Microhabitat associations and seedling bank dynamics in a neotropical forest

We conducted a rigorous test of tropical tree seedling microhabitat differentiation by examining microhabitat associations, survival and growth of established seedlings of ten tropical tree species representing a four-factor gradient in seed size. Eight microhabitat variables describing soil and light conditions were measured directly adjacent to each of 588 seedlings within twelve 10×100 m belt transects at Paracou, French Guiana, and at 264 reference points along the transects. From these measurements, we defined three principal components describing soil richness, soil softness and canopy openness. Six of ten species (in 9 of 30 total cases) were distributed non-randomly with respect to microhabitat along at least one principal component. However, few species demonstrated clear microhabitat specialization. All shifts in distribution relative to reference points were in the same direction (richer, softer soil). Furthermore, of 135 pairwise comparisons among the species, only 7 were significantly different. More than three-fourths of all seedlings (75.3%) survived over the 2-year monitoring period, but survival rates varied widely among species. In no case was the probability of survival influenced by any microhabitat parameter. Relative height growth rates for the seedlings over 2 years varied from –0.031 cm cm^–1 year^–1 (Dicorynia guianensis, Caesalpiniaceae) to 0.088 cm cm^–1 year^–1 (Virola michelii, Myristicaceae). In only 4 of 30 cases was height growth significantly associated with one of the three principal components. Because the conditions in this study were designed to maximize the chance of finding microhabitat differentiation among a group of species differing greatly in life history traits, the lack of microhabitat specialization it uncovered suggests that microhabitat partitioning among tropical tree species at the established seedling stage is unlikely to contribute greatly to coexistence among these species.     

Ant-nest soil and seedling growth in a neotropical ant-dispersed herb

Summary A major hypothesis concerning the benefits of myrmecochory, seed dispersal by ants, to plants is that ant nests are nutrient-enriched microsites that are beneficial to seedling growth. We experimentally test this hypothesis for a neotropical myrmecochore, Calathea ovandensis, asking two questions: 1) is soil of nests of a seed-dispersing ant chemically or structurally distinct from surrounding soils, and 2) do seedlings grow better in soil collected from ant nests than in randomly collected soil? We found that although ant-nest soil was significantly enriched in nitrate-nitrogen, magnesium, iron, manganese, cadmium and percent organic matter compared to randomly collected soil, seedling growth was not significantly improved by ant-nest soil.     

The effects of elevated CO2 on seed production and seedling recruitment in a sheep-grazed pasture

Seed production and seedling recruitment were measured over 2 years under ambient (360 ppm) and elevated (475 ppm) atmospheric CO₂ in a free air carbon dioxide enrichment (FACE) experiment, carried out in a sheep-grazed pasture on dry, sandy soil in New Zealand. In both years elevated CO₂ led to more dispersed seeds of the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and T. subterraneum and the herbs Hypochaeris radicata and Leontodon saxatilis. The increased seed dispersal in A. odoratum, H. radicata, Leontodon saxatilis and T. repens reflected both more inflorescences per unit area and more seeds per inflorescence under elevated CO₂. The increased seed dispersal in Lolium perenne, P. pratensis and T. subterraneum was due solely to more inflorescences per unit area. The number of seedlings that emerged and survived to at least 7 months of age was increased by elevated CO₂ for H. radicata, Leontodon saxatilis, T. repens and T. subterraneum in both years and for A. odoratum and Lolium perenne in the first year. For species where increased seedling recruitment was noted, there was a significant positive correlation between seed production in summer and seedling emergence in the following autumn and winter, and sowing 200 extra seeds per species m^-2 resulted in more seedlings compared to unsown controls. Elevated CO₂ did not affect seedling survival in any species. There was no measurable effect of elevated CO₂ on canopy and soil surface conditions or soil moisture at the time of seedling emergence. The results suggest the dominant effect of elevated CO₂ on seedling recruitment in this pasture was an indirect one, reflecting effects on the number of seeds produced. The biomass of H. radicata, Leontodon saxatilis, T. repens and T. subterraneum in the above-ground vegetation was greater under elevated than ambient CO₂. However, the size of individual seedlings and mature plants of these four species was unaffected by elevated CO₂. The results indicate an important way elevated CO₂ influenced plant species composition in this pasture was through changes in the pattern of seedling recruitment.     

