Influence of temperature and rainfall on fruit and seed production of Arbutus unedo L.

Correlations were found between certain climatic parameters and fresh weight, dry weight and seed number of the fruit of Arbutus unedo L. In five stations in central Italy. Temperature and rainfall were found to affect fruit production. For the parameters fresh weight and number of seeds, the first months of development were important and fresh weight was also influenced by rainfall in the last monthS. Dry weight did not show statistically significant correlation with any one of the considered parameterS. However, the pattern of temperature and rainfall over the 12-month period of fruit ripening determined the characteristics of the fruit.     

Fire,canopy cover and seedling dynamics in Mediterranean shrubland of northeastern Spain

Abstract. Seedling establishment and survival of the main woody species of early-successional shrubland in northeastern Spain were studied from 1992 to 1995 with emphasis on the importance of vegetation cover (existence of open areas and the situation beneath the plant canopy) and microhabitat (occurrence of stones, litter and bare soil). In the absence of fire, vegetation cover (measured at a scale of 30cm × 30 cm) was not correlated with seedling emergence of most species, nor with growth and survival of seedlings up to one year old. Seedlings older than one year showed a similar pattern: their density was not significantly different in both kinds of habitats. The emergence of seedlings was mainly associated with the presence of mature plants – which can provide seeds – and with the absence of unsuitable microhabitats (large stones, deep litter and bare soil). In April 1994 a wildfire burned the study area. This enabled a study of the pattern of post-fire establishment. After the wildfire, seedling emergence of several species increased and most species produced seedlings with higher survival and growth rates than in the period before the fire. Pre-fire cover, however, did not show significant effects on post-fire seedling dynamics in most cases.     

Germination responses of <Emphasis Type="Italic">Erica andevalensis</Emphasis> to different chemical and physical treatments

Erica andevalensis Cabezudo & Rivera is a threatened edaphic endemic species of Andalusia (SW Spain). Under natural conditions, the plants produce a very large number of small seeds (0.3–0.4 mm) but very few seedlings survive. Different treatments (high temperature, cold pre-treatment, nitrogen salts, and gibberellic acid applications) were tested to assess germination patterns in different populations and to determinate the most favorable conditions for germination. Gibberellic acid was provided in five different concentrations from 0 to 400 ppm GA₃, while nitrogen was applied as 10 mM of either KNO₃ or NH₄NO₃. The effect of pH on germination was also tested. The species always showed a low germination rate (6.50–22%) that was not stimulated either by 1 or 4 months in dry cold pre-treatment, nitrogen application, acid pH medium, or by high temperature (80°C for 10 min); although gibberellic acid application (100–400 ppm) significantly enhanced germination. The highest percentage of germination (41.6%) was achieved with a mean germination time to start germination (t ₀) of 7.6 ± 0.54 days when the seeds were subjected to 400 ppm gibberellic acid treatment. The population origin did not have a significant effect on germination percentage.     

The effects of drought and fire on seed and seedling dynamics in a tropical seasonal forest in Thailand

Seed production and seedling dynamics were studied for 6 years(1992–1997) with particular emphasis on the effects of the drought andfire in a natural tropical seasonal forest at Mae Klong Watershed ResearchStation, Kantchanaburi, western Thailand. The number of seed produced andemerged seedlings varied greatly among years and species. Most tree speciesproduced very few seeds in 1994 and many in 1995 and 1996, particularly,Shorea siamensis Miq. which displayed the most seedinghabit. Some seedlings, Shorea, Dipterocarpus alatus Roxb.,and Dipterocarpus turbinatus Gaertn. f., emerged at theendof the dry season (late April, after the first rain but when soils still have alow soil moisture content), while the majority of seedlings emerged at thebeginning of the rainy season. After a fire disturbed the plot in April 1996,many species increased their seedling emergences, especially Berryaammonilla. Seedlings of most species less than a year old showed arelatively low mortality in the rainy season, but those ofPterocarpusmacrocarpus Kurz had high mortality. The mean survival rate for allseedling species in this forest was quite low (24.1% y^–1), anddifferent values existed for the rainy and the dry season (11.5, and 6.1%month^–1, respectively). This suggests that different specieshave adaptations related to the season of seedling emergence and resistance todrought in this tropical seasonal forest community. The traits of tree speciesare classified in terms of their traits in the early stages of regeneration;seed and/or seedling bank, resprouting, and drought resistance. Most specieshave adapted to fire and/or drought by resprouting, seed bank, and/or seedlingbank, although the few species which occur mainly in mesic evergreen forestshave less adapted to these environments. The demographic variations in seed andseedling stages may contribute to the coexistence of these species in thisspecies-rich dry tropical forest.     

