Evaluating models to assess the distribution of Buxus balearica in southern Spain

Question: Which is the best model to predict the habitat distribution of Buxus balearica Lam. in southern Spain? Location: Málaga and Granada, Spain, across an area of 38 180 km². Methods: Prediction models based on 17 environmental variables were tested. Six methods were compared: multivariate adaptive regression spline (MARS), maximum entropy approach to modelling species' distributions (Maxent), two generic algorithms based on environmental metrics dissimilarity (BIOCLIM and DOMAIN), Genetic Algorithm for Rule‐set Prediction (GARP), and supervised learning methods based on generalized linear classifiers (support vector machines, SVMs). To test the predictive power of the models we used the Kappa index. Results: Maxent most accurately predicted the habitat distribution of B. balearica, followed by MARS models. The other models tested yielded lower accuracy values. A comparison of the predictive power of the models revealed that climate variables made the highest contributions among the environmental variables studied. The variables that made the lowest contributions were the insolation models. To examine the sensitivity of the models to a reduction in the number of variables, a test showed that accuracy of over 0.90 was maintained by applying just three climatic variables (spring rainfall, mean temperature of the warmest month, and mean temperature of the coldest month). Maps derived from the algorithms of all models tested coincided well with the known distribution of the species. Conclusions: Model habitat prediction is a preliminary step towards highlighting areas of high habitat suitability of B. balearica. These data support the results of previous research, which show that MaxEnt is the best technique for modelling species distributions with small sample sizes.     

祁连山5种典型灌丛土壤生态化学计量特征

土壤C、N、P生态化学计量特征是体现生态系统变化过程的重要依据。该研究以分布于祁连山北麓中段的甘青锦鸡儿、鲜黄小檗、金露梅、鬼箭锦鸡儿、吉拉柳等5种典型灌丛群落为研究对象,通过野外调查采样,测定土壤有机C、N、P含量,分析了不同灌丛群落土壤C、N、P含量及其生态化学计量比的垂直分布特征,并探讨了各指标间的耦合关系,为祁连山地区土壤-植物养分关系及退化植被恢复与重建提供理论依据。结果表明:(1)祁连山北麓中段5种灌丛群落土壤有机C(45.29 g/kg)、全N(3.85 g/kg)、全P含量(0.70 g/kg)较高,均高于全国平均水平。(2)相关性分析表明,土壤有机C与全N之间呈极显著正相关(P0.01),但土壤有机C与全P以及全N与全P之间没有明显的相关性,且土壤C、N含量变化几乎完全同步。(3)各灌丛类型土壤有机C、全N含量均表现为表层高于下层,而全P含量土层间差异不显著;各灌丛群落土壤有机C、全N含量均随土层加深呈现"倒金字塔"的分布格局,但不同灌丛类型全P含量随土层加深的变化规律却不一致,且土层对全P含量的影响不显著。(4)5种灌丛群落土壤整体C∶N、C∶P、N∶P(11.29、66.99、5.67)均低于全国平均水平,各灌丛群落土壤C∶P、N∶P随土层加深均呈不同程度的下降趋势,但不同灌丛群落土壤C∶N随土层加深的变化规律有所不同。(5)土壤有机C、全N与C∶N、C∶P、N∶P之间均具有极显著的二次函数关系(P0.01),土壤全P与C∶N、C∶P、N∶P之间关系不显著。研究认为,祁连山北麓中段土壤C∶N和P含量的稳定性较高(CV=3.99%和2.66%),C∶P、N∶P是判断研究区限制性养分的重要指标,祁连山北麓中段灌丛群落土壤主要受N素的限制。     

