Low Fertility Sites Research Articles

Effects of eutrophication on salt marsh root and rhizome biomass accumulation

The root:shoot ratio of grassland plants may be lower in high fertility sites than in low fertility sites as plants modify their root, rhizome and shoot morphologies (and masses) to suit prevail- ing nutrient availability. We conducted geographically diverse and regionally specific field sampling and measured above- and belowground plant biomasses in western Atlantic and Gulf of Mexico salt marshes to determine whether there is a similar morphological response in Spartina alterniflora, the dominant salt marsh plant. Coastal nutrient addition/enrichment, which is widespread and ongoing, may lower root and rhizome biomass, belowground production and organic accumulation in this spe- cies. Higher soil respiration and a lower Eh (redox potential) are expected additional soil property changes. The addition of P, more than of N, seems to reduce root and rhizome biomass accumulation. The cumulative effects of increased nutrient loadings on salt marshes may be to decrease soil eleva- tion and accelerate the conversion of emergent plant habitat to open water, particularly on the lower marsh. The recommendations for management practices intended to conserve coastal marshes include reducing nutrient loading to coastal zones, solving water quality problems with a multiple nutrient approach, and choosing monitoring metrics that are based on both above- and belowground plant biomass.

Cronología de la regeneración del bosque tropical seco en Santa Rosa, Guanacaste, Costa Rica. II. La vegetación en relación con el suelo

Tropical dry forest (TDF) succession was monitored in Santa Rosa, Costa Rica. We analyzed the effect of soil type on forest structure and diversity. Eight seasonally-dry TDF sites located along a successional chrono-sequence (10, 15, 20, 40, 60 and >100 years) were examined in relation to 17 soil pedons and six soil orders. Soils had moderate to high fertility and were classified as Entisols and Vertisols, although Mollisols, Alfisols, Inceptisols and Ultisols were also present. One-hundred and thirty 500 m2 plots were established, 20 plots in secondary and 10 plots in mature TDF sites. Diameter at breast height (dbh) and total tree height were measured for saplings (dbh > or = 1 and <5 cm), shrubs and trees (dbh > or = 5 cm). With the exception of two sites (40 and 60 years), soil type did not have significant effects on forest structure. However, tree diversity measured with Shannon-Wiener's H' and Fisher's alpha rarefaction curves, showed substantial differences among soil types, which became accentuated in mature forests. This pattern might be explained by non-random distributions of TDF trees, the scale of the study, the plot shape, and the use of systematic sampling designs. Low-fertility sites in general had higher species richness, consistent with idea that more restrictive soils reduce competition among trees and allow co-existence of species with contrasting growth rates. Changes in soil properties along a chrono-sequence of Entisols indicated that trees may experience more severe water stress as succession progresses, which may require adjustments in biomass allocation and phenological behavior of the dominant species. Our results suggest that edaphic specialization is more pronounced in mature TDF forests, and that most TDF trees are generalists in relation to soil type, highly tolerant to site heterogeneity, and show little physiological specializations in response to edaphic heterogeneity.

Elevated CO2 concentration affects leaf photosynthesis-nitrogen relationships in Pinus taeda over nine years in FACE

To investigate whether long-term elevated carbon dioxide concentration ([CO(2)]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis, carboxylation capacity and area-based leaf nitrogen concentration (N(a)) in Pinus taeda L. growing in a long-term free-air CO(2) enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (V(cmax)) and electron transport (J(max)) varied with N(a) under elevated [CO(2)]. In trees exposed to elevated [CO(2)] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (A(net-Ca)) and N(a) was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the relationships of both V(cmax) and J(max) with N(a) decreased in 1-year-old needles after up to 9 years of growth in elevated [CO(2)], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N ha(-1)) in the ninth year of exposure to elevated [CO(2)] restored the slopes of the relationships of V(cmax) and J(max) with N(a) to those of control trees (i.e., in ambient [CO(2)]). The J(max):V(cmax) ratio was unaffected by either [CO(2)] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO(2)] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with N(a) may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO(2)].

