Soil temperatures during bushfires in semi-arid,mallee shrublands

Abstract Soil temperatures were measured during 11 experimental fires in semi-arid mallee shrublands in central NSW. Sensors were placed at depths from 1–10 cm beneath the soil surface in three fuel types; litter beneath Eucalyptus shrubs, live hummocks of the grass Triodia irritans and litter beneath shrubs of Acacia species. Weights of these fuels per unit area were determined. Maximum soil temperature and its duration were related to fuel type and depth. Mean weights of Eucalyptus and Triodia fuels were similar (0.35 kg m^?2), while there was less Acacia fuel (0.1 kg m^?2). Highest maximum temperatures were registered under Eucalyptus litter (e.g. 140°C at 2 cm). Maximum temperatures under Triodia and Acacia litter were similar (e.g. 60–70°C at 2 cm). Durations were examined in two temperature classes (60–120 and > 120°C) chosen to represent threshold for stimulation of germination and mortality, respectively, of soil-stored seeds. Temperatures between 60 and 120°C were recorded only between 0–2 cm soil depth for Acacia and Triodia (one exception at 4 cm). No temperatures >120°C were recorded for these fuel types. Temperatures between 60 and 120°C were recorded to 5 cm depth under Eucalyptus fuels while putative lethal temperatures for seeds occurred occasionally at 0–2 cm depth. The results indicated greatest potential for stimulation of germination and death of buried seeds under Eucalyptus fuels, although the level of variability of temperature was highest under Eucalyptus fuels. Despite similar fuel loads, differences between temperatures under Eucalyptus and Triodia fuels reflected the influence of the depth of the fuel bed, with Triodia hummocks constituting a deep fuel bed and Eucalyptus litter a shallow fuel bed.     

Functional Group Identity Does not Predict Invader Impacts: Differential Effects of Nitrogen-fixing Exotic Plants on Ecosystem Function

The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits rather than broadly defined functional groups alone when predicting invader impacts.     

Allelopathic Effects of Tree Species on Some Soil Microbial Populations and Herbaceous Plants

The allelopathic potential of four tree species on soil microbial populations and some herbaceous plants (two understory species and one general biotest species) was investigated. Effects of three nonindigenous tree species, Eucalyptus globulus Labill, Pinus radiata D.Don and Acacia melanoxylon R.Br., on microorganisms participating in the cycle of nitrogen were evaluated, comparing them with those produced by the autochthonous Quercus robur L. Influence of the trees on Lactuca sativa L., Dactylis glomerata L. and Trifolium repens L. was also checked in bioassays. Cell numbers of Nitrosomonas sp. were negatively affected by Acacia and Eucalyptus stands, mainly during spring, when flowers are especially abundant on the ground. Proteolytic microorganisms were also negatively affected by Eucalyptus and Pinus stands, whilst Quercus stand did not show any toxicity. Soil bioassays showed clear inhibitory effects on germination and growth of understory plants, particularly soils from Eucalyptus and Acacia stands. The greatest effects had the soil from Acacia stand, which was phytotoxic during the whole period of germination and growth of understory plants. Allelopathic phenomena could be, at least partially, responsible of the low species diversity in the understory of the nonindigenous tree stands.     

Soil‐mediated effects on germination and seedling growth of coastal wattle (Acacia sophorae) by the environmental weed,bitou bush (Chrysanthemoides monilifera ssp. rotundata)

Bitou bush (Chrysanthemoides monilifera ssp. rotundata; Asteraceae) is a major woody weed that competes with the native legume Acacia sophorae in coastal ecosystems of eastern Australia. Three glasshouse experiments examined whether litter or soil from beneath bitou bush or Acacia plants could influence seed germination and seedling growth of A. sophorae. The presence of litter decreased seed germinability and this effect was greater for bitou bush litter than for Acacia litter. Shoot growth was increased by the addition of Rhizobium after 40 days, irrespective of soil type. After 78 days, shoot and root biomass were significantly lower for seedlings grown in bitou bush soil than for those grown in Acacia soil. There was a non‐significant trend towards a lower median population of Rhizobium in the soil beneath bitou bush than in that beneath Acacia. The results demonstrated a slight effect of bitou bush on the growth of A. sophorae, which could, however, be overshadowed by the judicious use of herbicides or fire for weed control and revegetation.     