Roller Chopping Effectively Reduces Shrub Cover and Density in Pine Flatwoods

Changes to fire regimes have resulted in excessive shrub growth and declines in the species rich herbaceous ground layer of pyric savanna and grassland systems worldwide, including the pine flatwoods of the Southern Coastal Plain of the United States. Prescribed burning and roller chopping during growing (April–October) and dormant (November–March) seasons are management practices promoted to reduce shrub invasion and increase herbaceous plant growth in flatwoods. However, relatively little is known about the seasonal effects these activities have on shrubs and herbaceous vegetation. We assessed the effects of prescribed burning and roller chopping on herbaceous and shrub characteristics in pine flatwoods and explored how grazing may mediate these treatments. We used a paired design, with comparison made between sampling locations randomly located within treated (e.g. burned) and adjacent untreated areas. Growing season burning was more effective at reducing shrub cover and height than dormant season burning. However, shrub re‐growth occurred the second year post‐burn. Roller chopping and roller chopping/burning combinations led to decreases in shrub cover and height for 2 years post‐treatment. Decreases in shrub density were seen on sites subject to growing season roller chopping and grazing. Decreases in herbaceous vegetation were observed following all treatments, possibly the result of grazing. If reductions in shrub density are required, growing season roller chopping in combination with grazing may be the only effective treatment. However, initial deferment from grazing following burning and roller chopping treatments may be necessary to permit re‐establishment and growth of forbs and graminoids.     

Forest Restoration in Urbanizing Landscapes: Interactions Between Land Uses and Exotic Shrubs

Preventing and controlling exotic plants remains a key challenge in any ecological restoration, and most efforts are currently aimed at local scales. We combined local‐ and landscape‐scale approaches to identify factors that were most closely associated with invasion of riparian forests by exotic shrubs (Amur honeysuckle [Lonicera maackii] and Tatarian honeysuckle [L. tatarica]) in Ohio, U.S.A. Twenty sites were selected in mature riparian forests along a rural–urban gradient (<1–47% urban land cover). Within each site, we measured percent cover of Lonicera spp. and native trees and shrubs, percent canopy cover, and facing edge aspect. We then developed 10 a priori models based on local‐ and landscape‐level variables that we hypothesized would influence percent cover of Lonicera spp. within 25 m of the forest edge. To determine which of these models best fit the data, we used an information‐theoretic approach and Akaike's information criterion. Percent cover of Lonicera was best explained by the proportion of urban land cover within 1 km of riparian forests. In particular, percent cover of Lonicera was greater in forests within more urban landscapes than in forests within rural landscapes. Results suggest that surrounding land uses influence invasion by exotic shrubs, and explicit consideration of land uses may improve our ability to predict or limit invasion. Moreover, identifying land uses that increase the risk of invasion may inform restoration efforts.     

Low seed availability may limit recruitment in grazed Atriplex vesicaria and contribute to its local extinction

Poor recruitment in Atriplex vesicaria Hewd exBenth (bladder saltbush) under sheep grazing in the chenopodshrublands of southern Australia contributes to a decline in the shrub'spopulation growth rate. This may lead to local extinction of the species overlarge areas around watering points. This study investigated whether low seedavailability may contribute to poor recruitment. It examined the incidence offlowering and seed bank size at sites distributed across a large grazedpaddock,and examined the longevity of seed in the soil. Grazing by sheep reduced theincidence of flowering and input to the seed bank. The proportion of shrubswithflowers increased with distance from water, showing the characteristicpiosphereresponse. Shrubs on grazed sites closer to water experienced extended periodswhen they failed to flower or flowered poorly. The seed bank was negligible atthree of the sites within 1650 m of water for all three years ofsampling. In contrast, the seed bank at the most distant site sampled (2800m from water) was small in 1990 (37 ± 5.1seeds/m²) but in 1991 and 1992 seed numberswere substantial (626 ± 315.2 seeds/m²and 318 ± 169.0, respectively). Soil seed was short-lived inthis study, with only 34% and 17% of the original seed remainingas viable ungerminated seed after 12 months for the under-canopy andexposed treatments respectively. Whilst recruitment may also be limited byaltered soil conditions due to grazing and trampling and the availability ofsafe sites, the results of this study suggest that low seed availability may bean important factor contributing to poor recruitment and may limit the abilityof the population to recover from the loss of established plants. Management ofgrazing must take into account the need for A. vesicariapopulations to flower and set seed on a regular basis.     

