Establishment of

We compared establishment of Douglas fir (Pseudotsuga menziesii) and Corsican pine (Pinus nigra) seedlings in kanuka (Kunzea ericoides) and manuka (Leptospermum scoparium) shrubland to test the hypothesis that Douglas fir, because of its greater shade tolerance, is better able to establish in woody communities than pine species. Seed of the conifer species was sown under a range of canopy covers at six sites, the cover being low-statured vegetation in openings between stands, stand edges, and moderate and dense canopies. After three growing seasons, survival of Corsican pine seedlings was greatest in the open and declined progressively as canopy cover increased. This contrasted with Douglas fir, where survival was greatest at the canopy edge. Survival of Douglas fir seedlings significantly exceeded that of Corscican pine seedlings under dense canopy positions. Seedling numbers of both species declined significantly with increasing leaf area index of manuka, but not kanuka stands, where seedling numbers were lower. Leaf area index of manuka stands accounted for substantially greater variation in number and survival of Corsican pine than Douglas fir seedlings. It is concluded that Douglas fir is better able to establish in shaded environments in woody communities than Corsican pine; however, further monitoring is required to confirm the long-term survival of both species under the moderate and dense canopy positions in this trial.     

The effect of canopy cover and seasonal change on host plant quality for the endangered Karner blue butterfly (Lycaeidesmelissasamuelis)

Larvae of the Karner blue butterfly, Lycaeidesmelissasamuelis, feed solely on wild lupine, Lupinusperennis, from the emergence to summer senescence of the plant. Wild lupine is most abundant in open areas but Karner blue females oviposit more frequently on lupines growing in moderate shade. Can differences in lupine quality between open and shaded areas help explain this disparity in resource use? Furthermore, many lupines are senescent before the second larval brood completes development. How does lupine senescence affect larval growth? We addressed these questions by measuring growth rates of larvae fed lupines of different phenological stages and lupines growing under different shade conditions. The habitat conditions under which lupines grew and plant phenological stage did not generally affect final larval or pupal weight but did significantly affect duration of the larval period. Duration was shortest for larvae fed leaves from flowering lupines and was negatively correlated with leaf nitrogen concentration. Ovipositing in areas of moderate shade should increase?second-brood larval exposure to flowering lupines. In addition, larval growth was significantly faster on shade-grown lupines that were in seed than on similar sun-grown lupines. These are possible advantages of the higher-than-expected oviposition rate on shade-grown lupines. Given the canopy-related trade-off between lupine?abundance and quality, maintenance of canopy heterogeneity is an important conservation management goal. Larvae were also fed leaves growing in poor soil conditions and leaves with mildew infection. These and other feeding treatments that we anticipated would inhibit larval growth often did not. In particular, ant-tended larvae exhibited the highest weight gain per amount of lupine eaten and a relatively fast growth rate. This represents an advantage of ant tending to Karner blue larvae.     

A native nitrogen-fixing shrub facilitates weed invasion

Invasions by exotic weedy plants frequently occur in highly disturbed or otherwise anthropogenically altered habitats. Here we present evidence that, within California coastal prairie, invasion also can be facilitated by a native nitrogen-fixing shrub, bush lupine (Lupinus arboreus). Bush lupines fix nitrogen and grow rapidly, fertilizing the sandy soil with nitrogen-rich litter. The dense lupine canopy blocks light, restricting vegetative growth under bushes. Heavy insect herbivory kills lupines, opening exposed nitrogen-rich sites within the plant community. Eventual re-establishment of lupine occurs because of an abundant and long-lived seed bank. Lupine germination, rapid growth, shading and fertilization of sites, and then death after only a few years, results in a mosaic of nutrient-rich sites that are available to invading species. To determine the role of bush lupine death and nitrogen enrichment in community composition, we examined nutrient dynamics and plant community characteristics within a site only recently colonized by lupine, comparing patches where lupines had recently died or were experimentally killed with adjacent areas lacking lupine. In experimentally killed patches, instantaneous pool sizes of exchangeable ammonium and nitrate nitrogen were higher than in adjacent sites free of lupine. Seedlings of the introduced grass Bromus diandrus accumulated 48% greater root biomass and 93% more shoot biomass when grown in a greenhouse in soil collected under experimentally killed lupines compared to B. diandrus seedlings grown in soil collected at least 1 m away from lupines. At the end of the spring growing season, total above-ground live plant biomass was more than twice as great in dead lupine patches as in the adjacent lupine-free grassland, but dead lupine patches contained 47% fewer plant species and 57% fewer native species. Sites where lupines have repeatedly died and reestablished during recent decades support an interstitial grassland community high in productivity but low in diversity, composed of mostly weedy introduced annual plants. In contrast, at a site only recently colonized by bush lupines, the interstitial grassland consists of a less productive but more diverse set of native and introduced species. We suggest that repeated bouts of lupine germination, establishment, and death can convert a rich native plant community into a less diverse collection of introduced weeds.     

