Causes of arrested succession in coastal dune forest

Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.     

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m^–2) than in the inland zone (0.76±0.78 crabs m^–2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.     

Rodent-favored cache sites do not favor seedling establishment of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China

Seed dispersal by scatter hoarders is widely assumed to involve directed dispersal, in which microhabitats selected for caching also benefit seedling establishment and growth. However, in many systems, this may not be true if animal-favored cache sites do not match the safe sites for plants, or if cache sites benefit the plant in one life stage and not another. Here, we investigated whether cache sites selected by rodents are favorable for seedling establishment and growth of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China. We tracked tagged seeds and compared the germination and growth of seedlings from rodent-cached seeds with that of naturally established seedlings in a secondary forest and shrubland stand. Rodents preferred to cache seeds under shrubs with medium canopy cover (31–60 %) in litter substrate in the secondary forest, and under shrubs with high canopy cover (>60 %) in soil or litter substrate in the shrubland stand, neither of which conveyed an advantage for seedling establishment. Although fewer caches were made along shrub edges, or under low canopy cover (≤30 %) in the secondary forest, or along shrub edges, open areas of grass, or under low canopy cover in the shrubland stand, these cache sites consistently contributed to higher survival rates. The microhabitats of grass, soil, or low canopy cover significantly promoted the emergence, survival, and growth of naturally established seedlings. Our results are best explained by the conflicting demands of rodents for caching seeds in more secure sites and P. armeniaca’s high-light requirements for seedling recruitment. We argue that the relationship between favored cache sites for seed hoarders and safe sites for plants will often not match but may still allow a reasonable rate of establishment and regeneration.     

Grass-to-tree facilitation in an arid grazed environment (Aïr Mountains, Sahara)

The scarcity of tree regeneration is a major concern for the long-term conservation of tree cover in the Aïr-Ténéré Nature Reserve (Sahara, Niger), and the causes of this shortage are poorly understood. We raised the hypothesis that livestock pressure is a key-factor in the mortality of tree seedlings and that facilitation processes among plants may enhance tree regeneration. We tested whether the tussock grass Panicum turgidum could facilitate the regeneration of a keystone tree species, Acacia tortilis var. raddiana, under a grazing regime. We transplanted 3-month-old seedlings of Acacia to plots representing different neighbourhood conditions (in areas without vegetation, amongst intact tussocks, among cropped tussocks, and under branches), which were either fenced or exposed to grazing by large herbivores. Survival and height of 160 transplants were recorded during one year. Acacia performance was much lower in the grazed plots, indicating that large herbivores had a strong negative impact on tree regeneration. Presence of Panicum tussocks had a positive effect on the height of seedlings, likely because they improved soil properties and water availability. No aboveground positive effect on Acacia was detected though the grass did reduce atmospheric aridity. We detected an indirect positive effect of Panicum on Acacia survival in the presence of herbivores when aridity and grazing pressure reached their maximum levels. We obtained evidence that indirect facilitation may occur in arid grazed areas, although it has been predicted to mainly exist in mesic competitive environments. Additionally, our results support the stress-gradient hypothesis against assumptions that competitive effects may dominate in extremely arid environments. Since most of the positive effect on Acacia survival is indirect (biotic grazing refuge) due to the presence of herbivores, transplantation of tree seedlings amongst Panicum may be a simple and low-cost technique for the restoration of Saharan ecosystems degraded by herbivore grazing.     

Gnawing damage by rodents to the seedlings ofFagus crenata andQuercus mongolica var.grosseserrata in a temperateSasa grassland-deciduous forest series in southwestern Japan

The effects of dwarf bamboo,Sasa, cover on the initial morrality of hardwood seedlings were investigated by transplanting 1-year-old beech (Fagus crenata) and current-year oak (Quercus mongolica var.grosseserrata) seedling to three different stands; old-growth beech and secondary oak forests withSasa undergrowth, and aSasa grassland in a grassland-forest series near the top of Mt Jippo, southwestern Japan. The most frequent cause of seedling morrality was gnawing of the stems by rodents. In the beech forest, the gnawing was more likely to occur underSasa cover, suggesting that it provides a good habitat for rodents on the beech forest floor. TheSasa under growth may thus play an imporrant role in regeneration of beech forest. In the oak floor, mortality of both species was low and only a little gnawing occurred during a year. However, no natural oak seedling were found in the forest even after a mast year. This may be because most of the acorns disappeated before establishment. The early-stage demography of hardwood seedling as oak may thus play an imporrant role in regeneration of oak forest. In theSasa grassland where the seed supply is small, almost all of the seedlings died fromo gnawing regardless of the presence ofSasa cover. These factors prevent the recruitment of a sizable seedling bank. Rodents may thus play an imporrant role in maintenance of theSasa grassland.     

