Modest enhancement of nitrogen conservation via retranslocation in response to gradients in N supply and leaf N status

Plant nutrient resorption, a ubiquitous mechanism of nutrient conservation, has often been proposed to be more pronounced in infertile than fertile habitats, and in species common to infertile compared to fertile habitats, because of the presumed advantage when nutrients are scarce. However, previous studies provide weak and inconsistent empirical support for these hypotheses, although few have examined intraspecific variation across well-quantified resource gradients. This study addresses intraspecific patterns of nutrient resorption for eight species across two N availability gradients on similar soils in an N-limited oak savanna ecosystem: a long-term fire frequency gradient with a negatively correlated N fertility gradient and a long-term N fertilization gradient. We hypothesized that both resorption proficiency (the minimum nutrient level retained in a senesced leaf) and efficiency (the proportional change in leaf nutrient concentration) would decrease with increasing soil N availability and plant N status. For the seven non-N fixers, either resorption proficiency or efficiency decreased modestly in treatments with higher N availability. In contrast, the legume Amorpha canescens Pursh had higher N levels in green and senesced leaves, and resorbed N much more weakly than the non-fixers, and did not respond in terms of proficiency or efficiency to soil N availability. Across all species and sites in each N fertility gradient, a scaling analysis showed greater resorption efficiency in plants with lower N concentrations. Our data suggest that species can have modest resorption responses reflective of soil nutrient availability and differences in resorption related to their N economy that represent mechanisms of nutrient conservation in nutrient-limited soils.     

Species richness of gall-forming insects in a tropical rain forest: correlations with plant diversity and soil fertility

We tested two hypotheses to explain changes in species richness ofgall-forming insects. The first hypothesis proposes that gall-forming insectspecies richness increases as more potential host–plant species areavailable. The second hypothesis implies that soil fertility affects plantcolonization by gall-forming insects. Seven sites, representing strongdifferences in vegetation and soil were chosen at the Lacandona tropical rainforest region, Chiapas, Mexico. Overall, we found 1522 individual plantsbelonging to 340 different plant species. From this, we found gall-forminginsects on 737 (43.9%) plants and on 74 (22%) of total plant species. We found asignificant negative correlation between gall-forming insect species richnessand species richness of plants, which does not support the hypothesis that plantspecies richness is an important factor in generating the radiation ofgall-forming insects. Using phosphorus as an indicator of soil fertility, wefound the lowest number of plants with gall-forming insects and the smallestgall-forming insect load per individual plant in the more fertile soil(alluvial). In contrast, the highest number of plants with galls and the highestgall-forming insect load per plant were found at a savanna-like vegetationsite, where the poorest soil was recorded. These results did not support thesoil fertility hypothesis in terms of species richness, but did with respect toabundance of plants with galls.     

Competitive dominance mediates the effects of topography on plant richness in a mountain grassland

Small-scale landforms influence plant species richness, but their mechanisms and effects in semi-natural dry grasslands have been poorly investigated. In this study we compared vascular plant richness, species composition, plant traits, soil properties and biomass nutrient content of convex (hillocks) and concave (hollows) karst landforms in a mountain pasture of the Central Apennines (Italy), at a small spatial scale (1 m² plots). We found hillocks had significantly higher species richness than hollows. On hillocks, smaller Specific Leaf Area and Lateral Width, together with greater allocation of resources to Below-Ground Organs, indicated lower water availability, whereas hollows had deeper (thus moister), more acidic and more fertile soils, with aboveground plant biomass displaying higher nutrient levels. Partial correlation and regression tree models suggested that fine-scale richness patterns were not directly determined by abiotic properties, but were rather the result of competition levels associated with the cover of Agrostis capillaris (=A. tenuis) – a calcifuge and drought-sensitive grass able to achieve dominance only in hollows. The higher functional convergence exhibited by hollows suggests that A. capillaris is a strong competitor both above- and below-ground, mediating the effects of topography by imposing a biotic filter. On hillocks, competition is released by lower levels of available soil water in summer and higher soil pH, resulting in higher species richness and a more functionally divergent assemblage.     