The steppes of Tierra del Fuego: floristic and growthform patterns controlled by soil fertility and moisture

Northern Tierra del Fuego is the insular extreme of the Magellanic steppe, a unique example of a cold-temperate oceanic grassland in South America. We surveyed this poorly known region with a landscape-based procedure to classify plant communities and investigate environmental gradients. Sampling sites were selected with the aid of aerial photos, and floristic composition, soil, topography and landform type were recorded in each site. Classification was achieved with Two-Way Indicator Species Analysis and gradients were analyzed with Detrended Canonical Correspondence Analysis. Two broad groups of communities were separated by the presence of typical acidophilous dwarf shrubs: acidophilous/mesotrophic and neutrophilous vegetation. Along the main floristic gradient, ericoid dwarf shrubs were replaced by forbs and grasses as soil acidity decreased. The acidic habitats were on Pleistocene glacial and alluvial deposits, and had coarse texture, poorly decomposed organic matter, and low base status. The eutrophic habitats were on Tertiary rocks of medium to fine texture and had well-decomposed organic matter and high base status. A second axis of variation was associated to latitudinal position and slope aspect, revealing a precipitation/evaporation gradient. Scrubs dominated by midheight shrubs, with species from the southern forests, were at the humid extreme (higher latitudes/south aspects); and short grass steppes, with taxa commonly found further North in the continent, at the xeric end (lower latitudes/north aspects). Although fertility and moisture seem to control the overall floristic gradient, the extreme vegetation types of the fertility gradient, both having a very simple structure, could be attributed to grazing disturbance. Floristics and ecology of the Fuegian steppe resemble to a certain degree some boreal oceanic systems.     

Towards an explanation of the causes of the rarity of two Tasmanian Spyridium species

Spyridium microphyllum (F. Muell. ex Reissek) Druce and Spyridium obcordatum (Hook. f) W. M. Curtis are narrow endemic shrubs which are confined to small parts of Tasmania, Australia, and are rare within their present ranges. Analysis of quadrat data from transects through populations of both species showed that at most individual sites the rare taxa occur in floristically and environmentally distinct situations. However, sites supporting the same species differ floristically and environmentally, with the only communality being a lack of dense tall shrub or tree cover. Both species are absent from many areas with closely similar physical environments and vascular plant species composition to those in which they occur. Thus, their current distributions cannot be explained by limitation to a rare habitat. Spyridium microphyllum appears to be susceptible to local extinction because it is an obligate seeder, is killed by fire, does not survive in taller, dense vegetation, and is not adapted for dispersal over the distances between its potential habitats. While S. obcordatum recovers vegetatively after fire, it also does not survive in dense taller vegetation, has limited dispersal ability and highly disjunct potential habitat. Both species may have been more widespread and competitive during cooler and drier times.     

A phenological hypothesis on the thermophilous distribution of Pistacia lentiscus L.

In this study we present evidences of the importance of the phenological pattern on the distribution limits of the dioecious Pistacia lentiscus L. This species, though displaying a thermophilous distribution, has been proved to resist low freezing temperatures during winter. We try to explain this apparent paradox by studying the effects of an extreme cold event that occurred in December 2001 on a natural population of P. lentiscus in the NE of the Iberian Peninsula. In previous phenological studies conduced over 4 years, no spring branch damage was noticeable among population individuals. Female individuals of this species present a prolonged phenophase development which extends until early winter with the conclusion of fruit set. Therefore, we hypothesize that female plants would be more affected than male ones in terms of the following: (1) vegetative and reproductive organs survival; and (2) next-year vegetative growth and fruit production. To test this hypothesis, we selected 225 adult individuals (113 females and 112 males) in April 2002, and estimated their crown volume, percentage of frozen branches and reproductive buds, and the amount of 2001 fruits frozen. In June 2002 we evaluated, in the same individuals, the percentage of vegetative buds flushed into shoots and of reproductive buds producing infrutescences. Branch mortality was significantly higher in female plants and females with increased frost-damage displayed a higher amount of frozen fruits. The loss of reproductive buds caused a decrease in 2002 fruit production, while 2002 vegetative growth was unaffected by the degree of frost damage. These results verify most of the predictions of our hypothesis. Moreover, they suggest that the limited distribution of P. lentiscus in the cold areas of Mediterranean climate could be more determined by the long extent of the phenological activity of the crown than by its frost tolerance during winter.     