The effects of a single hot summer fire on soil fauna and on litter decomposition in jarrah (Eucalyptus marginata) forest in Western Australia

An area of jarrah (Eucalyptus marginata Sm) forest was burnt with a hot fire (ca 600 kw/ha) in March 1975. The meso- and microarthropod faunas were sampled in May, June and July 1976 and the decomposer activity of the soil and litter was estimated using buried cotton strips. The growth of jarrah seedlings in pots was measured under different litter and nutrient regimes. Feeding experiments with Eucalyptus, Banksia and Bossiaea litter were carried out on Podykipus sp., a litter millipede common at the site of the fire. Burning reduced the numbers of arthropods, the proportion of juveniles and the proportion of fungal feeders in the micro-arthropod population. The rate of decomposition was also reduced. The seedlings grew most under leaf Utter and millipede faeces and least under leaf ash. Podykipus sp. preferred Bossiaea litter (high nutrient content) to either Eucalyptus or Banksia litter (low nutrient content).     

Eucalyptus recruitment in degraded woodlands: no benefit from elevated soil fertility

Grasses and forbs compete heavily with young tree seedlings for available resources, greatly reducing tree seedling establishment success. Soil nutrient enrichment associated with agricultural intensification can increase the growth of both herbaceous and woody lifeforms growing in isolation, but may change the balance of competitive advantage when growing together. The effects of nitrogen and phosphorus enrichment on pasture biomass and competition with two Australian grassy woodland trees (Eucalyptus albens and Eucalyptus microcarpa) was investigated in a field plot trial. Soil nutrients increased pasture biomass, but had no measurable effect on tree growth in our experiment. Competition from pasture species, even at low levels, led to high tree seedling mortality and greatly reduced tree seedling growth compared with pasture-free plots. However, when pasture-free plots were excluded from the analysis, tree seedling leaf area was not strongly correlated with herbaceous biomass. Tree seedling establishment was severely restricted even at the lowest levels of pasture biomass. We conclude that increased soil fertility resulted in a competitive advantage to the pasture, and does not improve tree seedling establishment when grown either with or without exotic herbaceous pasture (grassy understorey) species.     

The impact of mycorrhizal colonization upon nitrogen source utilization and metabolism in seedlings of Eucalyptus grandis Hill ex Maiden and Eucalyptus maculata Hook