Shrub Cover Influence on Seedling Growth and Survival Following Logging of a Tropical Forest

Logging in tropical forests may create large canopy openings. These gaps provide suitable conditions for some opportunistic shrubs and herbs to take advantage of the surge in resources and rapidly colonize disturbed sites. This dense plant cover may limit forest regeneration by interfering with tree seedling establishment, growth, and survival by altering the light and nutrients available to seedlings, modifying herbivore behavior, or a number of other factors. In Kibale National Park (Uganda), old logging sites are mainly covered by dense stands of Acanthus pubescens Engl., which appear to inhibit tree regeneration. We wanted to identify the ecological processes underlying this regeneration collapse. To do so, we designed a factorial experiment to evaluate the influences of herbivory and vegetation cover on the growth and survival of tree seedlings. We compared the survival and growth of transplanted tree seedlings in A. pubescens stands and logged forests, in the presence or absence of the understory vegetation layer (logged forest) or vegetation cover (A. pubescens), and with or without herbivory. We found no evidence to support the hypothesis that herbivory is significantly higher under dense A. pubescens cover. Seedling survival was not influenced by the environment. Seedling growth, however, was positively influenced by the removal of A. pubescens, suggesting that changes in resource availability associated with the presence of A. pubescens, may be important for regeneration. Our results suggest that sustained cutting of A. pubescens cover could foster the growth of established seedlings and could lead to tree regeneration and habitat restoration.     

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.     

The effects of size of opening in vegetation and litter cover on seedling establishment of goldenrods (Solidago spp.)

We investigated the effects of size of opening in the vegetation and litter cover on seedling establishment of two species of goldenrods (Solidago spp.) in an abandoned field in southwestern Michigan, U.S.A. Seeds of S. canadensis and S. juncea were sown into clipped plots, ranging from 0 cm (control, unclipped) to 100 cm in diameter, with and without litter. Seedling emergence, survival and growth were followed for one year. Soil moisture was not significantly different among the opening sizes, but, within a size, tended to be lower when litter was removed. Light intensity at the soil surface was positively related to opening size early in the growing season, but later in the growing season reached a maximum in intermediate-sized openings and then leveled off. Litter strongly inhibited seedling emergence in both species. Emergence of S. canadensis seedlings was lower in 0 and 10 cm openings than in the larger openings, while emergence of S. juncea seedlings was lower in the largest openings (100 cm) than in all the smaller openings. In contrast, seedling growth and probability of survival increased with diameter of opening for both species. Some seedlings of S. juncea did survive in complete vegetation cover (controls, 0 cm openings) while seedlings of S. canadensis survived only in openings of at least 30 cm diameter. Thus, S. juncea had a smaller minimum opening size for seedling establishment than S. canadensis, although both species grew and survived best in the largest openings made in the experiment.     

Restoring Perennial Warm‐Season Grasses as a Means of Reversing Mesophication of Oak Woodlands in Northern Mississippi

Fire suppression has removed an important ecological force previously responsible for shaping many plant communities throughout the world. Upland areas of north‐central Mississippi that have been protected from fire are now closed‐canopy forests including species known to be uncommon as bearing/witness trees in upland portions of the landscape (historically off‐site species) and sparse ground cover vegetation. Anecdotal evidence suggests that warm‐season grasses were prevalent in the understory of these communities, which could have provided more consistent fuel. We corroborate the historic presence of these grasses by looking at their natural co‐occurrence with oak regeneration (a requisite of self‐replacing stands of oaks found historically). Restoration of these communities has typically focused on burning and off‐site tree thinning. Utilizing a restoration experiment implementing these treatments, we found significantly reduced understory leaf litter in treatment areas. To test which variables associated with restoration treatments were most important for the survival of these grasses, we measured the effect of leaf litter removal and its interaction with environmental conditions on the survival of transplanted shoots. Survival of little bluestem increased with decreasing canopy density and decreasing leaf litter. Leaf‐litter removal did not increase survival, nor did it interact with either pre‐treatment leaf litter depth or canopy density. These results show that little bluestem benefits from conditions expected historically: increased light and possibly fire.     

