Climate warming could shift the timing of seed germination in alpine plants

Background and Aims

Despite the considerable number of studies on the impacts of climate change on alpine plants, there have been few attempts to investigate its effect on regeneration. Recruitment from seeds is a key event in the life-history of plants, affecting their spread and evolution and seasonal changes in climate will inevitably affect recruitment success. Here, an investigation was made of how climate change will affect the timing and the level of germination in eight alpine species of the glacier foreland.

Methods

Using a novel approach which considered the altitudinal variation of temperature as a surrogate for future climate scenarios, seeds were exposed to 12 different cycles of simulated seasonal temperatures in the laboratory, derived from measurements at the soil surface at the study site.

Key Results

Under present climatic conditions, germination occurred in spring, in all but one species, after seeds had experienced autumn and winter seasons. However, autumn warming resulted in a significant increase in germination in all but two species. In contrast, seed germination was less sensitive to changes in spring and/or winter temperatures, which affected only three species.

Conclusions

Climate warming will lead to a shift from spring to autumn emergence but the extent of this change across species will be driven by seed dormancy status. Ungerminated seeds at the end of autumn will be exposed to shorter winter seasons and lower spring temperatures in a future, warmer climate, but these changes will only have a minor impact on germination. The extent to which climate change will be detrimental to regeneration from seed is less likely to be due to a significant negative effect on germination per se, but rather to seedling emergence in seasons that the species are not adapted to experience. Emergence in autumn could have major implications for species currently adapted to emerge in spring.     

Changes in phenological events in response to a global warming scenario reveal greater adaptability of winter annual compared with summer annual arabidopsis ecotypes

Background and AimsThe impact of global warming on life cycle timing is uncertain. We investigated changes in life cycle timing in a global warming scenario. We compared Arabidopsis thaliana ecotypes adapted to the warm/dry Cape Verdi Islands (Cvi), Macaronesia, and the cool/wet climate of the Burren (Bur), Ireland, Northern Europe. These are obligate winter and summer annuals, respectively.MethodsUsing a global warming scenario predicting a 4 °C temperature rise from 2011 to approx. 2080, we produced F₁ seeds at each end of a thermogradient tunnel. Each F₁ cohort (cool and warm) then produced F₂ seeds at both ends of the thermal gradient in winter and summer annual life cycles. F₂ seeds from the winter life cycle were buried at three positions along the gradient to determine the impact of temperature on seedling emergence in a simulated winter life cycle.Key ResultsIn a winter life cycle, increasing temperatures advanced flowering time by 10.1 d °C^–1 in the winter annual and 4.9 d °C^–1 in the summer annual. Plant size and seed yield responded positively to global warming in both ecotypes. In a winter life cycle, the impact of increasing temperature on seedling emergence timing was positive in the winter annual, but negative in the summer annual. Global warming reduced summer annual plant size and seed yield in a summer life cycle.ConclusionsSeedling emergence timing observed in the north European summer annual ecotype may exacerbate the negative impact of predicted increased spring and summer temperatures on their establishment and reproductive performance. In contrast, seedling establishment of the Macaronesian winter annual may benefit from higher soil temperatures that will delay emergence until autumn, but which also facilitates earlier spring flowering and consequent avoidance of high summer temperatures. Such plasticity gives winter annual arabidopsis ecotypes a distinct advantage over summer annuals in expected global warming scenarios. This highlights the importance of variation in the timing of seedling establishment in understanding plant species responses to anthropogenic climate change.     

Critical periods for impact of climate warming on early seedling establishment in subarctic tundra

Climate warming is expected to shift bioclimatic zones and plant species distribution. Yet, few studies have explored whether seedling establishment is a possible bottleneck for future migration and population resilience. We test how warming affects the early stages of seedling establishment in 10 plant species in subarctic tundra. To zoom into the life phases where the effects of warming actually take place, we used a novel approach of breaking down the whole‐season warming effect into full factorial combination of early‐, mid‐, and late‐season warming periods. Seeds were sown in containers placed under field conditions in subarctic heath and were exposed to 3 °C elevation of surface temperature and 30% addition of summer precipitation relative to ambient. Heating was achieved with Free Air Temperature Increase systems. Whole‐season heating reduced germination and establishment, significantly in four out of 10 species. The whole‐season warming effect originated from additive effects of individual periods, although some of the periods had disproportionally stronger influence. Early‐germinating species were susceptible to warming; the critical phases were early summer for germination and mid summer for seedling survival. Graminoids, which emerged later, were less susceptible although some negative effects during late summer were observed. Some species with intermediate germination time were affected by all periods of warming. Addition of water generally could not mitigate the negative effects of whole‐season heating, but at individual species level both strengthening and amelioration of these negative effects were observed. We conclude that summer warming is likely to constrain seedling recruitment in open micro sites, which is a common seed regeneration niche in tundra ecosystem. Importantly, we described both significant temporal and species‐specific variation in the sensitivity of seedling establishment to warming which needs to be taken into consideration when modelling population dynamics and vegetation transitions in a warmer climate.     