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

AbstractIncreased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO2 enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO2 Enrichment experiment to determine the effects of elevated atmospheric CO2 concentration ([CO2]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO2], although L senescence for hardwoods was slowed. Elevated [CO2] enhanced pine L and the total canopy L (combined pine and hardwood species; P&lt;0.050); on average, enhancement following canopy closure was ∼16% and 14% respectively. However, variation in pine L and its response to elevated [CO2] was not random. Each year pine L under ambient and elevated [CO2] was spatially correlated to the variability in site nitrogen availability (e.g. r2=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO2]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO2] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO2] in moderate fertility sites, but are unable to respond to [CO2] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO2], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (∼3 g N inputs m−2 yr−1) may be sufficiently enhanced under elevated [CO2] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (∼14%).

Functional and performance comparisons of invasive Hieracium lepidulum and co‐occurring species in New Zealand

Abstract One of the key environmental factors affecting plant species abundance, including that of invasive exotics, is nutrient resource availability. Plant functional response to nutrient availability, and what this tells us about plant interactions with associated species, may therefore give us clues about underlying processes related to plant abundance and invasion. Patterns of abundance of Hieracium lepidulum, a European herbaceous invader of subalpine New Zealand, appear to be related to soil fertility/nutrient availability, however, abundance may be influenced by other factors including disturbance. In this study we compare H. lepidulum and field co‐occurring species for growth performance across artificial nutrient concentration gradients, for relative competitiveness and for response to disturbance, to construct a functional profile of the species. Hieracium lepidulum was found to be significantly different in its functional response to nutrient concentration gradients. Hieracium lepidulum had high relative growth rate, high yield and root plasticity in response to nutrient concentration dilution, relatively low absolute yield, low competitive yield and a positive response to clipping disturbance relative to other species. Based on overall functional response to nutrient concentration gradients, compared with other species found at the same field sites, we hypothesize that H. lepidulum invasion is not related to competitive domination. Relatively low tolerance of nutrient dilution leads us to predict that H. lepidulum is likely to be restricted from invading low fertility sites, including sites within alpine vegetation or where intact high biomass plant communities are found. Positive response to clipping disturbance and relatively high nutrient requirement, despite poor competitive performance, leads us to predict that H. lepidulum may respond to selective grazing disturbance of associated vegetation. These results are discussed in relation to published observations of H. lepidulum in New Zealand and possible tests for the hypotheses raised here.

Phenotypic Plasticity of Reproduction in Schizachyrium scoparium (Poaceae) Populations in Relation to Ecological History

Genetic differentiation in reproduction in the wide-ranging Schizachyrium scoparium (Poaceae) has been demonstrated in uniform gardens. However, the fine-tuning of flowering phenology and biomass allocation in relation to spatial and temporal fluctuations in the local environment is best accomplished by plastic responses to local variability. An earlier central New Jersey study suggested that S. scoparium populations in old fields of 2 to 40 years differed in plasticity. To test this apparent effect of ecological history on the development of different levels of plasticity, genotypes were collected from high- and low-fertility sites in New Jersey (forest biome) and in Oklahoma (grassland biome). Three greenhouse experiments manipulating light and nutrients were used to partition variation into genetic and environmental components. High light or high nutrients resulted in plasticity for increased biomass, greater reproductive allocation, and more tillers. Earlier flowering was induced by high light, but nutrient treatments had no effect. Populations were more likely to differ in plasticity across regions than within regions, and Oklahoma populations were consistently more plastic than New Jersey populations. In response to nutrients, populations from high-nutrient sites were often more plastic than those from low-nutrient sites. There were fewer differences in plasticity in response to light between high- and low-nutrient populations. The greater plasticity in Oklahoma populations is suggested to be the result of historically greater environmental unpredictability and K-selection factors such as density-dependent selection and greater competition for resources. A native grass population is more than just a Latin binomial. Evolutionary forces create an ecological unit unique and irreplaceable at the local level.