东灵山地区辽东栎幼苗的建立和空间分布

研究树木实生苗的建立对森林生态系统的保育和恢复具有重要意义,在北京东灵山地区调查了辽东栎(Quercus liaotungensis)实生和萌生幼苗在几种典型的植被类型中的空间分布以及辽东栎一年生实生幼苗在1个林窗梯度上的建立。选择几种典型的植被类型,研究辽东栎实生幼苗和萌生幼苗在森林中的空间分布。结果显示,在不同植被类型中辽东栎实生和萌生幼苗的密度存在空间差异,实生苗密度随幼苗年龄增大逐渐降低,种群的更新主要依靠萌生苗完成。在1个落叶阔叶林中选择1个林窗,在不同梯度(即林窗中间、林窗和树冠连接处,以及树冠下)上,播种辽东栎坚果,第二年秋季调查实生幼苗的2个生长指标和出苗率。结果显示,林窗对一年生实生幼苗的生长高度有影响。幼苗高度在林窗中比在树冠下大;林窗对辽东栎幼苗最长叶片长度和幼苗的出苗率没有影响。说明辽东栎实生幼苗的良好生长需要阳光比较充足的生境条件。本项研究结果建议,对辽东栎种群的更新,应该适当择伐一些较大个体,使森林形成一些林窗或林中空地,以利于辽东栎实生幼苗的建立,这样才能使辽东栎种群通过实生苗进行更好的自然更新。     

Leaf litter and irrigation can increase seed germination and early seedling survival of the recalcitrant‐seeded tree Beilschmiedia miersii

Although regeneration of recalcitrant‐seeded tree species can be affected by prolonged drought, especially in Mediterranean regions, little is known about the response of such species to varying site conditions. A field experiment was performed to determine the effect of irrigation and leaf litter cover on seed germination and early seedling survival of the vulnerable recalcitrant‐seeded tree Beilschmiedia miersii (Lauraceae). Two levels of irrigation (non‐irrigated and irrigated units) and three levels of leaf litter depth (0, 5 and 12 cm) were applied to 72 groups of 30 seeds along a wet ravine of the Coastal Range of Central Chile, equally distributed across sites with different levels of canopy cover. Seed germination was significantly increased by irrigation only under closed‐canopy cover, and by leaf litter cover (>5 cm) under all canopy conditions. The effect of leaf litter on germination increased with canopy openness, while the effect of irrigation did not show any tendency. Meanwhile, early seedling survival was significantly increased by irrigation under intermediate canopy cover, and by leaf litter (>5 cm) under closed‐canopy cover. As a result of its overall positive effect on germination, leaf litter should be maintained within B. miersii communities, particularly under intermediate to closed‐canopy conditions, where it can also increase early seedling survival, and both seed germination and early seedling survival might be increased through additional water inputs. The presence of leaf litter might help retaining such inputs, prolonging their effect on regeneration of B. miersii communities. We see this as a baseline assessment of regeneration and persistence that needs further testing on species with similar traits, given the expected increase in the frequency and length of dry periods into the future.     

Leaf litter input mediates tadpole performance across forest canopy treatments

Understanding the mechanisms limiting the distributions of organisms is necessary for predicting changes in community composition along habitat gradients. In many areas of the USA, land originally cleared for agriculture has been undergoing a process of reforestation, creating a gradient of canopy cover. For small temporary wetlands, this gradient can alter abiotic conditions and influence the resource base of wetland food webs by affecting litter inputs. As distributions of amphibians and many other temporary wetland taxa correlate with canopy cover, we experimentally manipulated shade levels and litter types in pond mesocosms to explore mechanisms limiting species performance in wetlands with canopy cover. Most differences between ponds were mediated by litter type rather than direct effects of shading. Although all three amphibian species tested are open-canopy specialists, spring peepers were the only species to show decreased survival in shaded ponds. Pond litter type generally had strong effects on growth and development rates, with tadpoles of two species in grass litter ponds growing to twice the size of, and metamorphosing 7 days earlier than, those in leaf litter ponds. Contrary to our initial hypothesis, shade level and litter type showed very few significant interactions. Our results indicate that the effects of shading cannot be considered in isolation of vegetation changes in pond basins when evaluating the effects of forest succession on temporary pond communities.     