Analysis of soil solution from forest sites dominated by Eucalyptus grandis and Eucalyptus maculata indicates that soluble forms of organic nitrogen (amino acids and protein) are present in concentrations similar to those of mineral nitrogen (nitrate and ammonium). Experiments were conducted to determine the extent to which mycorrhizal associations might broaden nitrogen source utilization in Eucalyptus seedlings to include organic nitrogen. In isolation, species of ectomycorrhizal fungi from northern Australia show varying abilities to utilize mineral and organic forms of nitrogen as sole sources. Pisolithus sp. displayed strongest growth on NH₄⁺, glutamine and asparagine, but grew poorly on protein, while Amanita sp. grew well both on mineral sources and on a range of organic sources (e.g. arginine, asparagine, glutamine and protein). In sterile culture, non-mycorrhizal seedlings of Eucalyptus grandis and Eucalyptus maculata grew well on mineral sources of nitrogen, but showed no ability to grow on sources of organic nitrogen other than glutamine. In contrast, mycorrhizal seedlings grew well on a range of organic nitrogen sources. These observations indicate that mycorrhizal associations confer on species of Eucalyptus the ability to broaden their resource base substantially with respect to nitrogen. This ability to utilize organic nitrogen was not directly related to that of the fungal symbiont in isolation. Seedlings mycorrhizal with Pisolithus sp. were able to assimilate sources of nitrogen (in particular histidine and protein) on which the fungus in pure culture appeared to grow weakly. Experiments in which plants were fed ¹⁵N-labelled ammonium were undertaken in order to investigate the influence of mycorrhizal colonization on the pathway of nitrogen metabolism. In roots and shoots of all seedlings, ¹⁵N was incorporated into the amide group of glutamine, and label was also found in the amino groups of glutamine, glutamic acid, γ-aminobutyric acid and alanine. Mycorrhizal colonization appeared to have no effect on the assimilation pathway and metabolism of [¹⁵N]H₄⁺; labelling data were consistent with the operation of the glutamate synthase cycle in plants infected with either Pisolithus sp. (which in isolation assimilates via the glutamate synthase cycle) or Elaphomyces sp. (which assimilates via glutamate dehydrogenase). It is likely that the control of carbon supply to the mycorrhizal fungus from the host may have a profound effect on both the assimilatory pathway and the range of nitrogen sources that can be utilized by the association.     

Restoration of an Ecosystem Function to Revegetation Communities: The Role of Invertebrate Macropores in Enhancing Soil Water Infiltration

Guidelines for revegetation in agricultural landscapes may not address restoration of ecosystem functions because management is focused on the replanting stage, although certain functions are delivered by organisms that colonize revegetation months or years later. We investigated the ecosystem function of water infiltration to tree root zones and channels, delivered by invertebrates that form soil macropores. We measured macropore density and infiltration rates at revegetation sites established on retired grazing land, in relation to site age, tree species composition, and geographical location, compared with adjacent matched pastures. Revegetated sites had significantly more macropores than pastures, and revegetation sites aged 11–20 years had more macropores than sites aged 3–5 and 6–10 years. Tree species had a marginal effect, with more macropores in sites with Acacia spp. and Eucalyptus spp. than those with Eucalyptus spp. only. Besides ants, the main groups of soil burrowers were mygalomorph and lycosid spiders and also ground‐nesting native bees. Infiltration rates in revegetation sites aged 11–20 years were double those of pastures and of 3–5 and 6–10 year sites. This is the first study to quantify the rate of recovery of an invertebrate‐driven soil hydrological ecosystem function following revegetation.     

Litter decomposition and nitrogen mineralization of soils in subtropical plantation forests of southern China,with special attention to comparisons between legumes and non-legumes

Litter decomposition and nitrogen mineralization were investigated in subtropical plantation forests in southern China. The CO₂ –C release from incubated litter and the forest floor of Acacia mangium, Acacia auriculaeformis, Eucalyptus citriodora, Pinus elliotii and Schima superba stands were used to estimate relative rates of litter decomposition. Decomposition was not positively correlated with litter nitrogen. E. citridora litter decomposed most rapidly and A. mangium litter most slowly, both with and without the addition of exotic nitrogen. Aerobic incubation and intact soil core incubation at 30 °C over a period of 30 days were used to assess nitrogen mineralization of six forest soils. Although there were differences in results obtained using the two methods, patterns between legume and non-legume species were the same regardless of method. All soils had pH values below 4.5, but this did not prevent nitrification. The dominant form of mineral nitrogen was nitrate for legume species and ammonium for non-legume species. The nitrogen mineralization potential was highest for soils in which legumes were growing.     

