Responses of soil biota and nitrogen availability to an invasive plant under aboveground herbivory

Plant and Soil - Recently, much attention has been paid to the plant-mediated effects of aboveground herbivory on soil ecosystems. However, studies about the herbivore-induced effects of invasive...     

Allocation strategies and seed traits are hardly affected by nitrogen supply in 18 species differing in successional status

Species performance depends on ecological strategies, revealed by suites of traits, conferring different relative ecological advantages in different environments. Although current knowledge on plant strategies along successional gradients is derived from studies conducted in situ, actually quantifying these strategies requires disentangling the effects of environmental factors from intrinsic differences between species.Here we tested whether allocation strategies and seed traits differ among successional stages and nitrogen levels. To this aim, we assessed biomass and nitrogen allocations and seed traits variations for 18 species, differing in life history and belonging to three stages of a Mediterranean old-field succession. These species were grown as monocultures in an experimental garden under limiting and non-limiting nitrogen supply.Early successional species allocated allometrically more nitrogen and proportionally more biomass to reproduction, and set more seeds than later successional species. Seed mass increased with successional status and was negatively related to seed number. Early successional species thus produced more but less-provisioned seeds, suggesting better colonization abilities. These patterns were not the sole consequence of the replacement of annuals by perennials along the successional gradient, since comparable trends were also observed within each life history. Allocation patterns were generally not altered by nitrogen supply and the higher nitrogen content in vegetative organs of plants grown under high nitrogen supply was not retranslocated from leaves to seeds during seed development.We therefore conclude that differences in plant ecological strategies in species characteristics from contrasting successional stages appear to be intrinsic properties of the studied species, and independent from environmental conditions.     

Responses of common and successional heathland species to manipulated salt spray and water availability

Coastal sandplain heathlands are a rare plant community in the northeastern United States. Salt spray and water availability are likely important factors determining heathland distribution. Field surveys and manipulative experiments were performed to examine heathland species' responses to salt spray and water availability. We surveyed field distributions of four typical heathland species: Solidago puberula, Solidago rugosa, Gaylussacia baccata, and Myrica pensylvanica. The distributions of two native tree species, Pinus rigida and Quercus ilicifolia, were also surveyed because they succeed into coastal heathlands with low disturbance frequency. We then manipulated salt spray and water in the field and measured species' water status, necrosis, and growth responses to the treatments. Predawn xylem pressure potential and necrosis were strongly affected by high salt spray and low water availability. Shoot elongation was also limited in S. puberula and S. rugosa grown in high salt, low water treatments. Gaylussacia baccata and Q. ilicifolia were particularly sensitive to high salt spray and low water, suggesting that they might excluded be from areas with those conditions. The interaction between salt spray and water availability could affect the landscape scale and should be incorporated into conservation management plans.     

Gap phase regeneration of tree species of differing successional status in a humid tropical forest of Kerala,India

Germination, establishment and growth of seedlings of tree speciesPalaquium ellipticum (primary),Actinodaphne malabarica (late secondary) andMacaranga peltata (early secondary) were studied in a humid tropical forest at Nelliampathy, in the Western Ghats of Kerala. While the primary species completed its germination within a brief period of 1.5 months, at the other extreme, early secondary species showed slow germination extending for about 5 months, the late secondary species falling in between. Although, all the species studied showed higher establishment and growth under gaps, the early secondary species were more responsive compared to the primary species. Primary species showed better establishment in undisturbed sites and natural gaps than under selection felled gaps; the reverse was true for late and early secondary species. Survival of seedlings increased with gap size, but sharply declined with gap age. Shoot/root ratio was consistently higher in the early secondary species than in the primary species.     

Plasticity responses of an invasive macrophyte species to inorganic carbon availability and to the interaction with a native species

The acorn barnacle Perforatus perforatus has a defined breeding temperature range and reproductive season that varies geographically. This study aims to investigate the influence of reproductive parameters of P. perforatus in species distribution ranges in the NE Atlantic. The hypothesis tested is that the breeding season of P. perforatus off NW Portugal begins earlier and is longer than at the northern distribution limit of this species, and that fecundity is higher in terms of number of broods per individual per breeding season. The span of the breeding season and fecundity indices were assessed based on the presence and maturation of ovigerous lamellae and correlated with temperature. Results showed that the breeding season in the NW Portuguese coast lasts over 10 months (February–November) and the number of broods was determined to be 9.2 ind/year. Temperature seems to be a primary factor determining the breeding season, but other factors, such as food availably, light and photoperiod, are also of great importance. However, the higher quantity of embryos produced in NW Portugal is not reflected in a higher abundance of settled adults in rocky shores. Contrarily, the species is particularly abundant in artificial substrata offshore.     

An invasive grass shows colonization advantages over native grasses under conditions of low resource availability

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C₄ perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12?weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2?weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.     

Seedling growth and soil nutrient availability in exotic and native tree species: implications for afforestation in southern China

Background and aims

The relationship between tree species and soil nutrient availability is critical for evaluating plantation succession and promoting forest restoration. This study was conducted to evaluate the impact of exotic and native tress species on soil nutrient availability.

