Regeneration pattern of primary forest species across forest-field gradients in the subtropical Mountains of Southwestern China

Evergreen broad-leaved forest is now gradually degraded and fragmented, and there is an increase in the amount of habitat edges as a result of long-term human activity. However, the role of edges in the regeneration of primary forest species is poorly understood. After 20 years of the edge creation, we analyzed primary forest species distribution and abundance, and changes in floristic composition, vegetation structure across forest-field gradients in Ailao Mountain, SW China. Our results revealed that there was a higher abundance and richness of primary species, late secondary species and thorny lianas at the distances 0–50 m than at the distances more than 50 m from the edge into the forest exterior. At the distances >50 m, no individuals of dominant canopy trees Lithocarpus xylocarpus, Castanopsis wattii, and L. jingdongensis were found, whereas the abundance of early pioneer shrub species and herbaceous cover was significantly greater. The richness of primary species showed a decrease with increasing distances from the forest edge to the exterior, particularly of medium-seeded primary species showing a drastic decrease. Moreover, no large-seeded primary species occurred at the distances >60 m. This study indicates that the forest edge as a buffer zone may be in favor of primary species regeneration. A dense shrub and herb layer, and seed dispersal may be the major factors limiting the forest regeneration farther from the forest edge. Therefore, to facilitate forest recovery processes, management should give priority to the protection of buffer zones of this forest edge.     

Decomposition of bamboo (Dendrocalamus hamiltonii Nees.) leaflitter in relation to age of jhum fallows in Northeast India

Summary Litter bags, containing bamboo (Dendrocalamus hamiltonii Nees.) leaf litter was used to compare the decomposition rates and microbial populations in litter under fallows of different ages. The age of the fallows were 0,5 and 10 years. All were developed after Jhum (shifting) cultivation. It was found that the fastest rate of decomposition occurred in the 10 year and the slowest in the 0 year old fallow. Changes in the litter components followed the following trend: sugar > amino acid > hemicellulose > cellulose > lignin. Great fluctuations from month to month were found in the number of microbes. They reached their maximum during the rainy season. However, there was little variation in the composition of fungal taxa. Species of Penicillium, Trichoderma, Aspergillus, Cladosporium, Alternaria, yeasts and a dark sterile mycelia were dominant.     

Response of clonal plasticity of Fargesia nitida to different canopy conditions of subalpine coniferous forest

The aim of this study is to explore the effects of canopy conditions on clump and culm numbers, and the morphological plasticity and biomass distribution patterns of the dwarf bamboo species Fargesia nitida. Specifically, we investigated the effects of canopy conditions on the growth and morphological characteristics of F. nitida, and the adaptive responses of F. nitida to different canopy conditions and its ecological senses. The results indicate that forest canopy had a significant effect on the genet density and culm number per clump, while it did not affect the ramet density. Clumps tended to be few and large in gaps and forest edge plots, and small under forest understory plots. The ramets showed an even distribution under the closed canopy, and cluster distribution under gaps and forest edge plots. The forest canopy had a significant effect on both the ramets’ biomass and biomass allocation. Favourable light conditions promoted ramet growth and biomass accumulation. Greater amounts of biomass in gaps and forest edge plots were shown by the higher number of culms per clump and the diameter of these culms. Under closed canopy, the bamboos increased their branching angle, leaf biomass allocation, specific leaf area and leaf area ratio to exploit more favourable light conditions in these locations. The spacer length, specific spacer length and spacer branching angles all showed significant differences between gaps and closed canopy conditions. The larger specific spacer length and spacer branching angle were beneficial for bamboo growth, scattering the ramets and exploiting more favourable light conditions. In summary, this study shows that to varying degrees, F. nitida exhibits both a wide ecological amplitude and high degree of morphological plasticity in response to differing forest canopy conditions. Moreover, the changes in plasticity enable the plants to optimize their light usage efficiency to promote growth and increase access to resources available in heterogeneous light environments. __________ Translated from Acta Ecologica Sinica, 2006, 26(12): 4019–4026 [译自: 生态学报]     