Consequences of disperser behaviour for seedling establishment of a mistletoe species

The dispersal process in plants links adults and their offspring. For frugivore‐dispersed plants, animal behaviour can have a strong effect on plant fitness. Many mistletoes are totally dependent on animals that deposit seeds on suitable hosts and particular branch diameters. We characterised the seed dispersal and seedling establishment of the mistletoe Tristerix corymbosus, which at our study site, is exclusively dispersed by the marsupial Dromiciops gliroides. Mistletoes’ fruits have a viscous pulp that remains in the seed even after dispersal. This substance adheres the seed to the host branch. We estimated host branch availability in the forest and seed deposition (faeces) by the marsupial in the study area. Specifically, the branch suitability factors we assessed were host identity, branch status (alive or dead), branch diameter, height, and canopy cover. Lodged faeces were individually marked and the number of seed deposited within these droppings was counted, and we recorded the number of seedlings with true leaves that had established after 1 year to estimate the probability of seedling establishment. Branch diameter and canopy cover had a significant positive effect on seed deposition probability. Seedling establishment probability decreased with the number of seeds deposited per faeces and with canopy cover. In general, the marsupial deposited mistletoe seeds in microsites that increase the chance of seedling establishment. Thus, the movement behaviour of the marsupial has a positive effect on the regeneration process of this mistletoe species.     

The regeneration of pioneer tree species under browsing pressure of Sika deer in an evergreen oak forest

The effects of Sika deer (Cervus nippon) browsing on the regeneration of pioneer species were studied in relation to canopy gaps in a warm temperate evergreen oak forest in Kasugayama, Nara City. Four study sites, three in canopy gaps and one under a closed canopy, were selected and each divided into fenced and unfenced plots. Under the closed canopy, seedlings of all the pioneer species died irrespective of browsing pressure. However, in the canopy gap sites, seedlings of the pioneer species could establish and grow well. The seedling survival ratio in the fenced plots in the canopy gaps was >60% 1 year after germination. However, in the unfenced plots, only <20% of the seedlings survived 1 year, with all dying within 3 years after germination. Thus, the regeneration of pioneer trees in this forest was strongly inhibited by deer browsing. Successful regeneration of a pioneer,Zanthoxylum ailanthoides, occurred for several years even after two major wind disturbances during the past 90 years. This may be due to less browsing pressure from the deer.     

Dung beetles as secondary seed dispersers in a temperate grassland

The two-phase dispersal event in which dung beetles move seeds after endozoochory is often assumed to be advantageous for plant regeneration. Because seeds are expected to end up in favourable and safe germination sites, it is considered as an example of directed dispersal. However, literature so far is restricted to tropical rain forest ecosystems, while data for temperate regions are lacking. In this study, the effect of dung beetles on seedling establishment of endozoochorically dispersed seeds is evaluated for a temperate grassland ecosystem. We performed a field experiment in which cages excluded dung beetles from horse and cattle dung samples with mixed-in grass seeds. Seed germination from these samples was significantly higher than that from samples which were accessible to dung beetles. This indicates that the effect of dung beetles on short-term seedling establishment was negative, which contrasts with the patterns found for large-seeded species used in tropical studies. This is most likely attributed to the lack of roller species and the larger depth at which tunneling Geotrupes species bury seeds.     

Understorey gaps influence regeneration dynamics in subtropical coastal dune forest

Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.     

