Nurse rocks influence forest expansion over native grassland in southern Brazil

Questions: Do small rocky outcrops favor forest plant colonization and establishment in grasslands? If so, what are the potential mechanisms involved in this process? Location: Araucaria forest and Campos grassland mosaic in southern Brazilian highlands (29°29′S, 50°12′W). Methods: We collected data on the density of forest woody species in plots located on rocky outcrops and in open fields subject to different management regimes. We evaluated the influence of outcrops on the density of forest plants ≤60‐cm tall, and the effects of other environmental variables and of site on plant density; we also used information on diaspore traits of the species to discuss the way in which plants disperse to the outcrops. Results: Rocky outcrops harbored a significantly higher density of forest plants than open fields, irrespective of site effect, and rock height was the best predictor for plant density on outcrops. Diaspores of all recorded species possess characteristics associated with dispersal by birds or mammals or by both. Conclusions: Small rocky outcrops markedly influence forest expansion by acting as perches for vertebrate dispersers and as nurse objects and safe sites for plants. Forest expansion starting in small outcrops possibly occurs as follows: perching of dispersers and increase of seed rain on rocks, promotion of better conditions of establishment for forest plants by nurse rocks, protection of plants sensitive to grazing and fire, and nucleation of forest vegetation.     

Climatic signals in tree‐rings of Araucaria angustifolia in the southern Brazilian highlands

Araucaria angustifolia (Bertol.) O. Kuntze (Araucariaceae) is a Neotropical tree, widely distributed in subtropical mountain rain forests and nearby natural grasslands of Southern Brazil. This species produces annual growth rings, but its dendroclimatic potential is barely known. In the present paper, the long‐term growth patterns of A. angustifolia were investigated using annual growth ring time series and association to climate over the last century. Wood cores of A. angustifolia trees growing in forest and grassland habitats were obtained with an increment borer. The cores were surfaced, measured and cross‐dated. The dated ring‐width time series were standardized and submitted to correlation and principal component analysis to verify growth trends among sites and trees. Growth‐climate relationships were investigated using correlation and regression analyses, comparing the ordination axes scores to regional time series of precipitation and temperature. Due to anatomical irregularities, mainly partial rings, only 35 out of 60 trees were cross‐dated. The correlation and ordination analyses showed common tree‐growth trends within and between sites, indicative of a regional environmental force determining inter‐annual cambial activity variation. Despite growing in distinct habitats and disturbance regimes, A. angustifolia trees share a common long‐term growth pattern, which is significantly related to thermal conditions during the current and previous growing seasons. Moreover, site‐specific characteristics may have influenced opposite growth responses and association to climate conditions between forest and grassland trees.     

Restoration of Araucaria Forest: The Role of Perches, Pioneer Vegetation, and Soil Fertility

The Araucaria forest is Brazil's highly threatened subtropical forest ecosystem that has been disappearing in recent decades. Restoration programs involving this forest type are scarce, and there is a lack of scientific information on how ecological processes such as competition, facilitation, and seed dispersal influence natural forest restoration. This work aims to investigate how use of perches to attract seed dispersers and the influence of pioneer vegetation and soil fertilization could affect the colonization of woody species in a degraded area. An experiment was conducted in an abandoned field where the natural establishment of seeds and seedlings of woody species was monitored under factorial combinations of the following treatments: (1) pioneer vegetation (presence and absence); (2) soil fertility (addition of NPK and control); and (3) perches (presence and absence). Seed and seedling abundance, seed and seedling species richness, and seedling mortality were recorded monthly during 12 months. Seed abundance and species richness were significantly greater in places with perches than in control plots. These results were consistent over the year and more pronounced when the surrounding forest produced a higher amount of fruit. Species richness and abundance of seedlings were significantly greater in places with perches than in control plots, and in places with vegetation than without. Soil fertility did not influence seedling establishment. Facilitation and seed dispersal are important factors affecting the colonization of woody species in this subtropical area. Nutrient availability neither regulates the facilitation process nor influences species replacement during the early stages of Araucaria forest succession.     

Seasonal Food Resources of the Squirrel Sciurus ingrami in a Secondary Araucaria Forest in Southern Brazil

In a field study carried out over one year in a secondary Araucaria forest in eastern ParanÝ State, Brazil, foraging was observed in a population of the squirrel Sciurus ingrami. Seeds of Araucaria and about ten more species of native and exotic trees were the main food items besides mushrooms and fruits. Not all seeds were consumed immediately. Especially in autumn and winter, a large portion was cached, in particular of the Araucaria seeds. The relevance of this behaviour, well known in squirrels, for seed dispersion and forest regeneration is discussed.     

