Effects of Fire on Seedling Diversity and Plant Reproduction (Sexual vs. Vegetative) in Neotropical Savannas Differing in Tree Density

Little is known about the effects of fire on the structure and species composition of Neotropical savanna seedling communities. Such effects are critical for predicting long‐term changes in plant distribution patterns in these ecosystems. We quantified richness and density of seedlings within 144 plots of 1 m² located along a topographic gradient in long‐unburned (fire protected since 1983) and recently burned (September 2005) savannas in Brazil. These savannas differ in tree density and canopy cover. Sites along the gradient, however, did not differ in species composition prior to the fire. In recently burned savannas we also evaluated the contribution of vegetative reproduction relative to sexual reproduction by quantifying richness and density of root suckers. Finally, we tested seed tolerance to pulses of high temperatures—similar to those occurring during fires on the soil surface and below—of five dominant savanna tree species. Seedlings were more abundant and diverse in unburned than in burned savannas. Seedling species composition differed among unburned and burned savannas probably reflecting early differences in root: shoot biomass allocation patterns. In recently burned savannas, root suckers were more abundant and diverse than seedlings. Relatively long exposures (>10 min) of temperatures of 90 °C reduced seed germination in all studied species suggesting a negative effect of fire on germination of seeds located at or aboveground level. Because vegetative reproduction contributes more than sexual reproduction in burned environments, frequent fires are likely to cause major shifts in species composition of Neotropical savanna plant communities, favoring clonally produced recruits along tree density/topographic gradients.     

Responses of root‐crown bearing shrubs to differences in fire regimes in Pinus palustris (longleaf pine) savannas: Exploring old‐growth questions in second‐growth systems

Question: What are the effects of fire season and intensity on resprouting of different root‐crown bearing shrub species in second‐growth Pinus palustris (longleaf pine) savannas? Location: northern Florida and eastern Louisiana, USA. Methods: In Florida, quadrats were burned biennially either during the dormant season or the growing season. In Louisiana, we applied intensity treatments to quadrats by manipulating ground‐cover fuels, just prior to biennial growing season fires. Maximum fire temperatures were measured, and stem densities were censused before and after fires in both regions. Results: After dormant season fires in Florida, stem densities were seven times greater than initial levels for Hypericum spp. In contrast, growing season fires reduced densities of H. brachyphyllum by 65%, but did not change densities of H. microsepalum. Only resprouting of H. microsepalum decreased with increased fire intensity. In Louisiana, fire intensity influenced Ilex vomitoria, but not Quercus spp. Following fires, stem densities oil. vomitoria were five times greater in fuel removal than fuel addition areas. Conclusions: Past use of dormant season fires likely contributed to increased abundances of some species of root‐crown bearing shrubs observed today in old‐growth savannas. Reintroduction of growing season fires will be effective in maintaining or decreasing stem densities, depending on species and fuel type. Genet mortality and stem density reductions appear most likely in areas at localized scales where tree falls and needle coverage create hotspots in Pinus palustris savannas.     

不同植物类群物种丰富度垂直格局分形特征的比较

格局和过程一直是生态学的核心问题,该文应用“非布朗运动随机分形”模型,在北京东灵山远离人为干扰地区的阴坡设置了两条宽2 m的样带,描述和比较了暖温带落叶阔叶林区乔木、灌木、草本以及总的植物物种丰富度在不同尺度上沿海拔梯度变异特征,以及相关的生态过程。研究结果表明:1)直接梯度法刻画了植物物种丰富度在海拔梯度上的总体变化趋势,并不能从中获得更多的信息。“非布朗运动随机分形”模型揭示了不同尺度上物种丰富度的空间变化并能和生态过程相联系,研究表明该模型适合对乔木、灌木和草本植物物种丰富度的垂直梯度格局特征进行描述,但对总的植物物种丰富度有一定的局限性。2)乔木物种丰富度在海拔梯度上的变化主要受两种生态过程的控制,小尺度上分形维数接近于2,主要受独立的不具有长程相关的生态过程控制,呈近随机分布;大尺度上分形维数接近于1,主要受自相关范围大、具有长程关联特性的生态过程控制,随海拔上升呈近似单调下降的趋势。灌木和草本物种丰富度在海拔梯度上的变化主要受3种生态过程的控制,小尺度和大尺度上的特征与乔木相似;但在中等尺度上,分形维数接近于1.5,它们以近似布朗运动的形式变化。虽然乔木和灌木物种丰富度在海拔梯度上变化的尺度范围相似,但决定它们的主要生态过程完全不同,或相同的生态过程以截然不同的方式作用于两种不同的植物类群。总的植物物种丰富度在整个尺度范围内呈现标度不变性,这可能是由于决定其变化的生态过程作用尺度紧密相关,也可能因为该文研究范围的局限性。     

