Litter production in two mangrove forests along the coast of Ghana.

Litter production is an important component of mangrove primary productivity and has been widely used as a measure of productivity. During the past years, studies have been carried out on aspects of mangrove ecology and biodiversity in Ghana. These lay the foundations for surveys in ecological processes including productivity. This study was aimed at evaluating the production of litter within the mangrove forests at the Kakum and Pra estuaries. Four 0.25 ha study plots were demarcated in each mangrove forest, within which litter production and mangrove structural parameters were measured. Records on meteorological parameterss surrounding the study areas were obtained from Tutiempo Network. It was observed that annual litter production rate varied significantly (t = 2.91, P < 0.05) between the two mangrove forests - Kakum mangrove forest recorded 9.60 t ha-1 y-1 while Pra mangrove forest recorded 10.72 t ha-1 y-1. Litter production also varied significantly within the sampling months and study plots (P < 0.05). Leaf litter accounted for 61.26-99.45% of litterfall in both forests. Mean DBH and height of mangrove trees per study plot ranged from 3.05 cm to 3.39 cm and 3.20 m to3.92 m respectively for the Kakum mangrove forest, while that of the Pra mangrove forests ranged between 3.12 cm and 4.06 cm, and 3.58m and 4.44 m respectively. It was observed that the meteorological and structural parameters explained very little of the litter variability within the two mangrove forests. Hence, other factors such as growth cycles, senescence and age might have influenced mangrove litter production more than those measured in this study. This calls for more studies into litter production in mangrove ecosystems in Ghana, to ascertain the various factors that contribute to litter production. This will help emphasise the importance of mangrove litter productivity and the need to restore and conserve our mangrove ecosystems.

Utilization and reproductive performance of gilts in large-scale pig farming system with different production levels in China: a descriptive study.

BACKGROUND: This study was to investigate the utilization and reproductive performance of gilts in large-scale pig farms. Data of this descriptive study included 169,013 gilts of 1540 gilts' batches on 105 large-scale pig farms from April 2020 to March 2021. According to the upper and lower 25th percentiles of piglets weaned per sow per year (PSY) during the research stage, pig farms were divided into three productivity groups: high-performing (HP), intermediate-performing (IP) and low-performing (LP) farms. On the basis of breeds, LP (LP-Total) farms was further divided into LP-breeding pig (LP-BP) and LP-commercial pig (LP-CP) groups. Average utilization, estrus and first mating data was collected from a total of 1540 gilts' batches. The age-related factors (introduction age, age at first estrus and age at first mating) and litter production (total number of piglets, number of piglets born alive and number of weaned piglets, as well as their proportion distribution) among HP and LP groups were compared. The litter production in different age groups were also analyzed. RESULTS: The introduction age, mortality and culling rate of HP farms were lower compared with LP farms. Total number of piglets per litter, number of piglets born alive per litter and number of weaned piglets per litter in HP farms were significantly more than those of LP groups, respectively. The proportion distribution peaks of litter production in HP farms were shifted about two more than those in LP groups, respectively; and the proportion of low litter production (eight per litter or less) was lower than that in LP groups. The results of different age groups showed that total number of piglets per litter and number of piglets born alive per litter in 220-279 d were the most, while that of 370 d was the least. CONCLUSIONS: The overall utilization and reproductive performance of gilts in HP farms was better than those of LP farms. The difference in utilization was reflected in introduction source, culling rate and mortality. While the age at first estrus and first mating, breeds and litter production were the main differences for reproductive performance.

Nutrient limitation of plant reproduction in a tropical moist forest.

