An ericoid shrub plays a dual role in recruiting both pines and their fungal symbionts along primary succession gradients

Relative importance of positive and negative interactions between plant species may change along disturbance and resource gradients. Positive interactions are suggested to prevail in low resource, low productivity (high stress) conditions and negative interactions in high resource availability. A dwarf shrub, mountain crowberry Empetrum nigrum ssp. hermaphroditum, is known to have allelopathic impacts on both Scots pine Pinus sylvestris and its ectomycorrhizal symbionts. We aimed to study if the outcome of Empetrum impacts on Scots pine changes along primary succession gradients on the dune shores of Bothnian Bay, in Finland, where abiotic stress gradually changes to biotic stress along the succession. We found that Empetrum may act as a facilitator despite its allelopathic effects, since the proportion of Scots pine seedlings established in Empetrum patches was higher than in patches without the shrub in early and mid succession stages, whereas patches without Empetrum were preferred in late succession. The amount of mycelial fungal biomass (ergosterol) in the soil in the vicinity of the seedling roots was higher in Empetrum patches than in patches without Empetrum and it increased along the succession gradient. Proportion of pine root tips colonised by suilloid morphotypes with abundant external mycelia and the diversity of ectomycorrhizal morphotypes were higher in mid successional stage in Empetrum patches compared to patches without Empetrum. Our results suggest that in the harsh physical conditions of the dune shore Empetrum facilitates pine seedling establishment in the early and mid stages of succession by providing mechanical and physical shelter whereas in late succession negative interactions (competition and allelopathy) between the shrub and the pine are dominating. To our knowledge we present the first finding that an ericoid mycorrhizal shrub could enhance both the performance of the ectomycorrhizal host tree and the tree's fungal symbionts.     

Population dynamics and growth patterns for a cohort of northern red oak (Quercus rubra) seedlings

Summary I studied the survival and development of a 1986 cohort of northern red oak (Quercus rubra L.) seedlings growing under a variety of overstory and microsite conditions in a northern hardwood forest dominated by northern red oak, red maple (Acer rubrum L.) paper birch (Betula papyrifera Marsh.), and scattered white pine (Pinus strobus L.). Fifty naturally regenerating seedlings of oak were randomly selected in each of three canopy classes: no overstory, partial overstory, and complete overstory. Growth and mortality were measured for six years. Seedling height growth decreased with overstory density, with less growth evident with even a partial overstory. Seedling survival also declined with overstory density and depended on microtopography to a lesser extent. After six years, 92% of the seedlings survived in the open, compared to 54% under the partial overstory, and 36% under the complete overstory. The open environment, in which woody and herbaceous regrowth formed a low canopy reducing light intensities to about 50% of full sunlight, provided a favorable site for the growth and survival of northern red oak.     

Nitrogen‐dependent recovery of subarctic tundra vegetation after simulation of extreme winter warming damage to Empetrum hermaphroditum

Vast areas of (sub)arctic tundra are dominated by the ericoid dwarf shrub Empetrum hermaphroditum. Recent experimental and observational data have shown that Empetrum can be damaged heavily by recurrent extreme winter warming. In addition, summer warming leads to increased soil N availability in tundra ecosystems. In a 7‐year experiment, I investigated the recovery of subarctic Empetrum‐dominated tundra vegetation using a factorial combination of various degrees of aboveground Empetrum removal (simulating the damaging effects of extreme winter warming) and N addition (simulating one of the effects of summer warming). After 7 years no new species had established in the plots. The growth of planted Betula nana seedlings was stimulated by Empetrum removal and reduced by N addition. This Empetrum‐dominated tundra ecosystem was resilient against severe disturbances. Only when Empetrum was 100% removed did it fail to recover, and only in combination with high N supply the subordinate species (notably Eriophorum vaginatum and Rubus chamaemorus, a graminoid and a forb) could benefit. In the 50% removal treatment Empetrum recovered in 7 years when no N was supplied and the cover of the subordinate species did not change. However, when N was added Empetrum recovered faster (in 4 years) and the subordinates decreased. When Empetrum was not removed and N was added, Empetrum even increased in abundance at the expense of the subordinate species. Thus, profound changes in tundra ecosystems can only be expected when Empetrum is very heavily damaged as a result of recurrent extreme winter warming and when soil N availability is increased as a result of summer warming. These changes in species composition upon extreme disturbance events may lead to a wide variety of ecosystem feedbacks and cascade processes as this tundra system is relatively species‐poor, and can be hypothesized to have low functional redundancy.     

