Effects of experimental manipulation of inflorescence size on pollination and pre-dispersal seed predation in the hummingbird-pollinated plant Ipomopsis aggregata

Large floral displays should theoretically provide advantages to plants through increased pollinator visitation and resulting fruit and seed set. However empirical tests of the response of pollinators to floral display size have been limited by a lack of direct experimentation, and the results of such studies have been equivocal. In addition, other selective agents such as pre-dispersal seed predators might modulate effects of floral display on pollination. By artificially altering flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators, as well destruction of fruits by a pre-dispersal seed predator (Hylemya: Anthomyiidae). In addition, we quantified the ultimate effects of flower number on female reproductive success. Plants with larger floral displays were most likely to be visited first in any given foraging bout (P < 0.01). As expected, plants with more flowers received more total flower visits. However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. In fact, a significantly greater percentage of flowers were visited on few-flowered plants. Plants did not compensate for our reduction in flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or seed weight. Pre-dispersal seed predation was greater for many- than for few-flowered plants (P < 0.001), but this did not offset the potential fitness gains of producing large displays. Our data support the hypothesis that large floral displays function primarily in long-distance attraction of pollinators, and enhance maternal success. Received: 21 March 1996 / Accepted: 24 October 1996     

Fruit production in Sorbus aucuparia L. (Rosaceae) and pre-dispersal seed predation by the apple fruit moth (Argyresthia conjugella Zell.)

Variation in fruit production and pre-dispersal seed predation by Argyresthia conjugella was studied in␣four populations of Sorbus aucuparia in northern Sweden.␣The number of infructescences, fruits per infructescence, consumed seeds and developed unattacked seeds per fruit were scored in marked trees from 1984 to 1990. The results showed that the number of fruits produced in each population determined the number of seed predators occurring in the host population, as the yearly number of seed predators was significantly and positively correlated with yearly number of fruits, in all but one population. The seed predators showed a delay in response to variation in number of fruits produced. This lag in response resulted in a large proportion of fruits being attacked and seeds consumed in a bad fruiting year that followed a good fruiting year, and vice versa. The proportion of fruits attacked and seeds consumed was largest in the population showing the greatest between-year variation in fruit production and lowest in the population showing the lowest between-year variation in fruit production. Furthermore, the individuals within the former population were synchronised, while they were not in the latter population. These results contradict one of the possible explanations of mast-seeding, where large synchronised between-year variation is supposed to reduce pre-dispersal seed predation. Instead, differences in attraction of the seed predator to differences in fruit crop size could explain the observed difference in seed predation between the two populations with opposite fruiting patterns. Within each population, irrespective of year, the proportion of fruits attacked and seeds consumed was independent of a tree's fruiting display. Therefore, trees with high fruit production, despite harbouring the largest number of seed predators, produced the largest number of developed seeds in absolute numbers, compared to trees that produced few fruits. Received: 25 February 1996 / Accepted: 30 November 1996     

Interspecific interactions and co-existence in dabbling ducks: observations and an experiment

We studied the possible role of resource limitation and interspecific competition in assemblages of dabbling ducks on breeding lakes in Finland and Sweden with observational and experimental data. After initial vegetation mapping and yearly censuses of ducks in 1985–1990, we collected observational data in 1991–1994 from 28 lakes with natural populations of mallard Anas platyrhynchos and teal A. crecca. Mallard and teal co-occur over vast areas in the Holarctic and they are the only breeding dabbling ducks on many oligotrophic lakes. Both species are migratory in our study regions, teal arriving later in spring than mallards. Log-linear model analysis of observational presence/absence data revealed a positive, not a negative, association between the species. This association was independent of habitat diversity as well as of lake size. Mallard-teal interaction was also studied in a cross-over introduction experiment in 32 other lakes in two years. Wing-clipped mallards were introduced to breeding lakes before the arrival of teal to induce resource limitation and interspecific competition, hypothesized to reduce lake use by teal. The density of mallard pairs on experimental lakes was 2.9–8.0 times higher than on controls, but there was no negative response by teal to the treatment. This is the first combined observational-experimental demonstration of lack of interspecific competition in waterfowl. Our results indicate that heterospecific attraction may affect species co-existence in dabbling ducks. Received: 28 October 1996 / Accepted: 13 January 1997     