Nutrient addition experiments indicate that nitrogen and phosphorus limit plant processes in many tropical forests. However, the long-term consequences for forest structure and species composition remain unexplored. We are positioned to evaluate potential long-term consequences of nutrient addition in central Panama where we have maintained a factorial nitrogen-phosphorus-potassium fertilization experiment for 21 yr and an independent study quantified the species-specific nutrient requirements of 550 local tree species. Here, we ask whether nutrients limit reproduction at the species and community levels. We also ask whether species-specific reproductive responses to nutrient addition are stronger among species associated with naturally fertile soils, which could contribute to a shift in species composition. We quantified species-level reproductive responses for 38 focal species in the 21st year of the experiment and community-level reproductive litter production for the first 20 yr. Species-level reproductive responses to nitrogen and potassium addition were weak, inconsistent across species, and insignificant across the 38 focal species. In contrast, species-level responses to phosphorus addition were consistently and significantly positive across the 38 focal species but were unrelated to species-specific phosphorus requirements documented independently for the same species. Community-level reproductive litter production was unaffected by nutrient addition, possibly because spatial and temporal variation is large. We conclude that phosphorus limits reproduction by trees in our experiment but find no evidence that reproductive responses to phosphorus addition favor species associated with naturally phosphorus-rich soils.

Data-driven estimates of global litter production imply slower vegetation carbon turnover.

Accurate quantification of vegetation carbon turnover time (τveg ) is critical for reducing uncertainties in terrestrial vegetation response to future climate change. However, in the absence of global information of litter production, τveg could only be estimated based on net primary productivity under the steady-state assumption. Here, we applied a machine-learning approach to derive a global dataset of litter production by linking 2401 field observations and global environmental drivers. Results suggested that the observation-based estimate of global natural ecosystem litter production was 44.3 ± 0.4 Pg C year-1 . By contrast, land-surface models (LSMs) overestimated the global litter production by about 27%. With this new global litter production dataset, we estimated global τveg (mean value 10.3 ± 1.4 years) and its spatial distribution. Compared to our observation-based τveg , modelled τveg tended to underestimate τveg at high latitudes. Our empirically derived gridded datasets of litter production and τveg will help constrain global vegetation models and improve the prediction of global carbon cycle.

Importance of overstorey attributes for understorey litter production and nutrient cycling in European forests.

BACKGROUND: In contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest, its share in annual litter production and nutrient cycling may be more important. Whether and how this functional role of the understorey differs across forest types and depends upon overstorey characteristics remains to be investigated. METHODS: We sampled 209 plots of the FunDivEUROPE Exploratory Platform, a network of study plots covering local gradients of tree diversity spread over six contrasting forest types in Europe. To estimate the relative contribution of the understorey to carbon and nutrient cycling, we sampled non-lignified aboveground understorey biomass and overstorey leaf litterfall in all plots. Understorey samples were analysed for C, N and P concentrations, overstorey leaf litterfall for C and N concentrations. We additionally quantified a set of overstorey attributes, including species richness, proportion of evergreen species, light availability (representing crown density) and litter quality, and investigated whether they drive the understorey's contribution to carbon and nutrient cycling. RESULTS AND CONCLUSIONS: Overstorey litter production and nutrient stocks in litterfall clearly exceeded the contribution of the understorey for all forest types, and the share of the understorey was higher in forests at the extremes of the climatic gradient. In most of the investigated forest types, it was mainly light availability that determined the contribution of the understorey to yearly carbon and nutrient cycling. Overstorey species richness did not affect the contribution of the understorey to carbon and nutrient cycling in any of the investigated forest types.

Carbon isotope ratio of leaf litter correlates with litter production in a mangrove ecosystem in South China.

As an important ecological process, litter production is generally recognized as being directly relevant to net primary productivity and carbon storage of mangrove ecosystems. In the present study, we made continuous, monthly assessment of litter production from 2010 to 2016 for five mangrove sites in Shenzhen Futian Mangrove Nature Reserve. Results showed that all mangrove locations displayed distinct seasonality in litter production, and that the alien species produced significantly more litters than the native species. Carbon isotope analysis revealed an interesting, strongly negative relationship between litter production and δ13C of leaf litter (δ13CLL) among the five studied sites. Although it has long been known that δ13C of plant leaves correlates with water use efficiency and some components of plant productivity, the observed δ13CLL-litter production linkage is novel, justifying future exploration of δ13CLL as an potential indicator of litter production and net primary productivity in mangrove ecosystems.

