Riparian forest composition affects stream litter decomposition despite similar microbial and invertebrate communities

Cross-boundary flows of energy and nutrients link biodiversity and functioning in adjacent ecosystems. The composition of forest tree species can affect the structure and functioning of stream ecosystems due to physical and chemical attributes, as well as changes in terrestrial resource subsidies. We examined how variation in riparian canopy composition (coniferous, deciduous, mixed) affects adjacent trophic levels (invertebrate and microbial consumers) and decomposition of organic matter in small, coastal rainforest streams in southwestern British Columbia. Breakdown rates of higher-quality red alder (Alnus rubra) litter were faster in streams with a greater percentage of deciduous than coniferous riparian canopy, whereas breakdown rates of lower-quality western hemlock (Tsuga heterophylla) litter were independent of riparian forest composition. When invertebrates were excluded using fine mesh, breakdown rates of both litter species were an order of magnitude less and were not significantly affected by riparian forest composition. Stream invertebrate and microbial communities were similar among riparian forest composition, with most variation attributed to leaf litter species. Invertebrate taxa richness and shredder biomass were higher in A. rubra litter; however, taxa evenness was greatest for T. heterophylla litter and both litter species in coniferous streams. Microbial community diversity (determined from terminal restriction fragment length polymorphisms) was unaffected by riparian forest or litter species. Fungal allele richness was higher than bacterial allele richness, and microbial communities associated with lower-quality T. heterophylla litter had higher diversity (allele uniqueness and richness) than those associated with higher-quality A. rubra litter. Percent variation in breakdown rates was mostly attributed to riparian forest composition in the presence of invertebrates and microbes; however, stream consumer biodiversity at adjacent trophic levels did not explain these patterns. Riparian and stream ecosystems and their biotic communities are linked through exchange and decomposition of detrital resources, and we provide evidence that riparian forest composition affects stream ecosystem catabolism despite similarities in microbial and invertebrate communities.     

Nonadditive effects of leaf litter species diversity on breakdown dynamics in a detritus-based stream

Since species loss is predicted to be nonrandom, it is important to understand the manner in which those species that we anticipate losing interact with other species to affect ecosystem function. We tested whether litter species diversity, measured as richness and composition, affects breakdown dynamics in a detritus-based stream. Using full-factorial analyses of single- and mixed-species leaf packs (15 possible combinations of four dominant litter species; red maple [Acer rubrum], tulip poplar [Liriodendron tulipifera], chestnut oak [Quercus prinus], and rhododendron [Rhododendron maximum]), we tested for single-species presence/absence (additive) or species interaction (nonadditive) effects on leaf pack breakdown rates, changes in litter chemistry, and microbial and macroinvertebrate biomass. Overall, we found significant nonadditive effects of litter species diversity on leaf pack breakdown rates, which were explained both by richness and composition. Leaf packs containing higher litter species richness had faster breakdown rates, and antagonistic effects of litter species composition were observed when any two or three of the four litter species were mixed. Less-consistent results were obtained with respect to changes in litter chemistry and microbial and macroinvertebrate biomass. Our results suggest that loss of litter species diversity will decrease species interactions involved in regulating ecosystem function. To that end, loss of species such as eastern hemlock (Tsuga canadensis) accompanied by predicted changes in riparian tree species composition in the southeastern United States could have nonadditive effects on litter breakdown at the landscape scale.     

杉木林凋落物产量、分解率与储量的关系

在福建三明采用2种方法测定27a生杉木林的杉木叶和小枝凋落物的分解率．采用网袋法(孔径0．5mm尼龙网袋)测定的杉木叶和小枝凋落物残留率Olson指数模型的分解系数(k值)分别为0．7692a^-1和0．2501a^-1，相应的第1年的分解率分别为53．66％和22．13％；另一种测定矗值的方法较为准确，是通过计算凋落物年产量与地面凋落物储量的比值得到，采用该方法计算的杉木叶和小枝凋落物的矗值分别为1．788a^-1和0．8622a^-1，分别是网袋法测定值的2．32和3．54倍，相应的第1年的分解率分别为83．27％和57．78％，分别是网袋法测定值的1．55倍和2．61倍．采用后一种方法测定的凋落物分解率可以解决尼龙网袋法测定分解速率结果偏低的问题．     

Patterns of litter disappearance in a northern hardwood forest invaded by exotic earthworms.

