处理中杉木种间竞争大于种内竞争,而大叶榉相反;随杉木在混栽处理中比例减少,其主要通过增加比叶面积,提高净光合速率,减少茎生物量积累来适应种间竞争关系;而大叶榉随其在混栽处理中比例的减少,显著增加叶面积和根长来提高资源利用率,减少地下资源分配,提高地上茎生物量积累。因此,树种混交比例将显著影响林木功能性状及其生物量积累,选择适宜混交比例对混交林可持续经营具有重要意义。     

Allometry of Salsola collina in response to soil nutrients,water supply and population density

The allometry of greenhouse‐grown Salsola collina Pall. in response to variation in soil nutrient content, water supply and population density has been compared. The results showed that the biomass allocation was size‐dependent. Root, stem, leaf and reproductive allocation showed a ‘true’ plasticity in response to soil nutrient variation. At low soil nutrient content, plants tended to allocate more biomass to the development of reproductive organs than to stem and leaf, but root allocation was consistent due to a tradeoff between the effects of plant size and soil nutrient content. The plasticity of stem allocation and reproductive effort was ‘true’, while the plasticity of root allocation was ‘apparent’, but there was no plasticity for leaf allocation in response to soil water variation. At lower soil water content, plants tended to allocate more biomass to the stem than to development of reproductive organs. With the exception of ‘apparent’ plasticity of root allocation, no plasticity was detected in biomass allocation when population density was varied.     

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。     

三种高寒植物幼苗生物量分配及性状特征对光照和养分的响应

为了解高寒植物幼苗对生境资源异质性的适应策略,以高寒草甸中常见的3种草本植物大耳叶风毛菊(Saussurea macrota)、甘西鼠尾草(Salvia przewalskii)和千里光(Senecio scandens)为材料,比较研究了这3种植物幼苗对不同光照和养分资源的响应。结果表明:光照和养分异质性显著影响了3种植物幼苗的性状特征和生物量分配,并存在一定的交互影响。随着光照的降低,3个物种的幼苗的生物量和根分配呈现降低趋势,但是其株高、比叶面积、叶分配、茎分配却逐渐升高。在低养分条件下,3个物种幼苗的总生物量、株高、比叶面积和叶分配均降低,而根分配均却显著增加。对于光照和养分资源异质性而言,光照异质性对高寒植物生物量分配和性状特征的改变具有更大的影响。喜阴物种大耳叶风毛菊和喜光物种甘西鼠尾草比中性生境物种千里光表现出了较大的性状特征和生物量分配的可塑性指数。     

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。     

Decline in gypsy moth (Lymantria dispar) performance in an elevated CO2 atmosphere depends upon host plant species

Plant species differ broadly in their responses to an elevated CO₂ atmosphere, particularly in the extent of nitrogen dilution of leaf tissue. Insect herbivores are often limited by the availability of nutrients, such as nitrogen, in their host plant tissue and may therefore respond differentially on different plant species grown in CO₂-enriched environments. We reared gyspy moth larvae (Lymantria dispar) in situ on seedlings of yellow birch (Betula allegheniensis) and gray birch (B. populifolia) grown in an ambient (350 ppm) or elevated (700 ppm) CO₂ atmosphere to test whether larval responses in the elevated CO₂ atmosphere were species-dependent. We report that female gypsy moths (Lymantria dispar) reared on gray birch (Betula populifolia) achieved similar pupal masses on plants grown at an ambient or an elevated CO₂ concentration. However, on yellow birch (B. allegheniensis), female pupal mass was 38% smaller on plants in the elevated-CO₂ atmosphere. Larval mortality was significantly higher on yellow birch than gray birch, but did not differ between the CO₂ treatments. Relative growth rate declined more in the elevated CO₂ atmosphere for larvae on yellow birch than for those on gray birch. In preference tests, larvae preferred ambient over elevated CO₂-grown leaves of yellow birch, but showed no preference between gray birch leaves from the two CO₂ atmospheres. This differential response of gypsy moths to their host species corresponded to a greater decline in leaf nutritional quality in the elevated CO₂ atmosphere in yellow birch than in gray birch. Leaf nitrogen content of yellow birch dropped from 2.68% to 1.99% while that of gray birch leaves only declined from 3.23% to 2.63%. Meanwhile, leaf condensed tannin concentration increased from 8.92% to 11.45% in yellow birch leaves while gray birch leaves only increased from 10.72% to 12.34%. Thus the declines in larval performance in a future atmosphere may be substantial and host-species-specific.     