Forest encroachment into a Californian grassland: examining the simultaneous effects of facilitation and competition on tree seedling recruitment

Competition and facilitation are both considered major factors affecting the structure of plant assemblages, yet few studies have quantified positive, negative, and net effects simultaneously. In this study, we investigated the positive, negative, and net effects of tree saplings on the encroachment of two tree species, Douglas fir (Pseudotsuga menziesii) and tanoak (Lithocarpus densiflora), into a coastal California grassland. The study involved three components: sampling the spatial distributions of P. menziesii and L. densiflora in the grasslands, a field experiment examining seedling survival in different grassland environments, and a greenhouse experiment examining the effects of soil moisture on early seedling performance. The field experiment was conducted over a 2-year period, using Pseudotsuga in 2002 and both species in 2003. Seedlings were separated into four treatment groups: those planted in open grassland, in shaded grassland, under artificial (plastic) conifer saplings, and under natural Pseudotsuga saplings. Air temperature, relative humidity, soil moisture, incident radiation levels and fog water inputs were measured for each treatment group in 2003. In the greenhouse experiment, Pseudotsuga and Lithocarpus seedlings were grown for 13 weeks in watering treatments simulating the summer soil moisture conditions of the open grasslands and under Pseudotsuga saplings. Surveys of naturally established seedlings found that Lithocarpus occurred only under Pseudotsuga saplings, while most Pseudotsuga seedlings were located near but not directly under conspecific saplings. In the field experiment, positive effects of tree saplings were much larger than negative effects, resulting in strong net facilitation of seedling establishment. Survival for both species was always higher under the plastic and live trees than in the open or shade plots. The primary mechanism facilitating seedling survival appeared to be increased soil moisture caused by input of fog precipitation coupled with reduced microsite evaporation. The greenhouse experiment further showed that soil moisture strongly affected seedling performance, with both species having much higher photosynthetic rates in the higher moisture treatment. In the lower moisture treatment, Pseudotsuga seedlings had higher photosynthetic rates and stomatal conductance than Lithocarpus, suggesting they may be able to better tolerate the environmental conditions found in the open grasslands. Our combined results suggest that rate and patterning of woody plant encroachment can be strongly influenced by facilitation and that fog precipitation may play a key role in plant interactions.     

Plaice nurseries: effects on recruitment

The function of coastal nursery areas is discussed in relation to the variability in North Sea plaice recruitment. Since 60% of the recruitment of juveniles originates from the Wadden Sea, special attention is paid to this area. The concentration of juveniles in a restricted area seems to evoke only adverse effects: an increased risk of food limitation and hence reduced growth, and an increased vulnerability to predation. Despite these expectations, growth of most of the plaice in the Wadden Sea has always been optimal within the wide range of year-class strength observed and depends only on ambient water temperature. The same situation is indicated for some British bays, where the growth of 0-group plaice is far lower than in the Wadden Sea, because of lower temperatures. Mortality through predation seems to be relatively low in the Wadden Sea and restricted to only a short period, because of the absence of almost all potential predators. In the more open British bays higher mortalities are found, probably due to the presence of a number of predatory fish species. The low variability in the recruitment of plaice might be the combined result of optimal growth with the absence of between-year fluctuations in predator abundance in the Wadden Sea. As a result, observed mortality only depends on prey abundance and is therefore density-dependent both within one year and especially between years, reducing variations in recruitment. This suggestion is supported by the situation in more open British bays: here too, growth is maximal, but the abundance of predators shows larger fluctuations between years and, as a result, greater fluctuations in mortality are observed.     

Germination and seedling development of two mistletoes,Amyema preissii and Lysiana exocarpi: Host specificity and mistletoe-host compatibility

Abstract Potential hosts had no effect on seed germination of the mistletoes, Amyema preissii and Lysiana exocarpi, but affected seedling holdfast formation and leaf emergence. For A. preissii, formation of the holdfast and emergence of leaves occurred most rapidly in seedlings on its usual host, Acacia nyssophylla. Among the six species inoculated with L. exocarpi seeds, formation of the holdfast was quickest on A. nyssophylla and Myoporum platycarpum (usual hosts). Emergence of leaves was quickest in seedlings on A. nyssophylla, and on two non-hosts, Eucalyptus oleosa and Exocarpos aphyllus. For both mistletoe species, seedling mortality was high during the first 3 months, with considerable variation among different host species. After 6 months, little or no mortality was recorded on seedlings growing on their usual hosts. In contrast, seedlings on non-host species suffered continuing mortality and none survived the study period, except for 13% of the L. exocarpi seedlings, which persisted on Geijera linearifolia in a moribund state. The growth of seedlings on host and non-host species varied markedly. After 35 and 47 months, the average length of the longest shoot of two cohorts of A. preissii seedlings was 22 and 75 cm, respectively. After 47 months, the average length of the longest shoot of L. exocarpi seedlings on A. nyssophylla, H. oleifolium and M. platycarpum was 57, 38 and 41 cm, respectively. For both mistletoe species, a proportion of individuals on host species flowered and subsequently set fruits within 3 years. In comparison, none of the plants sown on the non-host species grew beyond seedling stage. The inability of A. preissii and L. exocarpi to establish on non-host species was attributed to mistletoe-host incompatibility.     

