Consequences of shrub expansion in mesic grassland: Resource alterations and graminoid responses

Abstract. In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall‐grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C₄ grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass‐shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass‐shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall‐grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long‐term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall‐grass prairies is, therefore, a preferred management option.     

Diversity of native and alien vascular plant species of dry grasslands in central Europe

Question: Which factors determine diversity of native and alien vascular plant species in semi‐natural dry grasslands? Location: Northern limestone Alps to the southern rim of the Bohemian massif in northern Austria. Methods: In 70 randomly chosen dry grassland patches (0.008 ha ‐ 7 ha) we sampled a complete inventory of vascular plant species at each site. We analysed the correlation between species diversity of natives, archaeophytes (pre‐1500 aliens) and neophytes (post‐1500 aliens). We used GLM to study the relationship of species number (natives, neophytes, archaeophytes) to five explanatory variables (altitude, within habitat diversity, habitat diversity of adjacent areas, within land‐use diversity and land‐use in adjacent areas). Orthogonal components of these variables were derived with a PCA and used in the models. We also tested the influence of minimum residence time (MRT) and the covariables origin, mode of introduction and life form on the number of grassland sites with neophytes with analogous GLMs. Results: Native species diversity species was positively correlated with the species diversity of new, but not old invaders. GLM explains 70% of the variance in the number of native species. Patch size explained the largest part of the variation in the number of native species. PCA axes 1 and 3 were significantly related to the number of native species. Axis 1was related to on‐site habitat and land‐use diversity. The GLM of the archaeophyte diversity explains 18% of the variance. Altitude and presence of fields and grassland in the neighbourhood mainly explained archaeophyte species diversity. The GLM of neophyte diversity explains 12% of the variance. The number of neophytes was positively related to that of archaeophytes. Only PCA axis 3, which is mainly influenced by adjacent land‐use types, showed a relationship with neophytes. MRT, mode of introduction and region of origin (but not life form) were significantly related to the number of grassland sites invaded by neophytes, explaining 35% of the variance. Conclusion: Most factors governing native species diversity are not significantly related to alien species diversity. Additional determinants of the local scale diversity of alien species exist such as region of origin and historical factors (MRT, mode of introduction).     

Seed bank dynamics in tall‐tussock grasslands along an altitudinal gradient

Abstract. We studied the germinable soil seed bank of tall‐tussock grasslands along an altitudinal gradient in the mountains of central Argentina. We selected 10 sampling plots at three altitudinal levels (1200 m, 1600 m and 2200 m). We assessed the composition of the established vegetation and took ten compound soil samples (0 ‐ 5 cm depth) at each plot in autumn and spring. The soil samples were sieved, chilled, and incubated in a glasshouse to assess the composition of the seed bank. The similarity between the composition of the seed bank flora and that of the established vegetation was low throughout the gradient. Most species did not change their seed bank strategy along the gradient. Seed bank richness and density increased with altitude. Most species had a persistent seed bank at all altitudinal levels, and the proportion of such species increased with altitude. These results suggest that a cold climate directly and/or indirectly favours the formation of seed banks and seed persistence in the soil.     

The role of spatio‐temporal heterogeneity in the establishment and maintenance of Suaeda maritima in salt marshes

Abstract. The effects of disturbance and microtopography on the organization and dynamics of plant communities were studied in a European salt marsh located in the Bay of Mont St. Michel, France. The existence of seed trapping mechanisms was also tested. The study took place in the lower and middle marsh plant communities dominated by the perennials Puccinellia maritima and Halimione portulacoides, respectively and associated with the annual Suaeda maritima. Three treatments were used in series of plots placed in each community: (1) vegetation removal and root destruction to a depth of 10 cm and refilling, (2) non‐remnant herbicide treatment without vegetation removal and (3) creation of depressions (20 cm deep). These treatments were compared with adjacent control plots. The first year of the experiment showed that the perennials facilitated the establishment of Suaeda by trapping its seeds. Estimation of cover, density and biomass over 5 yr following the disturbances showed that in the first 2 yr Suaeda dominated the disturbed plots. Thereafter Suaeda was gradually eliminated by competitive exclusion after ca. 3 yr in the zone originally dominated by Puccinellia maritima and after 4 yr in the zone occupied by Halimione portulacoides. Depressions constituted refuge habitats for Suaeda by limiting competition with the perennials but also led to a high risk of mortality with temporal fluctuations in density. Despite a period of investigation limited to 5 yr, our study demonstrated that natural disturbances of various types occurred and influenced the dynamics of Suaeda, Halimione and Puccinellia. We deduced that natural disturbances and microtopography are responsible for the maintenance of the habitat in a state of non‐equilibrium by favouring the establishment of both spatial and temporal environmental heterogeneity. These conditions appear to be particularly favourable for the maintenance of annual species such as Suaeda maritima.     