Effects of neighbouring vegetation on eucalypt seedlings at two sites subject to different levels of abiotic stress

Abstract In spite of numerous studies on the effect of nutrient levels and/or standing crop on the intensity of resource competition the debate has not been resolved. Field studies that have used natural productivity gradients have generally supported the argument that competitive intensity and resource availability are positively correlated, whereas studies that have used artificial resource gradients have generally refuted the same argument. Here we report the results from study in which both approaches were used within the same system. We studied two species of eucalypt that occupy contrasting parts of the same landscape: Eucalyptus camaldulensis, found mostly along creek lines and in valleys with deep alluvial soils, and Eucalyptus microcarpa, found on hillsides and ridges with shallow soils. We studied the response of seedlings of the two species to the combined effects of competition and manipulated nutrient levels in a glasshouse experiment, and also investigated their responses to removal of neighbouring plants in the field. Eucalyptus microcarpa was less responsive to increased resource availability, which is consistent with one of the principal assumptions of Grime’s C‐S‐R model. In the glasshouse experiment both species of eucalypt responded in a qualitatively similar fashion to the combined effects of resource availability and competition: release from competition resulted in increased growth, but only in pots that received additional resources. In the field we found that neighbouring vegetation could severely limit the establishment of E. camaldulensis but the removal of neighbouring vegetation did not affect the performance of E. microcarpa seedlings. Eucalyptus camaldulensis seedlings suffered high levels of damage from herbivores. Our results thus generally support the predications of the C‐S‐R model, however, they indicate that the effects of competition and herbivory may be heavily confounded.     

Browsing-induced Effects on Leaf Litter Quality and Decomposition in a Southern African Savanna

We investigated the linkages between leaf litter quality and decomposability in a savanna plant community dominated by palatable-spinescent tree species. We measured: (1) leaf litter decomposability across five woody species that differ in leaf chemistry; (2) mass decomposition, nitrogen (N); and carbon (C) dynamics in leaf litter of a staple browse species (Acacia nigrescens) as well as (3) variation in litter composition across six sites that experienced very different histories of attack from large herbivores. All decomposition trials included litter bags filled with chopped straw to control for variation in site effects. We found a positive relationship between litter quality and decomposability, but we also found that Acacia and straw litter mass remaining did not significantly vary between heavily and lightly browsed sites. This is despite the fact that both the quality and composition of litter returned to the soil were significantly different across sites. We observed greater N resorption from senescing Acacia leaves at heavily browsed sites, which in turn contributed to increase the C:N ratio of leaf litter and caused greater litter N immobilization over time. This, together with the significantly lower tree- and herb-leaf litter mass beneath heavily browsed trees, should negatively affect decomposition rates. However, estimated dung and urine N deposition from both browsers and grazers was significantly greater at high- than at low-herbivory sites. We hypothesize that N inputs from dung and urine boost litter N mineralization and decomposition (especially following seasonal rainfall events), and thereby offset the effects of poor leaf litter quality at chronically browsed sites. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

季节性冻融期间土壤动物对高山草甸两种凋落叶失重的贡献

季节性冻融期间高山草甸凋落叶的分解可为生长季节植物生长提供必要的养分,对于维持生态系统物质循环和养分平衡具有重要作用。然而,土壤动物对凋落叶分解是否具有明显的贡献仍然缺乏一致认识。因此,以高山草甸代表性植物黄花亚菊(Ajania nubigena)和黑褐苔草(Carex atrofusca)凋落叶为研究对象,采用不同孔径凋落叶袋排除土壤动物的方法,研究冬季不同冻融时期(冻结前期、冻结期和融化期)土壤动物对凋落叶失重的贡献。整个季节性冻融期间土壤动物对黄花亚菊和黑褐苔草两种凋落叶失重率的作用分别为12.07%和4.03%,总贡献率分别为46.39%和24.14%。土壤动物对两种凋落叶失重率的作用均在融化期最大,而土壤动物对黄花亚菊凋落叶失重率的作用在冻结初期最小,土壤动物对黑褐苔草凋落叶失重率的作用在冻结期最小。整个季节性冻融期,土壤动物对凋落叶失重率的作用和贡献率与正积温和凋落叶初始C、N浓度和C/N比均呈显著的正相关关系。因此,季节性冻融期间土壤动物对高山草甸凋落叶分解具有明显的贡献,但这些过程受冻融格局和凋落叶初始质量的调控。     