Gap formation, microsite variation and the conifer seedling occurrence in a subalpine old-growth forest, central Japan

To evaluate the effects of canopy gaps and forest floor microsites (soil, fallen logs, root-mounds, buttresses and stumps) on regeneration of subalpine forests, the gap regeneration and seedling occurrence of conifers (Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia) were studied in two stands of a subalpine old-growth forest, central Japan. The percentage of gap area to total surveyed area was 11.2–11.3% in the stands. Gap regeneration was not common for P. jezoensis var. hondoensis and T. diversifolia. In contrast, gap regeneration by advanced regeneration was relatively common for Abies. Seedling occurrence of P. jezoensis var. hondoensis and T. diversifolia was restricted on elevated surfaces such as stumps and root-mounds, while Abies seedlings could occur on soil as well as on elevated surfaces. Rotten stumps were the most favorable microsites for conifer seedling occurrence, which covered small area in the forest floor. Although canopy gaps were not always favorable for seedling occurrence, all conifer seedlings were larger under canopy gaps than under closed canopy. Canopy gaps and forest floor microsites clearly affected seedling occurrence and growth of conifers. This suggests that regeneration of conifers is related to the difference of growth advantage under canopy gaps and favorable microsites for seedling occurrence.     

Multiple Recruitment Limitation Causes Arrested Succession in Mediterranean Cork Oak Systems

Lack of tree regeneration and persistency of species-poor shrublands represent a growing problem across Mediterranean evergreen oak forests. What constrains forest regeneration is poorly understood, and restoration attempts have been largely unsuccessful. We assessed the contribution of four different mechanisms of tree recruitment limitation (that is, source, dispersal, germination, and establishment) in a cork oak (Quercus suber) system in southern Portugal. Using a combination of field studies and experiments, we quantified seed production, seed removal and dispersal, seed survival and germination, seedling establishment and survival, as well as cork oak natural regeneration for the three dominant vegetation types in this system (Cistus ladanifer shrubland, oak forest, and oak savanna). We found that all four forms of cork oak recruitment limitation were significantly more severe in shrublands than in oak forests and savannas, so that oak seedling recruitment in shrubland was impeded in multiple ways. Our results explain why transitions from shrublands to oak savannas and forests are extremely difficult, and that the release from arrested succession in this system requires the simultaneous relief of multiple constraints on recruitment limitation in the early life history of oaks. These results have important implications for the restoration and conservation of Mediterranean oak systems.     

Factors Related to the Recovery of Subalpine Woodland on Mauna Kea,Hawaii1

We measured mature tree and sapling density, tree associations, crown size, age structure, recovery from ungulate browsing, and grass cover at four study sites in two types of subalpine woodland on Mauna Kea volcano, island of Hawaii. Beginning in 1981, introduced ungulates were reduced in number to allow regeneration of Sophora chrysophylla (mamane) in habitat supporting the endangered Hawaiian finch, Loxioides bailleui (palila). We found Sophora regeneration at all four study sites, but regeneration was higher in mixed species woodland with codominant Myoporum sandwicense (naio) than in areas where Sophora dominated. Regeneration of Myoporum was uniformly very low in comparison. Invasive grass cover, which suppresses Sophora germination, was highest in mid-elevation woodland where Sophora dominated. The distribution of mature and sapling Sophora were both related to study site, reflecting previous ungulate browsing and uneven recovery due to grasses. Densities of Sophora snags were not different among any of the sites, suggesting a more even distribution in the past. Selective browsing before ungulate reduction may have favored Myoporum over Sophora, leading to high densities of mature Myoporum in codominant woodland. After ungulate reduction, however, we found no pattern of competitive inhibition by Myoporum on regeneration of Sophora. Reduction of Myoporum is not likely to enhance habitat for Loxioides as much as supplemental plantings of Sophora, glass control, and continued ungulate eradication. Mid-elevation Sophora woodland areas, where Loxioides forage and nest in high densities, would benefit the most from these management actions.     