Multivariate path analysis of the relationships between seedling regeneration and environmental factors beneath a dwarf bamboo understory

Seedling emergence and establishment are fragile processes that determine the direction and structure of forest succession and regeneration. However, seedling emergence and establishment are easily affected by biotic and abiotic (environmental) factors. A dense and expanding understory of dwarf bamboo is one such important factor that can seriously hinder the seedling regeneration. We conducted a field experiment to investigate the emergence and establishment of canopy tree seedlings under artificially controlled densities of dwarf bamboo. We found that understory dwarf bamboo obstructed seedling emergence but reduced the death of seedlings. Although understory dwarf bamboo reduced the median retention time of seedlings, dense bamboo increased the mean survival time of seedlings. Our results suggest that understory dwarf bamboo has multiple selectivities for tree seedling emergence and establishment: high‐density dwarf bamboo was beneficial to evergreen species but lower‐density of bamboo was conducive to the survival of deciduous species, it means the dwarf bamboo potentially alters successional trajectories of forest communities. Path analysis revealed that the most important factors affecting tree seedling emergence and death were the abundance of seeds in the seed bank and the density of emerged seedlings, and that the soil temperature promoted seedling emergence but increased seedling death, the thickness of litter limited seedling emergence, and the leaf area index of the bamboo canopy limited seedling death. The present study suggests that dwarf bamboo can directly alter the microenvironment, significantly reducing light levels and soil temperature but increasing the thickness of litter and soil humus, thereby indirectly impacting the regeneration of tree seedlings. Our results indicate that various factors affected seedling emergence, and there were complex indirect relationships among these factors. In general, biological factors had a stronger influence on tree seedling regeneration than environmental factors.     

Importance of seed size for the establishment of seedlings of five deciduous broad-leaved tree species

Effects of seed size and phenology on the establishment of five deciduous broad-leaved tree species were examined in deciduous woodland. Treatments included absence and presence of litter in the forest understory, a small gap, and a large gap. Seedling emergence of large-seeded speciesQuercus mongolica var.grosseserrata andAcer mono was not reduced by accumulation of litter in the forest understory, but was promoted in the large gap where litter was less. Seedling emergence of small-seeded species,Alnus hirsuta, Cercidiphyllum japonicum andBetula platyphylla var.japonica, was reduced by the litter in almost all of the sites. Seedlings of large-seeded species avoid shade stress phenologically by unfolding all of their large leaves in a short period before canopy closure in the forest understory. These species had little mortality after seedling emergence. In contrast, small-seeded species have a longer duration of leaf emergence, shorter leaf longevity, and rapid leaf turnover in all the sites. These seedlings attained similar height to those of the large-seeded species at the end of the second year in the large gap, but survival and height growth rate decreased after canopy closure in the forest understory. We suggest that the importance of seed size in determining seedling establishment largely depends on the relationships between seasonal changes of environmental conditions and phenological traits of seedlings, which are related to seed size.Abbreviations Ah Alnus hirsuta - Am Acer mono - Cj Cercidiphyllum japonicum - Bp Betula platyphylla var.japonica - Qm Quercus mongolica var.grosseserrata     

Establishment of microscale vegetation pattern in maritime chaparral after fire

We quantified microscale pattern in vegetation and seed assemblages along a 24 m transect before and for two years following a controlled burn in chamise (Adenostoma fasciculatum) chaparral in central coastal California. Our objective was to document scale-dependent correlation between pre-burn seed assemblages, pre-burn canopy cover, microtopography, soil temperatures during burning, post-burn seed assemblages and post-burn vegetation. Scale-dependent correlations among pre- and post-burn seed densities, maximum soil temperatures during burning, microtopography and post-burn vegetation were measured based on two-term local covariance analysis.Seed distribution varied among species prior to fire, with seeds of some annual species concentrated in gap areas. Maximum soil temperatures during burning ranged from less than 50 °C to 225 °C, and were generally lowest in gaps in the pre-burn canopy. These gaps were associated with local topographic depressions. After burning, readily germinable seeds were concentrated in or near gaps in the pre-burn canopy. Germination of different species was variously enhanced, diminished or unchanged by the passage of fire. Post-burn vegetation was very patchy, with some areas nearly devoid of seedlings and other areas, especially pre-burn canopy gaps, supporting numerous seedlings. Seedling recruitment patterns in the second year were generally highly correlated with patterns in pre-burn seed banks and first year vegetation. Although many species exhibited similar recruitment patterns, several different mechanisms may have been responsible for the origin of those patterns.     