Effects of a warmer climate on seed germination in the subarctic

Background and Aims

In a future warmer subarctic climate, the soil temperatures experienced by dispersed seeds are likely to increase during summer but may decrease during winter due to expected changes in snow depth, duration and quality. Because little is known about the dormancy-breaking and germination requirements of subarctic species, how warming may influence the timing and level of germination in these species was examined.

Methods

Under controlled conditions, how colder winter and warmer summer soil temperatures influenced germination was tested in 23 subarctic species. The cold stratification and warm incubation temperatures were derived from real soil temperature measurements in subarctic tundra and the temperatures were gradually changed over time to simulate different months of the year.

Key Results

Moderate summer warming (+2·5 °C) substantially accelerated germination in all but four species but did not affect germination percentages. Optimum germination temperatures (20/10°C) further decreased germination time and increased germination percentages in three species. Colder winter soil temperatures delayed the germination in ten species and decreased the germination percentage in four species, whereas the opposite was found in Silene acaulis. In most species, the combined effect of a reduced snow cover and summer warming resulted in earlier germination and thus a longer first growing season, which improves the chance of seedling survival. In particular the recruitment of (dwarf) shrubs (Vaccinium myrtillus, V. vitis-idaea, Betula nana), trees (Alnus incana, Betula pubescens) and grasses (Calamagrostis lapponica, C. purpurea) is likely to benefit from a warmer subarctic climate.

Conclusions

Seedling establishment is expected to improve in a future warmer subarctic climate, mainly by considerably earlier germination. The magnitudes of the responses are species-specific, which should be taken into account when modelling population growth and migration of subarctic species.Key words: Climate change, cold stratification, dwarf shrubs, germination percentage, incubation temperature, mean germination time, seedling establishment, seeds, Silene, subarctic species, Vaccinium, warming     

Species-specific flowering phenology responses to experimental warming and drought alter herbaceous plant species overlap in a temperate–boreal forest community

Background and AimsWarmer temperatures and altered precipitation patterns are expected to continue to occur as the climate changes. How these changes will impact the flowering phenology of herbaceous perennials in northern forests is poorly understood but could have consequences for forest functioning and species interactions. Here, we examine the flowering phenology responses of five herbaceous perennials to experimental warming and reduced summer rainfall over 3 years.MethodsThis study is part of the B4WarmED experiment located at two sites in northern Minnesota, USA. Three levels of warming (ambient, +1.6 °C and +3.1 °C) were crossed with two rainfall manipulations (ambient and 27 % reduced growing season rainfall).Key ResultsWe observed species-specific responses to the experimental treatments. Warming alone advanced flowering for four species. Most notably, the two autumn blooming species showed the strongest advance of flowering to warming. Reduced rainfall alone advanced flowering for one autumn blooming species and delayed flowering for the other, with no significant impact on the three early blooming species. Only one species, Solidago spp., showed an interactive response to warming and rainfall manipulation by advancing in +1.6 °C warming (regardless of rainfall manipulation) but not advancing in the warmest, driest treatment. Species-specific responses led to changes in temporal overlap between species. Most notably, the two autumn blooming species diverged significantly in their flowering timing. In ambient conditions, these two species flowered within the same week. In the warmest, driest treatment, flowering occurred over a month apart.ConclusionsHerbaceous species may differ in how they respond to future climate conditions. Changes to phenology may lead to fewer resources for insects or a mismatch between plants and pollinators.     