Fine-scale spatial heterogeneity and seed size determine early seedling survival in a desert perennial shrub (<Emphasis Type="Italic">Ericameria nauseosa:</Emphasis> Asteraceae)

In heterogenous landscapes, seeds settle in some microsites more readily than others, independently of whether those microsites are suitable for germination and subsequent growth, and survival of seedlings. Wild seeds and seedlings of Ericameria nauseosa var. oreophila in the sand-dune ecosystem near Mono Lake, California show both concordance and conflict in where seeds are retained and where seedlings flourish. Using a field experiment, we followed performance of seeds and seedlings (i.e., the rate of germination and successful emergence, initial seedling size, seedling growth and survival over first growing season), by planting seeds of known sizes in pre-identified microsites (windward and lee aspects of interspaces between shrubs, under shrub canopies, and under snag canopies, respectively). Heavier seeds were more likely to germinate and emerge successfully and resulted in initially larger seedlings than lighter seeds. These initially larger seedlings subsequently remained larger over the growing season and lived longer than initially smaller seedlings. Independent of seed and initial seedling size, seedlings that germinated under the canopy of adult shrubs grew larger and survived longer than seedlings that germinated in interspaces (open space with little or no vegetation) or under snags (dead adult shrubs). Seedlings grown under the canopy of adult shrubs experienced significantly less solar radiation and wind-deposited sand than seedlings grown in interspace or snag microsites. Sand burial more than 1 cm was lethal for seedlings. The two variables of seed size and microsite type had by far greater impact on eventual growth and survival of seedlings than did aspect, and the effects of the former two variables were independent of each other. This study amplifies the body of work on E. nauseosa at this site demonstrating that the earliest events in the life history of this long-lived perennial shrub appear to persist through time despite the harsh and variable environment.     

Facilitation and interference underlying the association between the woody invaders Pyracantha angustifolia and Ligustrum lucidum

Questions: 1. Is there any post‐dispersal positive effect of the exotic shrub Pyracantha angustifolia on the success of Ligustrum lucidum seedlings, as compared to the effect of the native Condalia montana or the open herbaceous patches between shrubs? 2. Is the possible facilitation by Pyracantha and/or Condalia related to differential emergence, growth, or survival of Ligustrum seedlings under their canopies? Location: Córdoba, central Argentina. Methods: We designed three treatments, in which ten mature individuals of Pyracantha, ten of the dominant native shrub Condalia montana, and ten patches without shrub cover were involved. In each treatment we planted seeds and saplings of Ligustrum collected from nearby natural populations. Seedlings emerging from the planted seeds were harvested after one year to measure growth. Survival of the transplanted saplings was recorded every two month during a year. Half of the planted seeds and transplanted saplings were cage‐protected from rodents. Results: Ligustrum seedling emergence did not differ among treatments while growth was significantly higher in the absence of shrub cover. Sapling survival was significantly higher under the canopy of Pyracantha, intermediate under Condalia, and lowest in the absence of shrub cover. Caging did not affect growth but enhanced seedling emergence and sapling survival. Conclusion: The differential sapling survival in the shrub canopy treatments is consistent with natural sapling distribution. Pyracantha and, less so, Condalia, has a nurse‐plant effect on Ligustrum. This results from contrasting effects of the shrubs on different stages of the life cycle of Ligustrum: no effect on seedling emergence, negative on seedling growth, and positive on sapling survival. This suggests that efforts to control the expansion of Ligustrum over the landscape should tackle Pyracantha as well.     