Lignin Degradation in Foliar Litter of Two Shrub Species from the Gap Center to the Closed Canopy in an Alpine Fir Forest

To understand the effects of forest gaps on lignin degradation during shrub foliar litter decomposition, a field litterbag experiment was conducted in an alpine fir (Abies faxoniana) forest of the eastern Tibet Plateau. Dwarf bamboo (Fargesia nitida) and willow (Salix paraplesia) foliar litterbags were placed on the forest floor from the gap center to the closed canopy. The litterbags were sampled during snow formation, snow coverage, snow melting and the growing season from October 2010 to October 2012. The lignin concentrations and loss in the litter were measured. Over 2 years, lignin loss was lower in the bamboo litter (34.64–43.89%) than in the willow litter (38.91–55.10%). In the bamboo litter, lignin loss mainly occurred during the first decomposition year, whereas it occurred during the second decomposition year in the willow litter. Both bamboo and willow litter lignin loss decreased from the gap center to the closed canopy during the first year and over the entire 2-year decomposition period. Compared with the closed canopy, the gap center showed higher lignin loss for both bamboo and willow litter during the two winters, but lower lignin loss during the early growing period. Additionally, the dynamics of microbial biomass carbon during litter decomposition followed the same trend as litter lignin loss during the two winters and growing period. These results indicated that alpine forest gaps had significant effects on shrub litter lignin loss and that reduced snow cover during winter warming would inhibit shrub lignin degradation in this alpine forest.     

Mixed leaf litter effects on decomposition rates and soil microarthropod communities in an oak–pine stand in Japan

Rates of decomposition, and soil faunal abundance and diversity associated with single-species and mixed-species litters were studied in a litter bag experiment in an oak–pine forest. We used two canopy species of leaf litter, pine and oak, and one shrub species, Sasa, and compared decomposition rates, and soil microarthropod abundance and community structure of oribatid mites in the litter bags. Mass loss of single species decreased in the order: oak > pine > Sasa. While the total mass loss rates of mixed litter were intermediate between those of the constituent species, enhancement of mass loss from the three-species mixture and from mixed slow-decomposing litters (pine and Sasa) was observed. Faunal abundance in litter bags was higher in mixed-species litter than in those with single-species litter, and species richness of oribatid mites was also higher in the three-species mixed litter. Faunal abundance in single-species litter bags was not correlated with mass loss, although enhancement of mass loss in mixed litter bags corresponded with higher microarthropod abundance. Habitat heterogeneity in mixed litter bags seemed to be responsible for the more abundant soil microarthropod community.     

乌蒙山峨眉栲、华木荷群落特征研究

乌蒙山位于中国西部半湿润常绿阔叶林与东部湿润常绿阔叶林交界区,是云南省唯一分布峨眉栲、华木荷群系(Formation Castanopsis platyacantha,Schima sinensis)的地区,该群系处于中国西部常绿阔叶林向东部常绿阔叶林过渡的地带,群落更加复杂多样。为了更深入地了解该群系的群落特征,同时找出其与东部湿润常绿阔叶林的联系,该文对该类型群落进行了数量分类研究,分析了聚类归并的各群落类型的乔灌优势种特征(径级结构、高度结构和重要值),同时采用DCA,CCA和DCCA排序分析进行群落特征分异的环境解释,并与四川峨眉山同类群落进行特征对比。结果表明:(1)乌蒙山峨眉栲、华木荷群落可划分为6个类型:峨眉栲群落,峨眉栲、华木荷群落,峨眉栲、十齿花群落,水青冈(Fagus longipetiolata)、华木荷、峨眉栲群落,华木荷、硬斗石栎(Lithocarpus hancei)群落,十齿花(Dipentodon sinicus)、华木荷群落。(2)乌蒙山峨眉栲、华木荷群落中,峨眉栲大树比例较高,华木荷小苗数量巨大,灌木层常以竹类为优势种。(3)海拔、坡度和太阳年辐射值是影响群落分布的主导因素。(4)峨眉山同类群落中优势种种群呈稳定型或增长型,而乌蒙山峨眉栲、华木荷群落中,峨眉栲和水青冈呈现下降型的特点。     

