Demographic consequences of drought in the herbaceous perennial Cryptantha flava: effects of density,associations with shrubs,and plant size

The demographic consequences of a severe drought year were examined for two experimental plantings of the herbaceous desert perennial Cryptantha flava(Boraginaceae) in northeastern Utah, United States. A total of 6680 nutlets were planted individually or in clusters of four both under shrubs and in open microhabitats within two natural populations. Survival, growth, and flowering as a function of density and microhabitat were followed for 7 years, including 1 year when precipitation just before and during the growing season was 74.5% below normal. The design permitted assessment of how intraspecific density and shrub cover affect demographic response to drought. Mortality increased and flowering decreased dramatically during drought but neither varied with density or between shrub and open microhabitats. For plants growing under shrubs, survival (at Site 1) and growth (at Site 2) varied with shrub species. Average aboveground plant size also decreased during drought. Population size hierarchies were rearranged because larger plants lost leaf rosettes while many smaller plants grew. Density and microhabitat affected plant performance in non-drought years but more often at Site 1 than at Site 2. Individuals growing alone often were more likely to flower and/or produced more inflorescences when they did flower than did individuals growing with at least one other plant. However, for 2 years, survival rates at Site 1 were higher for plants growing in clumps than for single individuals. Shrubs also had mixed effects on plant performance. In some years, survival was higher under shrubs, but at Site 1 plants in the open often were more likely to flower and/or produced more inflorescences. Thus despite severe demographic consequences of drought, the study provided no evidence that intraspecific competition, interference by shrubs, or facilitation by shrubs increases under limited soil water.     

Pinus monophylla establishment in an expanding Pinus‐Juniperus woodland: Environmental conditions,facilitation and interacting factors

Abstract. The tree species comprising Pinus‐Juniperus woodlands are rapidly expanding into shrub‐grasslands throughout their range. Observational studies indicate that establishment is facilitated by nurse plants, but little information exists on the mechanisms involved. I examined both abiotic and biotic factors influencing Pinus monophylla establishment in Artemisia tridentata steppe with expanding populations of P. monophylla and Juniperus osteosperma. I determined soil water contents, temperatures, and nutrient characteristics for the primary establishment microhabitats, i.e. under Pinus, under Juniperus, under Artemisia, tree interspace and sage interspace, and evaluated the emergence and survival response of two seedling cohorts over a 3‐yr period for the different microhabitats. I also examined the effects of seed burial and predation on seedling establishment. Microhabitats under trees and shrubs had higher extractable P and K, higher organic matter, total nitrogen and cation exchange capacity than interspace microhabitats. Soil water contents (0–15 cm) were lower in interspaces than under shrubs or trees due to dry surface (0–5 cm) soils. Soil temperatures (at 1 and 15 cm) were lowest under trees, intermediate under shrubs, and highest in interspaces. Timing and rate of seedling emergence were temperature dependent with the order of emergence paralleling mean growing season temperatures: tree interspace = shrub interspace > under shrub > under Juniperus under Pinus. Seed burial was required for rooting and the highest emergence occurred from depths of 1 and 3 cm indicating that caching by birds and rodents is essential and that animals bury seeds at adequate if not optimal depths for emergence. Seedlings required microenvironmental modification for survival; all seedlings, including those that emerged from seeds and transplants, died within the first year in interspace microhabitats. Survival in under‐tree or under‐shrub microhabitats depended on soil water availability and corresponded closely to soil water contents over the 3‐yr study. Under‐shrub microhabitats had more favourable soil and micro‐environmental characteristics than under‐tree microhabitats and had the highest seedling life spans for the first‐year seedling cohort. Predation of Pinus seedlings by rodents was a significant cause of mortality with caged transplants exhibiting life spans that were 74% longer overall than uncaged transplants. Emergence and survival of P. monophylla within the expanding woodland were dependent upon a complex set of interacting factors including growing season conditions, microhabitat characteristics, and animal species.     