A field study was conducted to evaluate the effects of exotic earthworm invasions on the rates of leaf litter disappearance in a northern hardwood forest in southcentral New York, USA. Specifically, we assessed whether differences in litter quality and the species composition of exotic earthworm communities affected leaf litter disappearance rates. Two forest sites with contrasting communities of exotic earthworms were selected, and disappearance rates of sugar maple and red oak litter were estimated in litter boxes in adjacent earthworm-free, transition, and earthworm-invaded plots within each site. After 540 days in the field, 1.7-3 times more litter remained in the reference plots than in the earthworm-invaded plots. In the earthworm-invaded plots, rates of disappearance of sugar maple litter were higher than for oak litter during the first year, but by the end of the experiment, the amount of sugar maple and oak litter remaining in the earthworm-invaded plots was identical within each site. The composition of the earthworm communities significantly affected the patterns of litter disappearance. In the site dominated by the anecic earthworm Lumbricus terrestris and the endogeic Aporrectodea tuberculata, the percentage of litter remaining after 540 days (approximately 17%) was significantly less than at the site dominated by L. rubellus and Octolasion tyrtaeum (approximately 27%). This difference may be attributed to the differences in feeding behavior of the two litter-feeding species: L. terrestris buries entire leaves in vertical burrows, whereas L. rubellus usually feeds on litter at the soil surface, leaving behind leaf petioles and veins. Our results showed that earthworms not only accelerate litter disappearance rates, but also may reduce the differences in decomposition rates that result from different litter qualities at later stages of decay. Similarly, our results indicate that earthworm effects on decomposition vary with earthworm community composition. Furthermore, because earthworm invasion can involve a predictable shift in community structure along invasion fronts or through time, the community dynamics of invasion are important in predicting the spatial and temporal effects of earthworm invasion on litter decomposition, especially at later stages of decay.     

Decomposition of diverse litter mixtures in streams

In view of growing interest in understanding how biodiversity affects ecosystem functioning, we investigated effects of riparian plant diversity on litter decomposition in forest streams. Leaf litter from 10 deciduous tree species was collected during natural leaf fall at two locations (Massif Central in France and Carpathians in Romania) and exposed in the field in litter bags. There were 35 species combinations, with species richness ranging 1-10. Nonadditive effects on the decomposition of mixed-species litter were minor, although a small synergistic effect was observed in the Massif Central stream where observed litter mass remaining was significantly lower overall than expected from data on single-species litter. In addition, variability in litter mass remaining decreased with litter diversity at both locations. Mean nitrogen concentration of single- and mixed-species litters (0.68-4.47% of litter ash-free dry mass) accounted for a large part of the variation in litter mass loss across species combinations. For a given species or mixture, litter mass loss was also consistently faster in the Massif Central than in the Carpathians, and the similarity in general stream characteristics, other than temperature, suggests that this effect was largely due to differences in thermal regimes. These results support the notion that decomposition of litter mixtures is primarily driven by litter quality and environmental factors, rather than by species richness per se. However, the observed consistent decrease in variability of decomposition rate with increasing plant species richness indicates that conservation of riparian tree diversity is important even when decomposition rates are not greatly influenced by litter mixing.     

Hierarchical regulation of nitrogen export from urban catchments: interactions of storms and landscapes.

