The mycorrhizal colonization of six wetland plant species at sites differing in land use history

 Five wetland prairie sites and six native plant species in western Oregon were examined to determine patterns of vesicular-arbuscular mycorrhizal fungal (VAMF) colonization. The sites differed in type and intensity of past land use. VAMF colonization was tested in situ on seedlings from both field-sown seeds and from transplants. Colonization was measured as the percentage of root length with arbuscles or vesicles. All species (Deschampsia cespitosa, Downingia elegans, Eriophyllum lanatum, Hordeum brachyantherum, Microseris laciniata, and Plagiobothrys figuratus) became colonized by VAMF during the study. This is the first report of mycorrhizal colonization of these important native species. All sites supported mycorrhizal colonization of some of the experimental species. Average VAMF colonization ranged from 58% to 92% but was unrelated to subjective rankings of land use intensity. These results suggest that VAMF inoculum at all sites was sufficient to support revegetation by at least some species of native plants. Accepted: 16 September 1999     

Land use and life history limit migration capacity of eastern tree species

Aim

Temperate tree species overwhelmingly responded to past climate change by migrating rather than adapting. However, past climate change did not have the modern human‐driven patterns of land use and fragmentation, raising questions of whether tree migration will still be able to keep pace with climate. Previous studies using coarse‐grained or randomized landscapes suggest that dispersal may be delayed but have not identified outright barriers to migration. Here, we use real‐world fragmented landscapes at the scale of forest stands to assess the migration capacity of eastern tree species.

Location

Eastern U.S.A.

Time period

Present day to 2100.

Major taxa studied

Eastern U.S. trees.

Methods

We simulated dispersal over 100 years for 15 species common to the mid‐Atlantic region and that are predicted to gain suitable habitat in the northeast. In contrast to previous studies, we incorporated greater realism with species‐specific life histories and real‐world spatial configurations of anthropogenic land use. We used simulation results to calculate dispersal rates for each species and related these to predicted rates of species habitat shift.

Results

Our simulations suggest that land use in the human‐dominated east‐coast corridor slows species dispersal rates by 12–40% and may prevent keeping pace with climate. Species most impacted by anthropogenic land use were often those with the highest predicted species habitat shifts. We identified two major dispersal barriers, the Washington DC metropolitan area and central NY, that severely impeded tree migration.

Main conclusions

Patterns of anthropogenic land use not only slowed migration but also resulted in effective barriers to dispersal. These impacts were exacerbated by tree life histories, such as long ages to maturity and narrow dispersal kernels. Without intervention, the migration lags predicted here may lead to loss in biodiversity and ecosystem functions as current forest species decline, and may contribute to formation of novel communities.     

浙江省典型天然次生林主要树种空间分布格局及其关联性

松阔混交林和常绿阔叶林是亚热带地区天然次生林代表性的森林类型,通过研究主要树种分布变动趋势,可以有效地指导森林经营措施。在该地区4个1hm~2典型样地,在0—30m尺度范围内综合分析了主要树种的空间分布格局及种间关联性,并对同一树种分发育阶段在不同森林群落中的种群空间分布格局进行了比较,以探讨亚热带地区天然次生林群落空间格局形成和种群维持机制。样地1和2为松阔混交林,其中样地1的马尾松密度较低,样地3和4为常绿阔叶林。研究结果表明:(1)以完全随机模型为零假设时,样地1的主要种群在小尺度(10 m)呈聚集分布,随尺度增加呈随机分布;样地2—4的主要种群在所有尺度呈聚集分布,随尺度增加聚集强度逐渐减弱;以异质泊松模型为零假设时,4个样地的主要种群在大部分尺度呈随机分布;(2)青冈和苦槠的小树(5.0 cm≤DBH10.0 cm)在4样地的所有尺度以聚集分布为主,大树(DBH≥10.0 cm)在松阔混交林样地呈随机分布趋势,但在常绿阔叶林样地青冈在0—20 m尺度、苦槠在所有尺度呈聚集分布;(3)松阔混交林中建群种马尾松和其他树种的种间关联性,在样地1的小尺度为负相关,随尺度增加为不相关,在样地2的所有尺度为负相关;常绿阔叶林中建群种青冈和其他树种的种间关联性在在样地3的小尺度为负相关,随尺度增加为不相关,在样地4的所有尺度为负相关;所有伴生树种的种间关联性以负相关为主。结果说明,种群空间分布格局及其关联性随群落结构和空间尺度的不同而出现变化,在松阔混交林和常绿阔叶林群落格局形成中除了扩散限制和生境异质性以外,密度制约机制在松阔混交林中发挥了重要作用,而在常绿阔叶林中其作用随着树木生活史阶段的提高而减弱。     

