外来植物入侵对土壤氮有效性的影响

外来植物入侵对土壤氮循环和氮有效性的影响是入侵成功或进一步加剧的重要原因。通过对比相同研究地点入侵区域和无入侵区域的土壤原位氮状态差异, 探讨了外来植物入侵对土壤氮有效性的影响程度和生理生态学机制。基于107篇相关研究文献数据的整合, 发现植物入侵区域相对于无入侵区域土壤总氮、铵态氮、硝态氮、无机氮、微生物生物量氮含量显著增加, 增幅分别为(50 ± 14)%、(60 ± 24)%、(470 ± 115)%、(69 ± 25)%、(54 ± 20)%。土壤硝态氮含量增幅较大反映硝化作用增强, 这可能增加入侵植物硝态氮利用以及喜硝植物的共存。温带地区植物入侵后土壤的硝态氮含量增幅显著高于亚热带地区。固氮植物入侵后土壤的总氮和无机氮含量增幅均显著高于非固氮植物入侵。木本和常绿植物入侵后土壤的总氮含量增幅分别高于草本和落叶植物入侵; 而土壤铵态氮含量的增幅没有显著差异且与固氮入侵植物占比无明显关系; 然而硝态氮含量的增幅普遍较高且与固氮入侵植物占比显著正相关。外来入侵植物固氮功能以及凋落物质量和数量是影响土壤氮矿化和硝化过程的关键因素。该研究为理解外来植物入侵成功和加剧的机制以及入侵植物功能性状与土壤氮动态之间的关系提供了新的见解。     

Effects of exotic plant invasion on soil nitrogen availability

外来植物入侵对土壤氮循环和氮有效性的影响是入侵成功或进一步加剧的重要原因。通过对比相同研究地点入侵区域和无入侵区域的土壤原位氮状态差异, 探讨了外来植物入侵对土壤氮有效性的影响程度和生理生态学机制。基于107篇相关研究文献数据的整合, 发现植物入侵区域相对于无入侵区域土壤总氮、铵态氮、硝态氮、无机氮、微生物生物量氮含量显著增加, 增幅分别为(50 ± 14)%、(60 ± 24)%、(470 ± 115)%、(69 ± 25)%、(54 ± 20)%。土壤硝态氮含量增幅较大反映硝化作用增强, 这可能增加入侵植物硝态氮利用以及喜硝植物的共存。温带地区植物入侵后土壤的硝态氮含量增幅显著高于亚热带地区。固氮植物入侵后土壤的总氮和无机氮含量增幅均显著高于非固氮植物入侵。木本和常绿植物入侵后土壤的总氮含量增幅分别高于草本和落叶植物入侵; 而土壤铵态氮含量的增幅没有显著差异且与固氮入侵植物占比无明显关系; 然而硝态氮含量的增幅普遍较高且与固氮入侵植物占比显著正相关。外来入侵植物固氮功能以及凋落物质量和数量是影响土壤氮矿化和硝化过程的关键因素。该研究为理解外来植物入侵成功和加剧的机制以及入侵植物功能性状与土壤氮动态之间的关系提供了新的见解。     

Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals

Understanding processes that underlie ecological resistance to weed invasion is critical for sustainable restoration of invaded plant communities. Experimental studies have demonstrated that invasive nitrophilic annuals can be controlled by addition of carbon to reduce soil nitrate concentrations, sometimes leading to enhanced establishment of native plants. However, effects of carbon supplements on soil nitrate are temporary, and the longer-term value of carbon supplementation as a restoration tool is dependent on the resistance of the re-established ecosystem to repeat invasion. We investigated whether re-established swards of the tussock grass Themeda australis (R.Br.) Stapf (a natural understorey dominant in mesic grassy woodlands of SE Australia) could suppress soil nitrate concentrations, and through this or other means, could impart ongoing resistance to exotic invasion in restored woodlands. In a remnant invaded by exotic annuals, we applied three plot treatments (carbon supplements, annual spring burns and untreated control) and two seed treatments (± Themeda seed) in a replicated, factorial design. Within 3 years, successful establishment of Themeda swards on burnt and carbon-supplemented plots was associated with a reduction in soil nitrate to levels comparable with non-invaded, Themeda-dominated reference sites in the region (<3 mg/kg), and significantly reduced exotic cover compared with unseeded plots. By contrast, on plots not seeded with Themeda, soil nitrate increased after cessation of carbon addition and exotic cover returned to levels comparable with untreated control plots, despite a high cover of other native perennial grasses. Few persistent effects of carbon supplements or spring burning on soil nutrients were evident 9–19 months after cessation of these treatments. Results suggest that Themeda is a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals.     