When the same is not the same: phenotypic variation reveals different plant ecological strategies within species occurring in distinct Neotropical savanna habitats

Variations in abiotic characteristics such as soil water availability and fertility impose different selective pressures on plant populations. This may produce intraspecific variability in functional traits, even at a fine spatial scale. We investigated whether functional traits related to water-use efficiency, resource-retention strategy, soil nutrient acquisition, and fire tolerance differ in species that occur in two different habitats of Brazilian Cerrado: rocky savannas and savanna woodlands. Rocky savannas occur over sandstone, quartzite outcrops and have shallow nutrient-poor and low-moisture rocky soils, while savanna woodlands occur over well-drained and deep soils with frequent fire regimes. We measured nine functional traits of 40 tree species that occur in both habitats. Rocky savanna individuals exhibited a greater water-use efficiency strategy. The resource-retention strategy in rocky savanna individuals was corroborated by lower adult maximum height. However, despite the lower nutrient availability in rocky savanna soils, we only detected lower leaf phosphorus content in individuals from this habitat. Furthermore, individuals from both habitats had equally thick bark, suggesting that the fire-defense strategy is related to a stable, rather than plastic trait. Overall, our results highlight the central role of contrasting soil water availability patterns in driving phenotypic plasticity within species. We conclude that savanna species are responding to water and nutrient availabilities, via plasticity in traits related to the resource-retention strategy, and preparing for future fires, via uniformly thick bark. Wide plant distribution in contrasting habitats is possible for species that can shift ecological strategies to survive in nutrient- and water-limited habitats such as rocky savannas.     

Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.     

Coarse root spatial distribution determined using a ground-penetrating radar technique in a subtropical evergreen broad-leaved forest, China

Coarse roots play a critical role in forest ecosystems and both abiotic and biotic factors affect their spatial distribution. To some extent, coarse root density may reflect the quantity of root biomass and biotic competition in forests. However, using traditional methods (e.g., excavation) to study coarse roots is challenging, because those methods are time-consuming and laborious. Furthermore, these destructive methods cannot be repeated in the same forests. Therefore, the discovery of non-destructive methods for root studies will be very significant. In this study, we used a ground-penetrating radar technique to detect the coarse root density of three habitats (ridge, slope and valley) and the dominant tree species (Castanopsis eyrei and Schima superba) in a subtropical forest. We found that (i) the mean of coarse root density for these three habitats was 88.04 roots m^?2, with roots being mainly distributed at depths of 0–40 cm. Coarse root densities were lower in deeper soils and in areas far from the trunk. (ii) Coarse root densities differed significantly among the three habitats studied here with slope habitat having the lowest coarse root density. Compared with S. superba, C. eyrei had more roots distributed in deeper soils. Furthermore, coarse roots with a diameter >3 cm occurred more frequently in the valleys, compared with root densities in ridge and slope habitats, and most coarse roots occurred at soil depths of 20–40 cm. (iii) The coarse root density correlated negatively with tree species richness at soil depths of 40–60 cm. The abundances of the dominant species, such as C. eyrei, Cyclobalanopsis glauca, Pinus massoniana, had significant impacts on coarse root density. (iv) The soil depth of 0–40 cm was the “basic distribution layer” for coarse roots since the majority of coarse roots were found in this soil layer with an average root density of 84.18 roots m^?2, which had no significant linear relationships with topography, tree species richness, rarefied tree species richness and tree density. Significant relationships between coarse root density and these factors were found at the soil depth of 40–60 cm, which was the “potential distribution layer” for coarse root distribution.     

Removal of vertebrate-dispersed fruits in vegetation on fertile and infertile soils

Summary The hypothesis that more plant species with vertebrate-dispersed fruits occur on fertile soils because there is a greater probability of fruit removal from the parent plant was tested at 16 sites around Sydney, Australia. Removal rates from artificial fruit spikes were two and a half times greater on fertile than infertile soil sites, although this was not quite statistically significant. High variability in removal rate between sites was evident irrespective of fertility. Most removal occurred during the day indicating that birds were important consumers, rather than nocturnal mammals. Bird abundance and diversity did not differ between soil types. More frugivorous species were found in plant communities growing on fertile soil. Two models could explain the patterns observed. Firstly, plants with vertebrate-dispersed fruits could be favoured on fertile soils because of a high abundance of frugivorous birds accomplishing seed dispersal. Alternatively, plants with vertebrate-dispersed fruits could be favoured on fertile soil sites for some other reason and frugivorous birds could be attracted to these areas of abundant food. The correlation between soil fertility and the percentage of vertebrate-dispersed fruits was stronger than the correlation between soil fertility and removal rates and suggests that the second model is more likely to be true. Frugivorous birds are unlikely to be responsible for the high percentage of species with vertebrate-dispersed fruits in fertile soil environments.     