The effects of two abundant ant species on soil nutrients and seedling recruitment in Brazilian Atlantic Forest

Ants can influence soil fertility and the spatial distribution of seeds, with possible effects on seedling recruitment. The ant species Pachycondyla striata Fr. Smith, 1858 and Odontomachus chelifer (Latreille, 1802) co-occur in many forest areas in the Neotropics. We assessed soil fertility and seed bank structure in soil samples close and distant (control) from ant nests in forest fragments. We also assessed the richness and abundance of seedlings on nests and control sites. In soil samples from ant nests, the concentration of phosphorus and potassium were respectively 55.6% and 36% higher than in control sites. Aluminium was 11–15% lower in soil samples from ant nests. In the greenhouse, soils from ant nests had higher plant abundance and species richness, but the same species composition in comparison with control sites. Although more plants emerged from soil samples of O. chelifer nests, in the field, the density and richness of seedlings were similar for the two ant species studied. Seedlings in the nest sites were, on average, 1.8 times more abundant and 1.6 times richer in species than in control sites. Our results showed that ant species can play a key role in seedling recruitment in forest fragments, where other animals with equivalent and positive effects, such as mammals, are missing.     

Plant recruitment in the High Arctic: Seed bank and seedling emergence on Svalbard

Abstract. Composition and density of the soil seed banks, together with seedling emergence in the field, were examined on Svalbard. 1213 soil samples were collected from six drymesic habitats in three regions representing various stages of colonization from bare moraines to full vegetation cover and spanning a range of typical nutrient and thermal regimes. Of the 165 vascular plant species native to Svalbard, 72 were present as mature plants at the study sites and of these 70% germinated seed. Proglacial soil had 12 seedlings per m², disturbed Dryas heath 131, intact Dryas heath 91, polar heath 715, thermophilic heath 3113, and a bird cliff 10437 seedlings. Highest seed bank species richness was at the thermophilic heath (26 species). Seedlings of 27 species emerged in the field, with fewer seedlings in disturbed habitats (60 seedlings per m²) than in intact Dryas heath (142), suggesting that an absence of ‘safe sites’ limited seedling establishment in disturbed habitats. Measurement of seedling emergence in the field increased awareness of which species are able to germinate naturally. This may be underestimated by up to 31% if greenhouse trials alone are used, owing partly to unsuitability of greenhouse conditions for germination of some species and also to practical limitations of amount of soil sampled. Most thermophilic species failed to germinate and some species present at several sites only germinated from the thermophilic heath seed bank, suggesting that climate constrains recruitment from seeds in the High Arctic.     

Microsite and elevation zone effects on seed pilferage,germination, and seedling survival during early whitebark pine recruitment

Tree recruitment is a spatially structured process that may undergo change over time because of variation in postdispersal processes. We examined seed pilferage, seed germination, and seedling survival in whitebark pine to determine whether 1) microsite type alters the initial spatial pattern of seed caches, 2) higher abiotic stress (i.e. higher elevations) exacerbates spatial distribution changes, and 3) these postdispersal processes are spatially clustered. At two study areas, we created a seed distribution pattern by burying seed caches in microsite types frequently used by whitebark pine's avian seed disperser (Clark's nutcracker) in upper subalpine forest and at treeline, the latter characterized by high abiotic environmental stress. We monitored caches for two years for pilferage, germination, and seedling survival. Odds of pilferage (both study areas), germination (northern study area), and survival (southern study area) were higher at treeline relative to subalpine forest. At the southern study area, we found higher odds of 1) pilferage near rocks and trees relative to no object in subalpine forest, 2) germination near rocks relative to trees within both elevation zones, and 3) seedling survival near rocks and trees relative to no object at treeline. No microsite effects were detected at the northern study area. Findings indicated that the microsite distribution of seed caches changes with seed/seedling stage. Higher odds of seedling survival near rocks and trees were observed at treeline, suggesting abiotic stress may limit safe site availability, thereby shifting the spatial distribution toward protective microsites. Higher odds of pilferage at treeline, however, suggest rodents may limit treeline recruitment. Further, odds of pilferage were higher near rocks and trees relative to no object in subalpine forest but did not differ among microsites at treeline, suggesting pilferage can modulate the spatial structure of regeneration, a finding supported by limited clustering of postdispersal processes.     