Validating the use of woodrat (Neotoma) middens for documenting natural invasions

Aim Plant macrofossils in fossil woodrat (Neotoma) middens are the primary source of information on late Quaternary biogeographical history of plants in arid and semi‐arid regions of North America. Macrofossil records from middens are playing particularly important roles in documenting spatial and temporal patterns of plant migrations and invasions since the last glacial maximum 21,000 years ago. However, relatively few actualistic studies comparing contents of modern middens with surrounding flora and vegetation have been carried out. The primary aim of this study is to assess the reliability of midden assemblages in detecting the presence of tree, shrub and several other plant species growing on the surrounding landscape. The secondary aims are to determine whether probability of occurrence of species in middens is related to abundance in vegetation, and whether representation of individual species in middens is contingent on presence/absence or abundance of other species. Location Our five study sites were bedrock escarpments or canyons in the central Rocky Mountains (north‐eastern Utah, central Wyoming and south‐central Montana). All sites were in conifer woodland or forest/woodland variously dominated by Juniperus osteosperma, J. scopulorum, Pinus edulis, P. flexilis and Pseudotsuga menziesii. Neotoma cinerea is the only woodrat species in the region. Methods Macrofossil assemblages from 59 modern middens (all showing clear signs of recent or ongoing woodrat activity) were compared with floristic composition of vegetation within 50 m of the middens, and with percent cover of vegetation within 30 m of the middens. Results Coniferous trees and shrubs were well‐represented in middens, occurring consistently even when abundance in the local vegetation was very low. Juniperus osteosperma and J. scopulorum were particularly well‐represented, occurring in middens regardless of local abundance. Other conifers (P. edulis, P. flexilis, P. ponderosa, Pseudotsuga menziesii) were occasionally absent from middens when abundance in vegetation was low (< 20% canopy cover). Occurrence of dicot shrubs and graminoids was less consistent. Main conclusions Middens constructed by N. cinerea are highly reliable sensors of presence/absence of J. osteosperma and J. scopulorum, and hence can be used to infer invasions and past biogeographical distributions of these species. The middens are also reliable in registering populations of other conifers, although presence in middens may be contingent on local abundance. Additional comparative studies are needed to develop a sound empirical basis for using middens of N. cinerea and other species to infer past presence/absence of plant species on the landscape, and to explore the vegetation‐sensing properties of midden assemblages.     

Plant–microbial competition for nitrogen increases microbial activities and carbon loss in invaded soils

Many invasive plant species show high rates of nutrient acquisition relative to their competitors. Yet the mechanisms underlying this phenomenon, and its implications for ecosystem functioning, are poorly understood, particularly in nutrient-limited systems. Here, we test the hypothesis that an invasive plant species (Microstegium vimineum) enhances its rate of nitrogen (N) acquisition by outcompeting soil organic matter-degrading microbes for N, which in turn accelerates soil N and carbon (C) cycling. We estimated plant cover as an indicator of plant N acquisition rate and quantified plant tissue N, soil C and N content and transformations, and extracellular enzyme activities in invaded and uninvaded plots. Under low ambient N availability, invaded plots had 77% higher plant cover and lower tissue C:N ratios, suggesting that invasion increased rates of plant N acquisition. Concurrent with this pattern, we observed significantly higher mass-specific enzyme activities in invaded plots as well as 71% higher long-term N availability, 21% lower short-term N availability, and 16% lower particulate organic matter N. A structural equation model showed that these changes were interrelated and associated with 27% lower particulate organic matter C in invaded areas. Our findings suggest that acquisition of N by this plant species enhances microbial N demand, leading to an increased flux of N from organic to inorganic forms and a loss of soil C. We conclude that high N acquisition rates by invasive plants can drive changes in soil N cycling that are linked to effects on soil C.     

Did Native Americans influence the northward migration of plants during the Holocene?

Abstract Long‐distance plant dispersal explains the rapid northward migration of plant species during the Holocene but the mechanisms by which it occurred are poorly understood. Given that Native Americans spread numerous cultigens over thousands of kilometres during the late Holocene, I examined historical literature for evidence of non‐cultigen dispersal or cultivation in North America's eastern woodlands. Cultivation references are included because a strong relationship between dispersal and indigenous flora husbandry is assumed. Sixty‐seven texts describing Native American lifestyle, cultural activities, and land management reported some form of plant use. Most accounts, however, focus on cultigen production or the use of indigenous flora for medicine or food without mention of dispersal. Twenty‐four of the texts described the trade, transport, or cultivation of plants indigenous to eastern North American woodlands. Most accounts focus on the informal production of food plants, especially trees and shrubs. Confounding these reports was clear evidence of observer bias, limited botanical knowledge, acculturation, and secrecy by Native American informants. Because of these shortcomings, the likelihood of widespread long‐distance plant dispersal by Native Americans could not be determined using historical literature. This activity was either not widespread or was not observed by, or revealed to, Europeans. To adequately test the Native American plant dispersal hypothesis, direct evidence from other sources (e.g. archaeobotancial data) will be required.     