Use of Shrubs as Nurse Plants: A New Technique for Reforestation in Mediterranean Mountains

Common techniques currently used for afforestation in the Mediterranean basin consider the pre‐existing vegetation (mainly shrubs) as a source of competition for trees, and consequently it is generally eliminated before planting. Nevertheless, it has been demonstrated that woody plants can facilitate the establishment of understory seedlings in environments that, like the Mediterranean area, are characterized by a pronounced dry season. In this study, we experimentally analyze the usefulness of shrubs as nurse plants for afforestation of two native conifers, Pinus sylvestris L. (Scots pine) and Pinus nigra Arnold (black pine). Two‐year‐old seedlings were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method used in afforestation programs), (2) under individuals of Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Pine survival was remarkably higher when planted under individuals of the shrub S. lavandulifolia (54.8% for Scots pine, 81.9% for black pine) compared with open areas (21.5% for Scots pine, 56.8% for black pine; chi square, p < 0.05). The survival of both pines was also higher when planted on the north side of spiny shrubs, although the survival on the south side was similar to that found in open areas. In addition, pine growth was not inhibited when planted in association with shrubs. This pattern appears to result from the combination of abiotic conditions imposed by the presence of a nurse shrub, which leads to improvement in seedling water status and therefore reduced summer mortality by drought. The results show that the use of shrubs as nurse plants is a technique that offers both economic and ecological advantages, in terms of savings in labor and plant material and reduced and even negligible impact on the pre‐existing vegetation.     

Short delay in timing of emergence determines establishment success in Pinus sylvestris across microhabitats

BACKGROUND AND AIMS: The date of emergence may have far-reaching implications for seedling performance. Seedlings emerging early in the growing season often have a greater rate of survival or grow better if early emergence provides advantages with respect to an environmental cue. As a result, the benefits of early emergence may be lost if the environmental stress creating the differences among cohorts disappears. The experimental manipulation under field conditions of the factors that constitute the main sources of stress for seedling establishment is thus a straightforward method to evaluate the impact of date of emergence on seedling establishment under realistic conditions. METHODS: Two field experiments were performed to analyse the effect of emergence date on survival and first-year growth of Scots pine seedlings in natural mountain forests in south-east Spain. Two main environmental factors that determine seedling success in these mountains were considered: (1) microhabitat type (monitoring the effect of date of emergence in the three most common microhabitats where seedlings recruit); (2) summer drought (monitored by an irrigation treatment with control and watered sampling points). KEY RESULTS: Overall, early emergence resulted in a higher probability of survival and better growth in the two experiments and across microhabitats. However, the reduction in summer drought did not diminish the differences observed among cohorts: all cohorts increased their survival and growth, but early cohorts still had a clear advantage. CONCLUSIONS: Date of emergence determines establishment success of Pinus sylvestris seedlings, even if cohorts are separated by only a few days, irrespective of the intensity of summer drought. The experimental design, covering a gradient of light intensity and soil moisture that simulates conditions of the regeneration niche of Scots pine across its geographical range, allows the results to be extrapolated to other areas of the species. Date of emergence is thus likely to have a large impact on the demography of Scots pine across its geographical range.     

Restoration of Dry Afromontane Forest Using Pioneer Shrubs as Nurse-Plants for Olea europaea ssp. cuspidata

Shrubs are often considered competitive barriers for seedlings planted in reforestation programs, although they can facilitate tree recruitment, especially in ecosystems under high abiotic stress. An alternative reforestation technique using pioneer shrubs as nurse‐plants for Olea europaea ssp. cuspidata was tested in exclosures in northern Ethiopia. Seedlings were planted in three different microhabitats, and their survival was monitored. The microhabitats were bare soil patches between shrubs, patches under the dominant shrub Acacia etbaica, and patches under Euclea racemosa, an evergreen shrub, which supports the majority of naturally established Olea recruits. The ability of shrubs to offer protection against browsing goats was tested experimentally. Controlled shading was used to determine whether solar irradiation causes seedling mortality in environments without water stress. Data were analyzed using Kaplan–Meier survival analysis, Kruskal–Wallis analysis of variance (ANOVA), and one‐way ANOVA. Olea survival was significantly higher and shoot damage by goats was lower when planted under shrub cover compared to bare soil patches, particularly under Euclea canopies, although high shade levels reduced seedling performance. Reduction of solar radiation by shrub canopies and thus control of soil–water evaporation and seedling transpiration most likely controlled the observed facilitation. Planting under shrubs may increase seedling survival and assist regeneration of dry Afromontane vegetation. Preserving pioneers also reduces soil erosion and conserves biodiversity. Excluding livestock is essential for Olea woodland restoration and allows persistent but morphologically modified Olea shrubs to develop vigorous regrowth. Facilitative processes are guiding principles for assisted forest restoration, but above‐average rains may be critical to restore higher biomass levels in semiarid areas.     