Early Influence of Wood Ash Application on the Carbohydrate Reserves and Composition in Needles of Scots pine {Pinus sylvestris)

Wood ash treatment can probably increase forest productivity on low fertility sites. However, the resulting effect on the carbohydrate concentration as the main carbon and energy reserve in trees is little studied. In 2000, a square of 0.1 ha sandy soil below a 19-year-old Scots pine (Pinus sylvestris) plantation was treated with raw fly ash (0.5 kg.m-2); untreated square was used as control. One year after the treatment, carbohydrates (glucose, fructose, sucrose, maltodextrins, starch and excess bound fructose) were analyzed enzymatically from current-year and one-year-old needles. The ratio K/N in needles suggested an improved balance between these elements in treated trees, in which the K concentration was higher. The largest relative differences (50% of control) were observed in glucose and fructose in summer. The squares did not differ in the concentration of the accumulated carbohydrate reserves in needles during low temperature stress in winter and before the growth of new shoots in spring. During the vegetation period decreased levels of soluble carbohydrates and starch were observed (max 70% of the control value around 100 mg total carbohydrates g-1 dry mass). Because the experiment was designed without true replicates, reasons for the observed differences require further study.

Fertilization×genotype interaction in Pinus radiata open pollinated families planted in three locations in Galicia (NW Spain)

A Pinus radiata progeny trial was established in 2003 on three low fertility sites in Galicia to study the relevance of genotype×fertilization interaction. At each site, twenty seven open pollinated families derived from plus trees selected in mature plantations in Galicia (NW Spain) were planted under 9 fertilization treatments following a split-plot design. One year after planting, P and Mg fertilization significantly increased height growth whereas N did not affect or even significantly reduced height growth. Fertilization×site interaction was not significant indicating parallel responses to fertilization in the three sites. Fertilization × genotype interaction was significant in only one site, suggesting some genetic variation in the nutrient use efficiency among the studied genotypes. However, the lack of a significant interaction in the other two sites, and the lack of consistence among the genotypic stability in relation to fertilization across sites, hinder the possibility of breeding in relation to the nutrient efficiency of the genotypes. Indications for further research are given.

Intra-specific variation in cell wall constituents of needle age classes of Pinus sylvestris in relation to soil fertility status in Southwest England

First, second and third year needles were collected from the same branches of young Scots pine trees growing on soils of two different status (high and low fertility sites) that varied in mineral nutrient concentrations and N mineralisation potential. All needle age classes were analysed for total carbon, acid detergent fibre (ADF), lignin, cellulose, phenylpropanoid derivatives (PPD) of lignin, sugar constituents of non-crystalline, hydrated cellulose and hemicellulose, and nutrient concentrations (N, P, K, Ca, Mg and Mn). Significant intra-specific variation in the litter quality variables in relation to soils of high and low fertility was found in the second and third year needles, whereas there were no differences in the other cell wall constituents and mineral elements of the first year needles. The second and third year needles from the low fertility soil contained higher concentrations of ADF, lignin, cellulose, sugar constituents of non-crystalline, hydrated cellulose and hemicellulose, and phenylpropanoid derivatives (PPD) of lignin, but lower concentrations of N, P and Mg than the same needles from the high fertility and fertilised soils. The results in the present study indicate that under different soil fertilities, needle age classes show significant variations in the cell wall constituents and mineral elements, and suggest that this can result in significant variation in litter quality and decomposition rates.

Nursery fertilization and tree shelters affect long-term field response of Acacia salicina Lindl. planted in Mediterranean semiarid conditions