Leaf litter contributes more to soil organic carbon than fine roots in two 10-year-old subtropical plantations.

Although many reports assume a strong relationship between plant litter inputs and soil organic carbon (SOC) accumulation, there is limited experimental evidence in support of this relationship. The Detritus Input and Removal Transfer (DIRT) experiment was conducted in two subtropical plantations, an Acacia crassicarpa monoculture (AC) and a Eucalyptus urophylla monoculture (EU), to assess the importance of leaf litter and fine roots for SOC content over a 6-year period. The SOC content in 0-10 cm soil layer was significantly reduced by litter removal (NL) or by removal of both litter and roots (NI), but the influence of root removal (NR) was only marginal. The AC forest had a much higher annual rate of litter production and a lower decomposition rate than the EU forest, but carbon release from plant litter did not significantly differ between the two forests. These results help explain why the SOC content did not differ between the two forests. Our findings suggest that plant leaf litter is more important than roots in maintaining the balance of the SOC pool in subtropical forest plantations.

Can mango orchards rehabilitate degraded areas by nutrient cycling?

Degradation of areas reduces nutrient cycling in the environment, and mango tree (Mangifera indica L.) cultivated in orchard could act as an alternative for degrading areas rehabilitation. The aim of this study was to evaluate the nutrients contribution to soil via litter in mango orchard, which was used as an alternative for degrading areas rehabilitation in the northeast of Pará, Brazil. The study was performed in the forest, mango orchard, and degrading area located in Salinópolis, Pará, Brazil. The total production and fractions (leaves, branches, reproductive parts, and miscellaneous), as well as the nutrient production and their contribution to the soil and litter decomposition were monitored. Mango trees have produced litter in similar quantities in the forest, 7.06 and 8.95 Mg ha-1 yr-1, respectively. However, mango orchard has denoted litter production seasonality, which was concentrated in less rainy season. Leaves fraction has presented larger contribution to litter total production. Moreover, the contribution of nutrients coming from mango orchard to the soil via litter was like forest. The decomposition was more rapid in mango orchard during the rainy season, when it took 5.51 months to decompose 50% of litter. Mango orchard may be used as an alternative to the rehabilitation of degrading areas due to its contribution to nutrient cycling and soil protection.

Plant responses to fertilization experiments in lowland, species-rich, tropical forests.

We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.

Canopy leaf area of a mature evergreen Eucalyptus woodland does not respond to elevated atmospheric [CO2] but tracks water availability.

Canopy leaf area, quantified by the leaf area index (L), is a crucial driver of forest productivity, water use and energy balance. Because L responds to environmental drivers, it can represent an important feedback to climate change, but its responses to rising atmospheric [CO2] and water availability of forests have been poorly quantified. We studied canopy leaf area dynamics for 28 months in a native evergreen Eucalyptus woodland exposed to free-air CO2 enrichment (the EucFACE experiment), in a subtropical climate where water limitation is common. We hypothesized that, because of expected stimulation of productivity and water-use efficiency, L should increase with elevated [CO2]. We estimated L from diffuse canopy transmittance, and measured monthly leaf litter production. Contrary to expectation, L did not respond to elevated [CO2]. We found that L varied between 1.10 and 2.20 across the study period. The dynamics of L showed a quick increase after heavy rainfall and a steady decrease during periods of low rainfall. Leaf litter production was correlated to changes in L, both during periods of decreasing L (when no leaf growth occurred) and during periods of increasing L (active shedding of old foliage when new leaf growth occurred). Leaf lifespan, estimated from mean L and total annual litter production, was up to 2 months longer under elevated [CO2] (1.18 vs. 1.01 years; P = 0.05). Our main finding that L was not responsive to elevated CO2 is consistent with other forest FACE studies, but contrasts with the positive response of L commonly predicted by many ecosystem models.