Scale-specific correlations between habitat heterogeneity and soil fauna diversity along a landscape structure gradient

Habitat heterogeneity contributes to the maintenance of diversity, but the extent that landscape-scale rather than local-scale heterogeneity influences the diversity of soil invertebrates—species with small range sizes—is less clear. Using a Scottish habitat heterogeneity gradient we correlated Collembola and lumbricid worm species richness and abundance with different elements (forest cover, habitat richness and patchiness) and qualities (plant species richness, soil variables) of habitat heterogeneity, at landscape (1 km²) and local (up to 200 m²) scales. Soil fauna assemblages showed considerable turnover in species composition along this habitat heterogeneity gradient. Soil fauna species richness and turnover was greatest in landscapes that were a mosaic of habitats. Soil fauna diversity was hump-shaped along a gradient of forest cover, peaking where there was a mixture of forest and open habitats in the landscape. Landscape-scale habitat richness was positively correlated with lumbricid diversity, while Collembola and lumbricid abundances were negatively and positively related to landscape spatial patchiness. Furthermore, soil fauna diversity was positively correlated with plant diversity, which in turn peaked in the sites that were a mosaic of forest and open habitat patches. There was less evidence that local-scale habitat variables (habitat richness, tree cover, plant species richness, litter cover, soil pH, depth of organic horizon) affected soil fauna diversity: Collembola diversity was independent of all these measures, while lumbricid diversity positively and negatively correlated with vascular plant species richness and tree canopy density. Landscape-scale habitat heterogeneity affects soil diversity regardless of taxon, while the influence of habitat heterogeneity at local scales is dependent on taxon identity, and hence ecological traits, e.g. body size. Landscape-scale habitat heterogeneity by providing different niches and refuges, together with passive dispersal and population patch dynamics, positively contributes to soil faunal diversity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.     

Determinants of bryophyte species composition and diversity on the Great Alvar of Öland,Sweden

Factors driving the species richness and distribution of bryophytes are poorly studied and not well understood, particularly in grasslands. We analysed the occurrence of bryophyte species and variation in species richness across 674 plots (0.5?m?×?0.5?m) in alvar vegetation (grassland on limestone pavement with thin or no soil) on Öland (Sweden) in relation to substrate characteristics and chemistry, inundation frequency, grazing pressure and geographical variables. We found 148 taxa, including 11 nationally red-listed ones. Species richness per plot was significantly associated with substrate type, positively associated with pH and grazing intensity, but negatively associated with soil depth. However, richness of species typical of, or restricted to, alvar habitats responded differently to richness of species more common in other habitats. Typical alvar species were favoured by high pH, shallow soil and low phosphate availability, while generalists preferred relatively low pH, higher phosphate availability and organic or mull soil types. Distance from the alvar margin had only weak effects. Concerning the effects on individual species and community composition, inundation frequency and pH were found to have the largest effects, although other factors (substrate type, soil depth, bare soil, bare stone, phosphate availability and grazing pressure) were more important for some individual species, stressing the importance of microsite variability and variability in management for regional species richness. From a conservation perspective, it is concluded that grazing is generally positive whilst factors increasing phosphate availability may disadvantage the typical alvar species, and proximity to the alvar margin is not a major problem.     