A fungal endophyte slows litter decomposition in streams

1. Phyllosphere interactions are known to influence a variety of tree canopy community members, but less frequently have they been shown to affect processes across ecosystem boundaries. Here, we show that a fungal endophyte (Rhytisma punctatum) slows leaf litter decomposition of a dominant riparian tree species (Acer macrophyllum) in an adjacent stream ecosystem. 2. Patches of leaf tissue infected by R. punctatum show significantly slower decomposition compared to both nearby uninfected tissue from the same leaf, and completely uninfected leaves. These reduced rates of decomposition existed despite 50% greater nitrogen in infected tissues and may be driven by slower rates of decomposition for fungal tissues themselves or by endophyte–hyphomycete interactions. 3. Across a temperate forest in the Pacific Northwest, approximately 72% of all A. macrophyllum leaves were infected by R. punctatum. Since R. punctatum infection can influence leaf tissue on entire trees and large quantities of leaf litter at the landscape scale, this infection could potentially result in a mosaic of ‘cold spots’ of litter decomposition and altered nutrient cycling in riparian zones where this infection is prevalent.     

Leaf litter and irrigation can increase seed germination and early seedling survival of the recalcitrant‐seeded tree Beilschmiedia miersii

Although regeneration of recalcitrant‐seeded tree species can be affected by prolonged drought, especially in Mediterranean regions, little is known about the response of such species to varying site conditions. A field experiment was performed to determine the effect of irrigation and leaf litter cover on seed germination and early seedling survival of the vulnerable recalcitrant‐seeded tree Beilschmiedia miersii (Lauraceae). Two levels of irrigation (non‐irrigated and irrigated units) and three levels of leaf litter depth (0, 5 and 12 cm) were applied to 72 groups of 30 seeds along a wet ravine of the Coastal Range of Central Chile, equally distributed across sites with different levels of canopy cover. Seed germination was significantly increased by irrigation only under closed‐canopy cover, and by leaf litter cover (>5 cm) under all canopy conditions. The effect of leaf litter on germination increased with canopy openness, while the effect of irrigation did not show any tendency. Meanwhile, early seedling survival was significantly increased by irrigation under intermediate canopy cover, and by leaf litter (>5 cm) under closed‐canopy cover. As a result of its overall positive effect on germination, leaf litter should be maintained within B. miersii communities, particularly under intermediate to closed‐canopy conditions, where it can also increase early seedling survival, and both seed germination and early seedling survival might be increased through additional water inputs. The presence of leaf litter might help retaining such inputs, prolonging their effect on regeneration of B. miersii communities. We see this as a baseline assessment of regeneration and persistence that needs further testing on species with similar traits, given the expected increase in the frequency and length of dry periods into the future.     

Species shift drives decomposition rates following invasion by hemlock woolly adelgid

Insect and disease outbreak is an important cause of selective species removal and accompanying functional change in North American forests. Outbreak of hemlock woolly adelgid, Adelgies tsugae– HWA, is causing selective removal of eastern hemlock Tsuga canadensis at a regional scale. Impacts of outbreak‐caused canopy mortality and shifts in dominant species on litter decay were compared across sites that range in HWA‐caused canopy damage and subsequent canopy dominance by black birch Betula lenta. Senescent litter from eastern hemlock, black birch, and equal litter mixes were decomposed in the field for 36 months within nine sites in Connecticut and Massachusetts USA. Mass loss and % N accumulation of black birch was 65% and 52% greater compared to eastern hemlock. In contrast, outbreak related canopy damage increased litter mass loss by 11.5% in high mortality stands relative to uninfested stands but canopy damage had no impact on % N dynamics. Non‐additive effects of litter mixing influenced chemical dynamics of decaying litter; black birch accumulated less N and eastern hemlock accumulated more N compared to each species decaying alone. However, these changes offset and mixed litter bags overall showed no differences in N dynamics compared to values from each species decaying alone. In eastern hemlock stands invaded by hemlock woolly adelgid, canopy damage influences the rates and dynamics of decay but species differences between hemlock and black birch leaf litter are the dominant mechanisms of decomposition changes and a long‐lasting driver of increased N cycling rates. Species shifts may be the dominant driver of altered ecosystem processes for other insect outbreaks, particularly when replacement species have very different characteristics regulating decomposition and N cycling.     