Effects of fire on forest dynamics in southern Switzerland

Abstract. We analysed the influence of fire on species composition, floristic diversity and indicator values of the vegetation in forests of the Canton Ticino (South Switzerland). Both frequency of fire and time since last fire were taken into account using variance analysis. Results show a strong interaction of these two parameters, i.e. the reaction of the vegetation after a fire depends on the frequency of past fires. A rapid recovery is generally observed after fire, involving both herbaceous and woody species. However, a long-term tendency towards floristic and edaphic impoverishment is observed with increasing frequency of fires. This phenomenon appears to be linked to the progressive dominance of a few fire-tolerant species.     

The role of seed banks in vegetation dynamics and restoration of dry tropical ecosystems

Abstract. This paper reviews studies on seed banks in tropical dry vegetation and, for comparison, some wet tropical and dry subtropical vegetation. A first general conclusion is that tropical seed banks are smaller than those in temperate ecosystems. Many studies are devoted to only one species, among which are several Acacia species, both tropical and subtropical, some of which reach densities in the seed bank of up to 10000 seeds/m². The lesser importance of the seed bank strategy in tropical species may be related to higher risks of seed loss through higher mortality (fire, predation, pathogens etc.), or to intermittent germination occasions in relation to ephemeral favourable conditions (e.g. rains during the dry season). Regarding vegetation dynamics, the importance of seed banks and regeneration from seeds is most pronounced in larger forest gaps. Examples are discussed of rapid succession to forest of savannas after protection from fire, which is partly due to regeneration from the seed bank. The significance of seed banks in vegetation restoration projects is outlined and the need for additional sowing of seeds of important species underlined.     

The role of seed depth,litter, and fire in the seedling establishment of amphicarpic peanutgrass (Amphicarpum purshii)

Summary Amphicarpum purshii is an annual grass which mostly grows in disturbed areas of the New Jersey Pine Barrens, USA. It is amphicarpic, producing spikelets (and seeds) both above and below the soil surface. Previous research has shown that subterranean seed production ensures reproduction in the event of a major disturbance such as fire and results in rapid post-burn colonization of these sandy habitats. The effects of fire, litter, and seed depth were further examined by planting subterranean seeds at four depths in 16 litter-covered flats buried at ground level and comparing plants arising from burned flats with those in undisturbed litter-covered flats. At 0 and 1 cm depth, rates of seedling emergence were lowest in burned flats. Surface-sown seeds produced seedlings more likely to desiccate. Sowing depth had a greater influence on most measured characters than burning treatments. The mean depth of subterranean seed placement by Amphicarpum is 3.5 cm and this coincides with the seed depth from which plants showed the greatest height growth, shoot biomass, and reproductive output. In a second experiment, subterranean seeds on the bare soil surface in clay pots were more likely to lose viability and less likely to germinate than seeds protected by litter or burial in soil. In addition to providing protection from fire, placement of seeds below ground in the sandy habitat of peanutgrass provides conditions more suitable for seed survival and subsequent seedling establishment.     