Atmospheric nitrogen deposition explains patterns of plant species loss

Atmospheric nitrogen (N) deposition across Europe increased substantially from the 1950s to the 1990s. Targeted surveys suggest a negative correlation between N deposition and species richness within quadrats in sensitive habitats. However, it remains unclear whether plant species losses at national recording scales are correlated with nitrogen deposition. We relate plant species losses before 1987 in Great Britain to reduced and oxidized N deposition, land use change and climate change. The mean Ellenberg fertility (N) indices of plant species lost in each 100 km² cell before 1987 was compared with those of species that were recorded between 1987 and 1999. In 45% of squares, indices of species lost were significantly lower than those for species present after 1986. For 17%, primarily upland, squares, the opposite effect was found. A generalized least squares regression model, with difference in the mean Ellenberg N index between samples as the dependent variable, showed that higher deposition of reduced N was significantly associated with selective loss of species with a lower index. Arable land use and change in arable land use also demonstrated this positive relationship. Rough grazing, change in rough grazing, change in pasture and change in annual precipitation showed negative effects. Difference in Ellenberg R index was highly correlated with difference in Ellenberg N and was negatively correlated with oxidized N deposition, suggesting that the lack of a significant effect of oxidized N deposition on Ellenberg N was because it had effects through both acidification and eutrophication, while the effect of reduced N deposition was primarily through eutrophication. Our results suggest that N deposition, along with land use and precipitation changes, has been a significant driver of local plant extinctions. With N deposition increasing in many parts of the world, local extinctions of plant species may be experienced in other regions.     

Temporal and spatial patterns of seed dispersal in two Cistus species (Cistaceae)

Cistus species are obligate seeding, early colonizers that follow disturbance, particularly fire, in Mediterranean ecosystems. We studied seed release, seed dispersal and soil seed populations in stands of Cistus ladanifer and C. libanotis. Seed release started in mid- to late summer (C. ladanifer) or in early autumn (C libanotis), and continued for a very extended period: 8-10 months in C. ladanifer, and for a mean of 16 months in C. libanotis. The xerochastic capsules of both species released seeds by successive dehiscence of the locules. All capsules begin to dehisce simultaneously at the start of the seed release period, but in C. libanotis capsule fragmentation replaced dehiscence early in the seed release period. In plants of both species, seed shadows were characterized by a peak of density beneath the plant canopy and a very short tail of much lower densities, indicating that seeds are concentrated beneath mother plants when dispersed. Nevertheless, in late May, at the onset of the fire season, soil seed densities beneath plant canopies were low compared with densities expected from seed shadows, but were apparently high enough to allow recovery of the stands if a disturbance, such as fire, had taken place. Seed-eating Bruchidae in summer and granivorous ants during the seed release period were apparently the main causes of seed losses. Results suggest that in both Cistus species, the staggered seed release could constitute an efficient risk-reducing trait. The plant pool of seeds existing throughout most of the year could be a relevant component of Cistus seed banks.     

Spatio‐temporal colonization patterns of forest plant species in a mixed deciduous forest

Abstract. An integrated analysis of the colonization patterns of forest plant species was carried out in a 34‐ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and environmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient enriched soils where much light can reach the ground. Next, the importance of these human‐altered environmental conditions for the colonization of forest plant species was assessed relative to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species‐specific spatio‐temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ‘ancient forest plant species’) are best explained by a combination of spatio‐temporal isolation, soil type, pH and the (non‐)cover of Urtica dioica. By contrast, spatio‐temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ‘ancient forest plant species’ is hampered by a combination of dispersal‐ and recruitment limitation and that the relative importance of both factors is species‐specific.     