Water relations and growth of shrubs before and after fire in a semi-arid woodland

Summary Plant water relations and shoot growth rate of shrubs resprouting after fire or unburnt were measured in a semi-arid poplar box (Eucalyptus populnea) shrub woodland of eastern Australia. In vegetation unburnt for about 60 years, the dawn xylem water potential (_x) of the dominant shrub species was about-1.0 MPa when the soil was wet and-8.0 MPa when the soil was very dry. At any one time, the dominant shrub species,Eremophila mitchellii, E. sturtii, Geijera parviflora andCassia nemophila, were similar in _x butAcacia aneura andDodonaea viscosa were consistently higher in _x than this group when the soil was moist and lower when the soil was dry. The dominant tree species,Eucalyptus populnea andE. intertexta, appeared to have access to additional water beneath the hardpan which is located 60–80 cm below the surface. When shrubs were under extreme water stress (_x of-8 MPa), the trees had a _x of-3 to-3.6 MPa. Following a fire, both _x and leaf stomatal conductance (g _s) of resprouting shrubs were higher for about 5 years than comparable-aged unburnt vegetation, with relative differences in _x increasing with drought stress. Elongation rate of resprouts was positively linked to prefire shrub height in 3 of 4 species. However, shrubs resprouting after high intensity fires had substantially higher rates of shoot elongation than after low intensity fires which were in turn higher than for foliar expansion of unburnt shrubs. It is concluded that the growth rate of resprouting shrubs is primarily determined by physiological/ morphological factors associated with plant size but is also assisted by greater availability of water and possibly nutrients for a period after fire.     

Effects of Differences in Prescribed Fire Regimes on Patchiness and Intensity of Fires in Subtropical Savannas of Everglades National Park, Florida

Abstract We investigated effects of fire frequency, seasonal timing, and plant community on patchiness and intensity of prescribed fires in subtropical savannas in the Long Pine Key region of Everglades National Park, Florida (U.S.A.). We measured patchiness and intensity in different plant communities along elevation gradients in “fire blocks.” These blocks were prescribed burned at varying times during the lightning season and at different frequencies between 1995 and 2000. Fire frequency, seasonal timing, and plant community all influenced the patchiness and intensity of prescribed fires. Fires were less patchy and more intense, probably because of drier conditions and pyrogenic fuels, in higher elevation plant communities (e.g., high pine savannas) than in lower elevation communities (e.g., long‐hydroperiod prairies). In all plant communities fires became increasingly patchy and less intense as the wet season progressed and moisture accumulated in fuels. Frequent prescribed fire resulted in increased patchiness but a wider range of intensities; higher intensities appeared to result from regrowth of more flammable vegetation. Our study suggests that frequent early lightning season prescribed fires produce a wider range of post‐fire conditions than less frequent late lightning season prescribed fires. Our study also suggests that natural early lightning season fires readily carried through pine savannas and short‐hydroperiod prairies, but lower elevation long‐hydroperiod prairies functioned as firebreaks. Natural fires probably crossed these firebreaks only during drier years, potentially producing large landscape‐level fires. Knowledge of how patchily and intensely fires burn across a savanna landscape should be useful for developing landscape‐level fire management.     