Altitude and species identity drive leaf litter decomposition rates of ten species on a 2950 m altitudinal gradient in Neotropical rain forests

Identifying the environmental factors controlling litter decomposition is key to understanding the magnitude and rates of nutrient cycling in tropical forests, and how they may be influenced by climate variability and environmental change. We carried out a leaf litter translocation experiment in mature rain forest over a 2,520 m altitudinal gradient in Costa Rica. Leaf litter decomposition rates (k) of ten tree species, two dominant species from each ecosystem, plus two standard species, were calculated over 540 days in four life zones. k was lowest in montane with 0.83 per year and lower montane forests with 2.21 per year. k did not differ between lowland and premontane forests at 3.12 per year, in spite of the 3℃ difference of mean annual temperature between these life zones. k varied fourfold among species. Species decomposition rates ranked as follows, and were predictably related to leaf economic spectrum traits of the species: Acalypha communis (standard, fast decomposer)» Hyeronima oblonga > Alchornea latifolia, Quercus bumelioides, Jarava ichu (standard, slow decomposer)> Minquartia guianensis > Magnolia sororum > Vochysia allenii > Pourouma bicolor, Carapa guianensis. These two slowest-decomposing species were native premontane and lowland forest dominants, respectively, with tough, low-nutrient leaves. The ranking of species by k varied very little among life zones suggesting that decomposer organisms in very different ecosystems and environments react in similar ways to the litter quality in general. We conclude that while k decreases with temperature in rain forests on tropical mountains, bioclimatic zones defined as premontane may be “functionally lowland.” The effects of species identity on decomposition rates on tropical mountains are consistent and independent of environment for both standard and native species. Under climate change on these mountains, if moisture regimes do not change, decomposition rates will increase due to rising temperatures. Soil carbon storage may therefore decrease. Changes in the altitudinal distributions of currently dominant species will also affect this critically important biogeochemical process.     

Factors influencing the distribution and growth of dwarf bamboo, <Emphasis Type="Italic">Fargesia nitida</Emphasis>, in a subalpine forest in Wolong Nature Reserve,southwest China

Dwarf bamboos impose intense resource competition in subalpine coniferous forests, and their exclusive densities have crucial impacts on tree regeneration and understory species diversity. We studied the factors influencing the distribution and growth of dwarf bamboo, Fargesia nitida, in a subalpine forest in southwest China. TWINSPAN, based on an attribute matrix, could divide the subalpine forest into 11 sub-associations, and more clearly reflected ecological functional features of the subalpine forest than analysis based on a species matrix. TWINSPAN was also generally consistent with DCA ordination based on the attribute matrix. DCA and DCCA ordination showed relationships between the distribution of F. nitida population and environment factor. The first DCCA axis showed topography and disturbance gradients (except fallen trees and broken branches); the second DCCA axis showed canopy density and composition gradients. Stepwise multiple regression analyses showed that distribution (culm density and coverage) of F. nitida decreased significantly with landslide and slope aspect, and increased significantly with soil status. The light condition had positive effects on growth and size of bamboo. A stable environment in the northern slope and more broadleaved species dominating in canopy would increase the dwarf bamboo biomass. Thus, the disturbance regimes, the slope aspect and the BA of evergreen conifer trees can provide useful guidelines for the control and management of F. nitida populations, and in helping to understand the succession and regeneration of subalpine forest in this region.     