Implications of climate warming on seedling emergence and mortality of African savanna woody plants

Seedling emergence from scarified seeds and mortality of different seedling cohorts of five African savanna woody species (Acacia polyacantha, A. sieberana, Bauhinia thonningii, Dichrostachys cinerea and Ziziphus abyssinica) were studied under field conditions at a site in central Zambia. The study was conducted over a 4-year period, from 2003 to 2007. The objectives of the study were to determine climate factors that significantly influence seedling emergence rate and mortality in order to assess likely responses of the studied species to a warmer climate. Mean seedling emergence rate was 12% in D. cinerea and Z. abyssinica, 17% in B. thonningii, 47% in A. poyacantha and 62% in A. sieberana. Climate factors did not significantly affect seedling emergence in A. sieberana while temperature significantly influenced seedling emergence rate in the other species. Under a 1° warmer climate, seedling emergence rate was predicted to decline in A. polyacantha, B. thonningii and Z. abysssinica but is likely to increase slightly in D. cinerea. Time of seedling emergence during the wet season did not appear to affect seedling survival. Temperature also significantly influenced seedling mortality in all the studied species such that under a warmer climate, mortality was predicted to increase in A. sieberana and D. cinerea but decrease in A. polyacantha, B. thonningii and Z. abyssinica. As the studied species exhibited differential optimum temperature conditions for seedling emergence and seedling survival, they are likely to respond to climate warming in different but predictable ways. The results of the study are useful to forest management and development of climate change adaptation strategies in southern Africa.     

Inter-annual variability of seed rain and seedling establishment of two woody Mediterranean species under field-induced drought and warming

We aimed to assess the impact of warmer and drier climate change conditions on the seed rain and seedling establishment of Globularia alypum L. and Erica multiflora L., two dominant species in Western coastal Mediterranean shrublands. We performed a non-intrusive field experiment in which we increased the night-time temperatures and excluded spring and autumn rainfall. We monitored the seed rain over 5 years and the seedling recruitment over 9 years on these experimental plots. Seed rain of E. multiflora was enhanced by warming treatment in relation to control, and higher annual rainfall, while seed rain of G. alypum was increased by drought treatment in relation to control, dry years and higher minimum annual temperature. Annual rainfall enhanced the seedling emergence of both species, which also positively correlated with annual mean temperatures. Drought treatment significantly decreased seedling emergence for both species, which was higher in open areas than below vegetation cover. The seedling survival of both species diminished at closer distances to competing neighbours, and in G. alypum seedling survival was higher with lower annual mean temperatures and higher annual rainfall, but also in drought treatment, which have experienced vegetation cover decline. The study confirms that the increasing aridity in Mediterranean ecosystems would constrain the early stages of development in typical co-occurring shrubs. However, there are contrasting responses to climatic conditions between species recruitment, which might favour changes in vegetation through modification of species relative abundance.     

Global change impacts on arid zone ecosystems: Seedling establishment processes are threatened by temperature and water stress

Recruitment for many arid‐zone plant species is expected to be impacted by the projected increase in soil temperature and prolonged droughts associated with global climate change. As seed dormancy is considered a strategy to avoid unfavorable conditions, understanding the mechanisms underpinning vulnerability to these factors is critical for plant recruitment in intact communities, as well as for restoration efforts in arid ecosystems. This study determined the effects of temperature and water stress on recruitment processes in six grass species in the genus Triodia R.Br. from the Australian arid zone. Experiments in controlled environments were conducted on dormant and less‐dormant seeds at constant temperatures of 25°C, 30°C, 35°C, and 40°C, under well‐watered (Ψ_soil = −0.15 MPa) and water‐limited (Ψ_soil = −0.35 MPa) conditions. Success at three key recruitment stages—seed germination, emergence, and survival—and final seed viability of ungerminated seeds was assessed. For all species, less‐dormant seeds germinated to higher proportions under all conditions; however, subsequent seedling emergence and survival were higher in the more dormant seed treatment. An increase in temperature (35–40°C) under water‐limited conditions caused 95%–100% recruitment failure, regardless of the dormancy state. Ungerminated seeds maintained viability in dry soil; however, when exposed to warm (30–40°C) and well‐watered conditions, loss of viability was greater from the less‐dormant seeds across all species. This work demonstrates that the transition from seed to established seedling is highly vulnerable to microclimatic constraints and represents a critical filter for plant recruitment in the arid zone. As we demonstrate temperature and water stress‐driven mortality between seeds and established seedlings, understanding how these factors influence recruitment in other arid‐zone species should be a high priority consideration for management actions to mitigate the impacts of global change on ecosystem resilience. The knowledge gained from these outcomes must be actively incorporated into restoration initiatives.     