Effect of grass litter on seedling recruitment of the critically endangered Cistus heterophyllus in Spain

Cistus heterophyllus Desf. is an Iberian-north African endemic plant species with only one wild population left in Spain. A sowing experiment under controlled conditions was designed to assess both the seedling emergence rate and the growth fitness of C. heterophyllus on a litter layer of Brachypodium retusum (Pers.) Beau., which forms dense carpets around the remnant population of C. heterophyllus. In order to test the physical effects of B. retusum litter on Cistus seedling recruitment, six sowing treatments in pots were compared that combined three doses of litter (0, 0.6 and 1.1 kg m⁻²) and different physical locations of the seeds (above, below the litter layer or within a litter–peat mixture). B. retusum litter interfered with the seedling recruitment of C. heterophyllus, negatively affecting both seedling emergence and their early growth. Such an effect was more severe when the seeds were located above a 0.6 kg m⁻² litter layer, although seedlings from seeds sown under the litter showed a growth reduction ranging from 58% to 100% compared to the no-litter treatment. A significant negative effect of the litter–peat mixture on the number of leaves, leaf size and number of branches of seedlings was also inferred. These data provide experimental evidence on the serious threat that B. retusum litter represents for the conservation and regeneration of the last population of C. heterophyllus in Spain.     

Simulated effects of fire,dispersal and spatial pattern on competition within forest mosaics

Simulations representing tree locations on a rectangular grid (cellular automaton) imply that spatial patterns associated with fire, seed dispersal, and the distributions of plants and resources affect forest dynamics profoundly. Simulated fires ignited at random locations in a uniform environment create non-uniform habitats and lead to patches dominated by different vegetation types. Short-range seed dispersal promotes vegetation clumping; fires cause these clumps to coalesce into vegetation zones separated by sharp borders, especially across an environmental gradient. In simulation of competition within vegetation mosaics, tree populations with a competitive advantage still require the intervention of fire to eliminate rivals. Also, the availability of local seed sources enables established tree populations to exclude invaders, but fires can trigger sudden changes in the composition of such systems. In models of simple succession systems, climax vegetation tends to displace pioneer vegetation, even under harsh fire regimes.     

Allelopathic effects of <Emphasis Type="Italic">Ceratiola ericoides</Emphasis> (Empetraceae) on germination and survival of six Florida scrub species

Allelopathic inhibition of germination by Florida scrub plants has been demonstrated in the greenhouse and lab, but not in the field. We studied the allelopathic effects of Florida rosemary (Ceratiola ericoides) roots, leaves, and litter leachates on field germination and three-month survival of six Florida scrub species, three habitat generalists (Lechea deckertii, Palafoxia feayi, and Polygonella robusta), and three rosemary scrub specialists (Hypericum cumulicola, Lechea cernua, and Polygonella basiramia). We used AIC and model averaging to evaluate support for a series of non-exclusive hypotheses. Species varied in germination (2.7–24.6%) and survival (39.2–71%) percentages, and in their sensitivity to leachates. Germination of scrub species was most negatively affected by leaf > root > litter leachates, although not all species followed the overall trend. Additional germination suppression by leachate combinations (relative to single leachates) was minimal. Sites did not vary in germination, but seedling survival did differ among sites. This study further documents the negative impact of Florida rosemary leachates on the germination of co-occurring plant species. Allelopathy may be partly responsible for bare sand gaps in Florida rosemary scrub, and therefore be one of the forces structuring Florida rosemary scrub ecosystems.     

Comparative studies on acorn and seedling dynamics of fourQuercus species in an evergreen broad-leaved forest

The dynamics and regeneration probabilities ofQuercus acuta, Q. gilva, Q. salicina, andQ. sessilifolia were studied in a 0.48 ha plot in an old-growth evergreen broad-leaved forest, Kagoshima prefecture, southwestern Japan. The canopy was well closed and had only one small gap. Seventeen tree species occurred in the canopy. The fourQuercus species ranked second to fifth in basal area. Excluding the trees originated from sprouts,Q. acuta and Q. gilva lacked trees at the subcanopy layer, andQ. salicina andQ. sessilifolia had some trees at the same layer. For eachQuercus species, the amount of fallen acorns fluctuated from year to year. The number of current year seedlings was positively correlated with the number of fallen acorns in the previous year. A disproportion between the spatial distribution of fallen acorns and that of established seedlings has been shown. The establishment of seedlings was frequently observed where no acorns had fallen in the previous year. Saplings of four species survived for several years and formed sapling banks under the closed canopy. During the three years of the study, the mortality ofQ. salicina andQ. sessilifolia saplings was lower than that ofQ. acuta andQ. gilva. However, the differences in the average annual height growth of saplings between four species were not significant, and the most of them did not grow much. These results suggest that the saplings of the fourQuercus, especiallyQ. gilva andQ. acuta, have no chance to recruit to the canopy or subcanopy layer within a closed stand, and that some changes in the environment caused by gap formation are required for their recruitment.     