Post-dispersal seed predation and the establishment of vertebrate dispersed plants in Mediterranean scrublands

The post-dispersal fate of seeds and fruit (diaspores) of three vertebrate-dispersed trees, Crataegus monogyna, Prunus mahaleb and Taxus baccata, was studied in the Andalusian highlands, south-eastern Spain. Exclosures were used to quantify separately the impact of vertebrates and invertebrates on seed removal in relation to diaspore density and microhabitat. The three plant species showed marked differences in the percentage of diaspores removed, ranging from only 5% for C. monogyna to 87% for T. baccata. Although chaffinches (Fringilla coelebs) fed on diaspores, rodents (Apodemus sylvaticus) were the main vertebrate removers of seed and fruit. Two species of ant (Cataglyphis velox and Aphaenogaster iberica) were the only invertebrates observed to remove diaspores. However, the impact of ants was strongly seasonal and they only removed P. mahaleb fruit to any significant extent. While removal of seed by rodents was equivalent to predation, ants were responsible for secondary dispersal. However, their role was limited to infrequent, small-scale redistribution of fruit in the vicinity of parent trees. Rodents and ants differed in their use of different microhabitats. Rodents foraged mostly beneath trees and low shrubs and avoided open areas while the reverse was true of ants. Thus, patterns of post-dispersal seed removal will be contigent on the relative abundance and distribution of ants and rodents. Studies which neglect to quantify separately the impacts of these two guilds of seed removers may fail to elucidate the mechanisms underlying patterns of post-dispersal seed removal. The coincidence of both increased seed deposition by the main avian dispersers (Turdus spp.) and increased seed predation with increasing vegetation height suggested that selection pressures other than post-dispersal seed predation shape the spatial pattern of seed dispersal. Rather than providing a means of escaping post-dispersal seed predators, dispersal appears to direct seeds to microhabitats most suitable for seedling survival. Nevertheless, the reliance of most vertebrate-dispersed trees on regeneration by seed and the absence of persistent soil seed banks imply that post-dispersal seed predators may exert a strong influence on the demography of the plants whose seeds they consume. Even where microsites are limited, the coincidence of the most suitable microhabitats for seedling establishment with those where seed predation is highest provide a means by which selective seed predators can influence community composition. Received: 19 August 1996 / Accepted: 25 January 1997     

Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m⁻²), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m⁻²). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence. Received: 24 October 1996 / Accepted: 4 February 1997     

Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests. Received: 2 November 1996 / Accepted: 8 February 1997     

Determinants of seed production in Geranium maculatum

Summary We experimentally examined factors limiting seed production in two populations of the perennial woodland herb Geranium maculatum in central Illinois, USA. To test the pollinator-limitation hypothesis, we compared the seed production of plants whose flowers were supplementarily pollinated with outcross pollen to that of control plants receiving natural pollination only. To test if fruit production by early flowers suppresses fruit and seed formation by late flowers, a third group of plants was prevented from producing seed from the first 50% of the flowers to open (stigmas were excised at flower opening). Finally, to test if seed maturation and flower initiation are correlated with photosynthetic capacity, we performed a defoliation experiment in which either the stem leaves within the inflorescence, the stem leaves below the inflorescence, or the rosette leaves were removed during late flowering. Plants that reccived supplemental pollination produced 1.5–1.6 times more seeds than control plants. We found no difference between hand-pollinated plants and controls in mortality, flowering frequency or number of flowers produced in the year following the experiment. In both control and hand-pollinated plants, the fruit set and total seed production of early flowers were more than twice as high as those of late flowers. In one of the two populations, plants whose early flowers were prevented from setting seed produced significantly more seeds from their late flowers than did control plants. Seed predation was low and did not differ between early and late flowers. Leaf removal did not affect seed number or size in the year of defoliation, nor did it reduce survival or flower production in the subsequent year. This suggests that the plants were able to compensate for a partial defoliation by using stored resources or by increasing photosynthetic rates in the remaining leaves. Taken together, the results demonstrate that both pollinator activity and resource levels influence patterns of seed production in G. maculatum. While seed production was pollinatorlimited in both populations, a seasonal decline in resource availability was apparently responsible for the low seed production by late flowers.     