Germination and water dispersal of seeds from a threatened plant species Penthorum chinense

To conserve a threatened plant species (Penthorum chinense Pursh) in Japan, seed germination responses to pretreatment (imbibition and/or chilled), temperature and light, and seed dispersal by water were examined. The seeds collected from abandoned paddy fields in a warm temperate region, central Japan, germinated in light (14 h photoperiod; light 22°C, dark 21°C) after a moist-chilled treatment. After this pretreatment, the seeds germinated well at 10–25°C (optimum temperature 15°C), but did not germinate in darkness even at the optimum temperature. Most of the seeds floated on distilled water, but 20–60% of the seeds that were collected from several populations sank in distilled water, indicating dimorphism in seed dispersal by water. The floating and sunken seeds did not show significant differences in weight and germination rate within a population. The addition of a surface-active agent in distilled water submerged the seeds, indicating that the buoyancy of the seeds is attributable to an oil coating on the seed surface that enhances the interfacial tension on the seeds. Three times the number of seeds sank in river water collected from a rural area than in distilled water. A greater number of seeds also sank in water that had increasing concentrations of linear alkylbenzenesulfonate, which is a major component of synthetic detergents. This suggests that the water dispersal of this species is suppressed by surface-active agents, including detergents, in river water.     

Gut passage effect of the introduced red‐whiskered bulbul (Pycnonotus jocosus) on germination of invasive plant species in Mauritius

In Mauritius, many of the worst invasive plant species have fleshy fruits and rely on animals for dispersal. The introduced red‐whiskered bulbul (Pycnonotus jocosus) feeds on many fleshy‐fruited species, and often moves from invaded and degraded habitats into higher quality native forests, thus potentially acting as a mediator of continued plant invasion into these areas. Furthermore, gut passage may influence seed germination. To investigate this, we fed fleshy fruits of two invasive plant species, Ligustrum robustum and Clidemia hirta, to red‐whiskered bulbuls. Gut passage times of seeds were recorded. Gut‐passed seeds were sown and their germination rate and germination success compared with that of hand‐cleaned seeds, as well as that of seeds in whole fruits. Gut passage and hand‐cleaning had significant positive effects on germination of both species. Gut‐passed seeds of both C. hirta and L. robustum germinated faster than hand‐cleaned seeds. However, for L. robustum, this was only true when compared with hand‐cleaned seeds with intact endocarp; when compared with hand‐cleaned seeds without endocarp, there was no difference. For overall germination success, there was a positive effect of gut passage for C. hirta, but not for L. robustum. For both C. hirta and L. robustum, no seeds in intact fruits geminated, suggesting that removal of pulp is essential for germination. Our results suggest that, first, the initial invasion of native forests in Mauritius may not have happened so rapidly without efficient avian seed dispersers like the red‐whiskered bulbul. Second, the bulbul is likely to be a major factor in the continued re‐invasion of C. hirta and L. robustum into weeded and restored conservation management areas.     

Restoration of salt marshes in the Netherlands

The conquest of land from the sea has been a long tradition in the Netherlands. When salt marshes were high enough, they were embanked when it was economically feasible, and transformed into intensively exploited agricultural land. This resulted in the transformation of halophytic communities to glycophytic communities. Often as an alternative, a low levee, a summerdike was built, which greatly reduced the flooding frequency of the landward summerpolder, hence creating a sedimentation deficit therein. Such summerpolders now cover 1200 ha in the Netherlands, 2100 ha in NW-Germany and small areas in England. Due to continuous embankments, the present salt-marsh area is relatively small with respect to the tidal basins. Discussions have been started how to increase the salt-marsh area. Two options will be discussed, firstly de-embankment of summerpolders and maintenance of the protective seawall, secondly increase of the effects of saline seepage behind the seawall by top soil removal. Both options include the restoration of salt-marsh communities (target communities) in intensively agriculturally exploited sites that have been salt marshes before. From the few examples abroad and experiments it is discussed (1) to which extent the sedimentation deficit in summerpolders could be compensated for, (2) if the soil seed bank is likely to contribute to re-establishment of salt-marsh communities, (3) if the dispersal of propagules of halophytic plants will be possible by hydrochory when the summerdike is breached, (4) to what extent is dispersal by endozoochory through waterfowl important in case re-establishment in a saline seepage area behind the seawall without open connection to the sea is envisaged. Two case studies of de-embanked summerpolders in the Netherlands revealed that the sedimentation deficit can be counteracted by rapid sedimentation, provided enough transport is possible from the foreshore. Dispersal by incoming tidal water from the nearby salt-marsh source area into the target area is possible for many salt-marsh plant species. The rate of success seems to depend on the relative position of source area and target area. A case study in a saline seepage area after top soil removal in the Netherlands, showed that the number of viable seeds dispersed by droppings from waterfowl is limited. Hence the possibilities for restoration of inland halophytic plant communities seem much lower than after de-embankment of summerpolders.     