Light and tree size influence belowground development in yellow birch and sugar maple

The effects of light and tree size on the root architecture and mycorrhiza of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) growing in the understory of deciduous forests in southern Québec, Canada were studied. At the study site, small (<50 m²), medium (101–200 m²) and large (201–500 m²) canopy gaps were investigated. From within these gaps, 17 yellow birch and 23 sugar maple saplings from 40 to 600 cm in height were sampled. In both species, root biomass and morphological traits were strongly correlated with tree size, but only weakly with light availability. Increased root biomass was primarily allocated to coarse roots and secondarily to fine roots. Yellow birch roots were longer, had a larger area, more endings and branches and grew more rapidly than sugar maple roots. Mycorrhizal colonization increased with available light and declined with tree age in sugar maple and was positively associated with tree size in yellow birch. The study demonstrates that tree size is a very important determinant of how belowground systems acclimate to understory conditions.     

Competitive ability of two Brassica varieties in relation to biomass allocation and morphological plasticity under varying nutrient availability

Green cabbage (Brassica campestris, leafy variety) and turnip (Brassica campestris var. rapifera, rooty variety) were grown in both monocultures and mixtures at three nutrient levels to investigate their responses to nutrient availability with respect to biomass allocation, morphological plasticity and competitive ability. Their allocation parameters and leaf morphological traits were affected by both nutrient availability and developmental stage. Both of the varieties had a smaller biomass allocation to leaf blades, but a greater allocation to petioles at high nutrient levels. Root:shoot ratio (RSR) of green cabbage decreased with increasing nutrient availability, whereas that of turnip increased. Turnip had a smaller leaf blade weight ratio (LBWR) than cabbage, being compensated for by larger leaf area ratio (LAR) and specific leaf area (SLA). Leaf area ratio and SLA of both the varieties increased with increasing nutrient availability as did their mean dry weights. The mean dry weight of turnip was slightly greater than that of green cabbage in their respective monocultures, while that of green cabbage was greater than that of turnip in their 1:1 mixture. Therefore, green cabbage, having inherently greater biomass allocation to leaves, was generally more competitive than turnip with more biomass allocation to roots, especially at higher nutrient levels. However, within a variety, morphological plasticity (variation in LAR and SLA) was more important than the plasticity in biomass allocation (e.g. variation in RSR and LBWR) in determining competitive ability. The implication of our results is that competition models based on biomass allocation pattern alone may fail to predict competitive outcomes and that such models should also take morphological plasticity into full account.     

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.     

Size and Structure of Fine Root Systems in Old-growth and Secondary Tropical Montane Forests (Costa Rica)

The fine root systems of three tropical montane forests differing in age and history were investigated in the Cordillera Talamanca, Costa Rica. We analyzed abundance, vertical distribution, and morphology of fine roots in an early successional forest (10–15 years old, ESF), a mid‐successional forest (40 years old, MSP), and a nearby undisturbed old‐growth forest (OGF), and related the root data to soil morphological and chemical parameters. The OGF stand contained a 19 cm deep organic layer on the forest floor (i.e., 530 mol C/m²), which was two and five times thicker than that of the MSF (10 cm) and ESF stands (4 cm), respectively. There was a corresponding decrease in fine root biomass in this horizon from 1128 g dry matter/m² in the old‐growth forest to 337 (MSF) and 31 g/m² (ESF) in the secondary forests, although the stands had similar leaf areas. The organic layer was a preferred substrate for fine root growth in the old‐growth forest as indicated by more than four times higher fine root densities (root mass per soil volume) than in the mineral topsoil (0–10 cm); in the two secondary forests, root densities in the organic layer were equal to or lower than in the mineral soil. Specific fine root surface areas and specific root tip abundance (tips per unit root dry mass) were significantly greater in the roots of the ESF than the MSF and OGF stands. Most roots of the ESF trees (8 abundant species) were infected by VA mycorrhizal fungi; ectomycorrhizal species (Quercus copeyemis and Q. costaricensis) were dominant in the MSF and OGF stands. Replacement of tropical montane oak forest by secondary forest in Costa Rica has resulted in (1) a large reduction of tree fine root biomass; (2) a substantial decrease in depth of the organic layer (and thus in preferred rooting space); and (3) a great loss of soil carbon and nutrients. Whether old–growth Quercus forests maintain a very high fine root biomass because their ectomycorrhizal rootlets are less effective in nutrient absorption than those of VA mycorrhizal secondary forests, or if their nutrient demand is much higher than that of secondary forests (despite a similar leaf area and leaf mass production), remains unclear.     