Fire season and intensity affect shrub recruitment in temperate sclerophyllous woodlands

The season in which a fire occurs may regulate plant seedling recruitment because of: (1) the interaction of season and intensity of fire and the temperature requirements for seed release, germination and growth; (2) post-fire rainfall and temperature patterns affecting germination; (3) the interaction of post-fire germination conditions and competition from surrounding vegetation; and (4) the interaction of post-fire germination conditions and seed predators and/or seedling herbivores. This study examined the effects of different fire intensities and fire seasons on the emergence and survival of shrubs representing a range of fire response syndromes from a summer rainfall cool climate region. Replicated experimental burns were conducted in two seasons (spring and autumn) in 2 consecutive years and fuel loads were increased to examine the effects of fire intensity (low intensity and moderate intensity). Post-fire watering treatments partitioned the effects of seasonal temperature from soil moisture. Higher intensity fires resulted in enhanced seedling emergence for hard-seeded species but rarely influenced survival. Spring fires enhanced seedling emergence across all functional groups. Reduced autumn recruitment was related to seasonal temperature inhibiting germination rather than a lack of soil moisture or competition. In Mediterranean-type climate regions, seedling emergence has been related to post-fire rainfall and exposure of seeds to seed predators. We think a similar model may operate in temperate summer rainfall regions where cold-induced dormancy over winter exposes seeds to predators for a longer time and subsequently results in recruitment failure. Our results support the theory that the effect of fire season is more predictable where there are strong seasonal patterns in climate. In this study seasonal temperature rather than rainfall appears to be more influential.     

Winter Climate Limits Subantarctic Low Forest Growth and Establishment

Campbell Island, an isolated island 600 km south of New Zealand mainland (52°S, 169°E) is oceanic (Conrad Index of Continentality  = −5) with small differences between mean summer and winter temperatures. Previous work established the unexpected result that a mean annual climate warming of c. 0.6°C since the 1940''s has not led to upward movement of the forest limit. Here we explore the relative importance of summer and winter climatic conditions on growth and age-class structure of the treeline forming species, Dracophyllum longifolium and Dracophyllum scoparium over the second half of the 20^th century. The relationship between climate and growth and establishment were evaluated using standard dendroecological methods and local climate data from a meteorological station on the island. Growth and establishment were correlated against climate variables and further evaluated within hierarchical regression models to take into account the effect of plot level variables. Winter climatic conditions exerted a greater effect on growth and establishment than summer climatic conditions. Establishment is maximized under warm (mean winter temperatures >7 °C), dry winters (total winter precipitation <400 mm). Growth, on the other hand, is adversely affected by wide winter temperature ranges and increased rainfall. The contrasting effect of winter warmth on growth and establishment suggests that winter temperature affects growth and establishment through differing mechanisms. We propose that milder winters enhance survival of seedlings and, therefore, recruitment, but increases metabolic stress on established plants, resulting in lower growth rates. Future winter warming may therefore have complex effects on plant growth and establishment globally.     

Potential Impacts of Future Warming and Land Use Changes on Intra-Urban Heat Exposure in Houston,Texas

Extreme heat events in the United States are projected to become more frequent and intense as a result of climate change. We investigated the individual and combined effects of land use and warming on the spatial and temporal distribution of daily minimum temperature (T_min) and daily maximum heat index (HI_max) during summer in Houston, Texas. Present-day (2010) and near-future (2040) parcel-level land use scenarios were embedded within 1-km resolution land surface model (LSM) simulations. For each land use scenario, LSM simulations were conducted for climatic scenarios representative of both the present-day and near-future periods. LSM simulations assuming present-day climate but 2040 land use patterns led to spatially heterogeneous temperature changes characterized by warmer conditions over most areas, with summer average increases of up to 1.5°C (T_min) and 7.3°C (HI_max) in some newly developed suburban areas compared to simulations using 2010 land use patterns. LSM simulations assuming present-day land use but a 1°C temperature increase above the urban canopy (consistent with warming projections for 2040) yielded more spatially homogeneous metropolitan-wide average increases of about 1°C (T_min) and 2.5°C (HI_max), respectively. LSM simulations assuming both land use and warming for 2040 led to summer average increases of up to 2.5°C (T_min) and 8.3°C (HI_max), with the largest increases in areas projected to be converted to residential, industrial and mixed-use types. Our results suggest that urbanization and climate change may significantly increase the average number of summer days that exceed current threshold temperatures for initiating a heat advisory for metropolitan Houston, potentially increasing population exposure to extreme heat.     