Transplant stress limits establishment of newly planted seedlings in semiarid Mediterranean regions, which are characterized by very low precipitation and poor fertility soils. Nursery cultural regimes which influence stock quality, as well as silvicultural treatments applied at outplanting may affect the capacity of seedlings to establish successfully. We examined the influence of nursery mineral nutrition and application of individual tree shelters on 9-year seedling performance of the leguminous species, Acacia salicina Lindl., planted on a degraded site in southeastern Spain. Survival was significantly greater throughout the duration of the study for seedlings fertilized at high rates, while initial benefits to field growth associated with nursery fertilization diminished after 4 years. A significant relationship was established between P supplied in the nursery and both seedling survival and root dry weight after the first growing season ( R 2 = 0.68 and 0.77, respectively), though no relationship was detected for N. The capacity of this species to fix N through root nodulation apparently dictates that P fertility is relatively more important to initial establishment on low fertility sites characteristic of this region. Survival of protected seedlings became significantly greater than that of non-protected seedlings following an extended drought after the sixth year. Stem diameter was significantly greater for non-protected seedlings as of the fourth year but height was greater for protected seedlings throughout the study, reflecting differential carbon allocation within the sheltered environment. Our results suggest that mineral nutrient status of nursery stock (especially high P content) and tree shelters may positively affect long-term plantation establishment of A. salicina seedlings in semiarid Mediterranean climates.

Early growth and nutrient dynamics of planted Pinus banksiana seedlings after slash-pile burning on a boreal forest site

Containerized jack pine (Pinus banksiana Lamb.) seedlings were planted in 10 paired slash-pile-burn and control plots established on a recent clearcut in north eastern Ontario to assess early impacts of intensive fire on soils and plant growth and nutrition. Root zone soil and seedlings were sampled one growing season after planting. The control seedlings exhibited modest growth without gain in N content, indicating reliance on internal nutrient reserves for growth on this low-fertility site. Slash-pile burning reduced seedling mortality, and markedly increased height, rootcollar diameter, and biomass growth, as well as uptake of N, P, K, Ca and Mg. The response was attributed to greatly improved soil fertility following fire, characterized by increased pH, extractable NO3- and NH4+ , available P, and exchangeable K, Ca and Mg. Significant correlations between corresponding soil tests and seedling nutrient composition indicated that laboratory extractions effectively reflected nutrient availability to plants. The highest correlations were associated with N. Vector diagnosis identified N as the most limiting nutrient constraining growth on the burns, even though base cation increases (mainly from ash) were relatively larger than extractable N in the soil. These early responses in soils and plants to intensive fire may have long-term effects on plantation development. Key words: Slash pile, prescribed fire, soil fertility, seedling nutrition

Soil–plant relationships and tree distribution in old growth Nothofagus betuloides and Nothofagus pumilio forests of Tierra del Fuego

The evergreen Nothofagus betuloides and deciduous Nothofagus pumilio live close together in Tierra del Fuego. The fact that these two species rarely form mixed forests suggests that the soil factor can contribute to the distribution of two species on a local scale. Most of these forests are undisturbed; therefore, soil characteristics may reflect both the influence of the species and of the dominant pedogenetic processes. In this paper, we aim to study how soil characteristics can affect nutrient cycling, the strategies of tree nutrition and the distribution of Nothofagus forests in Tierra del Fuego. Twenty-two soils in these two types of forests in contrasted topographic positions were sampled and analysed. The dominant soil processes were waterlogging in the evergreen forest and podzolisation in the deciduous one. Within each type of forest we observed large variability in soils. Nutrient-poor litter slowed down nitrogen cycling. However, phosphorus mobilisation was high in nutrient-poor environments because of low pH and waterlogging. Chemical and biological characteristics of the Oa horizon were able to discriminate between forest types while mineral soil characteristics were not. Biological activity of the organic horizons was limited by low pH and, in N. betuloides forests, also by waterlogging conditions. According to this pattern, N. betuloides grows in less fertile soils and thus has lower nutrient concentrations in leaves. In contrast, N. pumilio grows in soils with higher fertility and consequently has high nutrient content in leaves. Despite growing in low fertility sites, N. betuloides grows at a rate similar to that of N. pumilio. This suggests that the evergreen species is more efficient in using nutrients and better adapted to waterlogging conditions.