Importance of microhabitat and acorn burial on Quercus ilex early recruitment: non-additive effects on multiple demographic processes

This study investigates the effect of microhabitat and seed burial on the main demographic processes operating during the early recruitment of Quercus ilex, such as postdispersal seed predation, seed germination, and seedling emergence, survival and growth. The effect of burial was positive over all the processes analysed in this study, since predation rate was lower (63.6% vs. 88%), whereas germination (53.1% vs. 21.8%) and emergence (32.0% vs. 5.5%) were higher for buried acorns. The quality of some microhabitats remained similar throughout the stages and processes studied. Thus, afforestation provided especially suitable microhabitats for oak establishment, since seed predation was lower, while germination, emergence and seedling survival were higher, than in any other microhabitat. By contrast, the quality of some microhabitats, such as open sites and Holm oaks, differed between recruitment stages. Acorns in open sites escaped predation and germinated easily, but most seedlings died due to summer drought. Similarly, although acorns under Holm oaks can germinate and survive drought, they cannot survive to postdispersal predators. This uncoupling results in a post-dispersal change in the spatial distribution of Q. ilex recruits. Furthermore, there were significant interactions between burial and microhabitat for some demographic processes. The recruitment was in afforestations high irrespective of burial, suggesting that burial is not as beneficial in high-quality habitats as it is in lower-quality ones. An accurate understanding of plant recruitment requires the determination not only of the direct effects of limiting factors but also the potential interactions occurring between them.     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

Comparative drought-resistance of seedlings of 28 species of co-occurring tropical woody plants

Quantifying plant drought resistance is important for understanding plant species' association to microhabitats with different soil moisture availability and their distribution along rainfall gradients, as well as for understanding the role of underlying morphological and physiological mechanisms. The effect of dry season drought on survival and leaf-area change of first year seedlings of 28 species of co-occurring woody tropical plants was experimentally quantified in the understory of a tropical moist forest. The seedlings were subjected to a drought or an irrigation treatment in the forest for 22 weeks during the dry season. Drought decreased survival and growth (assessed as leaf-area change) in almost all of the species. Both survival and leaf-area change in the dry treatment ranged fairly evenly from 0% to about 100% of that in the irrigated treatment. In 43% of the species the difference between treatments in survival was not significant even after 22 weeks. In contrast, only three species showed no significant effect of drought on leaf-area change. The effects of drought on species' survival and growth were not correlated with each other, reflecting different strategies in response to drought. Seedling size at the onset of the dry season had no significant effect on species' drought response. Our study is the first to comparatively assess seedling drought resistance in the habitat for a large number of tropical species, and underlines the importance of drought for plant population dynamics in tropical forests.     

Restoring Quercus pyrenaica forests using pioneer shrubs as nurse plants

Question: How to improve reforestation success of Quercus pyrenaica. Location: 1800 m a.s.L, southern Spain. Methods: One‐year‐old Quercus pyrenaica seedlings were planted using two treatments: (1) bare soil, using a 30‐cm diameter augur bit (conventional technique) and (2) under the canopy of a pioneer shrub, Salvia lavandulifolia, using a 12‐cm diameter augur bit. Survival and growth were monitored for six years. Our hypothesis is that the use of shrubs as nurse plants is an alternative technique of reforestation with higher success than traditional techniques, in which pre‐existing vegetation is usually considered a source of competition. The rationale for the study was that for environments with a dry season, pre‐existing vegetation buffers summer drought stress, ameliorates the water status of seedlings and thus usually increases seedling recruitment. Results: Quercus survival was 6.3 × higher when planted under individuals of the pioneer shrub as compared to open areas. Quercus seedlings under shrubs also had shoots 1.8 X longer, while the number of shoots per plant did not differ among treatments. The first summer was the period with the highest mortality (49.1% of seedlings). Summer drought was the main cause of mortality. Conclusions: The use of shrubs as nurse plants for Q.pyrenaica reforestation is a viable technique to increase establishment success. The technique could be similarly useful in other environments with a dry period and for other Quercus species. In addition, this technique offers the advantage of following natural succession, thus minimizing the impact in the community.     