Biodiversity bottleneck: seedling establishment under changing climatic conditions at the boreal–temperate ecotone

Climate change is increasing global temperatures, severe rainfall events, and the occurrence and severity of drought. Changes in global climate may have negative consequences for particular plant species and for biodiversity overall. In the short term, altered temperature and precipitation regimes may have the most severe effects on plant species near their range limits and in the earliest stages of plant development. To address these issues, we assessed seedling emergence, early survival, and growth of 18 boreal, temperate, and exotic woody species at the boreal–temperate forest ecotone in central Minnesota. We experimentally warmed forest plots to mimic projected warming by the end of the twenty-first century (+ 1.7 °C and + 3.4 °C). We also experimentally removed summer rainfall (~?42% reduction) to simulate drought conditions in this region. We found that emergence and survival of boreal and exotic species was lower in experimentally warmed plots. This was exacerbated by drought. Temperate species emergence and survival was largely unaffected by climate manipulations (on average). Conversely, temperate seedling growth was greater in warmer conditions, but only when paired with drought. We found that overall seedling species richness was reduced by warming, mostly due to lower boreal and exotic species emergence and survival (conifers were also strongly negatively affected across species-range groups). If temperate seedling emergence and survival does not compensate for loss of boreal species, these forests may experience loss of biodiversity (and associated ecosystem functions) in the future.     

Seedling emergence responds to both seed source and recruitment site climates: a climate change experiment combining transplant and gradient approaches

Seedling recruitment allows genetic recombination and production of dispersal units. Both the climate experienced by the source populations (seed source effect) and the weather experienced by the seeds during germination and seedling emergence (recruitment site effects) are important for seedling recruitment. Separating these effects in the field is essential to assess potential climate change impacts on plant population. We combine experimental seed transplant and gradient analyses to separate the effects of seed source and recruitment site temperature and precipitation for the seedling emergence of two alpine/lowland species pairs (Viola biflora/Viola palustris, Veronica alpina/Veronica officinalis). Combining these approaches allows us to compare local responses versus responses along environmental gradients, but also tests for local adaptation and/or pre-conditioning effects (adaptive seedling emergence responses). Veronica officinalis emergence increased with increasing seed source temperature in both the experimental and the gradient approaches, and showed adaptive seedling emergence. Viola biflora, Viola palustris and Veronica alpina emergence decreased with recruitment site temperature in both approaches. Both Violas emergences increased with recruitment site precipitation, in both approaches for the alpine violet, and in the gradient approach for lowland one. Emergence was primarily affected by the environment of the recruitment site, whereas seed source climate and adaptive seedling emergence impacted recruitment in only one of our species. The responses to recruitment site temperatures were negative, whereas the response to seed source temperature was positive. Ignoring the distinctions between these different mechanisms can lead to erroneous conclusions regarding potential climate change impacts on plant recruitment.     

亚热带中山常绿阔叶林木本植物幼苗数量动态及其与生境的相关性

以百山祖自然保护区5 hm2永久样地150个幼苗监测站木本植物幼苗为研究对象,2008—2011年定期调查样方中幼苗的种类、数量、萌发、死亡等,探究亚热带中山常绿阔叶林幼苗种类组成、数量动态及其与生境的相关性。结果表明:1)百山祖样地在2008年至2011年出现的幼苗属于26科,40属,共53个物种,不同物种萌发时段有异;2)2009年样地幼苗存活比率为7.7%,2010年为-20.8%,2011年则是-0.3%,幼苗存活比率不高,种类和数量呈减少趋势;3)存活幼苗中有明显的优势物种,分别为光亮山矾(Symplocos lucida)、尖连蕊茶(Camellia cuspidata)、浙闽新木姜子(Neolitsea aurata var.undulatula)、尖叶菝葜(Smilax arisanensis)和短尾柯(Lithocarpus brevicaudatus),5个物种之和占幼苗总数比例50%;4)种子的萌发与生境有极显著的相关性,且与生境因子中水分关联最大;5)存活幼苗数与样站坡位、水分、落叶层厚度呈现显著相关性,水和光照是影响幼苗存活的主要因素。     

Plant community dynamics in a calcareous grassland under climate change manipulations