金佛山方竹经营对扁刺栲群落物种多样性和优势种种群结构的影响

为探讨金佛山方竹经营对扁刺栲群落物种多样性和优势种种群结构的影响,该研究以金佛山的扁刺栲自然群落和金佛山方竹经营群落为对象,分析了两类群落的物种组成特征、物种多样性及优势种种群结构。结果表明:(1)两种群落内共记录维管束植物84种,隶属于40科63属,以樟科、蔷薇科、壳斗科、山茶科物种为主; 扁刺栲为群落优势种,中华木荷和灰柯为次优势种,金佛山方竹为灌木层优势种,扁刺栲群落组成与亚热带其他地区的扁刺栲群落组成相似。(2)金佛山方竹经营群落的灌木层物种丰富度显著低于自然群落,草本层Shannon-Wiener指数和Pielou指数均显著低于自然群落。(3)扁刺栲和灰柯在自然群落中为稳定型种群,而在金佛山方竹经营群落中为衰退型种群,中华木荷在两类群落中均为衰退型种群。(4)自然群落中扁刺栲、中华木荷、灰柯种群的萌枝率、有萌个体率和幼苗相对优势度均高于金佛山方竹经营群落。综上认为,金佛山方竹经营对金佛山扁刺栲群落灌木层和草本层物种多样性产生了负面影响,并显著影响扁刺栲群落优势种种群更新和维持。     

Strong leaf morphological, anatomical, and physiological responses of a subtropical woody bamboo (Sinarundinaria nitida) to contrasting light environments

Dwarf bamboos are an important understory component of the lowland and montane forests in the subtropical regions of Asia and South America, yet little is known about their physiology and phenotypic plasticity in response to changing light environments. To understand how bamboo species adapt to different light intensities, we examined leaf morphological, anatomical, and physiological differentiation of Sinarundinaria nitida (Mitford) Nakai, a subtropical woody dwarf bamboo, growing in open and shaded natural habitats in the Ailao Mountains, SW China. Compared with leaves in open areas, leaves in shaded areas had higher values in leaf size, specific leaf area, leaf nitrogen, and chlorophyll concentrations per unit area but lower values in leaf thickness, vein density, stomatal density, leaf carbon concentration, and total soluble sugar concentration. However, stomatal size and leaf phosphorus concentration per unit mass remained relatively constant regardless of light regime. Leaves in the open habitat exhibited a higher light-saturated net photosynthetic rate, dark respiration rate, non-photochemical quenching, and electron transport rate than those in the shaded habitat. The results of this study revealed that the bamboo species exhibited a high plasticity of its leaf structural and functional traits in response to different irradiances. The combination of high plasticity in leaf morphological, anatomical, and physiological traits allows this bamboo species to grow in heterogeneous habitats.     

三个亚热带森林优势种凋落物非结构性碳水化合物含量的季节动态

非结构性碳水化合物(NSC)是凋落物中的易分解组分,在凋落物分解早期快速释放进入土壤并被微生物利用,参与森林土壤生物地球化学循环,因此新鲜凋落物中NSC变化规律是认识森林土壤碳和养分循环的关键之一。选取亚热带常绿阔叶林优势树种米槠(Castanopsis carlesii)和主要造林树种杉木(Cunninghamia lanceolata)、马尾松(Pinus massoniana)为研究对象,分析其新鲜凋落叶和凋落枝中NSC(可溶性糖和淀粉)含量的动态变化规律。结果表明：凋落物中NSC含量在不同月份表现出明显的时间动态,米槠、杉木和马尾松凋落叶和凋落枝中NSC含量总体上在11—12月呈上升趋势,而在2—6月呈缓慢下降趋势。不同类型的凋落物NSC含量存在显著差异,米槠、杉木和马尾松凋落叶中NSC含量分别为3.03%—3.56%、2.18%—4.37%、3.38%—4.89%,凋落枝中NSC含量分别为1.87%—4.22%、2.88%—4.28%、2.75%—5.27%,米槠和马尾松凋落叶中NSC含量高于凋落枝,而杉木凋落枝中NSC含量高于凋落叶。不同树种凋落物NSC含量差异显著,米槠和...     