Effect of seasonal flooding cycle on litterfall production in alluvial rainforest on the middle Xingu River (Amazon basin, Brazil) / Efeito da variação da inundação sazonal na produção de serrapilheira numa floresta aluvial do médio Rio Xingu (bacia do Amazonas, Brasil)

Abstract The assumption for this study was that litterfall in floodplain environments of the middle Xingu river follows a pattern of seasonal variation. According to this view, litterfall production (total and fractions) was estimated in four alluvial rainforest sites on the middle Xingu River over an annual cycle, and examined the effect of seasonal flooding cycle. The sites included two marginal flooded forests of insular lakes (Ilha Grande and Pimentel) and two flooded forests on the banks of the Xingu itself (Boa Esperança and Arroz Cru). Total litterfall correlated with rainfall and river levels, but whereas the leaf and fruit fractions followed this general pattern, the flower fraction presented an inverse pattern, peaking in the dry season. The litterfall patterns recorded in the present study were consistent with those recorded at other Amazonian sites, and in some other tropical ecosystems.

Resumo O pressuposto para este estudo foi que a produção de serrapilheira nos ambientes inundáveis do médio rio Xingu, segue um padrão de variação sazonal. Assim, se quantificou durante um ciclo anual a produção de serapilheira total e de suas frações, e se indagou a correlação entre o regime do nível do rio e a produção de serapilheira. Quatro ambientes de floresta ombrófila aluvial foram estudados: duas florestas inundáveis de lagoas insulares (Ilha Grande e Pimentel) e dois ambientes de floresta marginal no canal principal do rio Xingu (Boa Esperança e Arroz Cru). A produção de serapilheira total nos quatro ambientes mostrou um padrão sincrônico com a variação do nível do rio e com a inundação das áreas marginais. Por sua vez, foi evidente um aumento da produção de frutos durante a inundação local e de forma inversa uma maior produção de flores com o regime de seca. Os padrões registrados para os componentes de serapilheira do médio rio Xingu, confirmam os achados para outros ambientes amazônicos e tropicais.

Effect of seasonal flooding cycle on litterfall production in alluvial rainforest on the middle Xingu River (Amazon basin, Brazil)

Abstract The assumption for this study was that litterfall in floodplain environments of the middle Xingu river follows a pattern of seasonal variation. According to this view, litterfall production (total and fractions) was estimated in four alluvial rainforest sites on the middle Xingu River over an annual cycle, and examined the effect of seasonal flooding cycle. The sites included two marginal flooded forests of insular lakes (Ilha Grande and Pimentel) and two flooded forests on the banks of the Xingu itself (Boa Esperança and Arroz Cru). Total litterfall correlated with rainfall and river levels, but whereas the leaf and fruit fractions followed this general pattern, the flower fraction presented an inverse pattern, peaking in the dry season. The litterfall patterns recorded in the present study were consistent with those recorded at other Amazonian sites, and in some other tropical ecosystems.

Resumo O pressuposto para este estudo foi que a produção de serrapilheira nos ambientes inundáveis do médio rio Xingu, segue um padrão de variação sazonal. Assim, se quantificou durante um ciclo anual a produção de serapilheira total e de suas frações, e se indagou a correlação entre o regime do nível do rio e a produção de serapilheira. Quatro ambientes de floresta ombrófila aluvial foram estudados: duas florestas inundáveis de lagoas insulares (Ilha Grande e Pimentel) e dois ambientes de floresta marginal no canal principal do rio Xingu (Boa Esperança e Arroz Cru). A produção de serapilheira total nos quatro ambientes mostrou um padrão sincrônico com a variação do nível do rio e com a inundação das áreas marginais. Por sua vez, foi evidente um aumento da produção de frutos durante a inundação local e de forma inversa uma maior produção de flores com o regime de seca. Os padrões registrados para os componentes de serapilheira do médio rio Xingu, confirmam os achados para outros ambientes amazônicos e tropicais.