Blood flow in the carotid artery during breath-holding in relation to diving bradycardia

The present study investigated the mechanism of diving bradycardia. A group of 14 healthy untrained male subjects were examined during breath-holding either out of the water (30–33°C), in head-out immersion, or in whole-body submersion (27–29°C) in a diving pool. Blood velocity, blood volume flow in the carotid artery, diastolic blood pressure and electrocardiogram were measured and recorded during the experiments. The peak blood velocity increased by 13.6% (P < 0.01) and R-wave amplitude increased by 57.1% (P < 0.005) when the subjects entered water from air. End-diastolic blood velocity in the carotid artery increased significantly during breath-holding, e.g. increased from 0.20 (SD 0.02) m · s⁻¹ at rest to 0.33 (SD 0.04) m · s⁻¹ (P < 0.001) at 50.0 s in breath-hold submersion to a 2.0-m depth. Blood volume flow in the carotid artery increased by 26.6% (P < 0.05) at 30 s and 36.6% (P < 0.001) at 40 s in breath-hold submersion to a 2.0-m depth. Diastolic blood pressure increased by 15.4% (P < 0.01) at 60 s during breath-holding in head-out immersion. Blood volume flow, and diastolic blood pressure increased significantly more and faster during breath-holding in submersion than out of the water. There was a good negative correlation with the heart rate: the root mean square correlation coefficient r was 0.73 (P < 0.001). It was concluded that an increased accumulation of blood in the aorta and arteries at end-diastole and decreased venous return, caused by an increase in systemic peripheral resistance during breath-holding, underlies diving bradycardia. Accepted: 22 November 1996     

Single prenatal injections of melatonin or the D1-dopamine receptor agonist SKF 38393 to pregnant hamsters sets the offsprings' circadian rhythms to phases 180° apart

Pregnant Syrian hamsters with lesions of the suprachiasmatic nucleus (SCN) received single injections of melatonin or the D1-dopamine receptor agonist, SKF 38393 on day 15 of gestation (1 day before birth). Pups were weaned on postnatal day 20 and their freerunning activity rhythms recorded for 3–4 weeks. The pups' phases on the day of weaning were significantly clustered in both of the treatment groups, but the average phases differed by approximately 180°. The results demonstrate that a single prenatal stimulus is sufficient to set the phases of the hamsters' rhythms and that the phase established depends on the stimulus. Both c-fos mRNA and Fos protein were expressed in the fetal SCN after SKF 38393 injection but neither were expressed after melatonin injection. Simulations showed that a single stimulus could produce the observed synchrony from a population of uniformally distributed phases if the phase shifts were three to four times the magnitude of the adult hamster light phase response curve (PRC). A light pulse PRC mimicked the effect of an SKF 38393 injection and a dark-pulse PRC mimicked the effects of a melatonin injection. Together these results suggest that dopamine and melatonin either are, or mimic, maternal entraining signals that represent day and night. Accepted: 12 November 1996     

Floral abortion and pollination in four species of tropical mangroves from northern Australia