Effects of light and nutrient availability on dry matter and N allocation in six successional grassland species

Summary Recent discussions on determinants of competitive success during succession require the study of the combined effect of light and nutrient availability on growth and allocation. These effects can be used to predict the outcome of competition at changing resource availabilities. This work is part of a study on the successional sequence in permanent grassland starting after fertilizer application is stopped, but with continued mowing, in order to restore former species-rich communities. This yields a successional sequence which proceeds from grasslands with a high nutrient availability and a closed canopy, to grasslands with a low nutrient availability and an open canopy. If allocation is related to competitive ability, species from the productive stages would be expected to allocate more biomass and nitrogen to leaves, which could make them better competitors for light, while species from the unproductive stages would allocate more biomass to roots, which could make them better nutrient competitors. This study reports on growth, specific leaf area (SLA), vertical display of leaves, and allocation of biomass and nitrogen of six grassland species from this successional sequence at 16 combinations of light and nutrient supply. Species from the poorer successional stages reached a lower final dry weight than species from the richer stages, over all treatment combinations. The experimental design made it possible to test for unique effects of the resource ratio effect of light and nutrients on allocation characteristics. This resource-ratio effect was defined as the ratio light intensity/(light intensity + nutrient supply rate), using standardized levels for the treatments. The within-species variation (plasticity) in both allocation of dry matter and nitrogen was linearly related to this resource-ratio effect. Some interspecific differences in this relationship were found which could be related to the position of the species along the successional gradient. However, the range of plasticity in allocation pattern expressed within each species was much larger than the differences between species. It was concluded that allocation differences between these grassland species are relatively unimportant, given the large amount of plasticity in these traits. Interspecific differences in SLA and vertical stature seemed to be more important in explaining the position of species along the successional gradient.     

Competition between two grass species with and without grazing over a productivity gradient

Soil nutrient-level and herbivory are predicted to have opposing effects on the allocation pattern of the competitive dominant plant species. Lower stem and higher leaf allocation are favoured when plants are grazed, whereas a higher stem allocation is favoured at high nutrient levels. Grazing by hares and geese can prevent invasion of the tall Elymus athericus, into short vegetation of Festuca rubra, at unproductive stages of salt-marsh succession but not at more productive stages. We hypothesise that the negative effect of herbivory on Elymus decreases due to increasing soil nitrogen levels and shifts the competitive balance towards this species. We tested how simulated grazing and nitrogen availability affected the competitive balance between adult plants of both grass species in a greenhouse experiment. Elymus had a higher above-ground biomass production, invested relatively more in stem and root tissue and had a larger shoot length than Festuca. The above-ground relative yield of Elymus in mixtures of both species increased with increasing nitrogen levels. This indicates that Elymus was the superior competitor at high soil fertility. Although clipping removed relatively more biomass from Elymus than from Festuca and exceeded the observed biomass removal in field conditions, it did not change the competitive balance between both species. Decreasing effects of herbivory due to increasing nitrogen levels are not a likely explanation for the invasion of Elymus in productive marshes. The results suggest that once Elymus has established it can easily invade vegetation dominated by Festuca irrespective of grazing by herbivores such as hares and geese. Herbivory by small herbivores may mainly retard the invasion of this plant by influencing establishment itself.