Riparian leaf litter decomposition on pond bottom after a retention on floating vegetation

Allochthonous (e.g., riparian) plant litter is among the organic matter resources that are important for wetland ecosystems. A compact canopy of free‐floating vegetation on the water surface may allow for riparian litter to remain on it for a period of time before sinking to the bottom. Thus, we hypothesized that canopy of free‐floating vegetation may slow decomposition processes in wetlands. To test the hypothesis that the retention of riparian leaf litter on the free‐floating vegetation in wetlands affects their subsequent decomposition on the bottom of wetlands, a 50‐day in situ decomposition experiment was performed in a wetland pond in subtropical China, in which litter bags of single species with fine (0.5 mm) or coarse (2.0 mm) mesh sizes were placed on free‐floating vegetation (dominated by Eichhornia crassipes, Lemna minor, and Salvinia molesta) for 25 days and then moved to the pond bottom for another 25 days or remained on the pond bottom for 50 days. The leaf litter was collected from three riparian species, that is, Cinnamomum camphora, Diospyros kaki, and Phyllostachys propinqua. The retention of riparian leaf litter on free‐floating vegetation had significant negative effect on the carbon loss, marginal negative effects on the mass loss, and no effect on the nitrogen loss from leaf litter, partially supporting the hypothesis. Similarly, the mass and carbon losses from leaf litter decomposing on the pond bottom for the first 25 days of the experiment were greater than those from the litter decomposing on free‐floating vegetation. Our results highlight that in wetlands, free‐floating vegetation could play a vital role in litter decomposition, which is linked to the regulation of nutrient cycling in ecosystems.     

Non‐symbiotic nitrogen fixation during leaf litter decomposition in an old‐growth temperate rain forest of Chiloé Island,southern Chile: Effects of single versus mixed species litter

Heterotrophic nitrogen fixation is a key ecosystem process in unpolluted, temperate old‐growth forests of southern South America as a source of new nitrogen to ecosystems. Decomposing leaf litter is an energy‐rich substrate that favours the occurrence of this energy demanding process. Following the niche ‘complementarity hypothesis’, we expected that decomposing leaf litter of a single tree species would support lower rates of non‐symbiotic N fixation than mixed species litter taken from the forest floor. To test this hypothesis we measured acetylene reduction activity in the decomposing monospecific litter of three evergreen tree species (litter C/N ratios, 50–79) in an old‐growth rain forest of Chiloé Island, southern Chile. Results showed a significant effect of species and month (anova , Tukey's test, P < 0.05) on decomposition and acetylene reduction rates (ARR), and a species effect on C/N ratios and initial % N of decomposing leaf litter. The lowest litter quality was that of Nothofagus nitida (C/N ratio = 78.7, lignin % = 59.27 ± 4.09), which resulted in higher rates of acetylene reduction activity (mean = 34.09 ± SE = 10.34 nmol h^?1 g^?1) and a higher decomposition rate (k = 0.47) than Podocarpus nubigena (C/N = 54.4, lignin % = 40.31 ± 6.86, Mean ARR = 4.11 ± 0.71 nmol h^?1 g^?1, k = 0.29), and Drimys winteri (C/N = 50.6, lignin % = 45.49 ± 6.28, ARR = 10.2 ± 4.01 nmol h^?1 g^?1, k = 0.29), and mixed species litter (C/N = 60.7, ARR = 8.89 ± 2.13 nmol h^?1g^?1). We interpret these results as follows: in N‐poor litter and high lignin content of leaves (e.g. N. nitida) free‐living N fixers would be at competitive advantage over non‐fixers, thereby becoming more active. Lower ARR in mixed litter can be a consequence of a lower litter C/N ratio compared with single species litter. We also found a strong coupling between in situ acetylene reduction and net N mineralization in surface soils, suggesting that as soon N is fixed by diazotroph bacteria it may be immediately incorporated into mineral soil by N mineralizers, thus reducing N immobilization.     