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.     

Age structure and dynamics of Patagonian beech forests in Torres del Paine National Park,Chile

This study documents the stem size and age-structure in forests dominated by different species of Nothofagus in Torres del Paine National Park (51° S), in the Chilean Patagonian region. We also explored the relationship between the various types of Nothofagus forest and postglacial succession. Pioneer stands on moraine fields 1–2 km of the glacier front are dominated by Nothofagus betuloides and Nothofagus antarctica. Moraines appear to be first colonized by the evergreen N. betuloides, followed within 5–7 years by deciduous N. antarctica. Nothofagus antarctica may replace the former species and develop monospecific stands on glacial valleys. Most trees in the N. antarctica stand studied were older than 40 years and floods may cause a significant mortality of young trees. Recruitment from seed seems to be infrequent. Old-growth stands dominated by deciduous Nothofagus pumilio occupy more stable substrates, and probably represent the last stage of postglacial succession. This long-lived tree species had recorded ages over 200 years. The canopy of N. pumilio forests appears to be a mosaic of even-aged, old-growth patches. We propose that regeneration episodes follow the blowdown of a large portion of the canopy, with long intervals with little or no regeneration. Windstorms may be an important force influencing the regeneration of N. pumilio. Exogenous disturbances, such as floods and windstorms, are an integral part of the forest cycle in the Patagonian region.     

Limiting mechanisms in the regeneration of the Chilean matorral – Experiments on seedling establishment in burned and cleared mesic sites

The Chilean matorral is characterized by multispecific shrub clumps in dry areas but has a continuous canopy in wetter sites. It has been hypothesized that this difference is due to easier recolonization of open patches by shrub seedlings under more mesic conditions. Within the mesic range of the matorral we designed a field experiment to compare shrub seedling emergence, growth, and survival under the closed canopy of a secondary forest versus three types of open patches: burned, cleared of shrubs but with a herbaceous layer present, and clear without a herbaceous layer. After the first summer, survival of Quillaja saponaria seedlings was 75% in the burned site, 30% in the cleared patch without herbaceous vegetation, and 15% with herbaceous vegetation present, whereas there was 0% survival under the secondary forest canopy. After eight years, the percentages had dropped to: 22%, 12% and 3%, respectively. These results contrast strongly with the seedling establishment patterns in drier areas of the matorral where early seedling survival is higher under the shade of large shrubs. In the experimental mesic sites, seedlings did best on the burned site, not only in terms of survival, but also in terms of growth. After one year, seedling mean height was 10.2 cm in the burned site, whereas 3.8 cm and 5.3 cm in the cleared patches without and with herbaceous respectively. After eight years, mean height differences between treatments had increased further: 147.7 cm in the burned site, 40.3 cm in the cleared patch without herbaceous cover and 13 cm in the cleared patch with herbs. Our results indicate that the facilitative effect of nurse shrubs on seedling establishment found in dry ranges of the matorral is less important in more mesic sites. This difference may explain the continuous shrub cover in relatively mesic areas as opposed to the characteristic patchy structure of the matorral in its drier range.     

Patterns of Primary Succession on the Foreland of Coleman Glacier,Washington, USA

Patterns of community development vary among studied glacier forelands around the world. However, there have been few studies of primary succession on glacial forelands in temperate regions of North America. We described patterns in community composition, vegetation cover, diversity, and vegetation heterogeneity during primary succession on the foreland of Coleman Glacier, in Washington State, USA. Community composition changed rapidly with high turnover between age classes. Cover increased through succession as expected. Species richness and diversity were highest in early succession at small scales and in late succession at larger scales. At small scales, heterogeneity decreased in early succession but increased in mature sites. At larger scales, heterogeneity reached its lowest point earlier in succession. These scale-dependent patterns in diversity and heterogeneity differ from results of other studies of glacier forelands. We hypothesize that these patterns arise due to the development of a dense canopy of the deciduous shrub Alnus viridis followed by a dense canopy of Abies amabilis, Tsuga heterophylla, and Pseudotsuga menziesii.     