Conifer seedling distribution and survival in an alpine-treeline ecotone

The importance of seedling establishment to the position ofalpine-treeline is recognized, yet little is known about factorsaffecting the survival of seedlings of treeline conifers during their initialyears of growth and establishment. This establishment period may have thegreatest mortality of all life stages until death of mature trees by disease orfire. Spatial and temporal patterns in the distribution and survival ofseedlings of Picea engelmannii and Abieslasiocarpa were evaluated over four years in analpine-treeline ecotone of the Snowy Range, Wyoming, USA. Seedlings andsaplings of both species occurred most frequently near islands of adult trees.For P. englemannii, this appeared partly due to decreasedsurvivorship of young seedlings (< 5 cm height) with greaterdistance away from tree islands. Survival of emergents of P.engelmannii was 28% greater on the north compared to southsides of tree islands, 48% lower on south-facing slopes comparedto other aspects, and 70% greater with overhead cover such as treebranches. Survival of emergents was greater in microsites with grass cover(90% survival) compared to without ground cover (44% survival),but lowest in microsites surrounded, but not covered, by grass (19%).From 1994–1999, natural seedling emergence and survival washighest in 1995 (80% survival of 221 P.engelmannii,and 100% of seven A. lasiocarpa, in a sample areaof432 m²), when the smallest mean difference in dailymaximum and minimum temperatures occurred, and lowest in 1994 (30% ofseven P. engelmannii), when above-averagetemperatures were accompanied by low rainfall and clear skies. The growthseasons of 1994 and 1995 had among the lowest and highest precipitation of theprevious 30-year period, respectively. In an artificial seedingexperiment, less than 20% of seedlings of both species survived theirfirst complete year of growth. In the autumn of the second year, almost25% additional mortality was observed in the remaining experimentalseedlings when they were exposed to clear, cold skies without the normalprotection of snowcover. Both spatial and temporal patterns of seedlingsurvivalsuggest that exposure to high sunlight may exacerbate low-temperatureandwater stress in young conifer seedlings, inhibiting their establishment in thisalpine treeline.     

Effect of the grass Leersia hexandra on the dispersal,seed germination,and establishment of Pachira aquatica seedlings

1. In wetlands, hydrochory is one of the main mechanisms of seed dispersal and there is often synchrony between propagule production and the flood season. Different sources of disturbance can prevent seed dispersal to suitable sites, and if environmental conditions are not adequate for germination and seedling establishment, recruitment will be limited, affecting succession.
2. We worked in a disturbed tropical freshwater swamp where the native grass Leersia hexandra has dominated open areas, creating a grass matrix that surrounds patches of swamp forest. Leersia grows vigorously, forming cushions of dry matter that cover the soil, forming a potential obstacle to seed dispersal. We asked whether the vegetative growth of this grass prevents the entry and dispersal of seeds of the tropical swamp tree Pachira aquatica, thwarting seed germination and seedling establishment, and arresting succession. We set up transects in the grass matrix in two zones: close to the river (R) and bordering the tree patches or fragments (F). We quantified tree seed and seedling presence, survival and growth in situ and experimentally introduced seeds and seedlings in the field and monitored seed germination and the survival and growth of their seedlings, as well as that of transplanted seedlings.
3. There was a negative relationship between the number of seeds and established seedlings, and the distance to river or fragment (r = −0.86, p < 0.001 for zone R; and r = −0.77, p < 0.001 for zone F) and with the grass cushion (r = −0.68, p = 0.005 for zone R; and r = −0.66, p = 0.007 for zone F); the grass creating a barrier to dispersal. When seeds were sown after clearing the grass cushion, germination success was high, so this stage is not limited. The transplanted seedlings had better survival and a greater final height than the seedlings of the sown seeds. Grass cover had a negative effect on both types of seedlings. Seedling survival rates were inversely related to grass cover, showing that seedlings overgrown by grass had low survival rates. Flooding is a stress factor for seedlings and produced mortality, in addition to the effects of the grass.
4. Together, the field survey and the experiment show that succession is being arrested in two ways: (1) by limiting seed dispersal because the grass cushion slows the dispersal and penetration of seeds into the vegetation; and (2) by limiting seedling establishment because the grass competes for space and light. Our results show that even where the grass is native, slower growing, seed-dependent species may struggle to compete and establish. If grass cover is increasing, these swamps are very vulnerable to a decrease in area because it is very difficult for them to regenerate naturally.