海南岛霸王岭热带天然林景观中木本植物功能型分布的影响因素

为了探讨在热带天然林景观中不同因素对木本植物(限于乔木和灌木)功能型分布的影响,在对海南岛霸王岭的热带天然林进行样方调查的基础上,运用数量分类方法对热带天然林的木本植物进行了功能型划分,并运用冗余度分析(Redundancy analysis, RDA)分别探讨了功能型出现与否、功能型物种丰富度和功能型木本植物多度3个矩阵与环境、空间和干扰因素之间的关系。运用偏冗余度分析对影响功能型分布变化的环境、干扰、空间及其交互作用等因素进行了定量分解。结果表明:利用木材密度和潜在最大高度两个指标划分出的9个功能型,能较好的区分功能型间对生境的适应响应差异;因子分解表明,纯环境、纯人为干扰和混合的环境-人为干扰是影响3个功能型矩阵分布变化的主要因素,纯空间和与空间位置相耦合的因素相对较低;在诸多因子中,干扰类型、地形因子、土壤类型、砂砾含量、土层厚度以及经纬度坐标是影响功能型分布变化的主导因子;RDA排序分析表明,一般来讲,硬木功能型多在生境条件好和远离人为干扰的立地高发生。软木功能型则更多地出现于生境条件恶劣和人为干扰频繁的立地。而中等硬度功能型则生态幅度较宽。但除了软木灌木功能型外,其它功能型物种丰富度和木本植物多度多在土层深厚和生境条件较好的立地较高。     

Landscape patterns of understory composition and richness across a moisture and nitrogen mineralization gradient in Ohio (U.S.A.) Quercus forests

This study quantified relationships of understory vascular plant species composition and richness along environmental gradients over a broad spatial scale in second-growth oak forests in eastern North America. Species frequencies were recorded in 108 25 × 25 m plots in four study sites extending over 70 km in southern Ohio, U.S.A.. The plots were stratified into three long-term soil moisture classes with a GIS-derived integrated moisture index (IMI). In addition to the IMI, the environmental data matrix included eight soil and three overstory variables. Canonical correspondence analysis (CCA) indicated that variations in understory species composition were most strongly related to topographic variations in predicted moisture (IMI), N mineralization rate, nitrification rate, and soil pH. In addition, floristic variation at the regional scale was correlated with variations in soil texture, nitrification, pH, and PO₄ ^–, resulting from differences in the soil parent material complexes among sites. Species richness averaged 65 species/plot, and increased with moisture and fertility. Stepwise regression indicated that richness was positively correlated with N mineralization rate and nitrification rate, and inversely correlated with tree basal area. Greater richness on fertile plots was the largely the result of increasing forb richness. Forb richness per quadrat (2 m²) was most strongly and positively related to N mineralization rate. Conversely, richness of understory individuals of tree species was greatest on xeric, less-fertile plots. Our results describe general, broad-scale species-environment relationships that occurred at both the topographic scale (long-term moisture status and fertility) and the regional scale (geomorphological differences among the sites). Strong species richness-N mineralization correlations indicate an important link between below-ground processes and above-ground biodiversity. Because N availability was a strong correlate to vegetation patterns at a broad-scale, our results suggest that the increasing rates of atmospheric N deposition in the region could have a major impact on understory vegetation dynamics.     

Can the mass effect explain the mid-altitudinal peak in vascular plant species richness?

A mid-altitudinal peak in species richness is commonly observed and the mass effect (or source–sink effect) has been suggested as a possible cause. We test the importance of the mass effect for generating altitudinal patterns of plant species richness at two grain sizes using a simple estimate of sterility/fertility to indicate sinks and sources. To do this we identified species with fertile specimens (fertile species) and species with only sterile specimens (sterile species) in each sampling unit along altitudinal transects and assumed that the number of sterile species indicated the relative number of sink species, correspondingly that the number of fertile species indicated the relative number of source species when looking at the overall pattern of species richness along a transect. To evaluate this approach, we investigated the distribution of sterility and fertility of each species along the altitudinal transects. We found that sterile species are found more often at the edges and fertile species more often in the centre of the species altitudinal ranges than expected by chance. Using a fine grain, sterile species richness had a humped altitudinal pattern on all transects investigated at this scale, whereas using a coarse grain two of the three transects investigated had a humped pattern. At the fine grain, sterile species richness had a more pronounced peak than fertile species richness in two of the three transects investigated supporting the hypothesis of the mass effect, but this pattern did not persist at coarser grain. The observations at the fine grain are in accordance with the idea that the mass effect is important in shaping the mid-altitudinal peak in species richness, whereas the observations from the coarser grain are ambiguous.     