Methods

Four exotic species (Eucalyptus urophylla, E. tereticornis, Acaia auriculaeformis, A. mangium) and four native species (Castanopsis fissa, Schima superba, C. hystrix, Michelia macclurei) were planted and grown for one-year. Soil solution (DOC, DON, NH₄?N, NO₃?N) was sampled and analyzed during the study. After the experiment, soil properties were determined, and plant tissues were analyzed.

Results

DOC levels were greater in soils with trees planted than controls without trees. Compared to native species, exotic species had much faster growth rates and greatly reduced DON and NO₃?N concentrations. Exotic species always had less P concentrations in leaves and stems than native species. Furthermore, N-fixing A. auriculaeformis led to greater soil available P compared to other species.

Conclusions

Based on these findings, we provide some recommendations for afforestation practice. This study highlights that a better understanding of the pros and cons of exotic species would be beneficial to advance afforestation in China and the world.     

Shoot life span in relation to successional status in deciduous broad-leaved tree species in a temperate forest

In trees, leaf life span is closely related to successional status. Although leaves are attached to shoots, shoot life span has been insufficiently studied in the context of ecological systems. Interspecific variation in shoot survivorship was investigated over 27 months in 15 temperate hardwood tree species. Relationships between shoot architecture and shoot survival were also investigated. Shoot life span was shortest in early successional species, and longest in late successional species, in each of the families Betulaceae and Fagaceae. In Salicaceae, all of which were early successional species, shoot life span was longer in mountainous than in riparian species. Early successional or riparian species distributed longer shoots densely, even in proximal positions on mother shoots, resulting in mutual shading and consequent early and massive shoot shedding. By contrast, late successional or mountainous species concentrated shoots in distal positions, allowing shoots to receive equally favorable light, resulting in a longer life span. These results reveal close relationships between shoot life span and environmental resource availability or successional status and suggest a causal relationship between shoot shedding and shoot architecture.     

Phenotypic plasticity of reproductive traits in response to food availability in invasive and native species of nematode

Invasive species cause severe ecological and economic damage; however, the mechanisms underlying their successful invasion often remain elusive. In the case of Bursaphelenchus xylophilus, a global quarantine pest which invaded Asia and Europe, it has been suggested that this species possesses highly competitive abilities, which promotes its establishment and rapid spread. To explore biological traits that may explain its highly competitive abilities, we focused on expression of phenotypic plasticity in response to the food conditions experienced by the females during their development as juveniles in the invasive species B. xylophilus and native species Bursaphelenchus mucronatus. We report an unexpected significant difference of phenotypic trade-off between egg number and egg size in the invasive species B. xylophilus and native species B. mucronatus. This leads to superior propagation ability of invasive species, under high and low food conditions in culture. These effects reflect adaptive optimal resource allocation where more eggs are produced in favorable environments to enhance population viability. Furthermore, we show that B. xylophilus eggs hatched earlier than B. mucronatus when their parents experienced high food availability. Thus, this study revealed, for the first time, phenotypic plasticity of reproductive traits in B. xylophilus which empowers the species a competitive advantage relative to their native counterpart B. mucronatus when they are under different range of food availability. These results are a step towards answering the vital question of how an exotic invasive species exclude a native species from its original niche.     

Root plasticity of native and invasive Great Basin species in response to soil nitrogen heterogeneity

Soil nutrients are heterogeneously distributed in natural systems. While many species respond to this heterogeneity through root system plasticity, little is known about how the magnitude of these responses may vary between native and invasive species. We quantified root morphological and physiological plasticity of co-occurring native and invasive Great Basin species in response to soil nitrogen heterogeneity and determined if trade-offs exist between these foraging responses and species relative growth rate or root system biomass. The nine study species included three perennial bunchgrasses, three perennial forbs, and three invasive perennial forbs. The plants were grown in large pots outdoors. Once a week for 4 weeks equal amounts of ¹⁵NH₄ ¹⁵NO₃ were distributed in the soil either evenly through the soil profile, in four patches, or in two patches. All species acquired more N in patches compared to when N was applied evenly through the soil profile. None of the species increased root length density in enriched patches compared to control patches but all species increased root N uptake rate in enriched patches. There was a positive relationship between N uptake rate, relative growth rate, and root system biomass. Path analysis indicated that these positive interrelationships among traits could provide one explanation of how invasive forbs were able to capture 2 and 15-fold more N from enriched patches compared to the native grasses and forbs, respectively. Results from this pot study suggest that plant traits related to nutrient capture in heterogeneous soil environments may be positively correlated which could potentially promote size-asymmetric competition belowground and facilitate the spread of invasive species. However, field experiments with plants in different neighbor environments ultimately are needed to determine if these positive relationships among traits influence competitive ability and invader success.     