Soil C:N:P stoichiometry responds to vegetation change from grassland to woodland

Woody encroachment has been a major land cover change in dryland ecosystems during the past century. While numerous studies have demonstrated strong effects of woody encroachment on soil carbon (C), nitrogen (N), and phosphorus (P) storage, far less is known about the plasticity of soil C:N:P stoichiometry in response to woody encroachment. We assessed landscape-scale patterns of spatial heterogeneity in soil C:N:P ratios throughout a 1.2 m soil profile in a region where grassland is being replaced by a diverse assemblage of subtropical woody plants dominated by Prosopis glandulosa, an N₂-fixing tree. Woody species had leaf and fine root C:N:P ratios significantly different from grasses. Variation in soil C:N ratios in both horizontal and vertical planes was remarkably smaller than that of soil N:P and C:P ratios. Spatial patterns of soil C:N ratio throughout the profile were not strongly related to vegetation cover. In contrast, spatial patterns of soil N:P and C:P ratios displayed a strong resemblance to that of vegetation cover throughout the soil profile. Within the uppermost soil layer (0–5 cm), soil N:P and C:P ratios were higher underneath woody patches while lower within the grassland; however, this pattern was reversed in subsurface soils (15–120 cm). These results indicate a complex response of soil C:N:P stoichiometry to vegetation change, which could have important implications for understanding C, N, and P interactions and nutrient limitations in dryland ecosystems.     

Savannah woodland vegetation in the South‐East District of South Australia: the influence of evaporative aerodynamics on the foliage structure of the understorey invaded by introduced annuals

Abstract Evaporative aerodynamics determine the foliage projective cover of the understorey of perennial tussock grasses and associated perennial herbs in the savannah woodland dominated by Eucalyptus camalduknsis on gleyed podsolic soils in the Mediterranean climate of the South‐East District of South Australia. By the mid 1940s, winter‐spring evapotranspiration from the ‘thin’ leaves (with low leaf specific weight) of introduced annual plants was depleting surface soil water and thus reducing the annual growth of the summer‐growing savannah understorey; perennial herbs between the tussock grasses were the first to succumb to this competition. During spring, the percentage of the ground covered by the savannah understorey was increased by 10% in the subhumid zone to 30% in the humid zone as the pre‐European perennial herbs between the tussock grasses were replaced by introduced annuals. Application of phosphatic fertilizer to the understorey increased the growth of introduced annuals, which formed a dense stratum during their winter‐spring growing season, increasing evapotranspiration and leading eventually to the extinction of the native perennial grasses. When the savannah understorey, invaded by introduced annuals in the mid‐1940s, was converted to improved pasture, the percentage of ground covered by the seasonal foliage was increased by 20–30%; 100% coverage of overlapping foliage resulted in the humid zone.     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Plant invaders outperform congeneric natives on amino acids

As a key nitrogen (N) source, soil amino acids play an important role in plant N nutrition. However, how amino acids differentially influence the N use strategies of native and invasive plants remains unclear. We performed a potted experiment using five pairs of native and invasive plant congeners, which were subject to 23 N treatments (i.e., 20 protein primary amino acids, nitrate, ammonium, and control), each with 10 replicates. We determined their growth, biomass allocation, N use efficiency, and the growth advantage of plant invaders over their natives. Native and invasive plants used the same 18 amino acid N sources (i.e., a similar amino acid economics spectrum). The growth of plant invaders was invariably better than the growth of native plants, and this superior growth of invaders was linked to their higher root biomass allocation and greater N use efficiency. Additionally, invasive plants had a greater growth advantage on amino acid N than on inorganic N, so was this advantage greater on neutral amino acids than on acidic amino acids. These findings suggest that the differences in amino acid use strategies between invasive and native congeners could help to explain plant invasiveness, as indicated by a growth advantage.     