The role of cattle in maintaining plant species diversity in wet dune valleys

The succession of species-rich wetland vegetation in dune valleys into species-poor dwarf shrub vegetation was followed by means of permanent vegetation plots, in which the cover of vascular plant, moss and lichen species were recorded over a period of up to 33 years. Low density cattle grazing is an effective substitute for rabbits in stalling this succession, thus preserving the local plant species diversity. The influence of direct rabbit grazing pressure on the vegetation was studied in exclosures. The differences were significant, but not long-lasting after the exclosures had been opened again to the rabbits. Cladina lichen-rich Empetrum-heathland in the dunes is a stage in a probably cyclic succession that might be triggered by grazing. The total vascular plant, moss and lichen diversity of the dune ecosystem can be maintained by a combination of extensive cattle grazing and a regular but limited re-creation of pioneer situations.     

Post‐fire shrub recruitment in a semi‐arid grassland: the role of microsites

Question: (1) Which factors regulate post‐fire recruitment and spread of the shrub Senecio bracteolatus in Patagonian grasslands? (2) What is the role of the grass Stipa speciosa on S. bracteolatus establishment in the post‐fire succession? Location: Northwest Patagonia, Argentina. Methods: We studied the effect of fire on S. bracteolatus recruitment and density by comparing these variables between burned and unburned grasslands. In burned areas, we compared abiotic characteristics and seedling establishment under the canopy of grasses (S. speciosa) and in gaps (inter‐tussock areas). Post‐fire interactions between S. bracteolatus seedlings and S. speciosa were studied using field and greenhouse experiments. Results: Density of S. bracteolatus was higher in burned than in unburned areas. In burned sites, seedlings were more abundant under tussock grasses, whereas juveniles were more abundant in gaps. Tussocks generated more attenuated micro‐environmental conditions than gaps during stressful summers. Gaps were more abundant in burned sites, while “under tussock” microsites were more frequent in unburned sites. In burned areas, tussocks allowed higher establishment of seedlings (facilitation), but gaps allowed more seedling growth and higher persistence of juveniles. Conclusions: Fire promoted S. bracteolatus recruitment in Patagonian grasslands by increasing the availability of favourable gap microsites. Grass protection for shrub seedlings became negative with time, probably due to competition with grasses. Gaps led to better performance and persistence of shrub plants. Six years after fire, higher shrub recruitment and adult density (observed as a trend) in burned grassland provides an opportunity for potential S. bracteolatus invasion.     

Canopy manipulations of exotic Bitter Willow (Salix elaeagnos) forest for indigenous seedling recruitment: A pilot study

The invasive exotic tree species Bitter Willow (Salix elaeagnos; Salicaceae) has colonised areas of rank exotic grassland and has been found to contain indigenous seed, dispersed by frugivorous birds into the monospecific stands. This small pilot study examined whether indigenous seedlings that have germinated in the understorey of exotic Bitter Willow stands could be stimulated to establish through the creation of small‐scale canopy gaps. In Bitter Willow forest, four single Bitter Willow trees were poisoned to create canopy gaps. Light transmission and seedling regeneration of tree and shrub species were assessed beneath both the four manipulated and three comparable intact Bitter Willow canopies. Over 3 years, seedling height and density increased more beneath opened compared to intact Bitter Willow canopies. These results suggest that Bitter Willow can fill the roles of both a facilitative nurse and a perch tree. Larger‐scale canopy manipulation experiments of both Bitter Willow and other Salix species are needed to determine the full potential of canopy manipulations for forest restoration.     