Effect of seasonal flooding cycle on litterfall production in alluvial rainforest on the middle Xingu River (Amazon basin, Brazil) / Efeito da variação da inundação sazonal na produção de serrapilheira numa floresta aluvial do médio Rio Xingu (bacia do Amazonas, Brasil)

The assumption for this study was that litterfall in floodplain environments of the middle Xingu river follows a pattern of seasonal variation. According to this view, litterfall production (total and fractions) was estimated in four alluvial rainforest sites on the middle Xingu River over an annual cycle, and examined the effect of seasonal flooding cycle. The sites included two marginal flooded forests of insular lakes (Ilha Grande and Pimentel) and two flooded forests on the banks of the Xingu itself (Boa Esperança and Arroz Cru). Total litterfall correlated with rainfall and river levels, but whereas the leaf and fruit fractions followed this general pattern, the flower fraction presented an inverse pattern, peaking in the dry season. The litterfall patterns recorded in the present study were consistent with those recorded at other Amazonian sites, and in some other tropical ecosystems.

O pressuposto para este estudo foi que a produção de serrapilheira nos ambientes inundáveis do médio rio Xingu, segue um padrão de variação sazonal. Assim, se quantificou durante um ciclo anual a produção de serapilheira total e de suas frações, e se indagou a correlação entre o regime do nível do rio e a produção de serapilheira. Quatro ambientes de floresta ombrófila aluvial foram estudados: duas florestas inundáveis de lagoas insulares (Ilha Grande e Pimentel) e dois ambientes de floresta marginal no canal principal do rio Xingu (Boa Esperança e Arroz Cru). A produção de serapilheira total nos quatro ambientes mostrou um padrão sincrônico com a variação do nível do rio e com a inundação das áreas marginais. Por sua vez, foi evidente um aumento da produção de frutos durante a inundação local e de forma inversa uma maior produção de flores com o regime de seca. Os padrões registrados para os componentes de serapilheira do médio rio Xingu, confirmam os achados para outros ambientes amazônicos e tropicais.

Long-term differences in annual litter production between alien (Sonneratia apetala) and native (Kandelia obovata) mangrove species in Futian, Shenzhen, China.

Annual litter production in alien (Sonneratia apetala) and native (Kandelia obovata) mangrove forests in Shenzhen, China were compared from 1999 to 2010. S. apetala had significantly higher litter production than K. obovata, with mean annual total litter of 18.1 t ha(-1) yr(-1) and 15.2 t ha(-1) yr(-1), respectively. The higher litter production in S. apetala forest indicates higher productivity and consequently more nutrient supply to the estuarine ecosystems but may be more invasive due to positive plant-soil feedbacks and nutrient availability to this alien species. Two peaks were recorded in S. apetala (May and October), while only one peak was observed in K. obovata, in early spring (March and April). Leaf and reproductive materials were the main contributors to litter production (>80%) in both forests. These results suggest that the ecological function of S. apetala and its invasive potential can be better understood based on a long-term litter fall analysis.

Nutrient fluxes in litterfall of a secondary successional alluvial rain forest in Southern Brazil

During forest succession, litterfall nutrient fluxes increase significantly. The higher inputs of organic matter and nutrients through litterfall affects positively soil fertility and the species composition, which are essential components in forest restoration and management programs. In the present study, the input of nutrients to the forest soil via litterfall components was estimated for two sites of different development stages, in an early successional alluvial rain forest in Brazil. Litterfall returned to the soil, in kg/ha, ca. 93 N, 79 Ca, 24 K, 15 Mg, 6 P, 1.7 Mn, 0.94 Fe, 0.18 Zn, 0.09 Cu and 11.2 Al, in the site where trees were more abundant and had higher values of basal area. In the other area, where trees where less abundant and values of basal area were comparatively low, litterfall returned <50% of those amounts to the forest soil, except for Al. The amount of Al that returned to the soil was similar in both areas due to the high contribution of Tibouchina pulchra (82% of Al returned). Comparatively, high proportion of three dominant native tree species (Myrsine coriacea, T. pulchra and Cecropia pachystachya) explained better litter nutrient use efficiency (mainly N and P) in the site with the least advanced successional stage. Although litterfall of these species show lower nutrient concentrations than the other tree species, their nutrient fluxes were high in both sites, indicating a certain independence from soil essential nutrients. Such feature of the native species is very advantageous and should be considered in forest restoration programs. Rev. Biol. Trop. 59 (4): 1869-1882. Epub 2011 December 01.