Physiognomie development of Pseudotsuga forests in relation to initial structure and disturbance intensity

Abstract. Physiognomic patterns may vary significantly during succession despite a tendency for larger-growth forms to gradually replace smaller ones. Development of understory structure was observed for 25 yr after harvest of Pseudotsuga forests on two sites in the western Cascade Range, Oregon. We examine the influences of disturbance intensity and initial vegetation structure on the origin, direction, and rate of physiognomic change. Broad-scale changes in vegetation structure differed between sites. On Watershed 1, herbs dominated for 11 yr, after which shrubs became co-dominant. In contrast, Watershed 3 never exhibited a distinct, transitional shrub phase - herbs dominated for 18 yr, after which trees assumed co-dominance. The pattern and rate of physiognomic succession also varied among pre-disturbance plant communities and with disturbance intensity. Differences among communities largely corresponded with initial vegetation structure, reflecting the disturbance tolerance of forest herbs and shrubs. Canopy closure occurred most rapidly in the initially depauperate, but tree-dominated Coptis community. Along the disturbance gradient, shifts from herb to shrub dominance occurred earlier on burned than on unburned sites due to rapid development of invading shrubs, whose germination and establishment were stimulated by fire. However, subsequent transitions to tree dominance showed no clear relationship with disturbance intensity. These long-term trends suggest that pre-disturbance community structure and disturbance intensity are major determinants of physiognomic succession, but that their effects may be modified by historical or stochastic factors such as limited seed availability or local fluctuations in weather.     

Short‐term effects of herbicides and a prescribed fire on restoration of a shrub‐encroached pine savanna

Shrub encroachment occurring worldwide in savannas and grasslands has commonly been hypothesized to result from anthropogenically altered environments. Two disturbance‐based approaches to restoration have involved: (1) application of selective herbicides to reduce density/cover of shrubs; (2) reinstatement of natural fire regimes to generate environmental conditions favoring herbaceous species. We studied short‐term responses of native shrubs, vines, and grasses in a Louisiana pine savanna to herbicides followed by a prescribed fire and fire alone. Plots established in the summer, 2013, were hand‐sprayed in the fall with Imazapyr and Triclopyr, Triclopyr alone, or no herbicide, then prescribed burned the following spring. Numbers of species of shrubs and vines at scales of 1 and 100 m², numbers of stems and regrowth of stems produced by six common species of shrubs, and the number of flowering culms of perennial C₄ grasses were assessed postfire in 2014. Compared with fire alone, herbicides followed by fire resulted in (1) small reductions in species richness of shrubs and no effects on vines, (2) fewer stems comprising shrub genets, but similar postfire regrowth of resprouting shrub stems, and (3) fewer flowering culms of C₄ grasses. Underground storage organs of savanna shrubs and vines survived both aboveground disturbances. Thus, single applications of herbicides followed by fires reduced, but did not reverse shrub encroachment and negatively affected grasses. Because effects of herbicides overrode those of prescribed fires, these disturbances did not act synergistically, suggesting that reinstating natural fire regimes should be a priority in restoration of savannas and grasslands.     

Aboveground biomass allocation and water content relationships in Mediterranean trees and shrubs in two climatological regions in Israel

Thisstudy investigated the variation along basipetal gradients of the relationshipsbetween the foliage/wood allocation ratios of biomass and of water content, inMediterranean trees and shrubs, at two different locations along a climaticgradient. Understanding of the biomass allocation and water relations inMediterranean trees and shrubs provides useful information on growth patternsofthese species, and on resource dynamics of these plant communities. Twoexperimental sites were selected along a climatological transect that runs fromthe foothills of the Judean Hills to the northern Negev desert in Israel. Ateach site, 16 quadrats of 10 × 10 m (eight on south-facingslopes and eight on north-facing slopes) were marked. The aboveground biomassofdominant tree and shrub species were estimated. Main branches of trees andshrubs were cut, their foliage and wood biomass were separately weighed, andtheir respective water contents were determined. The species studied includedthe evergreen sclerophylls, Quercus calliprinos, Phillyrealatifolia and Pistacia lentiscus, and thesemi-deciduous species, Cistus creticus, Coridothymuscapitatus and Sarcopoterium spinosum. Theresults indicated that the foliage/wood ratio decreased from the periphery ofthe crown to the interior of the trees and shrubs: foliage biomass and waterwere mainly limited to the top 30 cm of the crown in all studiedspecies. Leaves had higher relative water contents than woody tissues in theupper part of the crown. However; when the whole tree or shrub was considered,the relative water content was found to be mostly allocated to the woodystructures. The results are discussed in terms of biomass allocation in variouslife forms of the eastern-Mediterranean plant communities and how they areaffected by slope aspect and climatic conditions.     

