Foliage litter turnover and earthworm populations in three beech forests of contrasting soil and vegetation types

Summary Leaf litter decomposition, levels of accumulated litter as well as the abundance and biomass of earthworms were measured in three mature beech forests in southern Sweden: one mor site, one poor mull site, and one rich mull site. The disappearance rate of beech litter, measured with litter bags, increased with increasing soil fertility. On the rich mull site, the disappearance rate was much higher than in the two other forests, due to the combined effects of higher earthworm activity, more favouable soil moisture conditions, and higher litter quality. Incubating the litter in finely meshed bags (1-mm mesh) to exclude macrofauna had a great effect on litter mass loss in the rich mull site, but it had only a minor effect in the other sites. Simultaneous incubations of local and transplanted leaf litter on the three study sites showed that the substrate quality of the litter increased in the order: mor site — poor mull site — rich mull site. Lignin, N, and P concentrations of the leaf litter failed to explain the observed differences in decomposition rates, and acid/base properties are suggested to be more important. Earthworm numbers per m² were 2.5 (1 species) in the mor, 40 (6 species) in the poor mull and 220 (9 species) in the rich mull forest. Soil chemical conditions, notably pH, were suggested as the main factors determining the inter-site differences in abundance and species composition of earthworms. The role of litter decomposition and earthworm activity in the accumulation of organic matter in the forest floor in different types of beech woodlands are discussed.     

Earthworm numbers,biomass and respiratory metabolism in a beech woodland-Wytham Woods,Oxford

Summary The mean annual population density of earthworms was found to be 164.6 m^-2 during a period of detailed study between October 1971 and September 1972. In a year of less detailed study between November 1972 and October 1973 the population density was 117.5 m^-2 (139.8 m^-2 when the type of extraction method was allowed for). Mean biomass densities in the two years of investigation were 41.0 g preserved wet wt m^-2 (1971–1972) and 38.6 (possibly 39.2) g preserved wet wt m^-2 (1972–1973).Comparison of the Brogden's Belt population and biomass densities with those reported from other woodlands indicates that soil type is more important than leaf litter type in determining the numerical abundance of earthworms. Population densities are lower in beechwoods on mor soils, mor soils also support significantly fewer species. As with numbers, mean biomass density in beechwoods on mor soils was significantly lower than that occurring in beechwoods on mull soils; the latter, in turn, being lower than those found in mixed deciduous woods on mull soils. Unlike population density biomass density is influenced by both soil and litter type, this is discussed by reference to mean body weights and food quality as reflected by tannin, nitrogen and carbohydrate content.The annual respiratory metabolism of the Brogden's Belt earthworms was calculated to be between 10.7 and 13.41 O₂ m^-2 a^-1, which is equivalent to between 4.1 and 5.1% of the total soil metabolism. A production/biomass ratio of 0.49–0.58 was estimated, as was a net population efficiency of 22%.     

Humus functioning types in evergreen coniferous forests of the French Inner Alps

Three humus forms that are widespread in the Inner Alps, a dysmull below Pinus sylvestris, an oligomull below Abies alba and a mor below Pinus cembra, were studied by following mineral nitrogen production and uptake and by a characterization of the pedofauna. Due to the production and uptake of nitrogen, mainly as nitrates and also due to the dominance of Lumbricids versus Arthropods, the oligomull was the sole humus form that functioned as a mull. Nevertheless relatively weak mineral nitrogen production and the dominance of endogeic soil-dwelling earthworms versus anecic earthworms demonstrated the moderate activity of this mull, which is representative of humus forms of old growth forests and cold climates. There was a discrepancy between the very low biological activity of the dysmull and its status of mull, while an observed bifunctioning between the different layers of this humus also differentiated it from the mor.     

Earthworm invasion into previously earthworm-free temperate and boreal forests

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.     

Nutrient cycling in two Danish beech forests growing on different soil types

In two mature Danish beech forests, one growing on fertile mull, the other on acid mor, cycles through vegetation of Ca, Mg, K, Na, Mn and P are followed. Nutrient content is measured in precipitation, throughfall, stem flow, litter and herbage as well as in soil pool (0–10 cm).
In both forests, Na is found to pass vegetation in highest rate compared to soil pool, On acid mor, trees accumulate amounts of nutrients comparable to the amounts falling in precipitation, but on fertile mull trees accumulate more of all nutrients than that which falls in precipitation, with the exception of Mn.
P seems to be the limiting growth factors for trees on acid mor, and none of the nutrients investigated seems to limit growth on fertile mull.     