Differential effects of lichens,mosses and grasses on respiration and nitrogen mineralization in soils of the New Jersey Pinelands

In the New Jersey Pinelands, severely disturbed areas often do not undergo a rapid succession to forest; rather, a patchy cover of lichens, mosses and grasses persists for decades. We hypothesized that these plant covers affect soil microbial processes in different ways, and that these effects may alter the successional dynamics of the patches. We predicted that the moss and grass covers stimulate soil microbial activity, whereas lichens inhibit it, which may in turn inhibit succession. We collected soil cores from beneath each type of cover plus bare soil within two types of highly disturbed areas—sites subjected to hot wildfires, and areas mined for sand. Organic matter (OM) content, soil respiration and potential N mineralization were measured in the cores. Soils under mosses were similar to those under grasses; they accumulated more OM and produced more mineral N, predominantly in the form of ammonium, than either the bare soils or the soils beneath lichens. Mineralization under lichens, like that of the bare soils but unlike the soils beneath mosses or grasses, was dominated by net nitrification. These patterns were reproduced in experimentally transplanted moss and lichen mats. Mosses appear to create high-nutrient microsites via high rates of OM accumulation and production of ammonium, whereas lichens maintain low-nutrient patches similar to bare soil via low OM accumulation rates and production of mineral N predominantly in the mobile nitrate form. These differences in soil properties may explain the lack of vascular plant invasion in lichen mats, in contrast to the moss-dominated areas.     

Effects of adult neighbour and gap size on seedling emergence and early growth of <Emphasis Type="Italic">Bromus inermis</Emphasis> Leyss

A field study was conducted to determine the effects of neighbour root exclusion and gap size on the seedling emergence and early growth of Bromus inermis. Seeds of B. inermis were added to artificially created gaps in an improved shortgrass steppe in northern China. Neighbour root exclusion was accomplished using PVC tubes sunk in the soil of gaps. Emergence and survival of seedlings were greater in all gaps than in the control (0-cm diameter), but better growth performance (height of seedling, leaves, tillers and dry weight per seedling) was observed only in large gaps (20- and 40-cm diameter) with neighbour root present and gaps without neighbour root presence. Seedling growth performance was improved by reducing root and shoot competition. Neighbour root exclusion provided a favourable environment for seedling establishment. Differences between gaps in light levels and topsoil temperature can explain the patterns of germination. These results confirm that B. inermis is a gap-enhanced species. Our study strongly suggests that asymmetric competition by adult neighbour plants negatively influences the establishment of B. inermis.     

Responses of mycorrhizal and non-mycorrhizal<Emphasis Type="Italic"> Erica cinerea</Emphasis> and<Emphasis Type="Italic"> Vaccinium macrocarpon</Emphasis> to<Emphasis Type="Italic"> Glomus mosseae</Emphasis>