东北地区森林凋落叶分解速率与气候、林型、林分光照的关系

在东北长白山、张广才岭、小兴安岭、大兴安岭的主要森林类型中设置26块样地,进行为期3a(2004—2006年)凋落叶分解实验,以研究气候、林型、林冠透光率对凋落叶分解速率的相对影响大小。结果表明,不同林型凋落叶分解速率依次为:落叶阔叶林针阔叶混交林落叶针叶林常绿针叶林岳桦林。对分解速率影响因素的分析表明,气候因子(热量和水分)对分解速率有较强的解释力,分别解释了分解常数k和分解95%所需时间(t95%)的55.5%和65.0%的变异。但是,气候对分解速率的影响在很大程度上是通过与林型、林冠透光率的协同作用而实现的,其独立解释力并不大(9%)。气候的变化导致林型(物种组成)的变化、进而影响分解速率,这一因素解释了分解参数变异的46.8%(k)和56.8%(t95%)。与此同时,气候和林型的变化还导致林冠透光率的变化,随着热量水平的上升林冠透光率下降、间接提高分解速率。这一因素分别解释了k值和t95%变异的23.9%和22.3%。研究结果表明,气候对凋落叶分解的影响主要是通过对物种组成、林冠结构(影响透光率)等生物因素的间接作用实现的。忽视这些生物因素、简单研究气候和分解速率的关系可能难以正确预测未来气候变化对凋落物分解的影响。     

Litter decomposition rate is dependent on litter Mn concentrations

A statistically significant linear relationship was found between annual mass loss of foliar litter in the late stages of decomposition and Mn concentration in the litter. We used existing decomposition data on needle and leaf decomposition of Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta var. contorta), Norway spruce (Picea abies (L.) Karst.), silver birch (Betula pendula L.), and grey alder (Alnus incana L.) from Sweden and Aleppo pine (Pinus halepensis Mill.) from Libya, to represent boreal, temperate, and Mediterranean climates. The later the decomposition stage as indicated by higher sulfuric-acid lignin concentrations, the better were the linear relationships between litter mass loss and Mn concentrations. We conclude that Mn concentrations in litter have an influence on litter mass-loss rates in very late decomposition stages (up to 5 years), provided that the litter has high enough Mn concentration. The relationship may be dependent on species as the relationship is stronger with species that take up high enough amounts of Mn.     

Cottonwood hybridization affects tannin and nitrogen content of leaf litter and alters decomposition

Cottonwoods are dominant riparian trees of the western United States and are known for their propensity to hybridize. We compared the decomposition of leaf litter from two species (Populus angustifolia and P. fremontii) and their hybrids. Three patterns were found. First, in one terrestrial and two aquatic experiments, decomposition varied twofold among tree types. Second, backcross hybrid leaves decomposed more slowly than those of either parent. Third, the variation in decomposition between F₁ and backcross hybrids was as great as the variation between species. These results show significant differences in decomposition in a low-diversity system, where >80% of the leaf litter comes from just two species and their hybrids. Mechanistically, high concentrations of condensed tannins in leaves appear to inhibit decomposition (r ²=0.63). The initial condensed tannin concentration was high in narrowleaf leaves, low or undetectable in Fremont leaves, and intermediate in F₁ hybrid leaves (additive inheritance). Backcross hybrids were high in condensed tannins and were not different from narrowleaf (dominant inheritance). Neither nitrogen (N) concentration nor the ratio of ash-free dry weight to N (a surrogate for carbon:nitrogen ratio) were significantly correlated with decomposition. The N content of leaf material at the end of each year’s experiment was inversely correlated with rates of litter mass loss and varied 1.6- to 2.1-fold among tree classes. This result suggests that hybrids and their parental species are used differently by the microbial community. Received: 7 April 1999 / Accepted: 2 November 1999