Establishment of Perennial Shrub and Tree Species in Degraded Eucalyptus salmonophloia (Salmon Gum) Remnant Woodlands: Effects of Restoration Treatments

Woodlands dominated by Eucalyptus salmonophloia (salmon gum) occur throughout the fragmented landscape of the southwestern Australian wheatbelt. These remnants are often degraded by livestock grazing and weed invasion and in many cases there is little or no understorey remaining and little or no regeneration of the dominant tree E. salmonophloia. There is a growing interest in developing techniques for restoring remnant woodlands. This study describes techniques for establishing seedlings of the dominant tree and perennial understorey species in E. salmonophloia (salmon gum) woodlands degraded by livestock grazing. The study tests the hypothesis that, in addition to the exclusion of livestock, management of weeds and reintroduction of plant species, restoration of plant species diversity will require techniques which mimic large‐scale disturbances, reduce soil compaction, and restore soil water infiltration to suitable rates. Five‐month‐old seedlings of the dominant tree E. salmonophloia and four commonly associated woody shrubs (Acacia hemiteles, Atriplex semibaccata, Maireana brevifolia, and Melaleuca pauperiflora) were planted into areas that differed with respect to grazing (–rabbit/ ?livestock and +rabbit/–livestock), tree canopy disturbance (+/–competition with tree canopy) and amelioration of soil compaction (+/–deep ripping). Following three growing seasons and two summers, the exclusion of rabbits had no significant effect on the survival and growth of planted species. As a consequence grazing treatments are pooled for the purposes of presenting the impacts of removing competition with adult trees and soil deep ripping. The removal of competition with adult E. salmonophloia trees significantly improved the survival of E. salmonophloia seedlings but did not improve survival of understorey species. Deep ripping the soil significantly improved the survival of both E. salmonophloia and the shrub A. hemiteles but did not improve the survival of other understorey species. In contrast to seedling survival, the removal of adult E. salmonophloia trees and deep ripping soil significantly increased the growth of all species. The results indicate that increasing levels of intervention will increase the chances of successfully restoring tree and understorey species diversity in degraded E. salmonophloia woodlands.     

Microclimate and seed bank under Pithecellobium keyense (Fabaceae) in a coastal scrubland

Microclimatic conditions affect seeds' survival, dormancy, and germination, which are determinants in vegetation succession and recovery. The semiarid coastal scrublands of Yucatan, Mexico are very important because of the environmental services they provide thus, studies are needed in disturbed sites in relation to seed viability and microclimate. In this study, we characterized the microclimate and seed bank under Pithecellobium keyense shrubs, during the rainy and dry seasons. We collected soil samples and measured microenvironmental variables under adult P. keyense at three different positions under the canopy of every studied individual (“open” or completely sunny, “half-shaded,” placed at the border of the canopy, and “fully shaded” placed under the canopy, next to the stem of the shrub). Photosynthetic photon flux density (PPFD), minimum night-time temperature, and maximum daytime temperature behaved differently among positions under the canopy both in rainy and dry seasons. During the rainy season, seed viability was no different among microenvironments, while in the dry, the highest was found in the fully shaded and the lowest in the open position. Microclimatic variables under the canopy of P. keyense had an effect on the probability of seed viability in Chuburná; and this effect was stronger in the dry season. Abstract in Spanish is available with online material.     

Ecosystem disturbance reduces the allelopathic effects of Empetrum hermaphroditum humus on tundra plants