Dwarf palms and cyclanths strongly reduce Neotropical seedling recruitment

Recruitment limitation, which occurs when species either fail to reach regeneration sites or the number of suitable regeneration sites is limited, has been proposed as an important factor that controls population dynamics and species coexistence in tropical moist forests. Dwarf palms in some Neotropical forests dominate the understory strata, and their umbrella-like architecture may intensify understory shade and make the site less suitable for seedling regeneration. In this study we surveyed seedlings under dwarf palms in four habitats at La Selva Biological Station, Costa Rica. We compared seedling recruitment in both 2001 and 2002 at non-palm microsites vs microsites under 1) tall dwarf palms, 2) short dwarf palms, and 3) short palms surrounded by dwarf palms. Light environments at non-palm and under-palm sites were quantified with hemispherical photos. Seedling growth, survivorship, density, and species richness all differed significantly among microsites, and were all greatest at non-palm sites and lowest at dense palm sites. Overall, seedling growth, survivorship, density, and species richness were 30%, 50%, 50%, and 40%, respectively, less at under-palm than at non-palm sites. The main restriction by dwarf palms occurred on post-cotyledonous and woody seedlings. Percent canopy openness differed among microsites, and was greatest at non-palm sites and lowest at dense palm sites. However, seedling responses were only weakly correlated with percent canopy openness. These results revealed significant reduction in seedling recruitment under individual dwarf palms, with greatest negative effects where a palm was surrounded by palms. Dwarf palms occur at high density (2611 individuals/ha) and cover 21% of the understory in the four habitats (alluvium, residual flat area, residual slope area, swamp) of the primary forest. Therefore, this negative impact by individual dwarf palms is significant at the landscape scale as well, based on the great dwarf palm density and coverage in the four habitats.     

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     

Ethiopian mistletoes: new species and combinations

Preparatory to the publication of the Flora of Ethiopia, the following new species and combinations are made: Loranthaceae: Plicosepalus robustus Wiens & Polh., P. ogadenensis M. Gilbert, P. acaciae (Zucc.) Wiens & Polh., P. meridianus (Danser) Wiens & Polh.; Oncocalyx angularis M. Gilbert, O. fischeri (Engl.) M. Gilbert, O. glabratus (Engl.) M. Gilbert, O. ugogensis (Engl.) Wiens & Polh., 0. schimperi (Hochst. ex A. Rich.) M. Gilbert, O. ghikae (Volkens & Schweinfurth) M. Gilbert, O. kelleri (Engl.) M. Gilbert; Erianthemum aethiopicum Balle ex Wiens & Polh.; Englerina woodfordioides (Schweinfurth) Balle ex M. Gilbert; Phragmanthera ahhallensis (Engl.) M. Gilbert, P. erythraea (Sprague) M. Gilbert, P. macrosolen (Steud. ex A. Rich.) M. Gilbert, P. regularis (Steud. ex Sprague) M. Gilbert, P. sarertaensis (Hutch. & Bruce) M. Gilbert, P. usuiensis (Oliver) M. Gilbert. Viscaceae: Viscum triflorum DC. ssp. nervosum (A. Rich.) M. Gilbert.     

鼠类对山杏种子存活和萌发的影响

山杏（Prunus armeniaca)是广布于北京山区阳坡的矮林或灌丛,易生存在土壤贫脊、干旱、或严重退化的山坡、裸地,对于保护生态环境,减少水土流失及促进山区经济发展均具有重要作用。有关鼠类在山杏更新中作用的研究较少,本研究主要是确定影响山杏更新的鼠类种类、鼠类对山杏种子的取食压力、人工埋藏种子及植被因素对种子存活和萌发的影响。于1997年10月3－4日,采用夹捕法同时调查取食花生和山杏种子的鼠类种类及种群密度。于1997年9月24－25日,通过在地表放置山桔种子,调查在鼠类取食下,地表种子消失的速度。于1997年10月20日,作种子埋藏实验,调查种子存活及萌发情况。于1997年10月26日,进行扣网实验,研究网内、外山杏种子存活及萌发情况。结果表明：大林姬鼠（Apodemus speciosus)、社鼠（Rattus confucianus),黑线姬鼠（Apodemus agrarius)为取食山杏种子的主要鼠种种类;与取食花生相比,社鼠更偏爱山杏种子,大林姬鼠对山杏种子喜食性略低,黑线姬鼠对二者喜食程度相近;山杏种子放置地表后1、10、20、30d的消失率分别为13.06%,64.46%,90.70%,96.69%;扣网实验说明,山杏在地表的萌发率极低,生长也慢;而埋入土层内可明显提高萌发率和生长率（若除去鼠类的取食,萌发率还要高）;人工埋藏实验表明,将山杏种子埋入土层5cm后,能够有效地减少鼠类对种子的取食;通过分析山杏种子萌发与植被的关系,发现山杏易在开阔、阳光充分的草丛中存活和生长,而不易在阴闭的灌丛下存活和生长。