Plant species richness belowground: higher richness and new patterns revealed by next-generation sequencing

Variation in plant species richness has been described using only aboveground vegetation. The species richness of roots and rhizomes has never been compared with aboveground richness in natural plant communities. We made direct comparisons of grassland plant richness in identical volumes (0.1 × 0.1 × 0.1 m) above and below the soil surface, using conventional species identification to measure aboveground richness and 454 sequencing of the chloroplast trnL(UAA) intron to measure belowground richness. We described above- and belowground richness at multiple spatial scales (from a neighbourhood scale of centimetres to a community scale of hundreds of metres), and related variation in richness to soil fertility. Tests using reference material indicated that 454 sequencing captured patterns of species composition and abundance with acceptable accuracy. At neighbourhood scales, belowground richness was up to two times greater than aboveground richness. The relationship between above- and belowground richness was significantly different from linear: beyond a certain level of belowground richness, aboveground richness did not increase further. Belowground richness also exceeded that of aboveground at the community scale, indicating that some species are temporarily dormant and absent aboveground. Similar to other grassland studies, aboveground richness declined with increasing soil fertility; in contrast, the number of species found only belowground increased significantly with fertility. These results indicate that conventional aboveground studies of plant richness may overlook many coexisting species, and that belowground richness becomes relatively more important in conditions where aboveground richness decreases. Measuring plant belowground richness can considerably alter perceptions of biodiversity and its responses to natural and anthropogenic factors.     

Cervid Exclusion Alters Boreal Forest Properties with Little Cascading Impacts on Soils

Large herbivores are capable of modifying entire ecosystems with a combination of direct (for example browsing/grazing, trampling, defecation) and indirect (for example affecting plant species composition that then alters soil properties) effects. With many ungulate populations increasing across the northern hemisphere it is important to develop a general theory for how these animals can be expected to impact their habitats. Here we present the results of an 8-year experimental exclusion of moose (Alces alces) from 15 recent boreal forest clear-cut sites in Central Norway. We used standard univariate techniques to describe the treatment effect on multiple forest and soil properties and combined this with a multivariate Bayesian network structure learning approach to objectively assess the potential mechanistic pathways for indirect effects on soils and soil fertility. We found that excluding moose had predictable direct effects, such as increasing the ratio of deciduous to coniferous tree biomass and the canopy cover and decreasing soil bulk density and temperature. However, we found no treatment effects on any measures of soil processes or quality (decomposition, nitrogen availability, C/N ratio, pH, nutrient stocks), and furthermore, we found only limited evidence that the direct effects had cascading (indirect) effects on soils. These findings oppose the commonly held belief that moose exclusion will increase soil fertility, but still highlights the strong ability of moose to directly modify forested ecosystems.     

Ecosystem-phase interactions: aquatic eutrophication decreases terrestrial plant diversity in California vernal pools

Eutrophication has long been known to negatively affect aquatic and terrestrial ecosystems worldwide. In freshwater ecosystems, excessive nutrient input results in a shift from vascular plant dominance to algal dominance, while the nutrient-species richness relationship is found to be unimodal. Eutrophication studies are usually conducted in continuously aquatic or terrestrial habitats, but it is unclear how these patterns may be altered by temporal heterogeneity driven by precipitation and temperature variation. The California vernal pool (CVP) ecosystem consists of three distinct phases (aquatic, terrestrial, and dry) caused by variation in climatic conditions. The purpose of this study was to test the hypothesis that resource addition during the aquatic phase results in increased algal abundance, which reduces vascular plant cover and richness of the terrestrial phase upon desiccation. We used mesocosms layered with CVP soil, in which treatments consisted of five levels of nitrogen and phosphorous added every 2 weeks. Resource addition increased available phosphorus levels and algae cover during the aquatic phase. Increased algal crusts resulted in decreased vascular plant percent cover and species richness. Few significant patterns were observed with individual plant species and total biomass. The phosphorus-plant richness relationship was not significant, but species composition was significantly different among the low and high treatment comparisons. These results highlight a neglected effect of eutrophication in seasonal habitats. Interactions among ecosystem phases clearly require more attention empirically and theoretically. Management and restoration of temporally heterogeneous habitat, such as the endemic-rich CVP, need to consider the extensive effects of increased nutrient input.     