Topsoil translocation for Brazilian savanna restoration: propagation of herbs,shrubs, and trees

Topsoil translocation has been used for vegetation restoration throughout the world, but it has been poorly tested within savannas. This study describes Brazilian savanna (cerrado) regeneration for the first 3 years following topsoil translocation. The topsoil was stripped from 2.5 ha of savanna and spread on 1 ha of an abandoned laterite quarry in the Federal District, Brazil. We assessed vegetation structure and species composition in 18 circular plots (3.14/m²) after 5 and 15 months and in 30 circular plots after 37 months. In the last floristic survey, the coverage of herbs was estimated using the step‐point method. To verify the source of regeneration, a total of 181 shrubs and trees were excavated over the first 2 surveys. After 3 years, 24, 40, and 21 species of herbs, shrubs, and trees, respectively, had been recorded by the surveys. Of the 33 families found, Fabaceae, Poaceae, and Asteraceae were the most representative. At 5 and 15 months, 91 and 83% of the individuals (shrubs and trees combined) were derived from resprouting, respectively. Shrub and tree stem density reached 3.2/m² at 5 months, but declined to 0.5/m² at 37 months. By the final survey, native and exotic grasses completely covered the ground. Topsoil translocation was effective for the propagation of native herbs, shrubs, and trees, despite the need to control invasive grasses. The large number of shrub and tree resprouts from roots suggests that the bud bank is an important component of the topsoil for savanna restoration.     

三江并流地区干旱河谷植物物种多样性海拔梯度格局比较

在滇西北三江并流地区典型干旱河谷段, 在怒江、澜沧江和金沙江的东、西坡共设置了6条海拔梯度样带, 通过标准样地的植物群落调查, 分析各条样带植物的物种丰富度、物种更替率的海拔梯度格局, 并比较了地理和植被变量对分布格局的解释。干旱河谷植被带位于海拔3,000 m以下, 以灌丛和灌草丛为主, 其在各河谷的分布上限自西向东依次升高。植物物种丰富度的分布主要与海拔、流域、经纬度和植被带有关, 沿纬度和海拔梯度升高而显著增加的格局主要表现在草本层和灌木层, 灌木物种丰富度还呈现自西向东显著增加的趋势。怒江的灌木和草本种物种丰富度显著高于金沙江和澜沧江, 三条江的乔木种丰富度差异则不显著。森林带的样方草本物种丰富度显著低于灌草丛带样方, 并且还拥有后者没有的乔木种。不同样带的植物物种更替速率呈现了不一致的海拔梯度格局, 但均在样带海拔下部的灌草丛群落与海拔上部森林群落之间的交错带出现峰值。森林-灌草丛植被交错带在怒江样带处于海拔1,900-2,100 m处, 在澜沧江河谷位于海拔2,300-2,400 m, 在金沙江河谷位于海拔2,700-2,900 m。所有海拔样带的森林段或灌草丛段相对于同一样带不同植被段之间的物种更替程度为最小, 不仅小于同一流域不同样带相同植被段之间物种更替率的均值, 更小于所有样带相同植被段之间的更替率均值。在三条河流6条海拔样带的12个植被带段之间的物种更替变化中, 空间隔离因素可以解释34.2%, 而植被类型差异仅能解释不到0.5%。本研究结果显示了环境差异对不同植被类型物种丰富度的首要影响, 和各河流之间的空间隔离对植物群落构建和物种构成的主要作用。     