In urban catchments of arid central Arizona, we investigate how the export of mineral and organic nitrogen (N) in storm runoff is regulated by interactions between local landscape characteristics and broader scale storm features. First, we test whether N export is more a function of (1) processes that affect N concentration in runoff or (2) the propensity of the catchment to convey rainfall as runoff. With data pooled across catchments, the mass of N in export (load) is determined by processes regulating runoff N concentration. There are exceptions when catchments are examined individually, where N load from some catchments is determined by the hydrologic responsiveness of the catchment. Second, we investigate the relationship between N export and catchment features. Loads per catchment area were greater from more impervious catchments, probably because impervious catchments held more N in a mobilizable phase and conveyed more rainfall as overland flow. Loads per area were lower from larger catchments, possibly owing to more N-retention hot spots in larger catchments. Catchments with the greatest N exports were those with commercial land use, and loads decreased as development became less prevalent or as residential replaced industrial land use. Third, we investigated how catchment features moderated direct responses of N export to storms. Export was less correlated with storm features in catchments that were larger, more pervious, and less industrial. Results support an "N build and flush" hypothesis, which purports that there is little biotic processing of N deposited to arid, urban surfaces with little organic matter. The rate and duration of deposition determine the size of the mobile N pool. Any amount of rainfall capable of generating overland flow would entrain nearly all mobilizable N and export it from the catchment. Nonetheless, these results suggest that, even with daunting seasonal and interannual variability in storm conditions, material export can be reduced by managing intrinsic catchment features.     

Incubation time, functional litter diversity, and habitat characteristics predict litter-mixing effects on decomposition

Plant diversity influences many fundamental ecosystem functions, including carbon and nutrient dynamics, during litter breakdown. Mixing different litter species causes litter mixtures to lose mass at different rates than expected from component species incubated in isolation. Such nonadditive litter-mixing effects on breakdown processes often occur idiosyncratically because their direction and magnitude change with incubation time, litter species composition, and ecosystem characteristics. Taking advantage of results from 18 litter mixture experiments in streams, we examined whether the direction and magnitude of nonadditive mixing effects are randomly determined. Across 171 tested litter mixtures and 510 incubation time-by-mixture combinations, nonadditive effects on breakdown were common and on average resulted in slightly faster decomposition than expected. In addition, we found that the magnitude of nonadditive effects and the relative balance of positive and negative responses in mixtures change predictably over time, and both were related to an index of functional litter diversity and selected environmental characteristics. Based on these, it should be expected that nonadditive effects are stronger for litter mixtures made of functionally dissimilar species especially in smaller streams. Our findings demonstrate that effects of litter diversity on plant mixture breakdown are more predictable than generally thought. We further argue that the consequences of current worldwide homogenization in the composition of plant traits on carbon and nutrient dynamics could be better inferred from long-duration experiments that manipulate both functional litter diversity and ecosystem characteristics in "hotspots of biodiversity effects," such as small streams.     

Increasing litter species richness reduces variability in a terrestrial decomposer system

Debate on the relationship between diversity and stability has been driven by the recognition that species loss may influence ecosystem properties and processes. We conducted a litterbag experiment in the Scottish Highlands, United Kingdom, to examine the effects of altering plant litter diversity on decomposition, microbial biomass, and microfaunal abundance. The design of treatments was fully factorial and included five species from an upland plant community (silver birch, Betula pendula; Scots' pine, Pinus sylvestris; heather, Calluna vulgaris; bilberry, Vaccinium myrtillus; wavy-hair grass, Deschampsia flexuosa); species richness ranged from one to five species. We tested the effects of litter species richness and composition on variable means, whether increasing litter species richness reduced variability in the decomposer system, and whether any richness-variability relationships were maintained over time (196 vs. 564 days). While litter species composition effects controlled variable means, we revealed reductions in variability with increasing litter species richness, even after accounting for differences between litter types. These findings suggest that higher plant species richness per se may result in more stable ecosystem processes (e.g., decomposition) and decomposer communities. Negative richness-variation relationships generally relaxed over time, presumably because properties of litter mixtures became more homogeneous. However, given that plant litter inputs continue to enter the belowground system over time, we conclude that variation in ecosystem properties may be buffered by greater litter species richness.     

Tree species effects on decomposition and forest floor dynamics in a common garden