Mycorrhizal status of some plants of the Araucaria forest and the Atlantic rainforest in Santa Catarina, Brazil

 In Brazil, the Araucaria forest and the Atlantic rainforest are two threatened ecosystems, with 10% or less of their original areas presently existing. To assess the mycorrhizal status in these forests, roots of 29 native species, belonging to 19 families, were collected throughout the year from different regions of Santa Catarina, Brazil. Roots were washed, and then cut in a cryo-microtome to seek ectomycorrhizal colonization. Other roots were stained before being examined for vesicular-arbuscular mycorrhizas (VAM). Patterns of colonization were identified and photographed. All plants presented evidence of vesicular-arbuscular mycorrhizal colonization. No evidence of ectomycorrhizal colonization was found. Vesicular-arbuscular mycorrhizal colonization patterns varied from single intracellular aseptate hyphae, coils, and/or appressoria, to vesicles and/or arbuscules. Results confirmed that VAM hosts are predominant in South American forests while ectomycorrhizas are extremely rare even among genera known as ectomycorrhizal in other regions of the humid tropics. Accepted: 27 August 2000     

Testing for the influence of niche and neutral factors on sapling community assembly beneath isolated woody plants in grasslands

Questions: Perching and nursing effects drive initial steps of forest expansion over grasslands. Nursing effect is obviously related to niche mechanisms, while perching effect is likely to result both from neutral and niche factors. This study assessed the effect of neutral and niche factors on species composition in sapling communities developing beneath isolated trees/shrubs (ITS) in grassland. Location: A mosaic of Campos grassland and Araucaria forest in São Francisco de Paula, southern Brazil (29°28′S, 50°13′W). Methods: We described sapling communities beneath 32 ITS using mean number of forest woody saplings of different species. We performed a stepwise canonical correspondence analysis (CCA) to select ITS traits that maximized the association with species composition. Then we evaluated the contribution of distance from seed source, ITS traits and distance‐structured ITS traits on sapling community assembly using a variation partitioning method based on CCA and partial CCA. Results: Sapling species composition was significantly explained by ITS traits (ITS dispersal mode, ITS growth form, crown area:ITS height ratio, crown area, ITS height and crown area:volume ratio). Distance from seed sources explained only a minor, non‐significant fraction of sapling species composition. Distance‐structured trait variation was negligible. Conclusions: Sapling community assembly beneath ITS was mostly explained by niche factors related to both nursing and perching effects. Dispersal limitation explained only a small fraction of variation in species composition beneath ITS, suggesting that neutral‐based perching effect had a minor role in community assembly.     

Do Terrestrial Tank Bromeliads in Brazil Create Safe Sites for Palm Establishment or Act as Natural Traps for Its Dispersed Seeds?

This study shows for the first time that terrestrial tank bromeliads from Brazilian restinga can act as natural traps for dispersed palm Euterpe edulis seeds. Such bromeliads, which are shade intolerant, gain benefits by limiting palm recruitment since they hinder canopy formation and, consequently, increase luminosity over its aggregates.     

Nurse plants vs. nurse objects: effects of woody plants and rocky cavities on the recruitment of the Pilosocereus leucocephalus columnar cactus

BACKGROUND AND AIMS: Most studies on cactus recruitment have focused on the role of woody plants as seedling facilitators. Although the spatial association of cacti with objects had been described, the mechanisms underlying this association remain unknown. The aims of this study were to identify which mechanisms facilitate the establishment of a columnar cactus under the shade and protection of objects and to compare these mechanisms with those involved in plant-plant facilitation. METHODS: Three split-split-plot field experiments were conducted to compare the effects of two microhabitats (inside rocky cavities and beneath plant canopies) on seed removal, germination, seedling survivorship and dry weight. Flat, open spaces were used as the control. For each microhabitat, the effect of seed or seedling protection and substrate limitation were explored; aboveground microclimate and some soil properties were also characterized. KEY RESULTS: The permanence of superficial seeds was greater inside rocky cavities than beneath woody plant canopies or on flat, open areas. Germination was similar in cavities and beneath plant canopies, but significantly higher than on flat, open areas. Seedling survivorship was greater beneath plant canopies than inside cavities or on flat, open spaces. CONCLUSIONS: The mechanisms of plant facilitation are different from those of object facilitation. There are seed-seedling conflicts involved in the recruitment of P. leucocephalus: nurse plants favour mainly seedling survivorship by providing a suitable microenvironment, while nurse objects mainly favour seed permanence, by protecting them from predators.     