Arthropod assemblages in a focal tree species (Eucalyptus microcarpa) depends on the species mix in restoration plantings

The composition of arthropods in trees has long been a topic of interest, with many studies exploring ways in which arthropod communities differ among tree species or entire forests. Few studies, however, have examined arthropods from trees in restoration plantings, and little is known about how different tree plantings might lead to different biodiversity outcomes. The aim of our study was to determine if a focal tree species hosted a different arthropod fauna depending on its context. We examined arthropod assemblages from the foliage of Eucalyptus microcarpa (Grey Box) trees present in two kinds of plantings: (i) simple plantings with only Eucalyptus trees, and (ii) mixed plantings with both Eucalyptus and Acacia trees. We examined the composition of the assemblages, and looked for associations between ant and psyllid (Hemiptera) diversity across each kind of planting. We found more species of psyllids, beetles and ants in E. microcarpa trees when they were grown in simple plantings compared with the mixed species plantings. We also found that psyllid richness and abundance was positively correlated with ant richness and abundance across all plantings. Our study shows that a more diverse context (i.e. a mixed planting) is not necessarily associated with a more diverse fauna within a focal tree species, and that simple Eucalyptus-only plantings may encourage higher numbers of insect herbivores. The apparent association between psyllids and ants suggests a complex interaction that may drive patterns in tree arthropod assemblages.     

Benefits and risks of biotic exchange between Eucalyptus plantations and native Australian forests

Australia is unique in having two highly diverse plant genera, Eucalyptus and Acacia, that dominate the vegetation on a continent‐wide scale. The recent shift in plantation forestry away from exotic Pinus radiata to native Eucalyptus species has resulted in much more extensive exchange of biota between native forest and plantation ecosystems than exchange in the past with plantations of exotic species. Growing numbers of hectares are being planted to Eucalyptus globulus across Australia, and plantations are providing resources and corridors for native biota. The present paper focuses on both the benefits and risks of having large‐scale forestry plantations of native species that are closely related to dominant native taxa in local forests. At least 85 species of insects have been recorded as pests of Eucalyptus plantations around Australia; the vast majority of these have been insects using the same host species, or closely related taxa, in native forests. Plantations of native species may also benefit from closely related local forests through the presence of: (i) the diverse array of ectomycorrhizal fungi favourable for tree growth; (ii) natural enemies harboured in native habitats; and (iii) recruitment of other important mutualists, such as pollinators. Exchanges work in two directions: plantations are also likely to influence native forests through the large amount of insect biomass production that occurs in outbreak situations, or through the introduction or facilitation of movements for insects that are not native to all parts of Australia. Finally, older plantations in which trees flower may exchange genes with surrounding forest species, given the high degree of hybridization exhibited by many Eucalyptus species. This is an aspect of exchange for which few data have been recorded. In summary, because of Australia’s unique biogeography, plantation forestry using eucalypt species entails exchanges with natural habitats that are unparalleled in scale and diversity in any other part of the world. More exchanges are likely as plantations occupy greater area, and as the time under cultivation increases.     

Establishment and growth of seedlings of Eucalyptus obliqua: Interactive effects of litter,water, and pathogens