Spatial and temporal variability during primary succession on tropical coastal sand dunes

Abstract. We studied primary succession on mobile tropical coastal sand dunes over an 8‐yr period. Every six months, we monitored changes in species composition and sand movement in permanent quadrats located on the windward slopes, the arms and the crests. Our results indicate that sand movement decreased over time but was significantly higher on the slopes and crests than on the arms. In all cases, there were seasonal fluctuations in sand movement which increased during the period with strong northerly winds and decreased during the rainy season. Sand movement was significantly correlated with species distribution. Plant cover and species richness increased at all three locations. Diversity increased on the slope, decreased on the arms and remained unchanged on the crest. However, the equitability values indicated the dominance of a few species, especially at the end of the study period. Temporal trends and species turnover rates differed among locations. Species turnover occurred first on the arms (1994), then on the crests (1998) and lastly on the slopes (at the end of the study period). In all cases the tall grass, Schizachyrium scoparium var. littoralis, became dominant over the endemic legume, Chamaecrista chamaecristoides var. chamaecristoides. Similar to temperate dunes, primary succession on tropical sand dunes was spatially and temporally heterogeneous.     

Relationships between species richness patterns in deciduous forests at the north Estonian limestone escarpment

Abstract. The flora of the deciduous forests at the base of the north Estonian limestone escarpment is species rich, with an exceptionally high number of rare bryophyte species. Relationships between species richness of bryophyte and herb layers and biotic and environmental conditions were studied, using General Linear Mixed Models. Human disturbance (waste deposit, tree damage etc) was significantly negatively correlated with species richness of both plant layers. Soil nitrogen content was negatively and soil water retention positively correlated with bryophyte species richness, while herb richness was unrelated to soil factors. After eliminating the effects of environment, negative correlations in species richness and cover between the bryophyte and herb layers were discovered on finer scales (1 m²), referring to biotic interactions. This relationship was obscured with the simple correlation analysis. On the other hand, the positive correlation in species pools between the bryophyte and herb layers (0.1 ha) was insignificant. The species pools of both bryophyte and herb layers were significantly positively correlated with the species richness of the tree layer. In summary, bryophyte and herb layer richness responded differently to environmental conditions, but human disturbance significantly decreased the richness of both layers. Due to the uniqueness and small area of these forests we recommend protection and restoration of disturbed sites.     

Habitat distribution models,spatial autocorrelation,functional traits and dispersal capacity of alpine plant species

Abstract. We evaluate the potential influence of disturbance on the predictability of alpine plant species distribution from equilibrium‐based habitat distribution models. Firstly, abundance data of 71 plant species were correlated with a comprehensive set of environmental variables using ordinal regression models. Subsequently, the residual spatial autocorrelation (at distances of 40 to 320 m) in these models was explored. The additional amount of variance explained by spatial structuring was compared with a set of functional traits assumed to confer advantages in disturbed or undisturbed habitats. We found significant residual spatial autocorrelation in the habitat models of most of the species that were analysed. The amount of this autocorrelation was positively correlated with the dispersal capacity of the species, levelling off with increasing spatial scale. Both trends indicate that dispersal and colonization processes, whose frequency is enhanced by disturbance, influence the distribution of many alpine plant species. Since habitat distribution models commonly ignore such spatial processes they miss an important driver of local‐ to landscape‐scale plant distribution.     

Burrowing activities of kangaroo rats and patterns in plant species dominance at a shortgrass steppe-desert grassland ecotone

Abstract. Our objective was to evaluate the effects of burrowing activities by banner-tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off-mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis-dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda-dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas. Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small-scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.     