贵州玉舍国家森林公园种子植物区系研究

玉舍国家森林公园位于贵州西部的六盘水市水城县南部。根据多年的考察、鉴定、资料整理和统计,有种子植物122科373属923种。通过对该区种子植物属的区系分析结果:有14个分布区类型17个变型。热带分布属105属,占总属数的30.97%,温带分布属223属,占总属数的65.78%。显示了该区温带成分的特性。植物区系成分复杂,起源古老,分布有许多单型属和古老的孑遗植物,共有珍稀植物19种,其中国家一级保护植物2种,国家二级保护植物8种,国家三级保护植物3种,尚未列入保护等级的有6种。     

Past century changes in Araucaria angustifolia (Bertol.) Kuntze water use efficiency and growth in forest and grassland ecosystems of southern Brazil: implications for forest expansion

Araucaria angustifolia (Bertol.) Kuntze is an indigenous conifer tree restricted to the southern region of South America that plays a key role in the dynamics of regional ecosystems where forest expansion over grasslands has been observed. Here, we evaluate the changes in intrinsic water use efficiency (iWUE) and basal area increment (BAI) of this species in response to atmospheric CO₂, temperature and precipitation over the last century. Our investigation is based on tree-rings taken from trees located in forest and grassland sites in southern Brazil. Differences in carbon isotopic composition ( δ ¹³C), ¹³CO₂ discrimination (Δ¹³C) and intracellular carbon concentration ( C _i ) are also reported. Our results indicate an age effect on Δ¹³C in forest trees during the first decades of growth. This age effect is not linked to an initial BAI suppression, suggesting the previous existence of nonforested vegetation in the forest sites. After maturity all trees show similar temporal trends in carbon isotope-derived variables and increasing iWUE, however, absolute values are significantly different between forest and grassland sites. The iWUE is higher in forest trees, indicating greater water competition or nutritional availability, relative to grassland, or both. BAI is also higher in forest trees, but it is not linked with iWUE or atmospheric CO₂. Nevertheless, in both forest and grassland sites A. angustifolia has had growth limitations corresponding to low precipitation and high temperatures observed in the 1940s.     

A surplus of seeds: High rates of post-dispersal seed predation in a flooded grassland in monsoonal Australia

Abstract Oryza meridionalis is an annual emergent wetland grass which produces between 26 and 260 kg seeds ha^-1 annually. Seed shed occurs at the end of the wet season, when the plains are usually still partially flooded. The juvenile recruitment of key native vertebrate species, such as the Magpie Goose (Anseranus semipalmata) and the Dusky Plains Rat (Rattus colletti), coincides with seed shed. This study investigated predation of O. meridionalis seeds at two sites on the South Alligator River floodplain in monsoonal Australia. The effects of inundation and the presence of a background density of seeds on seed removal were investigated by stratified sampling with respect to position down the topographic slope, to include the ‘dry’ floodplain margin, ‘damp’Oryza zone, and ‘flooded’Oryza zone. The effect of seed lot size on the proportion of seed removed was also investigated, and exclosures were used to identify the principal predator group. The proportion of seeds removed was not affected by the presence/absence of a background of seeds nor the number of seeds placed in experimental ‘lots’. The majority of seeds (75%) was consumed by vertebrate predators (most likely the abundant Dusky Plains Rat). Inundation afforded some protection from predation. Despite high losses of seeds exposed to predators, O. meridionalis is an abundant and widespread species on these floodplains, possibly because of the protection from predation afforded by inundation to those seeds which are shed into the water column. It is likely that there is a complex interaction between topography, rainfall and predator and prey relationships, which ultimately determines the importance of seed removal for the maintenance of populations of O. meridionalis. These high losses of seeds to predation have implications for wetland rehabilitation where seed broadcasting is proposed.     

