Differential responses of native and exotic coastal sage scrub plant species to N additions and the soil microbial community

Background and aims

Native shrub species of southern California have a long history of displacement by exotic annual herbs and forbs. Such invasions may be mediated by interactions with the microbial community and changes in the N cycle as a result of N pollution. However, the simultaneous effects of the soil microbial community status and N fertilization on dominant native and exotic plant species growth have not been thoroughly explored in this ecosystem.

Methods

Three species of native shrubs and of exotic annuals were grown in an orthogonal two-factor greenhouse experiment. To assess the importance of the soil microbial community pre-sterilized soils were inoculated with sterilized or non-sterilized field soil; to assess the importance of N type pots were fertilized with nitrate, ammonium or glycine solutions. Plant shoot and root biomass was measured after harvesting.

Results

The natives Artemisia californica and Eriogonum fasciculatum had lower growth in sterilized soil, suggesting microbial facilitation of these species, and E. fasciculatum higher growth with ammonia than either nitrate or glycine. Salvia apiana had equal growth under all conditions. The exotics Brassica nigra and Bromus madritensis grew equally in sterilized and unsterilized soil, and B. madritensis greater growth with ammonia fertilizer. Centaurea melitensis had greater growth in sterilized soil, and with either form of inorganic N.

Conclusions

These results highlight the importance of the soil microbial community in contributing to relative success of native vs. exotic species, and could inform restoration approaches for these species.     

Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m² were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m² in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m² in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low. At time of research, Robert D. Cox was graduate student.     

Drought in Southern California coastal sage scrub reduces herbaceous biomass of exotic species more than native species,but exotic growth recovers quickly when drought ends

Plant Ecology - Semi-arid regions with Mediterranean-type climates harbor exceptional biodiversity, but are increasingly threatened by invading exotic annual species and climatic changes, including...     

Distribution and conservation status of coastal sage scrub in southwestern California

Abstract. A landscape-based characterization of vegetation has been developed for southwestern California using satellite imagery, air photos, existing vegetation maps, and field data. Distribution maps of nine dominant coastal scrub species and 13 species assemblages that were identified by divisive information analysis have been analyzed to quantify spatial patterns of species co-occurrence. Three general distribution patterns are identified that suggest the Diegan, Ventaran and Riversidian Associations identified by other workers. Vegetation data have also been related to land ownership and management to assess the conservation status of upland plant communities. A large proportion of the mapped distribution of species and vegetation types is on private land, and several taxa show less than 4 % of mapped distribution in nature reserves. The analysis highlights the need to extend current conservation planning efforts into the northern part of the region to encompass areas where Salvia leucophylla is a frequent community dominant.     

Alternative causes of edge-abundance relationships in birds and small mammals of California coastal sage scrub

Changes in the distribution and abundance of bird and small mammal species at urban-wildland edges can be caused by different factors. Edges can affect populations directly if animals respond behaviorally to the edge itself or if proximity to edge directly affects demographic vital rates (an "ecotonal" effect). Alternatively, urban edges can indirectly affect populations if edges alter the characteristics of the adjacent wildland vegetation, which in turn prompts a response to the altered habitat (a "matrix" or "habitat" effect). We studied edge effects of birds and small mammals in southern Californian coastal sage scrub, and assessed whether edge effects were attributable to direct behavioral responses to edges or to animal responses to changes in habitat at edges. Vegetation species composition and structure varied with distance from edge, but the differences varied among study sites. Because vegetation characteristics were correlated with distance from edge, responses to habitat were explored by using independently-derived models of habitat associations to calibrate vegetation measurements to the habitat affinities of each animal species. Of sixteen species examined, five bird and one small mammal species responded to edge independently of habitat features, and thus habitat restoration at edges is expected to be an ineffective conservation measure for these species. Two additional species of birds and one small mammal responded to habitat gradients that coincided with distance from edge, such that the effect of edge on these species was expressed via potentially reversible habitat degradation.     

Effects of natural and anthropogenic gradients on native and exotic winter annuals in a southern California Desert

Native annual plant species constitute a large proportion of the plant diversity found in arid vegetation types within the southwestern United States; yet, little is known about controls on diversity patterns along natural and anthropogenic gradients. In this study we evaluated native species richness and exotic species cover across overlapping gradients of precipitation, wind, and N deposition in the Colorado Desert of southern California. Factors allowing native diversity to persist under high N deposition and high wind were also evaluated in a second, focused study at one end of the gradient. We found that gradients in precipitation, nitrogen deposition, and wind were the most important factors to native richness and exotic species cover across the landscape, while local heterogeneity in bare ground influenced richness and cover at the high deposition/windy, or high-disturbance, end of the gradient. Patterns of native diversity were evaluated across the gradients using non-metric multidimensional scaling, which showed diversity was split into two axes: one strongly correlated to precipitation and the other strongly correlated with disturbance factors. The disturbance factors were also positively associated with exotic grass and forb cover. In total, these results indicate that large-scale patterns in disturbance and exotic species cover negatively affect native annual plant species diversity but native species can also persist due to local heterogeneity.     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.     