     

Initial regenerative processes ofDistylium racemosum andPersea thunbergii in an evergreen broad-leaved forest

We studied the initial regenerative processes ofDistylium racemosum andPersea thunbergii, major canopy species, in an old-growth evergreen broad-leaved forest, Kagoshima, southwestern Japan. Although the two species coexisted, the patterns of seed production, seedling emergence, and sapling survival were different between them. During the observation (1989–1991),P. thunbergii produced large numbers of seeds in alternate years, whileD. racemosum had no mast year. The density of established seedlings was high and increased with mass seed production forP. thunbergii, but was fairly low forD. racemosum. The mortality of seedlings and saplings ofP. thunbergii was higher than that ofD. racemosum. In a closed stand, the sapling bank was maintained by dense seedling supply forP. thunbergii, and by extremely low mortality of individuals germinated before the beginning of observation forD. racemosum. Since saplings ofD. racemosum showed continuous height growth beneath the closed canopy, the possibility of recruitment into the upper layer seems to be high. The strategy ofP. thunbergii might be to wait for suitable conditions favoring sapling growth on various site by means of frequent and large seed productivity and wide dispersal of seeds. Thus both species might be able to coexist through environmental heterogeneity.     

Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

Plant cover plays a major role in shaping the nature of recruitment microsites through direct (resource mediated) and indirect (consumer mediated) interactions. Understorey plants may differentially affect seedling establishment, thus contributing to regeneration-niche separation among canopy tree species. We examined patterns of early tree seedling survival resulting from interactive effects of understorey bamboo (Chusquea culeou) and resident consumers in a mixed temperate Patagonian forest, Argentina. Newly germinated seedlings of Nothofagus dombeyi and Austrocedrus chilensis were planted in bamboo thickets and non-bamboo patches, with or without small-vertebrate exclosures. We found species-specific patterns of seedling survival in relation to bamboo cover. Nothofagus survival was generally low but increased under bamboo, irrespective of cage treatment. Desiccation stress accounted for most Nothofagus mortality in open, non-bamboo areas. In contrast, Austrocedrus survival was highest in non-bamboo microsites, as most seedlings beneath bamboo were killed by small vertebrates through direct consumption or non-trophic physical damage. There was little evidence for a negative impact of bamboo on tree seedling survival attributable to resource competition. The balance of simultaneous positive and negative interactions implied that bamboo presence facilitated Nothofagus early establishment but inhibited Austrocedrus recruitment via apparent competition. These results illustrate the potential for dominant understorey plants to promote microsite segregation during early stages of recruitment between tree seedlings having different susceptibilities to water stress and herbivory. We recognise, however, that patterns of bamboo–seedling interactions may be conditional on moisture levels and consumer activity during establishment. Hence, both biotic and abiotic heterogeneity in understorey environments should be incorporated into conceptual models of regeneration dynamics and tree coexistence in forest communities.     

Determinants of coastal treeline and the role of abiotic and biotic interactions

Recent die-off of coastal forests has been attributed primarily to the effects of sea level rise by correlation with tide-gage records. Due to the temporal and spatial scales involved, direct monitoring of sea level rise impacts is challenging and its attribution can be confounded by both land-use history and species interactions. Here we present experimental evidence for a micro-tidal, oligohaline estuarine system that the location of coastal treeline is determined by both environmental controls and positive and negative species interactions. We conducted field surveys and a transplant experiment to determine the controls on pine seedling establishment and survival along a salinity and flooding gradient. Using a two-way changepoint model we determined that sawgrass cover (Cladium jamaicense) and salinity concentrations interact to control natural pine regeneration (Pinus taeda). The long-term removal of sawgrass resulted in increased soil salinity and high rates of (planted) pine seedling mortality. In contrast, pine seedlings planted directly under sawgrass were able to survive at the same level as upland forest plots because of reduced salinity levels. This research provides evidence that sawgrass can facilitate pine seedling survival, but also suggests that either competitive exclusion by sawgrass or dispersal limitations prevent initial pine seedling regeneration. We propose that forest dynamics are closely coupled to fire, which consumes sawgrass biomass and reduces competition between pine seedlings and grass. Following fire, pine seedling establishment and the regrowth of sawgrass facilitates long-term pine seedling survival. Under this scenario, recent marsh invasion into coastal forests may not necessarily represent a permanent state change in locations where abiotic stress is not the only determinant for community composition.     