Ecophysiology of six selected shrub species in different plant communities at the periphery of the Atlantic Forest of SE-Brazil

The Atlantic rain forest of Brazil is one of the 25 biodiversity hotspots of the world. It is surrounded by distinct marginal plant communities which are floristically related to the rain forest. We performed an in situ ecophysiological study of six shrub and tree species which are abundant in one or more of these marginal ecosystems. They formed three pairs of taxonomically related species, which, however, differed in geographic distribution and/or habitat preference. We aimed to answer the following questions: (1) Do habitat generalists and specialists differ in ecophysiological behaviour? (2) Are there general trends responsive to site characteristics, irrespective of taxonomic affiliation? (3) In which variables between-site intraspecific variation is more often detected? Plants chosen were the Clusiaceae Calophyllum brasiliense Cambess (swamp forest specialist) and Rheedia brasiliensis (Mart.) Planch &; Triana (a generalist plant, occurring in swamp forests and dry, sandy coastal vegetation); the Myrsinaceae Myrsine parvifolia A.DC. (widespread in sandy plains) and Myrsine gardneriana A.DC. (widespread in high-altitude habitats); and the Euphorbiaceae Styllingia dichotoma Muell. Arg. (a coastal inselberg specialist) and Croton compressus Lam. (a generalist found in coastal inselbergs and dry forests). We compared measurements of photosynthesis (chlorophyll fluorescence parameters), stable isotope (δ¹³C, δ¹⁵N) and metabolic carbon and nitrogen compounds. Generalists and specialists did not always differ in ecophysiological behaviour: for instance, while widespread generalists such as M. parvifolia and C. compressus were ecophysiologically versatile, R. brasiliense appeared to perform below optimum in all sampled habitats. Light and water were abiotic factors which apparently explained most of the variation found. Overall, ecophysiological responses were often more clearly related to individual species rather than to taxonomic affinities in higher hierarchies (family, genus) or habitat. These results are discussed as a further indication of high biodiversity even in habitats marginal to the rain forest hotspot.     

Beringia as a glacial refugium for boreal trees and shrubs: new perspectives from mapped pollen data

Aim Beringia, far north‐eastern Siberia and north‐western North America, was largely unglaciated during the Pleistocene. Although this region has long been considered an ice‐age refugium for arctic herbs and shrubs, little is known about its role as a refugium for boreal trees and shrubs during the last glacial maximum (LGM, c. 28,000–15,000 calibrated years before present). We examine mapped patterns of pollen percentages to infer whether six boreal tree and shrub taxa (Populus, Larix, Picea, Pinus, Betula, Alnus/Duschekia) survived the harsh glacial conditions within Beringia. Methods Extensive networks of pollen records have the potential to reveal distinctive temporal–spatial patterns that discriminate between local‐ and long‐distance sources of pollen. We assembled pollen records for 149 lake, peat and alluvial sites from the Palaeoenvironmental Arctic Sciences database, plotting pollen percentages at 1000‐year time intervals from 21,000 to 6000 calibrated years before present. Pollen percentages are interpreted with an understanding of modern pollen representation and potential sources of long‐distance pollen during the glacial maximum. Inferences from pollen data are supplemented by published radiocarbon dates of identified macrofossils, where available. Results Pollen maps for individual taxa show unique temporal‐spatial patterns, but the data for each taxon argue more strongly for survival within Beringia than for immigration from outside regions. The first increase of Populus pollen percentages in the western Brooks Ranges is evidence that Populus trees survived the LGM in central Beringia. Both pollen and macrofossil evidence support Larix survival in western Beringia (WB), but data for Larix in eastern Beringia (EB) are unclear. Given the similar distances of WB and EB to glacial‐age boreal forests in temperate latitudes of Asia and North America, the widespread presence of Picea pollen in EB and Pinus pollen in WB indicates that Picea and Pinus survived within these respective regions. Betula pollen is broadly distributed but highly variable in glacial‐maximum samples, suggesting that Betula trees or shrubs survived in restricted populations throughout Beringia. Alnus/Duschekia percentages show complex patterns, but generally support a glacial refugium in WB. Main conclusions Our interpretations have several implications, including: (1) the rapid post‐glacial migration rate reported for Picea in western Canada may be over estimated, (2) the expansion of trees and shrubs within Beringia should have been nearly contemporaneous with climatic change, (3) boreal trees and shrubs are capable of surviving long periods in relatively small populations (at the lower limit of detection in pollen data) and (4) long‐distance migration may not have been the predominant mode of vegetation response to climatic change in Beringia.