Benefits of Using Shrubs as Nurse Plants for Reforestation in Mediterranean Mountains: A 4-Year Study

Shrubs are commonly considered competitors of planted seedlings in reforestation programs. However, shrubs can facilitate the establishment of understory seedlings in environments that, like Mediterranean‐type ecosystems, are characterized by harsh environmental conditions. In 1997, an experiment was set up in the Sierra Nevada mountains (southeast Spain) to test the use of shrubs as nurse plants for an alternative reforestation technique. Two‐year‐old seedlings of Pinus sylvestris and Pinus nigra were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method employed in reforestation programs), (2) under individuals of the shrub Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Seedlings were also distributed in plots with and without ungulates to test the effect of herbivore damage. We report here the results of survival and growth after four growing seasons, a time span long enough to draw robust conclusions concerning the suitability of this technique. Pine survival was remarkably higher when planted under individuals of S. lavandulifolia as compared with open areas (2.6 times for P. sylvestris and 1.8 for P. nigra). The survival of both pine species was also higher when planted on the north side of spiny shrubs, while mortality on the south side was similar to that found in open areas. The reduction of solar radiation by the canopy of shrubs was likely the main factor determining shrub facilitation. The growth of the pines differed among years. However, growth was not inhibited when planted with shrubs as compared with open areas in any of the years. Herbivore damage was low but was mostly concentrated in the leader shoot, exacerbating the deleterious effect of ungulate herbivores on pine growth. We conclude that the use of shrubs as nurse plants for reforestation is a viable technique to increase establishment success of reforestation in Mediterranean‐type ecosystems and that it might be similarly useful in other water‐stressed environments. In addition, this technique offers the advantage of following natural succession, thus minimizing the impact in the community.     

Sporadic rainy events are more critical than increasing of drought intensity for woody species recruitment in a Mediterranean community

The understanding of the impact of extreme climatic events under a global climate change scenario is crucial for the accurate forecast of future plant community dynamics. We have experimentally assessed the effect of drier and wetter summer conditions on the recruitment probabilities and the growth of seedlings from eight woody species representative of the most important functional groups in the community, pioneer shrubs, mid-successional shrubs and trees, across the main habitats in the study area (open habitat, shrubland, and forest). Our hypothesis proposes that wet summer conditions would represent a good opportunity for tree species regeneration, enhancing both forest maintenance and expansion. A drier summer scenario, on the other hand, would limit forest regeneration, and probably hinder the colonization of nearby habitats. We found a habitat effect on the emergence, survival, and final biomass, whereas different climate scenarios affected seedling survival and biomass. A wet summer boosted growth and survival, whereas greater drought reduced survival only in some cases. These results were modulated by the habitat type. Overall, shrub species presented higher survival and growth and were less affected by more severe drought, whereas some tree species proved to be extremely dependent on wet summer conditions. We conclude that the reduction in frequency of wet summers predicted for the coming decades in Mediterranean areas will have greater consequences for species recruitment than will increased drought. The different response of the species from the various functional groups has the potential to alter the composition and dominance of future plant communities.     

<Emphasis Type="Italic">Pinus nigra</Emphasis> Arn. ssp <Emphasis Type="Italic">salzmannii</Emphasis> early recruitment and initial seedling growth in warmer and drier locations: the role of seed and soil provenance

Current decreasing precipitation and increasing temperatures promote the likelihood of extreme drought events and may alter the recruitment capacities of tree species. Spanish black pine (Pinus nigra ssp. salzmannii) initial recruitment is being one of the most affected pine species by changing conditions with alterations in the future species distribution. In this context, a cross-exchange experiment was implemented using an outdoor nursery located in a warmer and drier location for testing different Spanish Black pine seeds and soil provenance combinations in relation to early recruitment and initial seedling growth. Soil and seeds were collected at a high (HA, 1641 m.a.s.l.) and low (LA, 1099 m.a.s.l.) altitude in Cuenca Mountains (Spain). Then, a cross-sown experiment using HA and LA soils and seeds was set up in an outdoor nursery, which is located in Albacete (704 m.a.s.l.). Soil quality, seedling emergence, seedling survival, initial seedling growth and total seedling dry mass were measured after one year. We found higher seed emergence and seedling survival by combining LA soil with LA seeds or HA soil with HA seeds. Seedlings from LA seeds with both soil origins and seedlings from HA seeds with LA soils allocated more biomass to roots than seedlings from HA sites growing in HA soils under drier and warmer conditions. These results support the idea that autochthonous provenances have the potential to adapt to changing climatic conditions in their habitats.     