Spatial connectivity and boundary patterns in coastal dune vegetation in the Circeo National Park,Central Italy

Abstract. Distribution patterns of coastal sand dune plant communities in the Circeo National Park (Central Italy) are quantified and compared by measuring spatial connectivity and richness of community boundaries along the dune profile. The purpose of this study is: (1) to evaluate patchiness and frequency of spatial links between communities; (2) to identify the communities most sensitive to disturbance; and (3) to predict probable changes due to modification of spatial zonation. Data were obtained using belt transects across the Holocene coastal dune zone. Vegetation zonation from the seashore to the foredune slacks is analysed in relation to chorological, phytosociological and life‐form types. We found that under relatively undisturbed environmental conditions communities formed a sequence (communities 1 to 7), with the exception of a replacement community, which occupied gaps in the perennial vegetation. The spatial distribution of some communities was reduced as a consequence of disturbance; others became fragmented in small patches or even disappeared. In coastal environments with strong, complex gradients, the existence of certain communities depends on specific links (neighbourhood effects) and high connectivity values do not indicate better conservation conditions.     

Soil nutrient fluxes and vegetation changes on molehills

Abstract. The hypothesis that mole burrowing activity alters soil nutrient fluxes and that, as a response to the new conditions, a specialized guild of species develops on the molehills, was tested in an area located in the southwestern Spanish Pyrenees, on a spectrum of montane grassland communities that varies from xeric to temporally waterlogged. Evidence for an association between disturbance and nutrient availability was reported for nitrogen. Mole‐disturbed soils had elevated amounts of inorganic nitrogen compared to soils in surrounding pastures. At the first stages of mound revegetation, changes in nitrate flushes and in species competitive relationships following disturbance appeared to facilitate the establishment of ruderal and non‐mycorrhizal species. The diversity of the whole grassland was enhanced by the existence of these sets of species, abundant on mounds and rarer in the pasture. However, the difference was mainly quantitative, as exclusive colonizers of molehills were not found.     

Spatial modelling of succession-disturbance dynamics in forest ecosystems: Concepts and examples

Over the last few decades it has become increasingly obvious that disturbance, whether natural or anthropogenic in origin, is ubiquitous in ecosystems. Disturbance-related processes are now considered to be important determinants of the composition, structure and function of ecological systems. However, because disturbance and succession processes occur across a wide range of spatio-temporal scales their empirical investigation is difficult. To counter these difficulties much use has been made of spatial modelling to explore the response of ecological systems to disturbance(s) occurring at spatial scales from the individual to the landscape and above, and temporal scales from minutes to centuries. Here we consider such models by contrasting two alternative motivations for their development and use: prediction and exploration, with a focus on forested ecosystems. We consider the two approaches to be complementary rather than competing. Predictive modelling aims to combine knowledge (understanding and data) with the goal of predicting system dynamics; conversely, exploratory models focus on developing understanding in systems where uncertainty is high. Examples of exploratory modelling include model-based explorations of generic issues of criticality in ecological systems, whereas predictive models tend to be more heavily data-driven (e.g. species distribution models). By considering predictive and exploratory modelling alongside each other, we aim to illustrate the range of methods used to model succession and disturbance dynamics and the challenges involved in the model-building and evaluation processes in this arena.     

Spatio‐temporal colonization patterns of forest plant species in a mixed deciduous forest

Abstract. An integrated analysis of the colonization patterns of forest plant species was carried out in a 34‐ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and environmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient enriched soils where much light can reach the ground. Next, the importance of these human‐altered environmental conditions for the colonization of forest plant species was assessed relative to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species‐specific spatio‐temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ‘ancient forest plant species’) are best explained by a combination of spatio‐temporal isolation, soil type, pH and the (non‐)cover of Urtica dioica. By contrast, spatio‐temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ‘ancient forest plant species’ is hampered by a combination of dispersal‐ and recruitment limitation and that the relative importance of both factors is species‐specific.     