Survival,growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

In herbaceous dominated patches and ecosystems, tree establishment is influenced partly by the ability of woody seedlings to survive and grow in direct competition with herbaceous vegetation. We studied the importance of season long wet and dry spells on the competitive interactions between herbaceous vegetation and oak seedlings along a light and nitrogen gradient in an infertile secondary successional grassland in central North America. We conducted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of light (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m^–1 yr^–1), and three levels of water input (low, medium and high). In addition, seedlings were grown with and without the presence of surrounding herbaceous vegetation under both light and all three water levels. Seedling survival, growth, and rate of photosynthesis were significantly affected by competition with herbaceous vegetation and these effects varied along the multiple resource gradient. Overall, seedling survival of both species was significantly greater in wetter and shaded plots and when surrounding herbaceous vegetation was removed and was lower in nitrogen enriched plots. We found that soil water was significantly affected by varying inputs of water, light, and the presence or absence of herbaceous vegetation, and that seedling survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry spells on tree seedling success in grasslands can be affected by light and soil nitrogen availability.     

Vegetative communities as indicators of ground beetle (Coleoptera: Carabidae) diversity

Accurately measuring biodiversity is essential for successful conservation planning. Due to biodiversity’s complexity, specific taxa are often chosen as indicators of patterns of diversity as a whole. Such taxa can include vegetation which can inform conservation decisions by demarcating land units for management strategies. For land units to be useful, they must be accurate spatial representations of the species assemblages present on the landscape. In this study, we determined whether land units classified by vegetative communities predicted the community structure of a diverse group of invertebrates—the ground beetles (Coleoptera: Carabidae). Specifically, that (1) land units of the same classification contained similar carabid species assemblages and that (2) differences in species structure were correlated with variation in land unit characteristics, including canopy and ground cover, vegetation structure, tree density, leaf litter depth, and soil moisture. The study site, the Braidwood Dunes and Savanna Nature Preserve in Will County, Illinois is a mosaic of differing land units. Carabid beetles were sampled continuously with pitfall trapping for 1 year (excluding winter) from September 2011 to November 2011 and from March 2012 to September 2012. Land unit characteristics were measured in July 2012. Nonmetric multidimensional scaling (NMDS) ordinated the land units by their carabid species assemblages into five ecologically meaningful clusters: disturbed, marsh, prairie, restoration, and savanna. The subset of land unit characteristics with the highest rank correlation with the NMDS ordination included soil moisture, leaf litter depth, percentage of canopy cover, and percentage of grass ground cover. Land units classified by vegetative communities effectively represented carabid species assemblages.     

Indirect effects of deer herbivory on local nitrogen availability in a coastal dune ecosystem

Herbivores can have indirect effects on local nutrient availability if their direct effects on plants lead to changes in the amount or chemical composition of litter reaching the soil surface. Using two exclosure experiments, we evaluated this possibility for black-tailed deer ( Odocoileus hemionus columbianus ) feeding on silver bush lupine ( Lupinus chamissonis ) in a coastal dune system in northern California. Our first experiment assessed the effects of deer herbivory on 360 lupines that were tracked from newly germinated seedlings in 1996 and 1997 until 2000, when most surviving individuals had reached reproductive age. Results from this experiment showed that browsing by deer significantly reduced growth rates and seed production of lupines and delayed the onset of reproductive maturity. Although deer had no effect on the density of litter accumulating underneath shrubs (g m⁻²), browsing significantly decreased C:N ratios of leaf tissue, due primarily to increased nitrogen content of leaves. Deer browsing also caused significantly increased net nitrogen mineralization rates in the soil under shrubs, although pools of ammonium and nitrate were uninfluenced. The second experiment examined 72 established bush lupines from 1997 to 2000, and showed that deer browsing significantly decreased seed production but had no effect on shrub growth. We also detected trends for browsed shrubs to have reduced pools of ammonium and nitrate underneath their canopies, although no such patterns emerged for nitrogen mineralization rates. Collectively, these data suggest that changes in litter quality, more so than litter quantity, contributed to the indirect effects of deer herbivory on local nitrogen dynamics. We hypothesize that deer browsing induced chemical changes in lupine plant tissue, which increased the nitrogen content of litter reaching the soil surface and subsequently increased rates of nitrogen mineralization.     