A comparison of vacuum sampling versus sweep-netting for arthropod biodiversity measurements in California coastal sage scrub

The recent growth of conservation biology has demanded that faster and more effective measures of biodiversity be utilized. Arthropods, due to high levels of diversity and their relative ease of capture, are often the subject of such surveys. The vacuum sampler, used quite often in the context of agricultural arthropod surveys, has never been adequately evaluated or compared to more traditional collection techniques in relatively complex ecosystems. The purpose of this study was to determine whether the vacuum sampler was more or less effective than a sweep-net in measuring arthropod biodiversity in California coastal sage scrub. The results show that significantly more individuals were collected by the vacuum sampler per unit effort for three out of six orders of arthropods examined. In addition, the vacuum sampler collected a significantly greater number of arthropod species than the sweep-net technique for two out of the six orders sampled. There were no significant differences in the number of species collected for the remaining four orders. We feel these findings are important for arthropod biodiversity studies utilized for conservation efforts as the vacuum sampler can attain a level of efficiency and sensitivity (with regard to species detection) that sweep-net techniques cannot. © Rapid Science Ltd. 1998     

Ecophysiology of exotic and native shrubs in Southern Wisconsin

Summary We compared seasonal trends in photosynthesis of two naturalized exotic shrubs (Rhamnus cathartica and Lonicera X bella) and two native shrubs (Cornus racemosa and Prunus serotina) in open and understory habitats in southern Wisconsin. We examined the relationships between resource availability and leaf photosynthetic performance in these four species. All four species had similar relationships between leaf nitrogen (N) content and photosynthetic rate, but the species differed in absolute leaf N content and therefore in photosynthetic rates. Maximum daily photosynthetic rates of all species were significantly correlated with leaf N content in the open habitat, but not in the understory, where low light availability was the major limitation to photosynthesis. Extended leaf longevity was important in the forest understory because it allowed shrubs to take advantage of high light availability at times when the overstory canopy was leafless. Early leaf emergence was more important than late senescence: from 27% to 35% of the annual carbon gain of P. serotina, R. cathartica, and L. X bella occurred prior to leaf emergence of C. racemosa, the species with the shortest leaf life span. Extended leaf longevity of exotic shrubs may help explain their persistence in the understory habitat, but it contributed relatively less to their annual carbon gain in the open habitat.     

California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.     

Ecosystem responses to water and nitrogen amendment in a California grassland

The world's ecosystems are experiencing simultaneous changes in the supply of multiple limiting resources. Two of these, water and nitrogen (N) can strongly limit grassland production and can affect community composition and biogeochemical cycles in different ways. Grassland ecosystems in California may be particularly vulnerable to current and predicted changes in precipitation and N deposition, and ecosystem responses to potential interactive effects of water and N are not well understood. Here, we show strong colimitation of plant production resulting from factorial addition of water and N. In addition, water and N addition in combination led to increased dominance of the two most abundant grass species, while N addition regardless of water availability led to decreased species diversity. Late season carbon (C) flux response to water addition depended on N. Only plots that received additional water, but not N, still showed net ecosystem C uptake at the end of the experiment. Our results suggest that grassland ecosystem response to N deposition will be strongly dependent on future precipitation patterns.     

模拟氮沉降对青藏高原地区油菜幼苗期生长和生理特性的影响

以甘蓝型油菜(青杂303)为研究材料,通过研究分析油菜幼苗期的生长和生理特性,旨在探讨油菜幼苗对青藏高原地区未来大气氮沉降的响应机制。实验以NH4NO3氮肥模拟大气氮沉降的NH4+、NO3-输入并分别喷施0、2.5、5.0 g/m2.a模拟青藏高原地区大气氮沉降增加的0倍、2倍、4倍背景。结果表明:施氮处理对油菜幼苗生长具有促进作用,油菜幼苗的地上部分干重、叶面积和株高等形态指标都发生了显著变化;随着施氮量增加油菜幼苗叶片的抗氧化酶活性增强,氮肥输入量为5.0 g/m2.a的处理组中,其幼苗叶片超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性与对照的相比均具有极显著差异。因此,反映出青藏高原地区生态系统中植物生长受到有效氮的限制,在外界有效氮输入下,对植物生长发育具有促进作用。     