How to tell a shrub from a tree: A life‐history perspective from a South African savanna

The ecological differences between ‘shrubs’ and ‘trees’ are surprisingly poorly understood and clear ecological definitions of these two constructs do not exist. It is not clear whether a shrub is simply a small tree or whether shrubs represent a distinct life‐history strategy. This question is of special interest in African savannas, where shrubs and trees often co‐dominate, but are often treated uniformly as ‘woody plants’ even though the tree to shrub ratio is an important determinant of ecosystem functioning. In this study we use data from a long‐term fire experiment, together with a trait‐based approach to test (i) if woody species usually classified as shrubs or trees in African savanna differ in key traits related to disturbance and resource use; and (ii) if these differences justify the interpretation of the two growth forms as distinct life‐history strategies. We measured for 22 of the most common woody plant species of a South African savanna 27 plant traits related to plant architecture, life‐history, leaf characteristics, photosynthesis and resprouting capacity. Furthermore we evaluated their performance during a long‐term fire experiment. We found that woody plants authors call (i) shrubs; (ii) shrubs sometimes small trees; and (3) trees responded differently to long‐term fire treatments. We additionally found significant differences in architecture, diameter‐height‐allometry, foliage density, resprouting vigour after fire, minimum fruiting height and foliar δ¹³C between these three woody plant types. We interpret these findings as evidence for at least two different life‐history‐strategies: an avoidance/adaptation strategy for shrubs (early reproduction + adaptation to minor disturbance) and an escape strategy for trees (promoted investment in height growth + delayed reproduction).     

Distribution of Vascular Plant Species Richness Along an Elevational Gradient in the Dongling Mountains, Beijing, China

Quantifying spatial patterns of species richness and determining the processes that give rise to these patterns are core problems In blodlveralty theory. The aim of the present paper was to more accurately detect patterns of vascular species richness at different scales along altitudinal gradients in order to further our understanding of biodlverslty patterns and to facilitate studies on relationships between biodiversity and environmental factors. Species richness patterns of total vascular plants species, including trees, shrubs, and herbs, were measured along an altitudinal gradient on one transect on a shady slope in the Dongling Mountains, near Beijing,China. Direct gradient analysis, regression analysis, and geostatistics were applied to describe the spatial patterns of species richness. We found that total vascular species richness did not exhibit a linear pattern of change with altitude, although species groups with different ecological features showed strong elevational patterns different from total species richness. In addition to total vascular plants, analysis of trees, shrubs, and herbs demonstrated remarkable hierarchical structures of species richness with altitude (i.e. patchy structures at small scales and gradients at large scales). Species richness for trees and shrubs had similar spatial characteristics at different scales, but differed from herbs. These results indicated that species groups with similar ecological features exhibit similar biodlveraity patterns with altitude, and studies of biodiversity based on species groups with similar ecological properties or life forms would advance our understanding of variations in species diversity. Furthermore, the gradients or trends appeared to be due mainly to local variations in species richness means with altitude. We also found that the range of spatial scale dependencies of species richness for total vascular plants, trees, shrubs, and herbs was relatively large. Thus, to detect the relationships betweenspecies richness with environmental factors along altitudinal gradients, it was necessary to quantify the scale dependencies of environmental factors in the sampling design or when establishing non-linear models.     

Avian community structure along elevational gradients in the northeastern United States

Summary Breeding birds were censused along four elevational gradients in the Adirondack Mountains, New York, and the Green Mountains, Vermont. The bird communities of the four gradients were basically similar in species composition, richness and amplitude patterns. Three measures of species diversity decreased with increasing elevation. Low-elevation communities contained higher proportions of rare species and the relative abundances conformed to the broken-stick distribution. At higher elevations the communities showed greater dominance and the dominance-diversity curves approached geometric series. The species characteristic of high-elevation communities had the broadest altitudinal distributions.The upper and lower distributional limits of most species were independent of one another except at ecotones where marked changes in vegetation structure occurred. On each mountain, slightly more than half of the species limits coincided with ecotones. This is a significantly greater proportion than has been found in similar studies of tropical forest bird communities. In further contrast to tropical communities, we found no convincing cases of altitudinal competitive exclusion between species. Interspecific competition in the past seems to have been translated primarily into differences in habitat selection by temperate forest birds.Many of the differences between temperate forest breeding bird communities and tropical ones can be understood in terms of the migratory nature of most of the temperate species and the lower species richness in temperature forests.     