Stand dynamics,regeneration patterns and long-term continuity in boreal old-growth Picea abies swamp-forests

Abstract. The occurrence of macrofossil charcoal (long axis > 0.5 mm) and Picea abies (Norway spruce) pollen in peat stratigraphies, in combination with size and age data from 2976 P. abies trees were used to analyse ecosystem continuity and stand-structure in ten old-growth swamp-forests in northern Sweden. All stands were dominated by P. abies, a species whose abundance increased westwards in Sweden between 3000 and 2000 yr B.P. In three stands no macrofossil charcoal was found and the maximum age of the peat, determined by ¹⁴C dating, varied from 1800 to 3600 yr B.P. In the other seven stands the number of levels containing charcoal varied from 1 to 23, but only between 1 and 7 levels were found after the appearance of spruce. Here the maximum age of the peat varied from 400 to 7900 yr B.P. The ten stands had an all-sized stand structure and a stand continuity of ca. 300 yr. The shape of the age structure was similar to an inverse J-curve. This indicates a continuous recruitment over time in a self-perpetuating ecosystem. In a short-term perspective (< 300 yr), the swamp-forests are characterized by individual trees continually emerging while others are dying. it is suggested that internal dynamics of continuous small-scale disturbances in combination with local site-specific factors determine the structure of these forests. in a long-term perspective, some of the present spruce swamp-forests within the northern boreal zone have functioned as true fire-free refugia since the establishment of P. abies populations while others have been affected by recurring fires, although not as frequently as forests on surrounding drier sites. The hypothesis that Scandinavian spruce swamp-forests in general have functioned as true longterm fire-free refugia is thus modified by the present results.     

陕西府谷自然保护区杜松种群的空间分布格局及其动态

采用方差/均值比率法和5个聚集度指标研究了陕西府谷自然保护区杜松(Juniperus rigida Sieb.etZucc.)种群15个样地的空间分布格局及动态特征。结果表明,杜松种群的空间分布格局为聚集分布,其v/m值、负二项参数、聚块性指数、丛生指数、平均拥挤度和Morisita指数分别为1.98、1.84、1.54、0.98、2.79和1.54。随着径级的增大,聚块性指数逐渐减小,种群的聚集程度也减小,Ⅶ级大树呈随机分布,该种群表现出明显的扩散趋势。调查发现,人为干扰对杜松种群的空间分布格局有较大影响。     

Spatio-temporal patterns in post-burn succession on Brittany heathlands

Heathlands in the Paimpont area, Brittany, were ravaged by numerous fires in the summer of 1976. The period in which the fires occurred, and their intensity had important consequences for recolonization processes. In the tall Ulex europaeus heathland described in this paper, fire occurred relatively late (mid-July), and was of high intensity. Colonisation was mainly from propagules, often from species foreign to the original heathland. Plant succession together with vertical and horizontal structure was studied in permanent plots over 13 yr. Three stages were distinguished. Vertical structure varied greatly among the three stages. It was very simple in the colonisation phase, became more complicated during the competition phase, but became simpler again in the stabilisation phase. This was particularly evident for the moss layer, which, 13 yr after the fire, was made up of Polytrichum commune alone. Mosses showed interspecific competition with size as the decisive factor: the large mosses gradually eliminated the smaller ones. Horizontal structure was studied using spatial autocorrelation, which provided some indication of the distinctive -ness and scale of spatial pattern. Some species were weakly aggregated such as Ceratodon purpureus, Polytrichum pili-ferum, and P. juniperinum, whileothers, like the P. formosum-P. commune complex, Agrostis curtisii and Ulex minor were strongly aggregated. In the current community, 13 yr after the fire, a mosaic can be distinguished with three components: (1) a moss layer of Polytrichum commune on which a herbaceous layer, dominated by Agrostis curtisii has developed; (2) a birch wood (both Betulapéndula and B. pubescens), which established on the Polytrichum - Agrostis combination; (3) patches of Ulex minor heathland, with a few Frica and Calluna plants. Extreme simplicity and stability are the two prominent features of the present vegetation state.     