Establishment of native grasses and their impact on exotic annuals in degraded box gum woodlands

Restoration goals often involve the addition of new species to resident, degraded communities but in box gum woodlands such restoration is often constrained by competition from persistent exotic annuals that control critical ecological processes. Nutrient reduction (via carbon addition) and seed bank depletion are two approaches to reduce competition from exotic annuals but to be effective these treatments must allow establishment of species such as native grasses. This experiment was conducted in two degraded Austrostipa understoreys in the box gum woodlands of south‐east Australia. It compares the effects of carbon addition (sugar), seed depletion (spring burning or spring grazing) and combinations of carbon addition and seed depletion treatments on the establishment of C3 and C4 native grasses, and measured the effects of their establishment on soil nitrate concentration and exotic annuals. Treatments that reduced exotic annual abundance did not increase initial germination of the C4 native grasses, Bothriochloa or Themeda. However, sugar increased seedling survival of Themeda and Bothriochloa and grazing increased seedling survival of Bothriochloa, presumably by reducing effects of exotic annuals. Poa and Rytidosperma (C3 native grasses) failed to establish. Although we were unable to detect any reduction in soil nitrate concentration, swards with successful recruitment of C4 grasses suppressed exotic annuals more than the Austrostipa‐only swards at one site (the other was affected by wildfire). Further, Austrostipa‐Themeda swards were more effective than Austrostipa‐Bothriochloa for suppressing exotics, pointing to a role for both functional and species identity in the degree of resistance conferred.     

Seed germination of exotic and native winter annuals differentially responds to temperature and moisture,especially with climate change scenarios

Seeds of winter annuals require a summer after-ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co-occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After-ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after-ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear-cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.     

本地和外来草本物种对水分条件时间异质性的可塑响应

极端气候导致的干旱和水淹事件频发，影响了外来植物和本地植物的生长。为了解外来种和本地种植物对干旱和水淹事件发生顺序的响应，探讨草本植物适应水分时间异质性的策略，该文以美国蒙大拿州西部4种本地植物和4种外来植物为研究对象，将所有植物分别进行持续湿润(对照，CK)、水淹-干旱(I-D)和干旱-水淹(D-I)处理，并观测一系列形态和生物量特征的变化。结果表明：(1)与CK相比，D-I和I-D处理均显著降低了外来种的总生物量(P<0.05)。(2)D-I显著降低了本地种早期总生物量、后期地下生物量和根冠比，但显著提高了其后期的相对生长(P<0.05)。(3)D-I处理显著降低了所有植物的地下-地上生物量关系的异速指数，外来种异速指数显著高于本地种(P<0.05)。综上认为，极端事件(水淹和干旱)的发生顺序能改变外来植物和本地植物的生物量分配，早期干旱比后期干旱更容易减少植物生物量的积累，但能促进本地种后期的生长；本地种在环境胁迫下不被降低的总生物量表现说明维持表型稳定的能力较强；D-I处理下本地种和外来种地上和地下生物量关系的分配方式不同。     

九寨沟两种常见藓类植物对模拟氮沉降的生理响应

为探讨氮沉降对九寨沟藓类植物的影响，该研究以当地优势藓类植物锦丝藓(Actinothuidium hookeri)和塔藓(Hylocomium splendens)为对象，以NH₄NO₃为氮源，设置对照(0 kg N·hm^-2·a^-1)、低浓度(20 kg N·hm^-2·a^-1)、高浓度(50 kg N·hm^-2·a^-1)3种处理，开展为期6个月的氮沉降模拟实验。结果表明：(1)氮沉降处理导致两种藓类植物的活性氧、丙二醛、叶绿素、脯氨酸和可溶性蛋白含量显著增加，同时锦丝藓过氧化氢酶、过氧化物酶、超氧化物歧化酶、抗坏血酸过氧化物酶活性增加。(2)对于生长旺期和生长末期的塔藓，氮沉降导致其过氧化物酶、过氧化氢酶、抗坏血酸过氧化物酶活性降低。(3)锦丝藓的综合隶属函数值随氮沉降浓度增大而增加，在生长旺期和生长末期，塔藓综合隶属函数值对氮沉降的响应存在差异。综上认为，两种藓类植物对氮沉降处理的生理响应存在差异，高浓度氮沉...     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.