Recruitment trade-offs and the evolution of dispersal mechanisms in plants

In this study we place seed size vs. seed number trade-offs in the context of plant dispersal ability. The objective was to suggest explanations for the evolution of different seed dispersal mechanisms, in particular fleshy fruits, wind dispersal and the maintenance of unassisted dispersal. We suggest that selection for improved dispersal may act either by increasing the intercept of a dispersal curve (log seed number vs. distance) or by flattening the slope of the curve. 'Improved dispersal' is defined as a marginal increase in the number of recruits sited at some (arbitrary) distance away from the parent plant. Increasing the intercept of the dispersal curve, i.e. producing more seeds, is associated with a reduction in seed size, which in turn affects the recruitment ability, provided that this ability is related to seed size. If recruitment is related to seed size there will be a recruitment cost of evolving increased seed production. On the other hand, a flattening of the slope by evolving dispersal attributes is likely to be associated with a fecundity cost. An exception is wind dispersal where smaller (and hence more numerous) seeds may lead to more efficient dispersal. We derive two main predictions: If recruitment is strongly related to seed size, selection for improved dispersal acts on the slope of the dispersal curve, i.e. by favouring evolution of dispersal attributes on seeds or fruits. If, on the other hand, recruitment is only weakly related to seed size (or not related, or negatively related), selection for improved dispersal favours increased seed production. Despite its simplicity, the model suggests explanations for (i) why so many plant species lack special seed dispersal attributes, (ii) differences in dispersal spectra among plant communities, and (iii) adaptive radiation in seed size and dispersal attributes during angiosperm evolution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reduced germination success of temperate grassland seeds sown in dung: consequences for post‐dispersal seed fate

• Endozoochory is one of the main drivers shaping temperate grassland communities by maintaining plant populations of its constituents and enabling plants to colonize new habitats. Successful endozoochorous dispersal implies that seeds not only get consumed and survive the digestive tract but are also able to develop into viable seedlings in a dung environment.
• We experimentally assessed the germination probability and timing of 15 annual and perennial temperate European grassland species in cattle and horse dung and in different climatic conditions (greenhouse and outdoor conditions).
• Interspecific variation in germinability and germination timing are found, while life strategy had only an effect on germination timing. We found adverse effects of both cattle and horse dung on the germination characteristics of all tested grassland species, but the effects of cattle dung were more pronounced. In comparison with the control treatment, fewer seeds emerged in dung and more time was needed to germinate. Also, germination metrics clearly differed between the artificial greenhouse and outdoor conditions, with generally a lower germinability in outdoor conditions.
• According to our results, a large cost seems to be associated with endozoochorous dispersal in this stage of the life cycle, as seed dispersal effectiveness strongly depends on the quality of the deposition site with a lowered survival and germination probability when seeds are deposited in dung.

     

The roles of seed mass and persistent seed banks in gap colonisation in grassland

Gaps in grassland created by animals are often sites for species regeneration. The persistent seed banks of ant-hills and surrounding soils in a calcareous grassland in southern Britain were compared and analysed in terms of seed mass and longevity. The relative abundance of species on ant-hills compared to the pasture was highly correlated with seed abundance in ant-hill soil. The abundance of plant species on ant-hills compared to the pasture was significantly negatively correlated with seed mass, and positively correlated with seed longevity. These results suggest that germination from the seed bank in ant-hill soils, as well as limited seed dispersal and vegetative invasion, contributes to the distinctive ant-hill vegetation. The same may apply to other gaps created by animals in grasslands.     

Exotic Guavas are Foci of Forest Regeneration in Kenyan Farmland

Fruiting trees in degraded areas are attractive for frugivorous birds and may become centers of regeneration. However, a number of tree species in degraded areas are exotic species. Thus, the question arises whether these exotic species can also act as foci for forest regeneration. In the farmland adjacent to Kakamega Forest, Kenya, we investigated the frugivore assemblage in, and seed rain and seedling establishment under, 29 fruiting exotic guava trees ( Psidium guajava ) at different distances to the forest. The results show that 40 frugivorous bird species visited guava trees. All of the seed and 82 percent of the seedling species found under the treecrowns were animal dispersed, 58 and 57 percent of them late-successional species, respectively. Path analysis revealed that the abundance of frugivorous birds, seeds, and seedlings did not decrease up to a distance of 2 km from the forest. Surprisingly, the abundance of frugivorous shrubland birds, animal-dispersed seeds, and late-successional seeds showed an increase with increasing distance from forest. Even though they are exotics, fruiting guava trees may have a positive effect on forest regeneration and might prove valuable for management plans concerning forest restoration.     