Carbon and nitrogen in soil and vegetation at sites differing in successional age

We studied vegetation and soil development during primary succession in an inland drift sand area in the Netherlands. We compared five sites at which primary succession had started at different moments in the past, respectively 0, 10, 43 and 121 years ago, and a site at which succession had not yet started. In the three younger sites the vegetation was herbaceous, whereas in the two older sites a pine forest had formed. Forest formation was accompanied by the development of an FH-layer in the soil, an increase in the amount of soil organic matter, and an increase in nitrogen mineralisation rate from 1.9 to 18 g N m^–2 yr^–1. Soil moisture content also increased, whereas pH showed a steady decrease with site age. The vegetation changed from a herbaceous vegetation dominated by mosses and lichens and the grass species Corynephorus canescens and Festuca ovina towards a pine forest with an understorey vegetation dominated by Deschampsia flexuosa and, at the oldest site, with dwarf shrubs Empetrum nigrum and Vaccinium myrtillus. At the same time the total amounts of carbon and nitrogen of the ecosystem increased, with a relatively stronger increase of the carbon pool. The establishment of trees during succession greatly affects the dynamics of the ecosystem, especially its carbon dynamics.     

Photosynthesis, photoinhibition, and nitrogen use efficiency in native and invasive tree ferns in Hawaii

Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m^-2 s^-1, and 5.8 and 3.6 µmol m^-2 s^-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (F_v/F_m), while the invasive tree fern recovered 90% and 86% of its dark-acclimated F_v/F_m when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.     

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade‐tolerant and mid‐successional, and Acer mono, shade‐tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F_v/F_m) of detached leaf discs exposed to about 2000 μmol m^?1 s^?1 photon flux density (PFD) for 2 h under controlled conditions (25 to 28 °C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light‐saturated photosynthetic rate (B. platyphylla, 9·87, 5·85 and 2·82; Q. mongolica, 8·05, 6·28 and 4·41; A. mono, 7·93, 6·11 and 5·08 μmol CO₂ m^?1 s^?1for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring ΔF/F′_m on the first day of transfer to natural daylight. Compared with ΔF/F′_m in AM, the lower ΔF/F′_m in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of ΔF/F′_m showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade‐grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.     

Some autecological characteristics of early to late successional tree species in Venezuela

The breadth of the continuum concept of strategy with respect to succession was tested on 21 tree and shrub species common in either unlogged or logged stands, respectively, in the Forest Reserve of Caparo, Venezuela, by examining morphological, physiological and population characteristics. Based on a preliminary abundance analysis, `early', `mid' and `late' successional species as well as `generalists' were distinguished. Early successional species, i.e. Ochroma lagopus, Heliocarpus popayanensis and Cecropia peltata were similar in many autecological aspects, e.g. monolayered leaf arrangement, orthotropic architectural models, no adaptive reiteration, clumped distribution, but differed in gap association and distribution along a drainage gradient. Mid-successional species established themselves both in large and small gaps (> 300 m[sup2 ]; 80–300 m[sup2 ]) and showed a clumped to regular distribution pattern in logged areas; they exhibited more diverse crown and leaf characteristics than early successional species. Late successional species established themselves only in small gaps and understorey, and showed a regular spatial pattern in undisturbed areas. All late successional species displayed architectural models with plagiotropic lateral axes and showed a multilayered leaf arrangement. Adaptive reiteration was a common feature of late successional species which could be further subdivided into large, medium-sized and small trees, indicating different light requirements at maturity. Generalists were common treelet and shrub species in both disturbed and undisturbed sites where they are also capable of completing their life cycle. The light compensation point (LCP) of an individual plant was strongly influenced by its crown illuminance. Large late successional species showed the widest range of LCP values, reflecting the increasing light availability with increasing height in mature forest. On the basis of many autecological characteristics, it was found (i) that there is in fact a continuum of species strategies with respect to succession even among early and mid-successional species and (ii) that the latter group of species showed the widest breadth of autecological traits, reflecting the heterogeneous environment in which they establish and mature.     

Carbon and nitrogen economy of 24 wild species differing in relative growth rate

The relation between interspecific variation in relative growth rate and carbon and nitrogen economy was investigated. Twentyfour wild species were grown in a growth chamber with a nonlimiting nutrient supply and growth, whole plant photosynthesis, shoot respiration, and root respiration were determined. No correlation was found between the relative growth rate of these species and their rate of photosynthesis expressed on a leaf area basis. There was a positive correlation, however, with the rate of photosynthesis expressed per unit leaf dry weight. Also the rates of shoot and root respiration per unit dry weight correlated positively with relative growth rate. Due to a higher ratio between leaf area and plant weight (leaf area ratio) fast growing species were able to fix relatively more carbon per unit plant weight and used proportionally less of the total amount of assimilates in respiration. Fast growing species had a higher total organic nitrogen concentration per unit plant weight, allocated more nitrogen to the leaves and had a higher photosynthetic nitrogen-use efficiency, i.e. a higher rate of photosynthesis per unit organic nitrogen in the leaves. Consequently, their nitrogen productivity, the growth rate per unit organic nitrogen in the plant and per day, was higher compared with that of slow growing species.