Fine-scale spatial heterogeneity and seed size determine early seedling survival in a desert perennial shrub (<Emphasis Type="Italic">Ericameria nauseosa:</Emphasis> Asteraceae)

In heterogenous landscapes, seeds settle in some microsites more readily than others, independently of whether those microsites are suitable for germination and subsequent growth, and survival of seedlings. Wild seeds and seedlings of Ericameria nauseosa var. oreophila in the sand-dune ecosystem near Mono Lake, California show both concordance and conflict in where seeds are retained and where seedlings flourish. Using a field experiment, we followed performance of seeds and seedlings (i.e., the rate of germination and successful emergence, initial seedling size, seedling growth and survival over first growing season), by planting seeds of known sizes in pre-identified microsites (windward and lee aspects of interspaces between shrubs, under shrub canopies, and under snag canopies, respectively). Heavier seeds were more likely to germinate and emerge successfully and resulted in initially larger seedlings than lighter seeds. These initially larger seedlings subsequently remained larger over the growing season and lived longer than initially smaller seedlings. Independent of seed and initial seedling size, seedlings that germinated under the canopy of adult shrubs grew larger and survived longer than seedlings that germinated in interspaces (open space with little or no vegetation) or under snags (dead adult shrubs). Seedlings grown under the canopy of adult shrubs experienced significantly less solar radiation and wind-deposited sand than seedlings grown in interspace or snag microsites. Sand burial more than 1 cm was lethal for seedlings. The two variables of seed size and microsite type had by far greater impact on eventual growth and survival of seedlings than did aspect, and the effects of the former two variables were independent of each other. This study amplifies the body of work on E. nauseosa at this site demonstrating that the earliest events in the life history of this long-lived perennial shrub appear to persist through time despite the harsh and variable environment.     

Effects of the time of drought occurrence within the growing season on growth and survival of Pinus ponderosa seedlings

Key message

A drought event during spring produces a stronger and long lasting decrease in growth of ponderosa pine seedlings than a summer drought event. However, survival is not differentially affected.

Abstract

Although there is certainty about the increasing frequency of extreme climatic events, the consequences of changing patterns of drought events within the growing season on the growth and survival of different species are much less certain. In particular, little knowledge is available on the differential effect on tree seedlings of a drought event at different times within the growing season. The objective of this study was to quantify the effect of a drought event imposed at different times over the growing season on the growth, survival and some related morphological and physiological variables of Pinus ponderosa seedlings from two seed sources. Four treatments were applied: control conditions; spring drought; summer drought and spring plus summer drought (SpSuD). A drought event in spring reduced stem growth and biomass accumulation in ponderosa pine seedlings during the occurrence of the drought and afterwards, even when plant water status had recovered. The lack of growth recovery could not be associated with loss of stem hydraulic conductivity or reduction in stomatal conductance after drought. However, the spring drought did not differentially affect plant survival, as was the case with prolonged drought in the SpSuD treatment. The summer drought event had a significant but much smaller impact on plant growth. Our results suggest different consequences of a drought event in spring or in summer in ponderosa pine seedlings. This knowledge may be relevant to understand and predict tree seedlings responses to changing patterns of drought events within the growing season in the framework of climatic change.     

Factors limiting early establishment of the Mediterranean grassland species Brachypodium retusum at disturbed sites

In Mediterranean dry grasslands, water availability and grazing strongly influence plant establishment. Although higher water availability and lower grazing pressure usually increase seedling recruitment in the beginning, higher competition may reduce seedling survival. In our study, we analysed this trade-off for the dominant species of Western Mediterranean steppes Brachypodium retusum which hardly recolonises restoration sites degraded by soil disturbance such as arable use.A split-plot experiment was set up on an ex-arable site in the “La Crau” steppe in order to test the effects of initial watering and sheep grazing on seedling recruitment, survival and early growth during two years. The watering treatment was applied in the first autumn and spring doubling the usual precipitation during this period and grazing was tested using exclosures. Survival and growth of seedlings established from field-sown seeds were compared to those of transplanted seedlings pre-grown in a growth chamber.Watering did not affect germination whereas the grazing effect was negative. Initial watering had a positive effect on survival only in grazed plots suggesting a compensatory effect under grazing disturbance. Significant main effects of watering on early growth disappeared in the second season. Grazing significantly reduced most measured growth traits whereas the effect on survival was only marginally significant. Planted seedlings (49%) survived better than seedlings emerged from field-sown seeds (36%). In conclusion, initial fencing increases establishment success of B. retusum in ecological restoration. Initial watering may help to mitigate negative grazing effects when fencing is not possible.     