林隙对香樟和枫香幼苗早期生长阶段功能性状的影响

探究林隙对不同需光性树种早期生长特征和功能性状的影响,对揭示林隙微生境影响次生林内幼苗更新机制具有重要意义。以亚热带次生林中耐荫常绿树种香樟和阳性落叶树种枫香幼苗为试验对象,研究大林隙(D/H介于1.5—2.0)、中林隙(D/H介于1.0—1.5)和小林隙(D/H介于0.5—1.0)对不同需光树种幼苗早期(1—3年生)生长特征和功能性状的影响。结果表明：(1)林隙大小对两种幼苗的生长均有显著影响。其中,中林隙可显著促进香樟2—3年生幼苗的生长,大林隙对枫香1—3年生幼苗的生长均具有显著促进作用。(2)对林隙环境因子与幼苗功能性状的关系进行冗余分析表明,香樟幼苗功能性状的变化与林隙土壤有机质含量、水解性氮含量、酸碱度和有效磷含量密切相关,而枫香幼苗功能性状则主要受林隙土壤酸碱度、有机质含量、水解性氮含量、土壤含水率、冠层透光率和土壤有效磷含量的影响。(3)维持较高的根重比、细根比根长、叶碳氮比和叶碳磷比是幼苗应对林隙环境影响的重要生理生态调节机制。     

Evaluating Dispersal Limitation in Passive Bottomland Forest Restoration

Dispersal limitation can retard natural establishment of desirable species on restoration sites, especially where landscapes are fragmented, but dispersal limitation is assumed to become less critical with time as early colonists become reproductively mature. Distribution patterns of recruiting trees in a 20‐year‐old passively restored bottomland in northeast Louisiana suggested persistent dispersal limitation in some bottomland hardwood species and influence of dense shrub patches on colonization. To test these hypotheses, we measured seed rain as a function of distance to seed source and association with shrub cover. Seed rain of the wind‐dispersed Fraxinus pennsylvanica was highest near the forest edge, except where mature recruits occurred. Although shrub presence did not influence dispersal of F. pennsylvanica, its negative influence on probability of occurrence in the sapling layer suggests that shrub cover may limit its regeneration. The bird‐dispersed Crataegus viridis and Ilex decidua were found in the seed rain and as reproductive individuals within the field; neither had a positive relationship with shrub presence. Dispersal of heavy‐seeded Quercus spp. and Carya aquatica was limited to within 20 m of the forest edge. These results imply that dispersal limitation is diminishing in wind‐ and bird‐dispersed species with maturation of in‐field recruits and that shrub patches may influence these patterns. Heavy‐seeded species, however, remain restricted to field edges that directly abut a seed source. If canopy closure by wind‐ and bird‐dispersed species precedes dispersal of heavy‐seeded species into the field, establishment of Quercus and Carya spp. may remain low for the foreseeable future.     

Seedling and Sprout Response to Slash-and-Burn Agriculture in a Tropical Deciduous Forest1

Survival and growth of seedlings and sprouts were assessed in three plots for 16 mo following the slashing and burning of a tropical deciduous forest in Jalisco, Mexico. We encountered a total of 47 species: 21 seedling species and 35 sprout species. Calliandra formosa and Piptadenia flava were the most common seedling species; Bursera arborea, Cordia alliodora, and Piptadenia constricta were the most common sprouts. Colubrina triflora, Diphysa occidentalis, and Cnidoscolus spinosus had limited sprouting ability. Twenty-six species were represented by one seedling or one sprout. Thirty-eight percent of the seedlings were tree species, 59 percent were shrub species, and 2 percent were vines species. In contrast, 86 percent of the sprouts were from trees and 13 percent from shrubs. One year after the initial measurements, 29 percent of the seedlings and 13 percent of the sprouts were dead. Each of the seedling means (number of stems/individual, height and diameter of the tallest stem, and elliptical crown area) was significantly smaller (P < 0.05) than that of sprouts at all three measurement periods, but relative growth rates were similar. Total canopy area of seedlings had a larger relative increase than did the canopy area of sprouts. The presence of seedlings increased species diversity compared to calculated diversity excluding seedlings. Timing of fruit dispersal in relation to the date of burning and the high number of Leguminosae species in the forest appeared to favor seedling establishment for some species.