We assessed natural rates of floral abortion in four common mangrove species from northern Australia and subsequently manipulated pollination experimentally. Sonneratia alba J. Smith exhibited the highest rate of fruit set of the four species (23%), indicating this mangrove was best able to utilise the natural pollination opportunities provided. Fruit set in S. alba appeared, however, to be pollinator limited, as large increases in fruit set occurred after manual cross-pollination of flowers. Avicennia marina (Forsk.) Vierh. had the highest rate of natural pollination, but fruit set was lower (15%) and appeared to be impeded by resource limitations. Although a range of insects visited Ceriops australis (C.T. White) Ballment, T.J. Sm & Stoddart, the rate of fruit set was low (3%) and the capacity for flower fertilisation limited, despite evidence of autogamy in this species. There was an indication of both resource and pollinator limitation in C. australis. Rhizophora stylosa Griff. exhibited limited fruit set (0.5%), possibly due to limiting maternal resources and the lack of adaptation of flowers to either animal or wind pollination. Large increases in fruit set were recorded after manual cross-pollination of R. stylosa flowers. R. stylosa and C. australis, characterised by resource rich propagules with long periods of development, both aborted a large proportion of propagules during the fruit maturation process.     

The effect of light environment, leaf area, and stored carbohydrates on inflorescence production by a rain forest understory palm

Variation in flowering by long-lived plants may be correlated with current resource availability. If, however, there are trade-offs between current and future reproduction, or between reproduction and storage or growth, then understanding variation requires a whole-plant, longer-term perspective. Inflorescence production by Calyptrogyne ghiesbreghtiana Linden ex. H. Wendl., an understory palm, was studied over 3 years. Annual inflorescence production varied greatly and was correlated with variation in plant size and light environment. There was no trade-off between past inflorescence production and the frequency of future inflorescence production. On the contrary, individuals that produced more inflorescences than predicted from their size and light environment tended to continue to do so in subsequent years also. I manipulated the resource environment of a subset of plants by removal of leaves and/or reproductive spikes. Leaf removal suppressed inflorescence production for the following 2 years, but spike removal had no effect. One year after leaf removal stored reserves were, on average, back to pre-treatment levels. There was, however, a negative effect of recent inflorescence production on storage. Plants with higher levels of storage had higher inflorescence production in the next 75 days. In C. ghiesbreghtiana the resource cost of reproduction is apparent in short-term variation in stored reserves. In contrast, annual inflorescence production does not follow a trade-off pattern between successive years, but consistently reflects both plant size and the light environment. Received: 20 October 1996 / Accepted: 25 January 1997     

Fruit tracking,frugivore satiation,and their consequences for seed dispersal

Vertebrate frugivore communities are easily satiated by abundant fruit supplies and, contrary to abiotic dispersal agents, typically disperse only part of the available seed pool. This frugivore satiation is likely to be a widespread phenomenon and should be an influential predictor of plants’ ability to disperse their offspring to suitable establishment sites; yet it has never been systematically quantified. Here I investigate patterns of fruit abundance, frugivore activity and frugivore satiation, and their consequences for seed dispersal in the fleshy-fruited tree Frangula alnus. Based on constant-effort seed trapping conducted over 3 years, I assess densities of total and frugivore-consumed seedfall across two spatial (within/between populations) and two temporal (within/between ripening seasons) scales. Furthermore, I examine relationships between fruit abundance and the amount of seeds that are actually dispersed away from fruiting trees. Frugivore activity tightly matched fruit abundance, although some differences existed between scales. This marked fruit tracking did not prevent a significant frugivore satiation, however, and only 53% of the available fruit crops were actually consumed. The extent of satiation varied most at the within-population level, likely due to the territorial behaviour of important frugivore species. In contrast, levels of satiation remained remarkably invariable through time, suggesting that frugivores behave as opportunists and closely adjust the composition of their diet to the available food supply. Overall, greater fruit abundance resulted in a higher proportion of seeds falling beneath fruiting trees, but it also helped increase the (absolute) number of seeds dispersed. This study shows that frugivore satiation can be an important phenomenon even when frugivores tightly track fruit abundance. Its negative effects on recruitment may be attenuated, however, if greater fruit crops help increase population-wide frugivore activity and the amount of seeds being dispersed to suitable establishment sites.     