Interactive effects of soil warming and fertilization on root production, mortality, and longevity in a Norway spruce stand in Northern Sweden

AbstractThe effects of soil warming and nitrogen availability on root production, longevity and mortality were studied using minirhizotrons in irrigation (C), fertilized (F), heated (H), and heated‐fertilized (HF) plots in a Norway spruce stand in northern Sweden from October 1996 to October 1997. Irrigation was included in all treatment plots. Heating cables were used to maintain the soil temperature in heated plots at 5°C above that in unheated plots during the growing season. A Kaplan–Meier approach was used to estimate the longevity of fine roots and Cox proportional hazards regression to analyze the effects of the H, F, and HF treatments on the risk of root mortality.The proportion of annual root length production contributed by winter–spring production amounted to 52% and 49% in heated plots and heated‐fertilized plots, respectively. The annual root length production in C plots was significantly higher than in other treatments, while the HF treatment gave significantly greater production compared with the F treatment. The risk of mortality (hazard ratio) relative to C plots was higher in H plots (358%) and F plots (191%). The interaction between heating and fertilizing was strongly significant. The increase in the risk of root mortality in combined fertilization and heating (103%) was lower than that in the H or F plots.The results show that nitrogen addition combined with warmer temperatures decreases the risk of root mortality, and fine root production is a function of the length of the growing season. In the future, fertilization combined with the warmer temperatures expected to follow predicted climatic change may increase root production in boreal forests at low fertility sites.

The use of light and electron microscopy to assess the impact of ozone on Norway spruce needles

Light (LM) and transmission electron (TEM) microscopy were used to study previously specified ozone symptoms in the foliage of Norway spruce. The three youngest green needle generations from twenty mature trees in two stands on sites of different soil fertility at Asa, southern Sweden, were sampled in 1999. The critical dose of ozone, expressed as AOT40, was 6362 ppb.h. LM showed ozone-specific symptoms: decreased chloroplast size with electron dense stroma advancing gradually from the outer to the inner cell layers, being most severe in the needle side facing the sky. The symptoms were expressed as ozone syndrome indices at the needle generation, tree and stand levels. The index had higher values at the low fertility site. TEM was used to confirm the LM results. The study shows that LM can be used for diagnosis of the impact of ozone on conifers in the field.

Slits in container wall improve root structure and stem straightness of outplanted Scots pine seedlings

Root structure and basal sweep were measured on 6-year-old Scots pine (Pinus sylvestris L.) trees at two sites with different soil fertility. Each site was planted with seedlings of identical origin after nursery cultivation in either solidwall container types with vertical ribs or in slitwall container types. Neither container design nor container volume affected tree height or stem diameter on the two sites. The transversal area of lateral roots was larger than the transversal area of bottom roots for the two container types at both sites. The proportion of bottom root transversal area to the total root transversal area was larger in the seedlings growing on the low fertility site than in those growing in the high fertility site for both container types. Seedlings cultivated in slitwall containers had a larger root area in proportion to stem diameter and had less root spiralling compared to the trees cultivated in solidwall containers. At the high fertility site, trees from the slitwall container type had straighter stem bases than seedlings grown in solidwall containers. At the low fertility site, differences in basal sweep formation were small between the container types. Reasons for this are discussed.

Collection, evaluation and classification of Greek populations of faba bean ( Vicia faba L.)

Fifty-five Greek Vicia faba L. populations, collected from diverse areas, were planted at two dry and low fertility sites for evaluation and classification. Yield evaluation, which was carried out by Principal Component Analysis (PCA) on the basis of seven yield traits, showed the number of pods per plant, number of ovules and seeds per pod, and branching from the basal nodes to be the most important traits for population evaluation regarding yield. For population classification, four dissimilarity coefficients (Manhattan, Average Taxonomic Distance, Euclidean distance and squared Euclidean distance) and four multivariate methods (PCA, UPGMA, Neighbor-joining and Principal Coordinate Analysis) were evaluated using fifteen morphological and seven yield traits. Neighbor-joining was chosen as the most suitable multivariate method. This method combined with PCA for the seven yield traits, placed the populations into six groups. As revealed by the application of PCA on all twenty-two traits the grouping was based mainly on pod characteristics, stem thickness, plant height, 1000 seed weight and branching from basal nodes. Based on the results of the present study, a model is proposed for conserving cross-pollinated species, such as faba bean.