Question: Do naturally occurring ecosystem disturbances interfere with allelopathic effects and alleviate growing conditions for plants typical of more productive ecosystem states? Location: An unproductive tundra ecosystem in Northern Fennoscandia. Methods: We designed a factorial phytotron experiment to monitor the growth of seedlings of two herbaceous species in undisturbed and disturbed humus of the dominant dwarf shrub Empetrum hermaphroditum, which releases allelopathic substances that accumulate in soil over time. Herbivore defecation and wildfire are two major ecosystem disturbances likely to alleviate effects of these allelopathic substances. Humus and seeds of herbaceous species were sampled from Empetrum‐dominated heaths. Disturbance effects on humus were simulated by adding reindeer faeces and by low‐severity burning. Results: The concentration of the allelopathic substance batatasin‐III in Empetrum humus was 0.32±0.16 (SE) μg g^?1, which is similar to previous reports from boreal areas. Reindeer faeces addition and burning treatments to Empetrum humus caused three‐ and five‐fold increases in growth of the herbaceous species Avenella flexuosa and Solidago virgaurea respectively. Changes in soil chemistry caused by the disturbance, i.e. increased pH and nutrient availability, likely interfered with allelopathic effects and improved conditions for growth. Conclusions: Humus from Empetrum‐dominated tundra areas is infertile for seedlings of local herbaceous plants. Simulated herbivore defecation and wildfire can alleviate this infertility. Based on our results, we discuss the potential role of these naturally occurring disturbances in switching vegetation composition from Empetrum to herbaceous dominance, i.e. from an unproductive to a more productive tundra ecosystem.     

Effects of pioneer shrubs on the recruitment of the fleshy‐fruited tree Olea europaea ssp. cuspidata in Afromontane savanna

Question: Is seedling recruitment of a fleshy‐fruited tree in degraded Afromontane savanna dependent on shelter from pioneer shrubs, and is shelter availability related to shrub traits? Location: Degraded montane savanna in northern Ethiopia (13°36′N, 39°21′E). Method: Nurse plants of Olea europaea ssp. cuspidata seedlings were recorded using T‐square plotless sampling and clustered according to shrub traits, using Ward's method after Principal Components Analysis. Facilitation was further examined through experimental planting and Kaplan‐Meier survival analysis. Results: Both in grazed and protected areas, Olea recruits were found exclusively under shrubs, primarily under Euclea racemosa although Acacia etbaica was more abundant. Olea recruitment is distributed randomly at landscape scale, but depends on shelter at patch scale. Shelter ability is related to shrub shape and species identity. Dense multi‐stemmed shrubs with a wide base and crown on a mulch‐rich mound are key recruitment foci. Euclea shrubs have these favoured traits and probably act as preferential perching sites for avian seed dispersers. Soil and organic matter accumulation under Euclea shrubs may also create favourable conditions for Olea germination and survival. Experimentally planted seedlings had a better chance for survival under Euclea. Conclusions: Olea regeneration is probably subject to both passive (disperser‐mediated) and active facilitation. Small changes of shrub traits can alter the suitability of a patch for Olea recruitment. Protection of shrubs can increase facilitation for seedlings, while pruning may reduce competition for saplings and thus enhance forest succession. Planting of raised Olea seedlings under Euclea shrubs in years with a good rainy season may further assist forest restoration.     

Competitive effects of the invasive shrub,Lonicera maackii (Rupr.) Herder (Caprifoliaceae), on the growth and survival of native tree seedlings

Invasive plants are often associated with reduced cover of native plants, but rarely has competition between invasives and natives been assessed experimentally. The shrub Lonicera maackii, native to northeastern Asia, has invaded forests and old fields in numerous parts of eastern North America, and is associated with reduced tree seedling density in Ohio forests. A field experiment was conducted to test the effects of established L. maackii on the survival and growth of transplanted native tree species. The experiment examined above-ground competition (by removing L. maackii shoots) and below-ground competition (by trenching around transplanted seedlings). The effects of above-ground competition with L. maackii were generally more important than below-ground competition, though both were detected. Shoot treatment was the key determinant for the survival of all species except P. serotina, whereas trenching only enhanced survival for A. saccharum caged and P. serotina, and only in the shoot removal treatment. For the surviving seedlings, L. maackii shoot removal increased growth of A. saccharum seedlings protected with cages, but actually reduced the growth of unprotected Q. rubra and A. saccharum seedlings, indicating that L. maackii shoots confer some protection from deer browsing. Significant interactions between root and shoot treatment on Q. rubra growth parameters, specifically greatest growth in the shoot present & trenched treatment, is attributed to protection from deer browsing combined with release from below-ground competition. Despite this protective function of L. maackii shoots, the overall effect of this invasive shrub is increased mortality of native tree seedlings, suggesting it impacts the natural regeneration of secondary forests.     