An Aspergillus nidulans nuclear protein, AnCP, involved in enhancement of Taka-amylase A gene expression, binds to the CCAAT-containing taaG2 , amdS , and gatA promoters

Using AnCP (Aspergillus nidulans CCAAT-binding protein) as a CCAAT-specific binding factor model, the possibility that one factor is able to recognize CCAAT sequences in several different genes in A.␣nidulans was examined. DNase I protection analysis showed that AnCP specifically bound to CCAAT sequence-containing regions comprising 21 to 36 bp of the taa, amdS and gatA genes. Furthermore, replacement of the CCAAT sequence with CGTAA was found to abolish the binding of AnCP and to have an inhibitory effect on taa promoter activity. This clearly demonstrates a positive function of the CCAAT element. However, amylase was induced by starch and repressed by glucose in a CCAAT-box disruptant, as in wild-type cells. Received: 28 June 1996 / Accepted: 7 October 1996     

The influence of pollen addition and ramet isolation on current sexual reproduction in a clonal herb

Canada mayflower (Maianthemum canadense Desf.), a rhizomatous, perennial herb, was the subject of a 2-year field experiment that examined two factors potentially affecting fruit and seed production: pollen addition and ramet isolation. Ramets were either open pollinated or overpollinated by hand to supplement natural levels. Rhizomes of the ramets were either severed, to prevent resource supplementation from the genet, or left intact. Ramets that were overpollinated matured more fruits and more seeds than ramets that were open pollinated. Thus, mayflower appears to have been pollen limited in both years. Ramets that were open pollinated and whose rhizomes were severed matured as many fruits, seeds, and seeds/fruit as ramets that were open pollinated and whose rhizomes were left intact. Ramets that were overpollinated and whose rhizomes were severed matured fewer fruits, seeds, and seeds/fruit than overpollinated ramets whose rhizomes were left intact. It appears that at natural levels of pollination, mayflower ramets are physiologically independent but as the level of pollen increases, mayflower ramets receive support from other parts of the genet. Received: 13 May 1996 / Accepted: 1 November 1996     

Changes in selected aspects of immune function in the leopard frog, Rana pipiens, associated with exposure to cold

The effect of exposure to low temperatures (5 °C) on lymphocyte proliferation, leukocyte populations, and serum complement levels was examined in the northern leopard frog, Rana pipiens. Proliferation of T lymphocytes in response to phytohemagglutinin stimulation was significantly decreased in frogs kept for 2, 3, and 5 months at 5 °C compared to that of animals kept at 22 °C. A significant increase in the average percentage of neutrophils and a decrease in the mean percentage of eosinophils was observed in the blood of frogs held for 5 months in the cold compared to animals held at 22 °C for the same length of time. Mean serum complement activity after 1 month at 5 °C was significantly reduced in comparison to animals held at 22 °C and was not detectable after 5 months in the cold. Recovery of complement levels at room temperature (22 °C) was also examined after cold exposure. Complement levels were significantly higher than controls (at 22 °C) in frogs returned to 22 °C for 7 and 14 days after 5 months in the cold. After frogs were held at 5 °C for 1 month, serum complement levels increased significantly within 2 days after returning to 22 °C and continued to rise 5 and 9 days after warming. Injections with Aeromonas hydrophila following a 5-week exposure to 5 °C failed to cause death or observable symptoms of disease in frogs that were returned to 22 °C. Accepted: 20 November 1996     

Axillary bud banks of two semiarid perennial grasses: occurrence, longevity, and contribution to population persistence