Durante la sucesión secundaria forestal, el flujo de nutrientes en la hojarasca se incrementa significativamente. Los altos ingresos de materia orgánica y nutrientes a través de la hojarasca afecta positivamente la fertilidad del suelo y la composición de especies, las cuales son componentes esenciales para programas de restauración forestal y de manejo. En el presente estudio, el ingreso de nutrientes a través de la hojarasca y sus componentes fueron estimados para dos sitios de una selva lluviosa atlántica aluvial en sucesión temprana. La cantidad anual de elementos que ingresan al suelo desde la vegetación más desarrollada (sitios con alta área basal y abundancia de árboles) fueron (en kg/ha): 93 N, 79 Ca, 24 K, 15 Mg, 6 P, 1.7 Mn, 0.94 Fe, 0.18 Zn, 0.09 Cu y 11.2 Al. Menos de la mitad de esas cantidades fueron aportadas por la vegetación menos desarrollada, excepto para el Al. La cantidad de Al aportada a este sitio fue similar a la contribución de la vegetación más desarrollada, debido a la contribución de: Tibouchina pulchra (82% de todo el Al aportado). La eficiencia en el uso de nutrientes de la hojarasca (principalmente de N y P) de la vegetación menos desarrollada fue superior a la eficiencia de una más desarrollada, debido a la alta proporción de tres especies nativas dominantes (Myrsine coriacea, Tibouchina pulchra y Cecropia pachystachya) en la primera. Si bien la hojarasca de esas especies presenta menor concentración de nutrientes que las otras especies arbóreas, el flujo de nutrientes fue mayor (para ambos sitios), marcando una cierta independencia de los nutrientes esenciales del suelo. Las cantidades de esas especies deberían ser consideradas con mayor atención para su uso en programas de restauración forestal.

Ciclos biogeoquímicos en bosques naturales y plantaciones de coníferas en ecosistemas de alta montaña de Colombia

Biogeochemical cycles in natural forest and conifer plantations in the high mountains of Colombia. Plant litter production and decomposition are two important processes in forest ecosystems, since they provide the main organic matter input to soil and regulate nutrient cycling. With the aim to study these processes, litterfall, standing litter and nutrient return were studied for three years in an oak forest (Quercus humboldtii), pine (Pinus patula) and cypress (Cupressus lusitanica) plantations, located in highlands of the Central Cordillera of Colombia. Evaluation methods included: fine litter collection at fortnightly intervals using litter traps; the litter layer samples at the end of each sampling year and chemical analyses of both litterfall and standing litter. Fine litter fall observed was similar in oak forest (7.5Mg ha/y) and in pine (7.8Mg ha/y), but very low in cypress (3.5Mg ha/y). Litter standing was 1.76, 1.73 and 1.3Mg ha/y in oak, pine and cypress, respectively. The mean residence time of the standing litter was of 3.3 years for cypress, 2.1 years for pine and 1.8 years for oak forests. In contrast, the total amount of retained elements (N, P, S, Ca, Mg, K, Cu, Fe, Mn and Zn) in the standing litter was higher in pine (115kg/ha), followed by oak (78kg/ha) and cypress (24kg/ha). Oak forests showed the lowest mean residence time of nutrients and the highest nutrients return to the soil as a consequence of a faster decomposition. Thus, a higher nutrient supply to soils from oaks than from tree plantations, seems to be an ecological advantage for recovering and maintaining the main ecosystem functioning features, which needs to be taken into account in restoration programs in this highly degraded Andean mountains. Rev. Biol. Trop. 59 (4): 1883-1894. Epub 2011 December 01.