Reproductive potential and seedling establishment of the invasive alien tree Schinus molle (Anacardiaceae) in South Africa

Schinus molle (Peruvian pepper tree) was introduced to South Africa more than 150 years ago and was widely planted, mainly along roads. Only in the last two decades has the species become naturalized and invasive in some parts of its new range, notably in semi‐arid savannas. Research is being undertaken to predict its potential for further invasion in South Africa. We studied production, dispersal and predation of seeds, seed banks, and seedling establishment in relation to land uses at three sites, namely ungrazed savanna once used as a military training ground; a savanna grazed by native game; and an ungrazed mine dump. We found that seed production and seed rain density of S. molle varied greatly between study sites, but was high at all sites (384 864–1 233 690 seeds per tree per year; 3877–9477 seeds per square metre per year). We found seeds dispersed to distances of up to 320 m from female trees, and most seeds were deposited within 50 m of putative source trees. Annual seed rain density below canopies of Acacia tortillis, the dominant native tree at all sites, was significantly lower in grazed savanna. The quality of seed rain was much reduced by endophagous predators. Seed survival in the soil was low, with no survival recorded beyond 1 year. Propagule pressure to drive the rate of recruitment: densities of seedlings and sapling densities were higher in ungrazed savanna and the ungrazed mine dump than in grazed savanna, as reflected by large numbers of young individuals, but adult : seedling ratios did not differ between savanna sites. Frequent and abundant seed production, together with effective dispersal of viable S. molle seed by birds to suitable establishment sites below trees of other species to overcome predation effects, facilitates invasion. Disturbance enhances invasion, probably by reducing competition from native plants.     

Inferring colonization history from analyses of spatial genetic structure within populations of Pinus strobus and Quercus rubra

Many factors interact to determine genetic structure within populations including adult density, the mating system, colonization history, natural selection, and the mechanism and spatial patterns of gene dispersal. We examined spatial genetic structure within colonizing populations of Quercus rubra seedlings and Pinus strobus juveniles and adults in an aspen-white pine forest in northern Michigan, USA. A 20-year spatially explicit demographic study of the forest enables us to interpret the results in light of recent colonization of the site for both species. We assayed 217 Q. rubra seedlings and 171 P. strobus individuals at 11 polymorphic loci using nine allozyme systems. Plant genotypes and locations were used in an analysis of spatial genetic structure. Q. rubra and P. strobus showed similar observed levels of heterozygosity, but Q. rubra seedlings have less heterozygosity than expected. Q. rubra seedlings show spatial genetic clumping of individuals on a scale to 25 m and levels of genetic relatedness expected from the clumped dispersion of half-siblings. In contrast, P. strobus has low levels of genetic relatedness at the smallest distance class and positive spatial genetic structure at scales < 10 m within the plot. The low density of adult Q. rubra outside the study plot and limited, spatially clumped rodent dispersal of acorns is likely responsible for the observed pattern of spatial genetic structure and the observed heterozygote deficit (i.e. a Wahlund effect). We attribute weaker patterns observed in P. strobus to the longer dispersal distance of seeds and the historical overlap of seed shadows from adults outside of the plot coupled with the overlap of seed shadows from younger, more recently established reproductive adults. The study demonstrates the utility of long-term demographic data in interpreting mechanisms responsible for generating contemporary patterns of genetic structure within populations.     