We studied the effects of tree species on leaf litter decomposition and forest floor dynamics in a common garden experiment of 14 tree species (Abies alba, Acer platanoides, Acer pseudoplatanus, Betula pendula, Carpinus betulus, Fagus sylvatica, Larix decidua, Picea abies, Pinus nigra, Pinus sylvestris, Pseudotsuga menziesii, Quercus robur, Quercus rubra, and Tilia cordata) in southwestern Poland. We used three simultaneous litter bag experiments to tease apart species effects on decomposition via leaf litter chemistry vs. effects on the decomposition environment. Decomposition rates of litter in its plot of origin were negatively correlated with litter lignin and positively correlated with mean annual soil temperature (MAT(soil)) across species. Likewise, decomposition of a common litter type across all plots was positively associated with MAT(soil), and decomposition of litter from all plots in a common plot was negatively related to litter lignin but positively related to litter Ca. Taken together, these results indicate that tree species influenced microbial decomposition primarily via differences in litter lignin (and secondarily, via differences in litter Ca), with high-lignin (and low-Ca) species decomposing most slowly, and by affecting MAT(soil), with warmer plots exhibiting more rapid decomposition. In addition to litter bag experiments, we examined forest floor dynamics in each plot by mass balance, since earthworms were a known component of these forest stands and their access to litter in litter bags was limited. Forest floor removal rates estimated from mass balance were positively related to leaf litter Ca (and unrelated to decay rates obtained using litter bags). Litter Ca, in turn, was positively related to the abundance of earthworms, particularly Lumbricus terrestris. Thus, while species influence microbially mediated decomposition primarily through differences in litter lignin, differences among species in litter Ca are most important in determining species effects on forest floor leaf litter dynamics among these 14 tree species, apparently because of the influence of litter Ca on earthworm activity. The overall influence of these tree species on leaf litter decomposition via effects on both microbial and faunal processing will only become clear when we can quantify the decay dynamics of litter that is translocated belowground by earthworms.     

Effects of urbanization and urban stream restoration on the physical and biological structure of stream ecosystems

Streams, as low-lying points in the landscape, are strongly influenced by the stormwaters, pollutants, and warming that characterize catchment urbanization. River restoration projects are an increasingly popular method for mitigating urban insults. Despite the growing frequency and high expense of urban stream restoration projects, very few projects have been evaluated to determine whether they can successfully enhance habitat structure or support the stream biota characteristic of reference sites. We compared the physical and biological structure of four urban degraded, four urban restored, and four forested streams in the Piedmont region of North Carolina to quantify the ability of reach-scale stream restoration to restore physical and biological structure to urban streams and to examine the assumption that providing habitat is sufficient for biological recovery. To be successful at mitigating urban impacts, the habitat structure and biological communities found in restored streams should be more similar to forested reference sites than to their urban degraded counterparts. For every measured reach- and patch-scale attribute, we found that restored streams were indistinguishable from their degraded urban stream counterparts. Forested streams were shallower, had greater habitat complexity and median sediment size, and contained less-tolerant communities with higher sensitive taxa richness than streams in either urban category. Because heavy machinery is used to regrade and reconfigure restored channels, restored streams had less canopy cover than either forested or urban streams. Channel habitat complexity and watershed impervious surface cover (ISC) were the best predictors of sensitive taxa richness and biotic index at the reach and catchment scale, respectively. Macroinvertebrate communities in restored channels were compositionally similar to the communities in urban degraded channels, and both were dissimilar to communities in forested streams. The macroinvertebrate communities of both restored and urban degraded streams were correlated with environmental variables characteristic of degraded urban systems. Our study suggests that reach-scale restoration is not successfully mitigating for the factors causing physical and biological degradation.     

Whole-system nutrient enrichment increases secondary production in a detritus-based ecosystem

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.     

Predicting richness effects on ecosystem function in natural communities: insights from high-elevation streams

. Despite the increased complexity of experimental and theoretical studies on the biodiversity-ecosystem functioning (B-EF) relationship, a major challenge is to demonstrate whether the observed importance of biodiversity in controlled experimental systems also persists in nature. Due to their structural simplicity and their low levels of human impacts, extreme species-poor ecosystems may provide new insights into B-EF relationships in natural systems. We address this issue using shredder invertebrate communities and organic matter decomposition rates in 24 high-altitude (3200-3900 m) Neotropical streams as a study model. We first assessed the effects of stream characteristics and shredder diversity and abundance on organic matter decomposition rates in coarse- and fine-mesh bags. We found the interaction term shredder richness x shredder abundance had the most significant impact on decomposition rates in the field, although water discharge may also play a role locally. We also examined the relative contribution of the three most abundant shredders on decomposition rates by manipulating shredder richness and community composition in a field experiment. Transgressive overyielding was detected among the three shredder species, indicating complementary resource use and/or facilitation. By integrating survey and experimental data in surface response analyses we found that observed B-EF patterns fit those predicted by a linear model that described litter decomposition rates as a function of increasing shredder richness and the relative abundance of the most efficient shredders. Finally, the validity of our approach was tested in a broader context by using two independent but comparable data sets from 49 French and Swedish streams showing more complex shredder community structure. Results revealed that richness and identity effects on decomposition rates were lost with increasing shredder community complexity. Our approach of combining experimental and empirical data with modeling in species-poor ecosystems may serve as an impetus for new B-EF studies. If theory can explain B-EF in low-diversity ecosystems, it may also have credibility in more complex ones.     