Contribution of amino compounds to dissolved organic nitrogen in forest soils

Dissolved organic nitrogen (DON) may play an important role in plantnutrition and nitrogen fluxes in forest ecosystems. In spite of the apparentimportance of DON, there is a paucity of information concerning its chemicalcomposition. However, it is exactly this chemical characterization that isrequired to understand the importance of DON in ecosystem processes. Theprimaryobjective of this study was to characterize the distribution of free aminoacidsand hydrolyzable peptides/proteins in the DON fraction of Oa horizon leachatesalong an extreme edaphic gradient in northern California. Insitu soil solutions were extracted by centrifugation from Oahorizonscollected beneath Pinus muricata (Bishop pine) andCupressus pygmaea (pygmy cypress) on slightlyacidic/fertile and highly acidic/infertile sites. DON accounted for 77 to99% of the total dissolved nitrogen in Oa horizon leachates. Nitrogen infree amino acids and alkyl amines ranged from 0.04–0.07 mgN/L on the low fertility site to 0.45–0.49 mg N/L onthe high fertility site, and accounted for 1.5 to 10.6% of the DON fraction.Serine, glutamic acid, leucine, ornithine, alanine, aspartic acid andmethylamine were generally the most abundant free amino compounds. Combinedamino acids released by acid hydrolysis accounted for 48 to 74% of theDON, suggesting that proteins and peptides were the main contributor to DON inOa horizon leachates. Together, nitrogen from free andcombined amino compounds accounted for 59 to 78% of the DON. Most of theDON was found in the hydrophobic fraction, which suggests the presence ofprotein/peptide-polyphenol complexes or amino compounds associated withhumic substances. Because free and combined amino acids can be an importantnitrogen source for some plants, soil DON may play an important role in plantnutrition and ecosystem function.

The role of trees in enhancing soil nutrient availability for native perennial grasses in open eucalypt woodlands of north-east Queensland

To investigate the effects of eucalypt trees on pasture in open eucalypt woodlands of north-eastern Queensland, 2 common native pasture species, Chrysopogon fallaxand Heteropogon contortus, were grown in soil collected from under tree canopies and inter-canopy areas. These soils were collected from 2 localities that differed in soil fertility. The objective was to determine whether trees enhanced soil nutrient levels beneath their canopies and whether such changes affected pasture productivity and quality. It was hypothesised that tree effects would be greater when overall soil fertility was low. Shade and water stress treatments, which aimed to simulate field conditions, were also imposed to investigate their independent and interactive effects on plant growth. Chemical analyses showed that nutrient levels were higher in under-canopy soils, and plants grown in under-canopy soil produced 42% more biomass than plants in outside-canopy soil. This increase in biomass was significantly greater when the soil was from the low fertility site than from the high fertility site. Leaf quality, in terms of N and P concentration and dry matter digestibility (DMD), was generally higher in plants in under-canopy soil. Shading reduced plant root biomass but had no effects on above ground biomass or leaf quality. Water stress reduced above and below ground biomass and increased leaf quality. Shading and water stress effects were greater in plants in the more fertile soil types. Simulated ‘under-canopy’ plants (shaded, water-stressed plants in under-canopy soil) produced more biomass and had higher leaf N concentration and DMD than simulated ‘outside-canopy’ plants (unshaded, well-watered plants in outside-canopy soil). In a pastoral context, trees in tropical woodlands are generally considered in terms of their competitive effect on pasture productivity. This study has demonstrated that soil nutrient availability is enhanced under eucalypt canopies. The biomass results indicate that this effect is greater when overall soil fertility conditions are low. These enhanced soil nutrient conditions influence forage productivity and quality. Such positive benefits should be considered when management decisions are taken to remove or kill trees in eucalypt woodlands.