La caída y descomposición de hojarasca representan los principales ingresos de materia orgánica y nutrientes a los ecosistemas forestales. Se estudiaron la caída y acumulación de hojarasca fina y el retorno de nutrientes, en un robledal (Quercus humboldtii) y en plantaciones de pino (Pinus patula) y ciprés (Cupressus lusitanica) en tierras altas de Colombia. La caída de hojarasca fina fue similar entre el robledal (7.5Mg/ha.a) y el pinar (7.8Mg/ha.a), y muy inferior en el cipresal (3.5Mg/ha.a). El mantillo representó 1.76, 1.73 y 1.3Mg/ha.a en el robledal, pinar y cipresal, respectivamente. El tiempo medio de residencia (TMR) del mantillo siguió la secuencia: cipresal (3.3 años)>pinar (2.1 años)>robledal (1.8 años). La cantidad de nutrientes retenidos en el mantillo siguió la secuencia: pinar (115kg/ ha)>robledal (78kg/ha)>cipresal (24kg/ha). Los menores TMR de nutrientes se presentaron para la hojarasca foliar del robledal, en su mayoría inferiores a 1.0 años. En términos de la función ecosistémica en el robledal los procesos estudiados son muy superiores vía provisión de nutrientes al suelo y regulación de los ciclos biogeoquímicos, aspectos que deben ser considerados previa implementación de programas de repoblamiento forestal.

CAÍDA DE HOJARASCA Y DINÁMICA DE NUTRIENTESEN PLANTACIONES DE Acacia mangium (MIMOSACEAE)DE ANTIOQUIA, COLOMBIA. / Litterfall and Nutrient Dynamics in Acacia mangium (Mimosaceae)Forest Plantations of Antioquia, Colombia

La producción de hojarasca, el retorno y la reabsorción de nutrientes, y la eficiencia en su uso, fueron estudiados durante un año en plantaciones de Acacia mangium estable-cidas en suelos degradados por minería aurífera en la región del Bajo Cauca colom-biano. La producción anual de hojarasca fina fue de 10,4 Mg ha-1 y estuvo dominada por la fracción foliar (54%), seguida del material reproductivo (24%), y en menor pro-porción por otros restos (6%) y hojas de otras especies (1,5%). Los mayores retornos de materia orgánica y nutrientes se presentaron en los sitios clasificados como de calidad alta, en tanto que la práctica de subsolado del suelo, previo establecimiento de las plantaciones, no mostró efectos significativos sobre estos. La hojarasca foliar mostró una concentración alta de N y consecuentemente, dados los altos valores de producción de esta fracción, un retorno potencial alto de N. El P, con baja concentración foliar y un bajo retorno potencial, además de los altos valores de los índices de eficiencia en su uso y de reabsorción foliar, fue el nutriente más limitante. Los altos valores de producción de hojarasca fina y de retorno potencial de nutrientes determinados en este es-tudio, muestran que la especie Acacia mangium tiene un gran potencial para la recupera-ción de áreas degradadas, a partir del restablecimiento de los ciclos biogeoquímicos.

Fine litter production, nutrient return, nutrient resorption, and nutrient use efficiency were studied during one year in Acacia mangium forest plantations in mining gold degraded soils at the Bajo Cauca region of Colombia. Annual fine litter production was estimated at 10.4 Mg ha-1 and it was dominated by the leaf fraction (54%), followed by the reproduc-tive material (24%) and to a lesser proportion by other debris (6%) and other species leaves (1.5%). The highest organic matter and nutrients returns were found on sites classified as high quality. Soil plowing realized previous Acacia mangium planting, did not show any significant effect on organic matter and nutrients returns. A. mangium leaf litter had a high N concentration and consequently, given the high leaf litter production values, it was found a high N return. By the opposite, leaf litter P content and P returns via litter fall were very low. The high values found for P retranslocation and P use efficiency indexes showed that P was the most limiting nutrient for the species. The high values of fine litter production and nutrient return via leaf litter indicate that A. mangium has a great capacity for degraded areas reclamation, as of the restoration of the biogeochemical cycles.