The occurrence, longevity, and contribution of axillary bud banks to population maintenance were investigated in a late-seral perennial grass, Bouteloua curtipendula, and a mid-seral perennial grass, Hilaria belangeri, in a semiarid oak-juniper savanna. Axillary buds of both species were evaluated over a 2-year period in communities with contrasting histories of grazing by domestic herbivores. A double staining procedure utilizing triphenyl tetrazolium chloride and Evan's blue indicated that both viable and dormant axillary buds remained attached to the base of reproductive parental tillers for 18–24 months which exceeded parental tiller longevity by approximately 12 months. Bud longevity of the late-seral species, B. curtipendula, exceeded bud longevity of the mid-seral species, H. belangeri, by approximately 6 months. Younger buds located on the distal portion of the tiller base were 3.2 and 1.4 times more likely to grow out than older proximal buds of B. curtipendula and H. belangeri, respectively. The percentage of older proximal buds, which included comparable portions of viable and dormant buds, that grew out to produce tillers following mortality of parental tillers was 6.0% for B. curtipendula and 8.4% for H. belangeri. In spite of the occurrence of relative large axillary bud banks for both species, the magnitude of proximal bud growth did not appear sufficient to maintain viable tiller populations. We found no evidence to support the hypothesis of compensatory bud growth on an individual tiller basis for either species. Grazing history of the communities from which the buds were collected did not substantially affect the number, status, longevity, or outgrowth of axillary buds on an individual tiller basis for either species. However, long-term grazing by domestic herbivores influenced axillary bud availability by modifying population structure of these two species. Bud number per square meter for B. curtipendula was 25% lower in the long-term grazed compared to the long-term ungrazed community based on a reduction in both tiller number per plant and plant number per square meter. In contrast, bud number per square meter for H. belangeri was 190% greater in the long-term grazed than in the long-term ungrazed community based on a large increase in plant density per square meter. Minimal contributions of axillary bud banks to annual maintenance of tiller populations in this mid- and late-seral species underscores the ecological importance of consistent tiller recruitment from recently developed axillary buds. Consistent tiller recruitment in grasslands and savannas characterized by intensive grazing and periodic drought implies that (1) bud differentiation and maturation must be remarkably tolerant of adverse environmental conditions and/or (2) tiller recruitment may resume from buds that mature following the cessation of severe drought and/or grazing, rather than from mature buds that survive these disturbances. These scenarios warrant additional research emphasis given the critical importance of this demographic process to tiller replacement in species populations and the maintenance of relative species abundance in grasslands and savannas. Received: 12 August 1996 / Accepted: 30 December 1996     

Winter production of CO2 and N2O from alpine tundra: environmental controls and relationship to inter-system C and N fluxes

Fluxes of CO₂ and N₂O were measured from both natural and experimentally augmented snowpacks during the winters of 1993 and 1994 on Niwot Ridge in the Colorado Front Range. Consistent snow cover insulated the soil surface from extreme air temperatures and allowed heterotrophic activity to continue through much of the winter. In contrast, soil remained frozen at sites with inconsistent snow cover and production did not begin until snowmelt. Fluxes were measured when soil temperatures under the snow ranged from –5°C to 0°C, but there was no significant relationship between flux for either gas and temperature within this range. While early developing snowpacks resulted in warmer minimum soil temperatures allowing production to continue for most of the winter, the highest CO₂ fluxes were recorded at sites which experienced a hard freeze before a consistent snowpack developed. Consequently, the seasonal flux of CO₂ –C from snow covered soils was related both to the severity of freeze and the duration of snow cover. Over-winter CO₂ –C loss ranged from 0.3 g C m⁻² season⁻¹ at sites characterized by inconsistent snow cover to 25.7 g C m⁻² season⁻¹ at sites that experienced a hard freeze followed by an extended period of snow cover. In contrast to the pattern observed with C loss, a hard freeze early in the winter did not result in greater N₂O–N loss. Both mean daily N₂O fluxes and the total over-winter N₂O–N loss were related to the length of time soils were covered by a consistent snowpack. Over-winter N₂O–N loss ranged from less 0.23 mg N m⁻² from the latest developing, short duration snowpacks to 16.90 mg N m⁻² from sites with early snow cover. These data suggest that over-winter heterotrophic activity in snow-covered soil has the potential to mineralize from less than 1% to greater than 25% of the carbon fixed in ANPP, while over-winter N₂O fluxes range from less than half to an order of magnitude higher than growing season fluxes. The variability in these fluxes suggests that small changes in climate which affect the timing of seasonal snow cover may have a large effect on C and N cycling in these environments. Received: 5 April 1996 / Accepted: 25 November 1996     

Application of a 15N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling. Received: 11 July 1996 / Accepted: 5 December 1996     

Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995