The effects of gap size and disturbance type on invasion of wet pine savanna by cogongrass, Imperata cylindrica (Poaceae)

Cogongrass is a nonindigenous species perceived to threaten native communities of the southeastern United States through modification of species composition and alteration of community processes. To examine how gap size and disturbance type influence the invasion of wet pine savannas by cogongrass, we performed three field experiments to evaluate the response of cogongrass seeds and transplanted seedlings to four different gap sizes, four types of site disturbance, and recent burning of savanna vegetation. Cogongrass germinated, survived, and grew in all gap sizes, from 0 to 100 cm in diameter. Similarly, disturbance type had no effect on germination or seedling and transplant survival. Tilling, however, significantly enhanced transplanted seedling growth, resulting in a tenfold increase in biomass over the other disturbance types. Seedling survival to 1 and 2 mo was greater in burned savanna than unburned savanna, although transplant survival and growth were not affected by burning. Results of this study suggest that cogongrass can germinate, survive, and grow in wet pine savanna communities regardless of gap size or type of disturbance, including burning. Burning of savanna vegetation may enhance establishment by improving early seedling survival, and soil disturbance can facilitate invasion of cogongrass by enhancing plant growth.     

Yellow Bush Lupine Invasion in Northern California Coastal Dunes I. Ecological Impacts and Manual Restoration Techniques

We studied the ecological effects of the invasion of coastal dunes by Lupinus arboreus (yellow bush lupine), an introduced species, and used the results to develop manual restoration techniques on the North Spit of Humboldt Bay. Vegetation and soil data were collected in five vegetation types representing points along a continuum of bush lupine's invasive influence. We collected data on the number and size of shrubs, vegetation cover, and soil nutrients. One set of plots was subjected to two restoration treatments: removal of lupine shrubs only, or removal of all nonnative vegetation and removal of litter and duff. Treatments were repeated annually for four years, and emerging lupine seedlings were monitored for three years. Prior to treatment, ammonium and nitrate were found to increase along the lupine continuum, but organic matter decreased at the extreme lupine end. Yellow bush lupine was not the most significant variable affecting variation in soil nutrients. After four years, nonnative grasses, including Vulpia bromoides, Holcus lanatus (velvet grass), Bromus spp. (brome), and Aira spp. (European hairgrass), were significantly reduced in those restoration plots from which litter and duff was removed. Native species increased significantly in vegetation types that were less influenced by lupine. By the third year, soil variables differed among vegetation types but not by treatment. Bush lupine seedling emergence was higher, however, in plots receiving the litter and duff removal treatment. Based on these results, we conclude that bush lupine invasion results in both direct soil enrichment and indirect enrichment as a result of the associated encroachment of other nonnative species, particularly grasses. Although treatment did not affect soil nutrients during the period of this study, it did reduce establishment of nonnative grasses and recruitment of new bush lupine seedlings. Restoration should therefore include litter and duff removal. In areas that are heavily influenced by lupine and contain few native propagules, revegetation is also required.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

光对热带雨林冠层树种绒毛番龙眼种子萌发及其幼苗早期建立的影响

绒毛番龙眼(Pometia tomentosa)为西双版纳热带季节雨林标志树种,被列为中国珍稀濒危植物。对其开展种子及幼苗生态学研究可为珍稀濒危树种的保护及当地森林恢复提供科学依据。试验采用遮荫试验和野外试验相结合的方法,研究光对绒毛番龙眼种子萌发及幼苗早期建立的影响。结果表明:在遮荫试验30%(林窗中心光)、10%(林窗边缘光)和3.5%(林下光)3个光处理及森林3种生境(林窗中心、林窗边缘和林下)条件下,种子萌发率差异无显著性差异,均达到95%以上,且平均萌发周期小于6 d。3个光处理下的幼苗生长、生物量分配模式及气体交换参数差异显著。30%光下绒毛番龙眼幼苗的根重比(RMR)和茎重比(SMR)最高,10%和3.5%光下幼苗的叶重比(LMR)最高,3.5%光下的叶面积比(LAR)显著高于30%光下。30%光下绒毛番龙眼幼苗的最大净光合速率(P_nmax)、暗呼吸速率(R_d)和光饱和点(I_sat)在3个光处理中均最大,光补偿点(I_comp)则无显著性差异。绒毛番龙眼幼苗具耐荫性,能够在低光环境下长期存活且能缓慢生长;同时具有适应林窗光环境生长的能力,如高的质量相对生长速率(RGR_M)和高度相对生长速率(RGR_H)。林窗的出现是绒毛番龙眼进行成功更新的必要条件,水分可能是限制其幼苗生境选择的另一环境因子。