桂西北喀斯特移民区土地利用方式对土壤养分的影响

利用典型喀斯特峰丛洼地主要土地利用方式（灌草地、次生林地、果园、牧草地、旱地）的土壤养分测定数据,研究土地利用变化对土壤养分的影响.结果表明：土壤有机碳、全氮、有效氮含量随土地利用强度的增加而降低,灌草地和次生林地比果园、牧草地和旱地土壤有机质含量高86%～155%、全氮含量高62%～119%、有效氮含量高66%～215%（P<0.05）;全磷和全钾含量主要受养分来源的影响,受土地利用方式影响较小;速效磷含量主要受施肥的影响,有效钾含量则主要受植被覆盖和水土流失状况的影响.土地利用方式是影响土壤有机碳、全氮、有效氮、有效磷、有效钾等养分含量变化的重要因素.粗放的农业耕作降低了土壤养分并引起土壤退化,而植被恢复等措施可以培肥土壤.因此,喀斯特区域应改变粗放的农业耕作方式,加强施用有机肥和平衡施肥,对坡度≥25°的坡耕地采取“退耕还林还草”措施,以恢复和重建喀斯特地区生态环境,实现土地资源的可持续利用.     

不同土地利用类型对丹江口库区土壤氮矿化的影响

氮(N)素是陆地生态系统净初级生产力的重要限制因子, 土地利用类型的变化对生态系统氮循环过程有着重要的影响。采用PVC顶盖埋管原位培养的方法, 对丹江口库区清塘河流域相邻的侧柏(Platycladus orientalis)人工林、人工种植灌木林地和农田3种土地利用类型的氮素矿化和硝化作用进行了研究。结果表明, 侧柏人工林、灌木林地和农田的NH₄⁺-N浓度(mg·kg^-1)依次为1.33 ± 0.20、1.67 ± 0.17和1.62 ± 0.13, 不同土地利用类型间的NH₄⁺-N浓度无显著性差异; 而3种土地利用类型下土壤NO₃^--N浓度(mg·kg^-1)差异显著, 农田NO₃^--N浓度(9.00 ± 0.73)显著高于侧柏人工林(1.27 ± 0.18)和灌木林地(3.51 ± 0.11)。NO₃^--N在灌木林地和农田中分别占土壤无机氮库的67.8%和84.8%, 是土壤无机氮库的主要存在形式; 而侧柏人工林中NO₃^--N和NH₄⁺-N浓度则基本相等。土壤硝化速率(mg·kg^-1·30 d^-1)从农田(7.13 ± 2.19)、灌木林地(2.56 ± 1.07)到侧柏人工林(0.85 ± 0.10)显著性降低。侧柏人工林、灌木林地和农田的矿化速率(mg·kg^-1·30 d^-1)依次为0.98 ± 0.12、2.52 ± 1.25和6.58 ± 2.29。矿化速率和硝化速率显著正相关, 但是矿化速率在不同的土地利用类型间差异不显著。培养过程中灌木林地和农田NH₄⁺-N的消耗大于积累, 氨化速率为负值, 导致灌木林地和农田矿化速率小于硝化速率。氮素的矿化和硝化作用受土壤含水量和土壤温度的影响, 并对土壤含水量更为敏感。土壤C:N与土壤矿化和硝化速率显著负相关。研究结果表明: 土地利用类型的变化会改变土壤微环境和土壤C:N, 进而会影响到土壤氮循环过程。     

Patterns of invasion of an urban remnant of a species-rich grassland in southeastern Australia by non-native plant species

Abstract. The invasion by non-native plant species of an urban remnant of a species-rich Themeda triandra grassland in southeastern Australia was quantified and related to abiotic influences. Richness and cover of non-native species were highest at the edges of the remnant and declined to relatively uniform levels within the remnant. Native species richness and cover were lowest at the edge adjoining a roadside but then showed little relation to distance from edge. Roadside edge quadrats were floristically distinct from most other quadrats when ordinated by Detrended Correspondence Analysis. Soil phosphorus was significantly higher at the roadside edge but did not vary within the remnant itself. All other abiotic factors measured (NH₄, NO₃, S, pH and % organic carbon) showed little variation across the remnant. Non-native species richness and cover were strongly correlated with soil phosphorus levels. Native species were negatively correlated with soil phosphorus levels. Canonical Correspondence Analysis identified the perennial non-native grasses of high biomass as species most dependent on high soil nutrient levels. Such species may be resource-limited in undisturbed soils. Three classes of non-native plants have invaded this species-rich grassland: (1) generalist species (> 50 % frequency), mostly therophytes with non-specialized habitat or germination requirements; (2) resource-limited species comprising perennial species of high biomass that are dependent on nutrient increases and/or soil disturbances before they can invade the community and; (3) species of intermediate frequency (1–30 %), of low to high biomass potential, that appear to have non-specialized habitat requirements but are currently limited by seed dispersal, seedling establishment or the current site management. Native species richness and cover are most negatively affected by increases in non-native cover. Declines are largely evident once the non-native cover exceeds 40 %. Widespread, generalist non-native species are numerous in intact sites and will have to be considered a permanent part of the flora of remnant grasslands. Management must aim to minimize increases in cover of any non-native species or the disturbances that favour the establishment of competitive non-native grasses if the native grassland flora is to be conserved in small, fragmented remnants.     