Responses of riparian trees and shrubs to flow regulation along a boreal stream in northern Sweden

1. Flow dynamics is a major determinant of riparian plant communities. Therefore, flow regulation may heavily affect riparian ecosystems. Despite the large number of dams worldwide, little specific information is available on the longitudinal impacts of dams on vegetation, for example how far downstream and at what degree of regulation a dam on a river can influence riparian woodlands. 2. We quantified the long‐term responses of riparian trees and shrubs to flow regulation by identifying their lateral distribution and habitat conditions along a boreal river in northern Sweden that has been regulated by a single dam since 1948. The regulation has reduced annual flow fluctuations, this effect being largest at the dam, downstream from which it progressively decreases following the entrance of free‐flowing tributaries. 3. We related changes in the distribution patterns, composition, abundance and richness of tree and shrub species to the degree of regulation along the river downstream from the dam. Regulation has triggered establishment of trees and shrubs closer to the channel, making it possible to measure ecological impacts of flow regulation as differences in vegetation attributes relative to the positions of tree and shrub communities established before and after regulation. 4. Trees and shrubs had migrated towards the mid‐channel along the entire study reach, but the changes were largest immediately downstream of the dam. Shrubs were most impacted by flow regulation in terms of lateral movement, but the effect on trees extended furthest downstream. 5. The species composition of trees progressively returned to its pre‐regulation state with distance downstream, but entrance of free‐flowing tributaries and variation in channel morphology and substratum caused local deviations. Species richness after regulation increased for trees but decreased for shrubs. The changes in species composition and richness of trees and shrubs showed no clear downstream patterns, suggesting that other factors than the degree of regulation were more important in governing life form.     

Seed limitation of woody plants in Neotropical savannas

The failure of seeds to arrive at all suitable sites (seed limitation) greatly affects plant distribution and abundance. In contrast to tropical forests, the degree of seed limitation in Neotropical savannas is unclear because empirical studies at the community level are scarce. We estimated seed limitation of 23 woody species from annual seed rain measurements along a tree density gradient in the savannas of Central Brazil. These savannas differ in tree density and canopy cover, from closed to open savannas, and are located along shallow topographic gradients. We also studied post-dispersal seed predation and removal of 17 representative woody species, and seed viability loss over time of 12 common woody species under dry-storage conditions. Annual seed rain was lower in open (410 seeds/m²) than in closed savannas (773 seeds/m²). Average seed limitation across woody species was higher than 80% along the tree density gradient. More than 60% of seeds of the studied woody species were predated or removed within 30–45 days in all savannah types. Seeds of most common woody species (66%) lost their viability in less than 12 months of dry storage. This study shows that Neotropical savannah woody plants are strongly seed-limited because of low and poor distribution of seeds among sites, post-dispersal seed removal, and short seed longevity. The high seed limitation of tree species in Neotropical savannas, particularly in open savannas, also may contribute to maintain their relatively low tree densities and help to explain the spatial variation of tree abundance along topographic gradients.     