Enchytraeids, lumbricids and soil arthropods in a northern deciduous woodland – a quantitative study

Soil/litter inhabiting annelids and arthropods were studied for one year in a mesic deciduous woodland, Andersby ängsbackar, one of the most northerly areas in Sweden to possess unbroken oak/hazel vegetation. Lumbricidae, Enchytraeidae, Collembola, Acari, Coleoptera, Diptera and Araneae were important faunal groups with regard to abundance, biomass and respiratory metabolism. Annual mean biomass (8 g dw m⁻²) and respiration (450 kJ m ⁻² yr⁻¹) of the joint annelid and arthropod community were dominated by lumbricids (76 and 50%, respectively). The biomass and respiration of the annelids and arthropods at Andersby were lower than in some other oak forests in northwestern Europe, probably because of lower soil temperature, lower litter supply and because some lumbricid and diplopod species were at their northern border of distribution.
Of the annual respiration, saprovores, fungivores, carnivores and root-feeders contributed 65, 19, 14 and 1.5%, respectively. The saprovore percentage was lower at Andersby than at other mull sites compared, probably as an effect of the northern climate, which might favour microbial saprovory. The fungivore and carnivore percentages were higher, which might be due to the presence of a litter layer throughout the year.     

Decomposition of cellulose,soil organic matter and plant litter in a temperate grassland soil

The formation of mor humus in an experimental grassland plot, which has been acidified by long-term fertiliser treatment, has been studied by comparing the rates of cellulose, soil organic matter and plant litter decay with those in an adjacent plot with near-neutral pH and mull humus. The decomposition of cellulose filter paper in litter bags of 5 mm, 1-mm and 45-μm mesh size buried at 3 to 4 cm depth the plots was followed by measuring the weight loss and changes in glucose content over a 6 month period. Soil pH was either 5.3 or 4.3. Decomposition of native soil organic matter and plant litter in soil from the same plots were followed using CO₂ evolution in laboratory microcosms. Cellulose weight loss at pH 5.3 was greatest from the 5-mm mesh bags and least from the 45-um mesh bags. At pH 4.3 there was little weight loss from bags and no significant differences in weight loss between bags with different sized mesh. There was, however, a reduction in the glucose content of the hydrolysed and derivatised filter paper with time. The decomposition rate of native soil organic matter in the low pH soil was increased to that observed in the less acid soil when the pH of the former was increased from 4.3 to 5.3. The increase in decomposition rate of added plant litter in the more acid soil as a result of CA(OH)₂ addition was only 60% of that observed in the soil with pH 5.3. These data support the hypothesis that the absence of soil animals and the restricted microbial decomposition in the acidic soil was responsible for mor humus formation.     

Comparative efficacy of three epigeic earthworms under different deciduous forest litters decomposition

An experiment was conducted during 1998-1999, in a deciduous forest located in the semi-arid tropics of central India, to evaluate the suitability of different forest litters as food material for the tropical epigeic earthworms i.e. Eisenia fetida (Savigny), Perionyx excavatus (Perrier) and Dicogaster bolaui (michaelsen). The aim was to examine the influence of these earthworms on the decomposition processes of three types of forest litters i.e. Tectona grandis (teak), Madhuca indica (mahua) and Butea monosperma (palas), on the maintenance of quality in a vermicomposting system, and to assess the effect of applications of in situ prepared vermicomposts on the growth of forest trees. The results indicated that T. grandis litter was the most suitable food material for the earthworms possibly because it contained high reserves of mineral nutrients. Comparisons of the survival and reproduction rates of the three epigeic earthworm species indicated that a higher reproduction rate was maintained for E. fetida compared to P. excavatus and D. bolaui in the decomposition of these forest litters. The rates of growth and population increases of E. fetida approximately doubled after 12 weeks of litter decomposition. The litter decomposition process was associated strongly with the quality of the materials and their chemical composition. Irrespective of earthworm inoculations, the levels of available nutrient such as NH(4)-N, NO(3)-N, available P and K increased significantly (pM. indica litter compost>B. monosperma litter compost. The mature decomposed litter had lower C/N ratios (11.3-24.8:1), water-soluble carbon (0.30-0.58%), water-soluble carbohydrates (0.35-0.71%) and larger cation exchange capacity/total organic carbon ratios than the values in the parent forest litter. The lignin content increased with maturation with a concomitant decrease in cellulose resulting in higher lignin/cellulose ratios.Application of all three vermicomposts to forest trees significantly improved their heights and diameters over those of control trees, although the increases were lower than those resulting from the chemical fertilizer applications. However, soil biological activities i.e. soil respiration, soil microbial biomass carbon and dehydrogenase activity were greater by application of vermicomposts over that after application of inorganic fertilizer in a new plantation of T. grandis.     