毛乌素沙地优势灌丛对两种松树幼苗的保育作用

选择毛乌素沙地优势灌丛柠条(Caragana intermedia)、紫穗槐(Amorpha fruticosa)和沙柳(Salix psammophila)作为保育植物,选择2年生油松(Pinus tabulaeformis)和樟子松(Pinus sylvestris)幼苗作为目标植物,将其分别种植在开阔地、柠条、紫穗槐和沙柳4种微生境下,连续3a监测油松和樟子松幼苗的存活和生长动态,从而确定毛乌素沙地优势灌丛是否对植物幼苗的定居有促进作用。结果表明:柠条、紫穗槐和沙柳冠层下光照强度、大气温度和土壤温度都显著低于开阔地,致使土壤湿度增高,油松幼苗总存活率均显著高于开阔地,而樟子松幼苗总存活率仅紫穗槐冠层下显著高于开阔地(P0.05)。紫穗槐冠层下油松和樟子松幼苗总存活率均最高(P0.05),外来种紫穗槐可选为保育植物。保育灌丛下松树幼苗的主枝生长良好,并未受到抑制,但第1年主枝生长长度最小。光照强度和土壤湿度是决定主枝生长长度的重要因素。保育灌丛高度和冠幅面积与油松和樟子松幼苗存活率呈显著正相关(P0.05)。固氮灌丛(紫穗槐和柠条)对松树幼苗存活有持续的促进作用,但柠条冠幅结构紧密,可能因为对光的竞争,减弱了促进作用。沙柳灌丛下松树幼苗的存活率较低。油松幼苗具有一定程度的耐阴性,樟子松幼苗喜光性强,保育灌丛下油松幼苗存活率显著高于樟子松幼苗(P0.05)。总的来说,保育灌丛对松树幼苗的定居有促进作用,保育植物技术可作为一种有效的恢复措施,应用在毛乌素沙地的植被恢复中来。但是,保育灌丛的选择要考虑是本土种或外来种,以及灌丛的高度、冠幅大小、冠幅结构和化学效应,恢复物种的选择要考虑其自身的生物学特性及其与保育植物的相互作用。     

Regional and local variation in seedling emergence,mortality and recruitment of a perennial herb in Mediterranean mountain habitats

Spatial patterns of seedling regeneration of woody Mediterranean plants are widely documented, with seedlings and juveniles growing almost exclusively under shrub and tree canopy. Neighbourhood habitat amelioration and consequent facilitative effects on recruitment have been extensively suggested as the major determinant of such spatial pattern for Mediterranean vegetation. Much less is known on recruitment patterns of perennial herbs. As herbs differ from woody plants in many relevant ecological aspects and in their life cycles, we would expect particularities in their recruitment dynamics. We analysed the spatial (regional, local and fine-scale) variation in environment (herb cover, litter depth, air temperature and irradiance) and its relationship with seedling emergence, mortality, and recruitment of the perennial herb Helleborus foetidus (Ranunculaceae) during 2 years. We conducted the study in three distant localities in south-eastern Iberian Peninsula (defining the regional scale). Local scale was defined in terms of microhabitat type based on three categories of vegetation cover (under evergreen shrubs or trees, under deciduous spiny-shrubs, and in open interspaces). Finally, fine-scale variation was defined in terms of 1 m² sampling plots. Our results showed regional, local and fine-scale variations in recruitment, with the major source of variation changing across scales. Seed input was important in determining differences in recruitment both at the largest (regional) and the finest scale. Environment had minor importance in shaping differences in recruitment at these two scales. At the local scale, variation between microhabitats was mainly related to differences in seedling survival through facilitative effects (alleviation of water and irradiance stresses) of shrub and tree cover, although such facilitative effect was not consistent in all localities or years. Our study thus points to a similarity in the spatial patterns of recruitment (and in the environmental factors shaping such patterns) between Mediterranean woody plants and perennial herbs, but much more effort is needed to assess the generality of such similarity.     