Accumulation in above-ground biomass and soil storage of mineral nutrients in pure and mixed plantations in a humid tropical lowland

As fast-growing, short-rotation plantations are being planted in the tropics on low fertility soils, the problem of sustaining soil fertility becomes an important management issue. Above-ground biomass, nutrient concentration of above-ground tree tissues, and soil nutrients were examined in two young plantations of eight indigenous tree species grown in pure and mixed designs in a low fertility site in the humid lowlands of Costa Rica. The goal was to assess the role of nutrient accumulation in above-ground biomass on potential site nutrient decline, and to draw recommendations to conserve site nutrients in the long term. In Plantation 1, Jacaranda copaia pure stands had higher above-ground tree N, P, and Mg than the other treatments, while Vochysia guatemalensis had the greatest accumulation of K and Ca. For J. copaia, stem harvest would remove about 54% of total above-ground tree N, but about 80% of P, K, Ca and Mg. For V. guatemalensis, stem harvest would remove less than 30% of N but from 50 to 60% of total above-ground tree Ca, K, Mg and P. Branches and foliage summed together were 25 to 35% of total above-ground tree biomass, but they generally represented about 50% of above-ground tree nutrients. In Plantation 2, the mixed stands had the highest above-ground nutrient content for all nutrients, and both the mixture and Terminalia amazonia pure stands had the highest stem P and Mg. Five years after planting, decreases in soil P, K and Ca were apparent in pure plots of the fastest growing species with the largest accumulation of nutrients in above-ground biomass, such as J. copaia and V. guatemalensis. However, in other cases, beneficial effects on some soil nutrients were noted: for example, increases in soil Ca under T. amazonia and Virola koschnyi, both species with high Ca content in foliage and high rates of annual litterfall. The mixed plots showed intermediate values for the nutrients examined, and even improved soil conditions, as for P in Plantation 1. This suggests that in mixed conditions it may take longer to deplete soil nutrients than in monospecific stands of fast-growing species. Results of continued sampling will be needed to assess the long term effects of plantation treatments on soil chemistry, especially near the end of the rotation (estimated at 12–15 years, depending on the species). The calculation of whole-stand nutrient budgets can help in the selection of tree species and plantation management strategies to favor nutrient recycling mechanisms and site nutrient conservation.

Juvenile tree growth rates and species sorting on fine‐scale soil fertility gradients in a Chilean temperate rain forest

AbstractAimWe sought to clarify the role of juvenile tree growth rates in species sorting along fine‐scale fertility gradients in the coast range forests of southern Chile.LocationIn montane forests in the coast range, the conifer Fitzroya cupressoides occupies the poorest soils, whereas dominance shifts to the evergreen angiosperms Nothofagus nitida and/or Weinmannia trichosperma on moderately infertile sites. On the most fertile sites, N. nitida is usually the sole overstorey dominant.MethodsWe examined the effects of soil depth and nitrogen availability on height growth and leaf nitrogen levels of the three common overstorey dominants. Sapling height growth rates and foliage nitrogen concentration were measured in 20 large gaps (&gt;0.1 ha), on sites of varied soil depth and potential N‐mineralization rates.ResultsHeight growth rates of Weinmannia and Nothofagus were strongly positively correlated with both soil depth and potential N‐mineralization. Height growth of Fitzroya was strongly correlated with soil depth, but not with N‐mineralization. Nothofagus and Weinmannia enjoyed large growth‐rate advantages over Fitzroya on the more fertile soils of the study area, and this advantage was reduced or nullified (but not reversed) on poor soils. This pattern was associated with species differences in nitrogen use: on fertile soils, Nothofagus and especially Weinmannia, achieved more height growth per unit of leaf N than Fitzroya, whereas differences were minimal on poor soils.Main conclusionsResults suggest that comparative height growth rates of juvenile trees, associated with nitrogen productivity differences, play a key role in determining eventual dominance on productive sites, but that traits other than growth (possibly related to nutrient retention) are more important on low fertility sites. Disturbance regimes appear to vary in relation to site fertility, and differential species responses to this superimposed disturbance gradient are also probably influential in determining overstorey composition.