Plant Biomass,Nutrient Concentration and Nutrient Storage in a Tropical Dry Forest in the South–west of Madagascar

Plant biomass, mineral composition and the amounts of nutrients in the different fractions of the vegetation were determined for a dense dry deciduous forest growing on light red sands in south-western Madagascar. Complete harvesting and soil coring were used to determine the above- and below-ground biomass respectively. The above-ground biomass, weighing 118 t ha⁻¹ (dry matter), was mostly (96%) made up of phanerophytes (woody trees and shrubs >25 cm tall). Dead material (litter and dead wood on the soil surface) represented 13.8 t ha⁻¹. These results fit well into the range of values reported for other tropical ecosystems. The below-ground biomass was 17.8 t ha⁻¹ giving a root/shoot ratio of 0.15. Rooting is superficial. The nutrient concentration in this dry forest on light reddish-brown sands is, as in other dry forests, considerably higher than that usually found for humid forests. Calcium is the most abundant element. The plant biomass Ca/K ratio is much higher than that of humid tropical forests. In spite of its high originality, this Madagascan dry forest has the same behaviour as other dry forests of the world.     

Controls over leaf litter decomposition in a mixed evergreen and deciduous broad-leaved forest,Central China

Plant and Soil - Our understanding of the determinants of leaf litter decomposition is lacking for mixed evergreen and deciduous broad-leaved forests compared with tropical and temperate forests....     

Distribution and impacts of invasive earthworms in Canadian forest ecosystems

In Canada it is generally accepted that most indigenous earthworms did not survive glaciation, and that the majority of the earthworms now inhabiting Canadian soils are relatively recent introductions of European origin. Although these exotic earthworms are generally considered to be beneficial in agricultural soils, their effects can be less benign in forested ecosystems. Studies have shown that invading earthworms can significantly alter the forest floor, affecting the distribution of carbon, nitrogen and other chemicals, roots, microbes and other elements of the soil fauna, and even understory vegetation. This paper summarizes the current distribution of exotic and native earthworm species in Canadian forests and draws on the results of studies of invasion patterns and environmental impacts in northern forests in North America and Europe to discuss potential outcomes for forests in Canada. The potential for variables such as temperature, pH, litter palatability and dispersal, to limit or promote the invasion of exotic earthworms in Canadian forests is discussed, and areas for future research are proposed. The same earthworm species that are invading forests in northern Europe and the US are also invading Canadian forests. Several species of exotic earthworms are already established in a wide range of deciduous and coniferous forest types, including the boreal. Evidence is presented to suggest that further expansion into Canadian forests is likely.     

Ecological and site historical aspects of N dynamics and current N status in temperate forests