This study investigates the effects of field manipulations of local climate to determine the potential impact of climate change on plant community dynamics in a calcareous grassland. The experimental site is located in a grassland at the Wytham estate, Oxfordshire, UK. The one hectare study area is within a 10 ha abandoned arable field on Jurassic corallian limestone. Two climate change scenarios were used: warmer winters with increased summer rainfall and warmer winters with summer drought. Plant cover and species richness were significantly increased in plots receiving supplemented summer rainfall, while the amount of litter was significantly reduced. Litter formation was significantly increased by winter warming and drought. The responses of the plant community to the climate manipulations were related to the life-history attributes of the dominant species. Seedling recruitment was limited by microsite availability, which also varied in the different climate manipulations. The results are discussed in terms of successional dynamics. They suggest that warmer winters may delay succession, as gap formation in the sward will provide sites for colonisation of annuals, thereby enabling their persistence in the sward. Under wetter conditions during summer, perennial grasses tend to close the sward, thereby inhibiting the establishment of later successional species.     

Evidence of Tree Species’ Range Shifts in a Complex Landscape

Climate change is expected to change the distribution of species. For long-lived, sessile species such as trees, tracking the warming climate depends on seedling colonization of newly favorable areas. We compare the distribution of seedlings and mature trees for all but the rarest tree species in California, Oregon and Washington, United States of America, a large, environmentally diverse region. Across 46 species, the mean annual temperature of the range of seedlings was 0.120°C colder than that of the range of trees (95% confidence interval from 0.096 to 0.144°C). The extremes of the seedling distributions also shifted towards colder temperature than those of mature trees, but the change was less pronounced. Although the mean elevation and mean latitude of the range of seedlings was higher than and north of those of the range of mature trees, elevational and latitudinal shifts run in opposite directions for the majority of the species, reflecting the lack of a direct biological relationship between species’ distributions and those variables. The broad scale, environmental diversity and variety of disturbance regimes and land uses of the study area, the large number and exhaustive sampling of tree species, and the direct causal relationship between the temperature response and a warming climate, provide strong evidence to attribute the observed shifts to climate change.     

Influence of warming on soil water potential controls seedling mortality in perennial but not annual species in a temperate grassland

In a water-limited system, the following hypotheses are proposed: warming will increase seedling mortality; elevated atmospheric CO2 will reduce seedling mortality by reducing transpiration, thereby increasing soil water availability; and longevity (i.e. whether a species is annual or perennial) will affect the response of a species to global changes. Here, these three hypotheses are tested by assessing the impact of elevated CO2 (550 micromol mol(-1) and warming (+2 degrees C) on seedling emergence, survivorship and establishment in an Australian temperate grassland from autumn 2004 to autumn 2007. Warming impacts on seedling survivorship were dependent upon species longevity. Warming reduced seedling survivorship of perennials through its effects on soil water potential but the seedling survivorship of annuals was reduced to a greater extent than could be accounted for by treatment effects on soil water potential. Elevated CO2 did not significantly affect seedling survivorship in annuals or perennials. These results show that warming will alter recruitment of perennial species by changing soil water potential but will reduce recruitment of annual species independent of any effects on soil moisture. The results also show that exposure to elevated CO2 does not make seedlings more resistant to dry soils.     

Rodent seed predation and seedling recruitment in mesic grassland

Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m⁻²) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m⁻² but had no impact at 50,000 seeds m⁻², presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation. Received: 2 March 1998 / Accepted: 28 September 1998     

Restoring Western (Basalt) Plains grassland. 2. Field emergence, establishment and recruitment following direct seeding

Summary   Both reservation of small remnants and ecological restoration of degraded areas will be crucial if the Victorian Western (Basalt) Plains grassland community is to be conserved in the long term. This study examined the potential of direct seeding as a technique for grassland restoration by recording the initial establishment and subsequent recruitment success of 64 (predominantly perennial) grassland species direct sown onto a constructed site. Forty-three (67%) of the sown species emerged and established during the 2-year study and a further three species were recorded in subsequent years. In the second year, 32 species increased their number either through seedling or vegetative recruitment and 30 species dispersed beyond their original sown plot. Seed size was not correlated with field emergence but life form did influence initial field success for some groups. The finding that many species are able to establish and recruit under the study conditions supports the need for further investigation of direct seeding in the restoration of grassland communities.