A soil nutrient gradient in Magellanic Empetrum heathlands

The relationship of environmental and soil-nutrient variables with the floristic composition of Empetrum rubrum heathlands and related communities from northern Tierra del Fuego were studied by means of ordination techniques. A floristic gradient was found which had its axis of major variation closely related to a gradient in soil trophic status. Increases in the densities of Empetrum and other cushion plants (Bolax gummifera, Azorella lycopodioides, Pernettya pumila) along the floristic gradient were: (a) associated with a lower number of species and total cover, and greater soil erosion; (b) positively correlated with the C/N ratio and aluminium content in the soil; and (c) negatively correlated with pH, calcium content and base saturation.Climate, lithology, habitat exposure, and the history of human activity would be key factors in developing the oligotrophic conditions that favoured the establishment of different Empetrum heathlands.     

Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration

Restoration of shrubs in arid and semi‐arid rangelands is hampered by low success rates. Planting shrub seedlings is a method used to improve success in these rangelands; however, it is expensive and labor intensive. The efficiency of shrub restoration could be improved by identifying microsites where shrub survival is greater. Bitterbrush (Purshia tridentata Pursh DC) is an important shrub to wildlife that has declined because of conifer encroachment, excessive defoliation, wildfires, and low recruitment. We investigated planting bitterbrush seedlings in western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroached shrublands after prescribed fire was used to control trees. Bitterbrush seedlings were planted in under (canopy) and between (interspace) former juniper canopies at five blocks and evaluated for three growing seasons. Bitterbrush survival was greater than 50% in the former canopy, but only 5% in the interspace microsite by the third growing season. Growth of bitterbrush was also greater in the former canopy compared with the interspace, potentially due to markedly less perennial vegetation in this microsite. Exotic annual grasses and annual forbs became prevalent in the former canopy in the second and third growing season, suggesting that soil resource availability was greater in this microsite. These results suggest that restoration success will vary by specific locations within a burned landscape and that this variability can be used to improve restoration efficiency. In this situation, bitterbrush restoration can be improved by planting seedlings in former canopy compared with interspace microsites.     

Impacts of the invasive exotic <Emphasis Type="Italic">Prosopis juliflora</Emphasis> (Sw.) D.C. on the native flora and soils of the UAE

The effects of the invasive exotic Prosopis juliflora shrubs on the natural plant communities and soil chemical characters were assessed in two regions of the United Arab Emirates (UAE). Five sites were selected subjectively: three in Sharja with 73 stands and two in Ras Al-Khima with 37 stands. Stands were located randomly within each site to cover density variation in Sharja and size variation in Ras Al-Khima. Density, frequency, richness and evenness of the associated annual and perennial species were studied in nine quadrats distributed under, at the margin and outside the canopy of a P. juliflora shrub located in the center of each stand. The results indicated that the effect of P. juliflora on the associated flora depends significantly on the density and size of the canopy. Larger individuals and greater densities have significantly greater negative impacts on the associated plants. All the studied community attributes were significantly lower under P. juliflora canopies than outside. Annuals were inhibited more than perennials. The number of annuals with significant reductions in density and/or frequency under P. juliflora canopies was significantly greater than the number of perennials. Density of more than 50% of the associated annuals was significantly inhibited under P. juliflora canopies. Density of P. juliflora seedlings was greater underneath the canopy of the same species than away from them, indicating little or no self allelopathic effect (auto-inhibition) under field conditions. P. juliflora ameliorated some soil characters, through significant pH reduction and increase in K, N and P and organic matters. Hence, plant diversity might be enhanced following eradication of P. juliflora.