Ecological developments during Holocene age and high atmospheric depositions since industrialization have changed the N dynamics of temperate forest ecosystems. A number of different parameters are used to indicate whether the forests are N‐saturated or not, most common among them is the occurrence of nitrates in the seepage water below the rooting zone. The use of different definitions to describe N saturation implies that the N status of ecosystems is not always appropriately assessed. Data on N dynamics from 53 different German forests were used to classify various development states of forest ecosystems according to the forest ecosystem theory proposed by Ulrich for which N balances of input – (output plus plant N increment) were used. Those systems where N output equals N input minus plant N increment are described as (quasi‐) Steady State Type. Those forests where N output does not equal N input minus plant N increment as in a ‘transient state.’ Forests of the transient state may lose nitrogen from the soil (Degradation Type) or gain nitrogen [e.g., from atmospheric depositions (Accumulation Type)]. Forest ecosystems may occur in four different N states: (a) (quasi‐) Steady State Type with mull type humus, (b) Degradation Type with mull type humus, (c) Accumulation Type with moder type humus, and (d) (quasi‐) Steady State Type with moder type humus. Forests with the (quasi‐) steady state with mull type humus in the forest floor (n= 8) have high‐soil pH values, high N retention by plant increment, high N contents in the mineral soils, and have not undergone large changes in the N status. Forests of the Degradation Type lose nitrogen from the mineral soil (currently degradation is occurring on one site). Most forests that have moder or mor type humus and low‐soil pH values, and low N contents in the mineral soil have gone through the transient state of organic matter loss in the mineral soils. They accumulate organic matter in the forest floor (accumulation phase, currently 21 sites are accumulating 6–21 kg N ha^?1 yr^?1) or have reached a new (quasi‐) steady state with moder/mor type humus (n= 15). N retention in the accumulation phase has significantly increased in soil with N deposition (r²= 0.38), soil acidity (considering thickness of the forest floor as indices of soil acidity, r²= 0.43) and acid deposition (sulfate deposition, r²= 0.39). Retention of N (4–20 kg N ha^?1 yr^?1) by trees decreased and of soils increased with a decrease in the availability of base cations indicating the important role of trees for N retention in less acid soils and those of soils in more acid soils. Ecosystem theory could be successfully applied on the current data to understand the dynamics of N in temperate forest ecosystems.     

Arsenate tolerance in Agrostis castellana and Agrostis delicatula

Experiments were made to elucidate why many field-layer plants in beech forests do not grow in mor (raw humus), a layer of organic matter in various stages of decay containing little or no mineral soil particles. Three possibilities were considered directly or indirectly: (i) tree root competition and litter shading, (ii) phytotoxic and nutrient complexing organic compounds in the soil solution, and (iii) high H-ion concentration in the solution. Rhizomes of eight species (Allium ursinum, Carex sylvatica, Convallaria majalis, Deschampsia flexuosa, Galium odoratum, Poa nemoralis, Stellaria holostea, Stellaria nemorum) were collected in late winter and used in a soil experiment to test reestablishment and growth in untreated mor (soil solution pH 3.6) and after pH was raised to 4.3 by addition of SrCO₃. A flowing solution experiment was used with five of the species mentioned. The solution was composed according to the soil solution of the untreated mor but lacking organic compounds. pH was adjusted to and maintained at 3.6 and 4.3. The experiments showed that even when root competition, litter shading and organic compounds were excluded as limiting factors, only Deschampsia flexuosa, and partly Convallaria majalis, could produce new roots in the untreated mor or in the pH 3.6 solution treatment. Some shoot biomass developed in all species except in Galium odoratum, though significantly less than at pH 4.3.It was concluded that high H-ion concentration of the soil solution precludes establishment and growth of many forest plants in beech forests developed on mor podzols.     

Coordination of aboveground and belowground responses to local‐scale soil fertility differences between two contrasting Jamaican rain forest types

There is growing interest in understanding how declining soil fertility in the prolonged absence of major disturbance drives ecological processes, or ‘ecosystem retrogression’. However, there are few well characterized study systems for exploring this phenomenon in the tropics, despite tropics occupying over 40% of the Earth's terrestrial surface. We studied two types of montane rain forest in the Blue Mountains of Jamaica that represent distinct stages in ecosystem development, i.e. an earlier stage with shallow organic matter and a late stage with deep organic matter (hereafter ‘mull’ and ‘mor’ stages). We characterized responses of soil fertility and plant, soil microbial and nematode communities to the transition from mull to mor and whether these responses were coupled. For soil abiotic properties, we found this transition led to lower amounts of both nitrogen (N) and phosphorus (P) and an enhanced N to P ratio. This led to shorter‐statured and less diverse forest, and convergence of tree species composition among plots. At the whole community (but not individual species) level foliar and litter N and P diminished from mull to mor, while foliar N to P and resorption efficiency of P relative to N increased, indicating increasing P relative to N limitation. We also found impairment of soil microbes (but not nematodes) and an increasing role of fungi relative to bacteria during the transition. Our results show that retrogression phenomena involving increasing nutrient (notably P) limitation can be important drivers in tropical systems, and are likely to involve aboveground–belowground feedbacks whereby plants produce litter of diminishing quality, impairing soil microbial processes and thus reducing the supply of nutrients from the soil for plant growth. Such feedbacks between plants and the soil, mediated by plant litter and organic matter quality, may serve as major though often overlooked drivers of long term environmental change.     