Implications of nonrandom seed abscission and global stilling for migration of wind‐dispersed plant species

Migration of plant populations is a potential survival response to climate change that depends critically on seed dispersal. Biological and physical factors determine dispersal and migration of wind‐dispersed species. Recent field and wind tunnel studies demonstrate biological adaptations that bias seed release toward conditions of higher wind velocity, promoting longer dispersal distances and faster migration. However, another suite of international studies also recently highlighted a global decrease in near‐surface wind speeds, or ‘global stilling’. This study assessed the implications of both factors on potential plant population migration rates, using a mechanistic modeling framework. Nonrandom abscission was investigated using models of three seed release mechanisms: (i) a simple drag model; (ii) a seed deflection model; and (iii) a ‘wear and tear’ model. The models generated a single functional relationship between the frequency of seed release and statistics of the near‐surface wind environment, independent of the abscission mechanism. An Inertial‐Particle, Coupled Eulerian‐Lagrangian Closure model (IP‐CELC) was used to investigate abscission effects on seed dispersal kernels and plant population migration rates under contemporary and potential future wind conditions (based on reported global stilling trends). The results confirm that nonrandom seed abscission increased dispersal distances, particularly for light seeds. The increases were mitigated by two physical feedbacks: (i) although nonrandom abscission increased the initial acceleration of seeds from rest, the sensitivity of the seed dispersal to this initial condition declined as the wind speed increased; and (ii) while nonrandom abscission increased the mean dispersal length, it reduced the kurtosis of seasonal dispersal kernels, and thus the chance of long‐distance dispersal. Wind stilling greatly reduced the modeled migration rates under biased seed release conditions. Thus, species that require high wind velocities for seed abscission could experience threshold‐like reductions in dispersal and migration potential if near‐surface wind speeds continue to decline.     

The underpinnings of land-use history: three centuries of global gridded land-use transitions, wood-harvest activity, and resulting secondary lands

To accurately assess the impacts of human land use on the Earth system, information is needed on the current and historical patterns of land‐use activities. Previous global studies have focused on developing reconstructions of the spatial patterns of agriculture. Here, we provide the first global gridded estimates of the underlying land conversions (land‐use transitions), wood harvesting, and resulting secondary lands annually, for the period 1700–2000. Using data‐based historical cases, our results suggest that 42–68% of the land surface was impacted by land‐use activities (crop, pasture, wood harvest) during this period, some multiple times. Secondary land area increased 10–44 × 10⁶ km²; about half of this was forested. Wood harvest and shifting cultivation generated 70–90% of the secondary land by 2000; permanent abandonment and relocation of agricultural land accounted for the rest. This study provides important new estimates of globally gridded land‐use activities for studies attempting to assess the consequences of anthropogenic changes to the Earth's surface over time.     

Small mammal controls on the climate‐driven range shift of woody plant species

Climate change is resulting in shifts in species’ ranges as species inhabit new climatically suitable areas. A key factor affecting range‐shifts is the interaction with predators. Small mammals, being primary seed predators and dispersers in forest ecosystems, may play a major role in determining which plant species will successfully expand and the rate at which range‐shifts will occur. Plants dispersing seeds beyond the species’ current range limits will encounter seed predators to which these seeds are novel; however, empirical studies of seed predator–novel seed interactions are lacking. The aims of our study were to: 1) quantify seed selection by small mammals presented with ‘novel’ seeds; 2) quantify the post‐selection fate of ‘novel’ seeds; and 3) identify seed traits that affect seed selection and post‐selection seed fate. We designed a field experiment exposing small mammal communities to novel seeds produced by plants expected to shift their ranges in response to climate change. We matched novel seeds with reference ‘familiar’ seeds and studied key steps defining interactions between small mammals and novel seeds. We found that the probability of selection of a novel seed varied among species and was, at times, higher than the selection probability of familiar seeds. Key traits that affected seed selection and the distance a seed was dispersed for caching were shell hardness and seed mass. We also found that 33% of dispersed seeds were cached in optimal germination sites (e.g. within fallen logs and buried under the leaf litter mat). Through seed emergence trials we found that emergence was higher for larger seeds, suggesting that the role of small mammals may be modulated by emergence rates. Our results suggest that the interaction between small mammals and novel seeds may have cascading effects on climate‐induced plant range shifts and community composition.