Mitigating the impact of oil‐palm monoculture on freshwater fishes in Southeast Asia

Anthropogenic land‐cover change is driving biodiversity loss worldwide. At the epicenter of this crisis lies Southeast Asia, where biodiversity‐rich forests are being converted to oil‐palm monocultures. As demand for palm oil increases, there is an urgent need to find strategies that maintain biodiversity in plantations. Previous studies found that retaining forest patches within plantations benefited some terrestrial taxa but not others. However, no study has focused on aquatic taxa such as fishes, despite their importance to human well‐being. We assessed the efficacy of forested riparian reserves in conserving freshwater fish biodiversity in oil‐palm monoculture by sampling stream fish communities in an oil‐palm plantation in Central Kalimantan, Indonesia. Forested riparian reserves maintained preconversion local fish species richness and functional diversity. In contrast, local and total species richness, biomass, and functional diversity declined markedly in streams without riparian reserves. Mechanistically, riparian reserves appeared to increase local species richness by increasing leaf litter cover and maintaining coarse substrate. The loss of fishes specializing in leaf litter and coarse substrate decreased functional diversity and altered community composition in oil‐palm plantation streams that lacked riparian reserves. Thus, a land‐sharing strategy that incorporates the retention of forested riparian reserves may maintain the ecological integrity of fish communities in oil‐palm plantations. We urge policy makers and growers to make retention of riparian reserves in oil‐palm plantations standard practice, and we encourage palm‐oil purchasers to source only palm oil from plantations that employ this practice.     

Grazer traits, competition, and carbon sources to a headwater-stream food web

We investigated the effect of grazing by a dominant invertebrate grazer (the caddisfly Glossosoma penitum) on the energy sources used by other consumers in a headwater-stream food web. Stable isotope studies in small, forested streams in northern California have shown that G. penitum larvae derive most of their carbon from algae, despite low algal standing crops. We hypothesized that the caddisfly competes with other primary consumers (including mayflies) for algal food and increases their reliance on terrestrial detritus. Because Glossosoma are abundant and defended from predators by stone cases, their consumption of algal energy may reduce its transfer up the food chain. We removed Glossosoma (natural densities >1000 caddisflies/m2) from five approximately 4 m2) stream sections during the summer of 2000 and measured responses of algae, invertebrate primary consumers, and invertebrate predators. The treatment reduced Glossosoma biomass by 80-90%. We observed a doubling in chlorophyll a per area in sections with reduced Glossosoma abundance and aggregative increases in the biomass of undefended primary consumers. Heptageniid mayfly larvae consumed more algae (as measured by stable carbon isotope ratios and gut content analysis) in caddisfly removal plots at the end of the 60-day experiment, although not after one month. We did not see isotopic evidence of increased algal carbon in invertebrate predators, however. Patterns of caddisfly and mayfly diets in the surrounding watershed suggested that mayfly diets are variable and include algae and detrital carbon in variable proportions, but scraping caddisflies consume primarily algae. Caddisfly and mayfly diets are more similar in larger, more productive streams where the mayflies assimilate more algae. Isotopic analysis, in combination with measurements of macroinvertebrate abundance and biomass in unmanipulated plots, suggested that a substantial portion of the invertebrate community (>50% of biomass) was supported, at least partially, by local algal carbon during midsummer. These data suggest that algae may be more important to community dynamics in headwater streams than their relatively low productivity would suggest. Through their high densities and relative invulnerability to predation, armored grazers may also affect community structure and flow of algal and detrital carbon in headwater streams.     