Temperature and substrate availability regulate soil respiration in the tropical mountain rainforests,Hainan Island,China

Aims Tropical forest plays a key role in global C cycle; however, there are few studies on the C budget in the tropical rainforests in Asia. This study aims to (i) reveal the seasonal patterns of total soil respiration (R T), litter respiration (R L) and soil respiration without surface organic litter (R NL) in the primary and secondary Asian tropical mountain rainforests and (ii) quantify the effects of soil temperature, soil moisture and substrate availability on soil respiration.Methods The seasonal dynamics of soil CO₂ efflux was measured by an automatic chamber system (Li-8100), within the primary and secondary tropical mountain rainforests located at the Jianfengling National Reserve in Hainan Island, China. The litter removal treatment was used to assess the contribution of litter to belowground CO₂ production.Important findings The annual R T was higher in the primary forest (16.73±0.87 Mg C ha^-1) than in the secondary forest (15.10±0.26 Mg C ha^-1). The rates of R T, R NL and R L were all significantly higher in the hot and wet season (May–October) than those in the cool and dry season (November–April). Soil temperature at 5cm depth could explain 55–61% of the seasonal variation in R T, and the temperature sensitivity index (Q 10) ranked by R L (Q 10 = 3.39)> R T (2.17)> R NL (1.76) in the primary forest and by R L (4.31)> R T (1.86)> R NL (1.58) in the secondary forest. The contribution of R L to R T was 22–23%, while litter input and R T had 1 month time lag. In addition, the seasonal variation of R T was mainly determined by soil temperature and substrate availability. Our findings suggested that global warming and increased substrate availability are likely to cause considerable losses of soil C in the tropical forests.     

Influences of evergreen gymnosperm and deciduous angiosperm tree species on the functioning of temperate and boreal forests

It has been recognized for a long time that the overstorey composition of a forest partly determines its biological and physical–chemical functioning. Here, we review evidence of the influence of evergreen gymnosperm (EG) tree species and deciduous angiosperm (DA) tree species on the water balance, physical–chemical soil properties and biogeochemical cycling of carbon and nutrients. We used scientific publications based on experimental designs where all species grew on the same parent material and initial soil, and were similar in stage of stand development, former land use and current management. We present the current state of the art, define knowledge gaps, and briefly discuss how selection of tree species can be used to mitigate pollution or enhance accumulation of stable organic carbon in the soil. The presence of EGs generally induces a lower rate of precipitation input into the soil than DAs, resulting in drier soil conditions and lower water discharge. Soil temperature is generally not different, or slightly lower, under an EG canopy compared to a DA canopy. Chemical properties, such as soil pH, can also be significantly modified by taxonomic groups of tree species. Biomass production is usually similar or lower in DA stands than in stands of EGs. Aboveground production of dead organic matter appears to be of the same order of magnitude between tree species groups growing on the same site. Some DAs induce more rapid decomposition of litter than EGs because of the chemical properties of their tissues, higher soil moisture and favourable conditions for earthworms. Forest floors consequently tend to be thicker in EG forests compared to DA forests. Many factors, such as litter lignin content, influence litter decomposition and it is difficult to identify specific litter‐quality parameters that distinguish litter decomposition rates of EGs from DAs. Although it has been suggested that DAs can result in higher accumulation of soil carbon stocks, evidence from field studies does not show any obvious trend. Further research is required to clarify if accumulation of carbon in soils (i.e. forest floor + mineral soil) is different between the two types of trees. Production of belowground dead organic matter appears to be of similar magnitude in DA and EG forests, and root decomposition rate lower under EGs than DAs. However there are some discrepancies and still are insufficient data about belowground pools and processes that require further research. Relatively larger amounts of nutrients enter the soil–plant biogeochemical cycle under the influence of EGs than DAs, but recycling of nutrients appears to be slightly enhanced by DAs. Understanding the mechanisms underlying forest ecosystem functioning is essential to predicting the consequences of the expected tree species migration under global change. This knowledge can also be used as a mitigation tool regarding carbon sequestration or management of surface waters because the type of tree species affects forest growth, carbon, water and nutrient cycling.     