应用遥感技术研究土地利用/土地覆盖变化及其对土壤侵蚀过程的影响--以印度北部Pali Gad山地流域为例

应用遥感技术评估了印度北部Pali Gad山地流域过去几十年里土地利用／土地覆盖变化及其造成的土壤侵蚀程度,并基于摩根参数模型（Morgan Parametric Model）的方法来测定土壤的侵蚀程度;结果表明,由于不同的坡向受到太阳光照的不同可以引起土地覆盖的变迁;海拔和坡度已不再是阻碍人们获取自然资源的因素,人们的活动范围正转移到更高的海拔和更陡峭的坡度;揭示了土地利用／土地覆盖变化对土壤侵蚀进程有着直接的影响。     

Competition for nitrogen between plants and soil microorganisms

Experiments suggest that plants and soil microorganisms are both limited by inorganic nitrogen, even on relatively fertile sites. Consequently, plants and soil microorganisms may compete for nitrogen. While past research has focused on competition for inorganic nitrogen, recent studies have found that plants/mycorrhizae in a wide range of ecosystems can use organic nitrogen. A new view of competitive interactions between plants and soil microorganisms is necessary in ecosystem where plant uptake of organic nitrogen is observed.     

Global aspects of C/N interactions determining plant-environment interactions

The atomic C:N ratio in photolithotrophs is a function of their content of nucleic acids, proteins, lipids, polysaccharides, and other organic materials, and varies from about 5 in some protein-rich microalgae to much higher values in macroalgae and in higher plants with relatively more structural and energy storage materials. These differences in C:N ratios among organisms means that there is more N assimilation by photosynthetic organisms in the oceans than on land despite the near equality of global photosynthetic C assimilation rates in the two environments. Aquatic organisms obtain inorganic C and inorganic N from the surrounding water. Terrestrial photolithotrophs obtain inorganic C, dinitrogen (by diazotrophy) and some combined N from the atmosphere, with the remaining combined N coming from the soil. The nitrogen cost of growth (biomass production rate per unit plant N) varies with the C:N ratio and specific growth rate of the organism. The C:N ratio of plants can be increased with no, or minimal, decrease in growth rate by switching from N-containing to N-free solutes involved in, for example, UV-B screening or by reducing the content of particular proteins. The water cost of growth (water lost per unit biomass gain) in terrestrial plants is a function of N supply and of C supply; water cost is lower with higher N and C availability. Water supply is also important in determining denitrification rates on land and on N (and C) fluxes from terrestrial to aquatic systems.     

Interactions between annuals and woody perennials in a Western Australian nature reserve

Abstract. We studied the interactions between woody perennial species and native and non-native annual species in a number of vegetation types within a nature reserve in the Western Australian wheatbelt. In particular, we examined the responses of annuals to perennial canopy removal, fire, soil disturbance and nutrient additions, and the effects of removal of annuals on perennial seedling regeneration. Experimental shrub removal significantly increased the abundance of annuals in a dense shrubland dominated by Allocasuarina campestris, but had no effect in a more open species-rich sandplain heath. Soil disturbance and nutrient addition in the heath area had no significant influence on annual abundance until three years after treatment. Fire had no clear effect on annual abundance in the heath within the reserve, but promoted a large increase in non-native species within an adjacent roadverge. A pattern of increased soil nutrient levels was accompanied by greatly increased non-native annual abundance beneath individual trees of Santalum spicatum. Exploratory laboratory bioassay experiments indicated that several woody perennials produced leachates that were capable of reducing the germination or growth of the introduced grass Avena fatua, indicating that allelopathy may be an important component of the interaction between the annual and perennial components. Within a woodland community, fire temporarily reduced the abundance of annual species and increased the establishment of perennial seedlings. Field experiments showed that annuals significantly reduced the survival of seedlings of the shrub Allocasuarina campestris. Our results indicate that intact native vegetation canopies effectively prevent invasion by non-native annuals, and that regeneration by native perennials is likely to be inhibited by the presence of an abundant annual cover.