Extremely high secondary production of introduced snails in rivers.

The functional importance of invasive animals may be measured as the degree to which they dominate secondary production, relative to native animals. We used this approach to examine dominance of invertebrate secondary production by invasive New Zealand mudsnails (Potamopyrgus antipodarum) in rivers. We measured secondary production of mudsnails and native invertebrates in three rivers in the Greater Yellowstone Area (Wyoming, USA): Gibbon River, Firehole River, and Polecat Creek. Potamopyrgus production was estimated by measuring in situ growth rates and multiplying by monthly biomass; native invertebrate production was estimated using size frequency and instantaneous growth methods. Mudsnail growth rates were high (up to 0.06 d(-1)) for juvenile snails and much lower for adult females (0.003 d(-1)). Potamopyrgus production in Polecat Creek (194 g x m(-2) x yr(-1)) was one of the highest values ever reported for a stream invertebrate. Native invertebrate production ranged from 4.4 to 51 g x m(-2) x yr(-1). Potamopyrgus was the most productive taxon and constituted 65-92% of total invertebrate productivity. Native invertebrate production was low in all streams. Based on a survey of production measures from uninvaded rivers, the distribution of secondary production across taxa was much more highly skewed toward the invasive dominant Potamopyrgus in the three rivers. We suggest that this invasive herbivorous snail is sequestering a large fraction of the carbon available for invertebrate production and altering food web function.     

Effects of Urbanization on Occupancy of Stream Salamanders

Abstract: Urban development is the most common form of land conversion in the United States. Using a before–after control‐impact study design, we investigated the effects of urbanization on larval and adult stages of southern two‐lined salamanders (Eurycea cirrigera) and northern dusky salamanders (Desmognathus fuscus). Over 5 years, we estimated changes in occupancy and probabilities of colonization and survival in 13 stream catchments after urbanization and in 17 catchments that were not urbanized. We also examined effects of proportion of urbanized area in a catchment and distance of the salamander population to the nearest stream on probabilities of colonization and survival. Before urbanization, adult and larval stages of the two salamander species occupied nearly all surveyed streams, with occupancy estimates ranging from 1.0 to 0.78. Four years after urbanization mean occupancy of larval and adult two‐lined salamanders had decreased from 0.87 and 0.78 to 0.57 and 0.39, respectively. Estimates of mean occupancy of larval northern dusky salamanders decreased from 1.0 to 0.57 in urban streams 4 years after urbanization; however, adult northern dusky salamander occupancy remained close to 1.0 in urban streams over 5 years. Occupancy estimates in control streams were similar for each species and stage over 5 years. Urbanization was associated with decreases in survival probabilities of adult and larval two‐lined salamanders and decreases in colonization probabilities of larval dusky salamanders. Nevertheless, proportion of impervious surface and distance to nearest stream had little effect on probabilities of survival and colonization. Our results imply that in the evaluation of the effects of urbanization on species, such as amphibians, with complex life cycles, consideration of the effects of urbanization on both adult and larval stages is required.     

岷江上游高山森林溪流非木质残体现存量与碳储量及其分配特征

森林溪流非木质残体的特征直接关系到流域源头水质环境以及森林生态系统碳及养分等物质的输出格局.为了解岷江上游水源源头高山森林生态系统溪流非木质残体的储量特征,于2013 年8 月高山森林溪流水量最大的季节,在研究区域海拔3 600 m 典型高山森林范围内,沿主河道两岸调查每条森林溪流的非木质残体储量,共找到18 条森林溪流汇入主河道,根据实地采样的可操作性和典型性,选择其中12 条溪流详细调查非木质残体储量,每条森林溪流从尽头到源头每隔10 m 设置一个长度为1 m,溪流实际宽度的样方（源头作为最后一个样方）.将样方内所有非木质残体全部采集,低温保存,迅速带回实验室,分别按照树皮、树叶和直径小于1 cm 树枝分离,65 ℃烘干至恒重,测定各组分现存量.然后,将样品粉碎过筛,采用重铬酸钾氧化法测定凋落物有机碳含量,以不同组分现存量与其碳含量计算各组分的碳储量.结果表明：（1）该区域森林溪流非木质残体总现存量和总碳储量分别为657.25 kg 和262.96 kg,单位面积现存量和碳储量为439.70 g·m^-2 和175.92g·m^-2;（2）各溪流中直径小于1 cm 树枝占非木质残体总现存量和总碳储量的69.76%和73.41%,其次为树叶,树皮比例最小且不足10%;（3）尽管溪流长度、面积和流量与非木质残体各组分单位面积现存量和碳储量均无显著相关关系,但显著影响溪流非木质残体总现存量和总碳储量及其在各组分的分配比例;（4）相对于树皮,凋落树叶现存量和碳储量所占比例在流量较小溪流中相对较大.这些结果为深入认识高山森林流域水环境及其在森林生态系统中的重要作用提供新的思路和一定的科学依据.     