We studied how leaf litter, water and fungal pathogens affect the establishment, survivorship, and growth of Eucalyptus obliqua using glasshouse and field experiments. In a glasshouse experiment, the presence of leaf litter of E. obliqua increased the establishment of seedlings when the seeds were placed beneath the litter, but dense litter layers reduced seedling emergence when seeds were placed on top. Results from a complementary experiment suggest that the effect of litter may be mediated by the high humidity microenvironment it creates, which may enhance imbibition of unburied seeds. In another glasshouse experiment, litter in combination with excessive water applications reduced seedling emergence and increased seedling mortality, but the addition of fungicide reverted these effects. Litter with lower water application or fungicide added increased seedling emergence. In all cases higher biomass in seedlings growing in litter treatments was associated with earlier emergence. In two recently burnt sites, the density of seedlings of E. obliqua was positively correlated to the presence of litter. Experimental addition of litter in the field reduced soil temperature during the summer, and together with watering, increased soil water content, and the water potential of established seedlings. However, neither treatment had any positive effect on their growth or survivorship. On the contrary, litter addition reduced the height and number of leaves, probably as a result of increased herbivory. We conclude that in Mediterranean-type systems litter may produce various effects on seedling establishment depending on their developmental stage, and the season.     

不同林龄尾巨桉人工林凋落物和土壤C、N、P化学计量特征

为正确认识桉树(Eucalyptus spp.)人工林凋落物和土壤C、N、P时空分配格局及两者间的关系,对5个林龄尾巨桉林分凋落物及土壤C、N、P含量及化学计量比进行测定分析。结果表明,凋落物的C含量均显著大于土壤,且不同林龄间凋落物C含量无显著差异,1年生人工林土壤表层(0~20 cm)的C含量显著小于3~7年生,其他土层C含量在不同林龄间差异不显著。凋落物的N含量均显著高于土壤,且1年生人工林显著大于其他林龄的,而土壤表层的N含量以7年生人工林最大,1年生的最小。凋落物的P含量除1年生人工林显著大于3年生外,其他林龄间均无显著差异,土壤的P含量在不同林龄间的差异均不显著。凋落物C∶N随林龄呈逐渐增大趋势,且显著大于土壤层。凋落物的C∶P和N∶P在不同林龄间的差异不显著,但均显著大于土壤层。凋落物的N含量与表层土壤的C、N含量呈极显著负相关,凋落物的C∶N与表层土壤的C∶P,N∶P呈显著正相关,表层土壤C、N积累受到凋落物N的制约。因此,在桉树人工林经营管理过程中如何降低凋落物分解的N限制性、提高养分传递效率及合理施肥显得十分重要。     

The sugar and nitrogen content of the gums of Acacia species in the Mountain Ash and Alpine Ash forests of central Victoria and its potential implications for exudivorous arboreal marsupials

Abstract Chemical analysis of the total sugar and total nitrogen content of Acacia dealbata, Acacia obliquinervia and Acacia frigescens gum exudate was completed. These trees were located within stands of 53 year old Mountain Ash, Eucalyptus regnans and Alpine Ash, Eucalyptus delegatensis forest in the Central Highlands of Victoria, southeastern Australia. Values for sugar content ranged from 24 to 68% per sample. Gum samples that were collected in E. regnans forests had a significantly lower (P<0.05) sugar content than those from stands dominated by E. delegatensis. Statistical analyses using Scheffe's S-test indicated that there was a significant difference in the sugar content of gums between A. dealbata and A. frigescens but not between A. dealbata and A. obliquinervia or A. obliquinervia and A. frigescens. Values for the nitrogen content of Acacia gum varied from 0.2 to 0.7% per sample. Statistical analyses revealed that nitrogen content was significantly influenced (P<0.05) by a combination of three interacting factors: (i) Acacia species; (ii) tree diameter; and (iii) forest type. Therefore, the findings of this study indicate that the sugar and nitrogen content of Acacia gum may vary between forest types and tree species. The gum of Acacia species is an important source of food for several species of arboreal marsupials, and differences in sugar and nitrogen content could be a factor potentially influencing the distribution and abundance of these animals.     