Seedling dynamics of some cotton grass tussock tundra species during the natural revegetation of small disturbed areas

The dynamics of seedling establishment, and growth of native cotton grass-tussock tundra species were studied during the natural revegetation of small bare areas excavated in tussock tundra. The seedlings of Eriophorum vaginatum spp. spissum and Carex bigelowii , two sedge species, established most successfully. Few seedlings of nonsedge species emerged with the small disturbances. Most seedling emergence occurred early in the first growing season following the excavation of the bare areas. The density of emerged seedlings was higher in bare areas excavated in the autumn preceding the observations, rather than in bare areas excavated at the start of the first season of observation. After two growing seasons, survival of seedlings was not affected by the time the seedling emerged within the growing season. Eriophorum vaginatum was the most abundant species because it: (1) had the highest seedling emergence rates, particularly early in the growing season: (2) produced more tillers per seedlings; and (3) had a higher growth rate per seedling than C. bigelowii .     

Microhabitat distribution of two Florida scrub endemic plants in comparison to their habitat-generalist congeners

Habitat-specialist species may be restricted to a narrower range of microhabitats than habitat-generalist species. We addressed this hypothesis by comparing microhabitats of two pairs of congeners that differ in habitat specificity and co-occur in one distinct habitat type, Florida rosemary scrub. We characterized microhabitats of rosemary scrub specialists, Polygonella basiramia and Lechea cernua, their habitat-generalist congeners, Polygonella robusta and Lechea deckertii, and random points in the rosemary scrub habitat. Plants of both habitat specialists occurred in microhabitats with significantly more bare sand than plants of habitat-generalist species and random points. Plants of all four species occurred in microhabitats that were farther from dominant shrubs, Ceratiola and Quercus spp., than random points. Seedlings of both habitat specialists grew larger in bare sand microhabitats, whereas ground lichens and litter did not affect seedling growth of the habitat generalists. As the time since fire increases, bare sand cover decreases, Ceratiola density increases, Quercus density remains constant, and shrubs become taller. Physical characteristics, such as soil temperature, soil carbon, and soil moisture, differ slightly with respect to microhabitat. Our results suggest that P. basiramia and L. cernua are specialized on bare sand microhabitats that characterize their preferred habitat, rosemary scrub. Microhabitat specialization may limit the distribution of these rare species.     

Quercus ilex recruitment in a drier world: A multi-stage demographic approach

There is a growing interest in understanding and forecasting the responses of plant communities to projected changes of environmental conditions. Multi-stage demographic approaches, where plant recruitment is explored across multiple and consecutive stages, are essential to obtain a whole overview of the consequences of increasing aridity on tree recruitment and forest dynamics, but they are still rarely used. In this study, we present the results of an experimental rainfall exclusion aimed to evaluate the impact of projected increasing drought on multiple stage-specific probabilities of recruitment in a key tree species typical of late-successional Mediterranean woodlands (Quercus ilex L.). We calibrated linear and nonlinear likelihood models for the different demographic processes and calculated overall probabilities of recruitment along a wide range of microhabitat conditions. Rainfall exclusion altered Q. ilex recruitment throughout ontogeny. Seed maturation, seedling emergence and survival and, to a lesser extent, post-dispersal seed survival were the most sensitive demographic processes to decreased rainfall. Interestingly, both the identity of the most critical stages for recruitment and their specific sensitivity to rainfall manipulation depended largely on the yearly pattern of precipitation. The microhabitat heterogeneity strongly determined the success of recruitment in the study species. The experimental increase in drought displaced the peak of maximum overall recruitment towards the low end of the light gradient, suggesting that the dependence on shrubs for an effective recruitment in Q. ilex could be intensified under future environmental scenarios. In terms of phenotypic plasticity, Q. ilex seedlings responded more strongly to light availability than to experimentally increased drought, which could reduce its ability to persist under on-going environmental conditions due to climate change. Results from this study provide a full picture of the ecological and functional consequences of the projected rainfall reduction on tree recruitment and forest dynamics in two years of contrasting precipitation.