An investigation was carried out on the mycorrhizal colonisation, growth and nutrition of two members of the Ericaceae in close proximity to an arbuscular mycorrhizal (AM) association. This was undertaken by separating mycorrhizal (EM) and non-mycorrhizal (NEM) Erica cinerea and Vaccinium macrocarpon from AM (inoculated by Glomus mosseae) and non-mycorrhizal (NAM) Plantago lanceolata using a 30 µm nylon mesh in a sand culture/pot system. Ericoid mycorrhizal colonisation by Hymenoscyphus ericae on root systems of E. cinerea and V. macrocarpon was in the range 14–22% and 58–69%, respectively. The presence of AM P. lanceolata had no effect on the ericoid mycorrhizal colonisation of E. cinerea and V. macrocarpon. NEM E. cinerea showed reductions in shoot biomass and shoot nitrogen concentrations after exposure to AM P. lanceolata after incubations of 6 and 9 weeks but there were no differences in dry mass, length, and nitrogen and phosphorus concentrations of the root systems between the treatment combinations. Reductions were also found, after incubations of 6 and 9 weeks, in shoot dry mass, leaf area and shoot nitrogen concentrations of NEM V. macrocarpon in the presence of AM P. lanceolata but no changes occurred in the length and dry mass of the root systems. There were no differences in maximum photosynthesis in V. macrocarpon between treatment combinations but NEM V. macrocarpon in the presence of AM P. lanceolata had the lowest transpiration rates and stomatal conductance and the highest nitrogen- and phosphorus-use efficiencies compared with the other treatment combinations. These results are discussed in relation to the type of interaction found in these compatible and incompatible mycorrhizal associations.     

Effects of sand burial depth and seed mass on seedling emergence and growth of Nitraria sphaerocarpa

A greenhouse experiment was conducted to test the effects of sand burial depth and seed mass on seedling emergence and growth of Nitraria sphaerocarpa. Seeds of Nitraria sphaerocarpa were sorted into three size-classes (large, medium, small) and artificially buried at 0, 1, 2, 3, 4, 5 and 6 cm depths in plastic pots filled with unsterilized sand. In the seven treatments, the percent emergence, seedling mass and seedling height, significantly affected by both burial depth and seed size, were highest at the optimal burial depth of 2 cm burial depth, and decreased with increasing burial depth in each seed size-class. Although seedling mass was usually greatest for large seeds and least for small seeds at each burial depth, little difference was observed in seedling height at shallow burial depths of 0–3 cm. In each seed size-class, with increasing burial depth, both root-mass ratio and aboveground stem-mass ratio decreased, while belowground stem-mass ratio increased. In each burial depth, with decreasing seed size, belowground stem-mass ratio increased, while root-mass ratio decreased.     

Effects of light competition and litter on the performance of Viola palustris and on species composition and diversity of an abandoned fen meadow

Theeffects of light competition and litter on seedling recruitment and theperformance of established individuals were examined in Violapalustris. This polycarpic perennial plant was a common component ofspecies-rich fen meadows in northwestern Germany until the middle of thiscentury, but today is considered to be regionally endangered. From summer 1996until summer 1998 a bi-factorial field experiment combining three standing croptreatments (mowing, thinning, control) with two litter layer treatments (litterremoval, no litter removal) was carried out in an abandoned fen meadow toinvestigate the effects of these factors both on V.palustris and on aboveground species composition and diversity.MANOVAs (multivariate analysis of variance) revealed significant treatmenteffects for the performance of V. palustris related toexperimental manipulation. Mowing increased vital attributes including thenumber of rosette leaves, the percentage of individuals with chasmogamous andcleistogamous flowers and the number of rhizomes. In contrast, the length of theleafstalk, mean leaf area and the length of rhizomes were negatively associatedwith mowing. Litter removal significantly increased the number of V.palustris seedlings. The removal of the litter layer already resultedin an increase in aboveground species richness in the first year of theexperiment. After three years of experimental manipulation, both abovegroundspecies richness and diversity were positively associated with mowing.Redundancy Analysis showed that typical wet meadow species (Cardaminepratensis, Lychnis flos-cuculi, Lotuspedunculatus) and species of mesotrophic fens (Violapalustris, Agrostis canina, Potentillapalustris) were positively influenced by mowing. It can be concludedthat V. palustris recruitment and growth is promoted by aregular disturbance regime on fen meadows. The conservation of many typicalspecies of wet meadows and fens in northwestern Germany depends on moderategrazing or cutting. These management types prevent successional changes whichlead to an increase in standing crop and the development of a litter layer, bothof which can cause local extinction.