Disappearance of overstorey and understorey litter in an open eucalypt forest

Litter accumulation dynamics and first year rates of disappearance were investigated for leaves of overstorey and understorey species in the mixed eucalypt forest in the Griffith University study area, Queensland, Australia. The average biomass of the litter layer, was 10.2 t/ha. The wood and overstorey leaf litter formed 62.7% of the accumulated litter biomass, and were spread continuously across the ground. The distribution of shrub litter was discontinuous, being concentrated in localized patches beneath individual plants. The litter bag and tethered leaf techniques were used to measure the rate of disappearance of overstorey and shrub leaves. The ‘pairedquadrat’ technique was used to measure the weight loss of the grasses. The small leaves of the dominant shrub, Pultenaea villosa Willd. disappeared most rapidly, followed by the overstorey leaves, grasses and Acacia leaves. Fragmentation by physical factors and litter fauna appeared to be the major factors responsible for the disappearance of the overstorey leaves during the first year of exposure. The data suggest that removal of leaf constituents by leaching and microbial decomposition were more important for the shrub litter than the overstorey leaves. Fractional disappearance rates (loss constants) obtained from the short term weight loss measurements, and calculated using the steady-state model of litter accumulation (k=L/X), overestimated the rate of litter disappearance and litter decomposition. For the overstorey leaves in particular, the loss constants also overestimated the rate of loss of material from the litter layer since the fragmented and consumed tissues accumulated in the fraction of comminuted fragments before moving into the humus/soil subsystem. A compartment model of the components of the litter layer in the mixed eucalypt forest is presented. It incorporates overstorey and understorey litter accession, accumulation and disappearance data. The adoption of a two dimensional decomposition/accumulation matrix is suggested as an appropriate framework within which to simulate the dynamics of the litter subsystem in mixed eucalypt forest ecosystems.     

Ecosystem effects of non-native earthworms in Mid-Atlantic deciduous forests

In many mid-Atlantic forests where both native and non-native earthworms exist, it is the non-native species that are the dominant component of the soil macrofauna. Few earthworm ecology studies, however, focus attention on these forest systems in order to determine the relative ecological roles and potential interactions of the native and non-native earthworms. In a series of field samplings and experimental manipulations we collected data on the effects of earthworms on below-and aboveground ecosystem processes. Earthworm abundance and the ecological processes measured were dynamic in space and time across the range of study sites. Leaf litter decay rates doubled at sites that had abundant non-native earthworms. Earthworms also altered the abundance of soil fungi, the activity of extracellular enzymes, soil respiration, and the growth of tree seedlings but the effects varied among sites depending on differences in land-use history and forest age. Red oak seedling growth was less at sites that had abundant earthworms but tulip poplar and red maple seedlings grew equally well with and without abundant earthworms. These preliminary results suggest that non-native earthworms have significant ecosystem effects, even in forests where native earthworms still occur. Land use history, however, plays an important role in determining what those effects will be, and these effects are likely to be dynamic, depending on the abundance of non-native earthworms.     

Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

Background and aims

The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δ¹⁵N).

Methods

Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δ¹⁵N.

Results

All foliar δ¹⁵N values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δ¹⁵N of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δ¹⁵N values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δ¹⁵N in the two forest types suggesting a changing uptake ratio of different N forms between forest types.

Conclusions

These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δ¹⁵N values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δ¹⁵N between the two forest types.     

Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis

Proper estimates of decomposition are essential for tropical forests, given their key role in the global carbon (C) cycle. However, the current paradigm for litter decomposition is insufficient to account for recent observations and may limit model predictions for highly diverse tropical ecosystems. In light of recent findings from a nutrient-poor Amazonian rainforest, we revisit the commonly held views that: litter traits are a mere legacy of live leaf traits; nitrogen (N) and lignin are the key litter traits controlling decomposition; and favourable climatic conditions result in rapid decomposition in tropical forests. Substantial interspecific variation in litter phosphorus (P) was found to be unrelated to variation in green leaves. Litter nutrients explained no variation in decomposition, which instead was controlled primarily by non-lignin litter C compounds at low concentrations with important soil fauna effects. Despite near-optimal climatic conditions, tropical litter decomposition proceeded more slowly than in a climatically less favourable temperate forest. We suggest that slow decomposition in the studied rainforest results from a syndrome of poor litter C quality beyond a simple lignin control, enforcing energy starvation of decomposers.We hypothesize that the litter trait syndrome in nutrient-poor tropical rainforests may have evolved to increase plant access to limiting nutrients via mycorrhizal associations.