桂北不同林龄桉树人工林土壤碳库管理指数和碳组分的变化特征

采用时空互代法,以广西北部低山丘陵地区不同林龄(1、2、3、4、5和8 a)桉树人工林为研究对象,探讨林龄对桉树人工林地土壤碳库管理指数的影响及其规律。结果表明:(1)随着林龄的增加,土壤有机碳总体表现为增加的趋势,1～8 a桉树土壤有机碳范围在5.79～15.57 g· kg^-1之间,随着土层的加深而降低; 0～40 cm土层土壤有机碳平均含量表现为8 a>5 a>3 a>4 a>2 a>1 a。(2)土壤非活性有机碳、碳储量随林龄和土层的变化规律与土壤有机碳基本一致。土壤活性有机碳含量大小依次表现为8 a>5 a>4 a>3 a>2 a>1 a,占土壤有机碳的比例随林龄变化无明显规律,8 a和其他林龄间均具有显著差异。(3)碳库管理指数随林龄增加整体呈上升趋势,8 a桉树人工林土壤碳组分含量及碳库管理指数均高于10 a对照马尾松林。碳库管理指数与土壤有机碳、非活性有机碳、活性有机碳、碳储量、碳库活度、全氮、容重呈极显著或显著的相关性,不同林龄和土层间碳库管理指数有差异性。适当延长桉树人工林的轮伐周期,减少人为对林地凋落物和林下植被的干扰,将有利于提高土壤的有机碳含量,进而改善土壤质量。     

Plantation age,understory vegetation,and species‐specific traits of target seedlings alter the competition and facilitation role of Eucalyptus in South China

The Chinese government is considering ways to reconstruct Eucalyptus plantation monocultures in order to improve their ecosystem services. However, the effects of competition or facilitation of Eucalyptus on native species during the reconstruction have not been identified, especially during the formation of novel ecosystems. To better understand these relationships, in this study we tested the effects of competition and facilitation of Eucalyptus on three target species (Schima superba, Michelia macclurei, and Castanopsis fissa), in Eucalyptus plantations with different ages and understory vegetation. The results showed that the survival and growth of S. superba and M. macclurei were not sensitive to plantation age and understory removal treatment. The survival of C. fissa in understory removal was significantly higher and the growth of C. fissa increased gradually with increasing plantation age. The relative neighbor effect (RNE) index for growth indicated that the relationship between Eucalyptus and target species switched from competition to facilitation with an increase in plantation age. A redundancy analysis (RDA) showed that survival was positively correlated with soil water content and soil phosphorus. Growth was positively correlated with soil organic matter and soil nitrogen and negatively correlated with soil bulk density. Our results indicate that plantation age, understory vegetation, and species‐specific traits of target seedlings could change the competition and facilitation roles of Eucalyptus. Higher survival and growth rates indicated that C. fissa was an appropriate species for the reconstruction of mature Eucalyptus plantations and understory removal will benefit the process.     

AMF对喀斯特土壤枯落物分解和对宿主植物的养分传递

为探索丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)在喀斯特土壤中养分利用机制,采用分室系统隔室装置(用20μm或0.45μm尼龙网双层隔离)对香樟(Cinnamomum camphora)幼苗进行幼套球囊霉(Glomus etunicatum)接种处理和施氮处理,并采用15N稳定同位素技术标记了黑麦草(Lolium perenne)枯落物作为土壤有机残体,幼苗生长15周后测定了隔室幼苗生长性状指标、氮、磷摄取量、植株和隔室土壤中的δ15N值、微生物量碳、微生物量氮以及菌丝体密度,结果表明:AMF具有腐生营养能力,促进了土壤枯落物的分解并吸收其释放的15N传递给宿主植物利用;香樟幼苗优先利用根际周围氮维持生长;在低氮状态下,香樟植株通过AMF菌丝体更多地利用了相邻隔室枯落物分解释放的15N;施加根际外源氮有利于AMF对隔室枯落物的分解,但降低了植株对枯落物氮的利用;根际高氮状态下植株的氮、磷摄取量较大;高养分状态下提高了相邻隔室微生物量碳、氮含量和菌丝体密度。