Relationship between soil nutrient availability and plant species richness in a tropical semi‐arid environment

Question: What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecox and Prosopis laevigata)? Location: Tehuacán‐Cuicatlán region, central Mexico. Methods: Plant richness was measured in three micro‐environments (below canopies of C. praecox, below canopies of P. laevigata and in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs. Results: Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness under P. laevigata canopies. The relationship between soil fertility and species richness was always positive except for total N, ammonium and net C mineralization rate under C. praecox, and for available P under P. laevigata. Conclusions: The sign of the relationship between soil fertility and species richness varies according to the nutrient and the micro‐environment. Positive relationships could result from between species complementarity and facilitation. Negative relationships could be explained by a specific limitation threshold for some soil resources (P and N for plants and C for the soil microbiota) which eliminate the possibilities of between species complementarity and facilitation above that threshold. As in all observational studies, these relationships should be considered only correlational.     

Land‐use Change Dynamics,Soil Type and Species Forming Mono‐dominant Patches: the Case of Pteridium aquilinum in a Neotropical Rain Forest Region

Deforestation and agricultural land degradation in tropical regions can create conditions for growth of perennial plant species forming mono‐dominated patches (MDP). Such species might limit forest regeneration, and their proliferation forces the abandonment of fields and subsequent deforestation to establish new fields. Therefore, identifying factors fostering MDP species is critical for biodiversity conservation in human‐modified landscapes. Here, we propose a conceptual framework to identify such factors and apply it to the case of Pteridium aquilinum (bracken fern), a light‐demanding species, tolerant of low soil fertility and fire. We hypothesize that bracken proliferation is promoted by land‐use changes that increase light availability, especially in sites with low soil fertility and land uses involving fire. We assessed this idea using agricultural fields in southeastern Mexico with different land‐use change histories and quantifying prevalence and cover of bracken. Five different land‐use change histories resulted from transitions among forest, crop, pasture, and fallow field stages. Of the 133 fields sampled, 71 percent had P. aquilinum; regression tree analysis indicated that 65 percent of inter‐field variation in prevalence and 90 percent in cover was explained by land‐use change history and soil type. Maximum prevalence, cover, and rates of increase in bracken were found on fields with low fertility sandy/clay soils, which had been used for crops and pasture, were frequently burned, and had high levels of light. Fields on fertile alluvial soil never used for pasture were bracken‐free. Agriculture promoting high light environments on less fertile soils is a major cause of bracken proliferation and likely that of other MDP species.     

Canopy cover type,and not fine-scale resource availability,explains native and exotic species richness in a landscape affected by anthropogenic fires and posterior land-use change

Anthropogenic fires and land-use change, including the conversion from native to exotic species canopies, are two major types of disturbances that strongly affect the functioning of forest ecosystems around the world. These disturbances alter the resource availability for plants, which may lead to changes in species richness. Here we examined the relative effects of canopy cover type, light availability and soil nutrient (N and P) availability on species richness, including invasive species, at different post-fire plant systems. Additionally, we tested the resource heterogeneity hypothesis (RHH) for plant diversity, which proposes that diversity is higher in habitats with spatially heterogeneous resources. We evaluated four different canopy cover types, including mature and second-growth Nothofagus pumilio forests, treeless prairie, Pinus sylvestris afforestations, all of which were converted from mature N. pumilio forests. Using generalized mixed-effects model correlations, we determined (1) the relative influence of canopy cover type, light and soil nutrient availability on understory species richness and (2) the relationship between species richness and resource heterogeneity. We found that canopy cover type was the factor that best explained species richness, much more than fine-scale light and soil nutrient availability. Additionally, we found that the more homogeneous the light environment the higher the number of exotic species (mainly found in the prairie where the highest light intensity occurred), which is contrary to what the RHH states. In conclusion, canopy cover type, a stand-scale driver, and not fine-scale resource (light, N and P) availability, was most important for explaining native and exotic (including invasive) species understory richness in a landscape affected by anthropogenic fires and posterior land-use change.