Relationships between shrubs and annual communities in a sandy desert ecosystem: a three-year study

A field study was designed to investigate the effect of shrubs on the dynamics and structure of annual plant communities in a sandy desert ecosystem. Densities of emerging and reproductive plants of all annual species were monitored in permanent quadrats located under shrubs and in open areas between shrubs during three successive years; a relatively dry one, a relatively wet one, and a very dry one. A total of 29 species were recorded in the study. Of these, 19 species did not show any evidence for differences in abundance between the two habitats. Nine species exhibited year-to-year variation in their responses to the shrub-opening gradient, being more common in a particular habitat during one year, and showing no response or even an opposite response during a different year. Only one species was consistently more abundant under shrubs throughout the whole study period, and no species was consistently more abundant in the openings. These findings contradict the hypothesis that annual species associated with desert shrub communities can be categorized into distinct groups based on their preference for shrubs vs. openings. Ordination analyses of community-level patterns indicated that annual communities inhabiting the openings were more stable than those inhabiting the shrub habitat. The main lesson from this study is the importance of long-term observations in studies attempting to characterize community-level responses to environmental gradients.     

Tree-grass co-existence in savanna: Interactions of rain and fire

The mechanisms permitting the co-existence of tree and grass in savannas have been a source of contention for many years. The two main classes of explanations involve either competition for resources, or differential sensitivity to disturbances. Published models focus principally on one or the other of these mechanisms. Here we introduce a simple ecohydrologic model of savanna vegetation involving both competition for water, and differential sensitivity of trees and grasses to fire disturbances. We show how the co-existence of trees and grasses in savannas can be simultaneously controlled by rainfall and fire, and how the relative importance of the two factors distinguishes between dry and moist savannas. The stability map allows to predict the changes in vegetation structure along gradients of rainfall and fire disturbances realistically, and to clarify the distinction between climate- and disturbance-dependent ecosystems.     

Savannas after afforestation: Assessment of herbaceous community responses to wildfire versus native tree planting

Afforestation and fire exclusion are pervasive threats to tropical savannas. In Brazil, laws limiting prescribed burning hinder the study of fire in the restoration of Cerrado plant communities. We took advantage of a 2017 wildfire to evaluate the potential for tree cutting and fire to promote the passive restoration of savanna herbaceous plant communities after destruction by exotic tree plantations. We sampled a burned pine plantation (Burned Plantation); a former plantation that was harvested and burned (Harvested & Burned); an unburned former plantation that was harvested, planted with native trees, and treated with herbicide to control invasive grasses (Native Tree Planting); and two old-growth savannas which served as reference communities. Our results confirm that herbaceous plant communities on post-afforestation sites are very different from old-growth savannas. Among post-afforestation sites, Harvested & Burned herbaceous communities were modestly more similar in composition to old-growth savannas, had slightly higher richness of savanna plants (3.8 species per 50-m²), and supported the greatest cover of native herbaceous plants (56%). These positive trends in herbaceous community recovery would be missed in assessments of tree cover: whereas canopy cover in the Harvested & Burned site was 6% (less than typical of savannas of the Cerrado), the Burned Plantation and Native Tree Planting supported 34% and 19% cover, respectively. By focusing on savanna herbaceous plants, these results highlight that tree cutting and fire, not simply tree planting and fire exclusion, should receive greater attention in efforts to restore savannas of the Cerrado.     

浑善达克沙地不同植物功能型光合作用和水分利用特征的比较

为了比较不同植物功能型在沙地生境下光合作用和水分利用效率的差异,测定了浑善达克沙地3种功能型的代表种的气体交换特征来比较它们的光合碳固定能力和水分利用状况。3个代表种的气体交换日变化结果表明乔木的光合速率和水分利用效率比草本和灌木的低,而蒸腾速率和气孔导度较高,经过中午的光合午休后,乔木的光合速率在下午没有恢复,而草本和灌木都有不同程度的恢复。在所测定的所有代表物种中,研究地全部的乔木(3种)和灌木(6种)以及典型的草本(2 5种) ,气孔导度与光合速率和蒸腾速率都成显著的正相关关系;另外,在同样的叶片水势情况下,乔木植物的气孔导度最低,在同样的蒸腾速率情况下,乔木植物的光合速率最低。这些结果表明乔木在CO2 同化和H2 O蒸腾平衡上具有低的水分利用效率。从这个角度考虑,我们认为在对沙地进行恢复时,一些草本和灌木种比乔木更合适     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.