Nitrogen turnover in the leaf litter and fine roots of sugar maple

In order to better understand the nitrogen (N) cycle, a pulse of 15NO3- was applied in 1998 to a sugar maple (Acer saccharum) dominated northern hardwood forest receiving long-term (1994-2008) simulated atmospheric N deposition. Sugar maple leaf litter and live fine-root 15N were quantified for four years prior to labeling and for 11 subsequent years. Continuous sampling of 15N following addition of the tracer enabled calculation of leaf litter and fine-root N pool turnover utilizing an exponential decay function. Fine-root 15N recovery peaked at 3.7% +/- 1.7% the year the tracer was applied, while leaf litter 15N recovery peaked in the two years following tracer application at approximately 8%. These results suggest shoots are primarily constructed from N taken up in previous years, while fine roots are constructed from new N. The residence time of N was 6.5 years in leaf litter and 3.1 years in fine roots. The longer residence time and higher recovery rate are evidence that leaves were a stronger sink for labeled N than fine roots, but the relatively short residence time of tracer N in both pools suggests that there is not tight intra-ecosystem cycling of N in this mature forest.     

Inhibition of decomposing leaf litter of Cinnamomum camphora on growth of Pharbitis nil and the alleviation effect of nitrogen application北大核心CSCD

In plant configuration of landscaping, herbaceous plant is often inter-planted with ornamental tree species. But unreasonable plant collocation may reduce the effectiveness of afforestation for the inhibition effect of tree on the growth of understory, and further more affect the greening effect. Camphor (Cinnamomum camphora) is widely planted in landscaping accompanying herbaceous plants including morning glory (Pharbitis nil). But camphor was reported to have allelopathic effect on its adjacent plant, therefore correct selection and collocation of plants has important significance to obtain good greening effect. This research aimed to study by a pot experiment the effects of decomposing leaf litter of Cinnamomum camphora on the growth, phenological traits of Pharbitis nil, as well as modification of these effects by nitrogen application. Three application rates of C. camphora leaf litter including 25, 50 and 100 g/pot (denoted by L25, L50 and L100, respectively) and a control (CK) were implemented. Nitrogen application began on the 34th d of decomposition, with 0.39 g urea being divided into three equal portions and added continuously to each pot. The results showed that (1) The height, basal diameter, leaf area and biomass production of Pharbitis nil were all inhibited sharply by exposure to the leaf litter, being decreased by 32.85%-83.78%, 5.23%-23.00%, 30.31%-58.47%, and 40.34%-84.54%, respectively, with the inhibition effect increasing both in intensity and stability with the increase of leaf litter. Such inhibition effect was obviously alleviated by exogenous nitrogen application. (2) The flowering dynamics of Pharbitis nil was greatly impacted by the leaf litter, with the flower initiation 2.5-10.0 d later and the flowering duration 6.3-11.4 d longer compared to the control. Although the leaf litter-treated plants exhibited more (0.5-3.3) flowers than the control, their quality decreased, and the hundred-grain weight of the seed decreased with the increase of leaf litter. However, the differences among treatments in the reproductive parameters mentioned above reduced after nitrogen fertilization. The results indicated that leaf litter of C. Camphora has a great allelopathic effect on morphological and reproductive growth of Pharbitis nil, which may be attributed to the release of phototoxic substances during the decomposition process.