Nitrogen losses from perennial grass species.

Nitrogen losses from plants may occur through a variety of pathways, but so far, most studies have only quantified losses of nutrients by above-ground litter production. We used 15N pulse labelling to quantify total nitrogen losses from above- and below-ground plant parts. Using this method we were able to include also pathways other than above-ground litter production. To test the hypothesis that species from nutrient-poor habitats lose less nitrogen than species from more fertile soils, six perennial grasses from habitats with a wide range of nutrient availability were investigated: Lolium perenne, Arrhenatherum elatius, Anthoxanthum odoratum, Festuca rubra, F. ovina and Molinia caerulea. The results of an experiment carried out in pots in a green-house at two fertility levels show that statistically significant losses occur through pathways other than above-ground litter production. In the low fertility treatment, most (70%) losses from L. perenne occurred by litter production, but in Ar. elatius, F. rubra, F. ovina and M. caerulea, more than 50% of labelled N losses took place by root turn-over, leaching or exudation from roots. When nutrient supply increased, the 15N losses in above-ground dead material increased in all species and in Ar. elatius, A. odoratum and F. rubra the 15N losses via other pathways decreased. Ranked according to decreasing turnover coefficient the sequence of species was: L. perenne, A. odoratum, F. rubra, F. ovina, Ar. elatius, M. caerulea. These results suggest that species adapted to sites with low availability of nutrients lose less nitrogen (including above- and below-ground losses) than species adapted to more fertile soils.

Nutrient use efficiency in evergreen and deciduous species from heathlands.

The nutrient (N, P) use efficiency (NUE: g g-1 nutrient), measured for the entire plant, of field populations of the evergreen shrubs Erica tetralix (in a wet heathland) and Calluna vulgaris (in a dry heathland) and the deciduous grass Molinia caerulea (both in a wet and a dry heathland) was compared. Erica and Calluna are crowded out by Molinia when nutrient availability increases. NUE was measured as the product of the mean residence time of a unit of nutrient in the population (MRT: yr) and nutrient productivity (A: annual productivity per unit of nutrient in the population, g g-1 nutrient yr-1. It was hypothesized that 1) in low-nutrient habitats selection is on features leading to a high MRT, whereas in high-nutrient habitats selection is on features leading to a high A; and that 2) due to evolutionary trade-offs plants cannot combine genotypically determined features which maximize both components of NUE.Both total productivity and litter production of the Molinia populations exceeded that of both evergreens about three-fold. Nitrogen and phosphorus resorption from senescing shoots was much lower in the evergreens compared with Molinia. In a split-root experiment no nutrient resorption from senescing roots was observed. Nutrient concentrations in the litter were equal for all species, except for litter P-concentration of Molinia at the wet site. Both Erica and Calluna had a long mean residence time of both nitrogen and phosphorus and a low nitrogen and phosphorus productivity. The Molinia populations showed a shorter mean residence time of N and P and a higher N- and P-productivity. These patterns resulted in an equal nitrogen use efficiency and an almost equal phosphorus use efficiency for the species under study. However, when only aboveground NUE was considered the Molinia populations had a much higher NUE than the evergreens.The results are consistent with the hypotheses. Thus, the low potential growth rate of species from low-nutrient habitats is probably the consequence of their nutrient conserving strategy rather than a feature on which direct selection takes place in these habitats.

A comparative study on nutrient cycling in wet heathland ecosystems : I. Litter production and nutrient losses from the plant.

The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g-1 biomass year-1). It is derived that in a plant community with two competing perennial plant populations, species1 will become dominant if the relative competition coefficient k 12 (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L 1/L 2), whereas species 2 will become dominant, if k 12 is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g-1 yr-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g-1 yr-1). The relative competition coefficient of Molinia with respect to Erica (k me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k me is below the critical ratio of the relative nutrient requirements of the two species (L m /L e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.