Growth ring response of two Atlantic Forest tree species pre- and post-dam operation in Southern Brazil

Hydropower plants are important sources of renewable energy, but the climatic impacts of their constructions remain poorly explored. Considering that tree growth analysis is a useful tool to identify environmental impacts, this study aimed at using climate records and tree-ring chronologies to understand possible local climate changes caused by the construction of a hydropower plant in the 1980s in the State of Paraná, Southern Brazil. Historical climatic data were obtained from the local meteorological station and surrounding municipalities and analyzed using ANOVA and means tests. The Pettitt test was additionally used to identify change-points in the meteorological data. Wood samples from a total of 60 trees from Araucaria angustifolia (Bertol.) Kuntze (Araucariaceae) and Cedrela fissilis Vell. (Meliaceae) were collected, and tree-ring chronologies were built using dendrochronological standard procedures. Chronologies for A. angustifolia and C. fissilis represented time periods from 1800 to 2016 and 1899–2015, respectively. Tree-ring growth responses to climatic variables were evaluated by adjusting generalized mixed linear models and the Spearman correlations. Our results evidenced that the hydropower plant altered the local climate, mostly influencing the hydrological cycle by increasing local rainfall, with monthly rain volumes being statistically higher than in other meteorological stations. Significant responses in the growth of A. angustifolia were found to be associated with the water level increase caused by the dam and of C. fissilis due to the increase in cloud cover.     

Genetic structure of Araucaria angustifolia (Araucariaceae) populations in Brazil: implications for the in situ conservation of genetic resources

The distribution of the genetic variation within and among natural populations of A. ANGUSTIFOLIA growing in different regions in Brazil was assessed at microsatellite and AFLP markers. Both markers revealed high gene diversity ( H = 0.65; AR = 9.1 for microsatellites and H = 0.27; P = 77.8 % for AFLPs), moderate overall differentiation ( RST = 0.13 for microsatellites and FST = 0.10 for AFLPs), but high divergence of the northernmost, geographically isolated population. In a Bayesian analysis, microsatellite data suggested population structure at two levels: at K = 2 and at K = 3 in agreement to the geographical distribution of populations. This result was confirmed by the UPGMA dendrogram based on microsatellite data (bootstrap support > 95 %). Non-hierarchical AMOVA revealed high variation among populations from different A POSTERIORI defined geographical groups. The genetic distance between sample locations increased with geographical distance for microsatellites ( R = 0.62; P = 0.003) and AFLPs ( R = 0.32; P = 0.09). This pattern of population differentiation may be correlated with population history such as geographical isolation and postglacial colonization of highlands. Implications of the population genetic structure for the conservation of genetic resources are discussed.     

地表水热要素在青藏高原草地退化中的作用

在全球气候变暖和频繁的人类活动影响下,青藏高原草地生态系统发生了生产力下降、生物多样性减少及生态功能退化等一系列现象。与传统观测技术相比,遥感技术具有大范围、快速和连续监测等优点,因此被广泛用于区域尺度的草地植被长时间序列监测。以往对青藏高原草原植被影响因子的研究多集中在气温与降水,而相比较于气温和降水,地表温度和土壤湿度直接作用于植物的根部,对植物种子的萌芽和植株的生长也都有着重要影响,所以地表温度和土壤湿度与植被生长的关系更加紧密。基于遥感技术,利用青藏高原草地区域的MODIS和AVHRR数据,选择草地植被覆盖度作为草地退化的遥感监测指标,建立了青藏高原草地退化遥感监测和评价指标体系,并对青藏高原2001-2017年的草地退化状况进行了遥感监测和评价。同时,利用遥感数据获取青藏高原区域尺度的地表温度和温度植被干旱指数数据,用于指示地表水热状况,最后基于回归方法分析了地表水热要素在青藏高原草地退化中的作用。结果表明：从2001-2017年,青藏高原植被退化程度空间差异明显,柴达木盆地和青海湖附近退化较为严重,喜马拉雅山脉北部、昆仑山脉南部、冈底斯山脉北部交汇的地区退化也较严重。在2001-2017年间,青藏高原草地未退化面积从50.60%上升到59.00%,说明青藏高原草地整体上在朝着改善的方向发展。2001-2017年内,青藏高原草地整体上大部分时间处于轻度退化状态,但是2001年和2015年这两个年份青藏高原草地退化整体上达到中等退化水平。通过回归分析发现,土壤湿度主导的对青藏高原草地的影响面积达到14.04%。地表温度主导的影响面积达到草地总面积的约36.61%。但地表温度与植被之间相互影响,且主要呈现负相关关系。其中,在温性草甸地区,当植被覆盖度较低时,地表温度正向影响植被生长。