A model of alpine species distribution in relation to snowmelt time and altitude

Abstract. The focus of this study is the response of species to time of snowmelt and altitude in alpine areas and an examination of changes in species response to snowmelt as altitude increases and temperature decreases. Transects (n= 43) were placed evenly along an altitudinal gradient at Finse, Hardanger‐vidda, western Norway, from ridges to late snowbeds. These gradients were systematically sampled (‘Repeated Gradient Analysis, RGA’) and an adjusted F‐test was used to determine repeated trends in species distribution along the transects. Of the 41 taxa analysed 22 showed a significant change in expected occurrence in response to time of snowmelt (when a site becomes free of snow) as altitude increased. Three types of response were observed: (1) no change in response: (2) increased occurrence as altitude increases, i.e. the taxon invades snow‐free sites as altitude increases, and (3) decreased occurrence as altitude increases, i.e. the taxon retreats from snow covered areas. It is suggested that the changes in response are due to both environmental factors (temperature related) and biological interactions. Decreases in expected occurrence are probably due to increased environmental severity as altitude increases (temperature related decreases). These species are represented by taxa preferring intermediate cover of snow. The invasion of earlier snow‐free sites is probably due to reduced competition from lee‐side taxa as altitude increases. A predictive model based on the species‐environmental relationships suggests that a 1°K temperature increase changes the limits of occurrence in response to time of snowmelt from 3 to 20 days for the different taxa.     

Vegetation patterns in heterogeneous landscapes: The importance of history and environment

Abstract. Throughout the eastern United States, plant species distributions and community patterns have developed in response to heterogeneous environmental conditions and a wide range of historical factors, including complex histories of natural and anthropogenic disturbance. Despite increased recognition of the importance of disturbance in determining forest composition and structure, few studies have assessed the relative influence of current environment and historical factors on modern vegetation, in part because detailed knowledge of prior disturbance is often lacking. In the present study, we investigate modern and historical factors that control vegetation patterns at Harvard Forest in central Massachusetts, USA. Similar to the forested uplands throughout the northeastern United States, the site is physiographically heterogeneous and has a long and complex history of natural and anthropogenic disturbance. However, data on forest composition and disturbance history collected over the past > 90 years allow us to evaluate the importance of historical factors rigorously, which is rarely possible on other sites. Soil analyses and historical sources document four categories of historical land use on areas that are all forested today: cultivated fields, improved pastures/mowings, unimproved pastures, and continuously forested woodlots. Ordination and logistic regressions indicate that although species have responded individualistically to a wide range of environmental and disturbance factors, many species are influenced by three factors: soil drainage, land use history, and C:N ratios. Few species vary in accordance with ionic gradients, damage from the 1938 hurricane, or a 1957 fire. Contrary to our expectation that the effects of disturbance will diminish over time, historical land use predicts 1992 vegetation composition better than 1937 composition, perhaps because historical woodlots have become increasingly differentiated from post-agricultural stands through the 20th century. Interpretations of modern vegetation must consider the importance of historical factors in addition to current environmental conditions. However, because disturbances such as land use practices and wind damage are complex, it is often difficult to detect disturbance effects using multivariate approaches, even when the broad history of disturbance is known.     

Spatial patterns of trees in natural forests of East-Central Europe

The analysis of spatial patterns is one of the ways to estimate the role of competition among trees in forest dynamics. Three hypotheses concerning distribution patterns in old-growth stands were tested: (1) fine-scale spatial patterns of trees are regular; (2) patterns do not differ significantly from a random distribution, and (3) spatial patterns at larger scales are clumped because of site heterogeneity. Old-growth forest stands in Poland and the Czech Republic were analysed with a modified Ripley K function, using distribution maps of tree stems. Fine-scale spatial patterns (with distances among trees not exceeding 15 m) were usually intermediate between random and regular. Trends towards a regular distribution occurred more often among dead than among live individuals. No significant relationships between tree species were found at smaller scales; however, at larger scales (distances from 15–25 m) negative associations between some species were found. This reflects site heterogeneity rather than any direct influence of one tree species upon another.     

Matching data sets from two different spatial samples

Abstract. Methods for coupling two data sets (species composition and environmental variables for example) are well known and often used in ecology. All these methods require that variables of the two data sets have been recorded at the same sample stations. But if the two data sets arise from different sample schemes, sample locations can be different. In this case, scientists usually transform one data set to conform with the other one that is chosen as a reference. This inevitably leads to some loss of information. We propose a new ordination method, named spatial‐RLQ analysis, for coupling two data sets with different spatial sample techniques. Spatial‐RLQ analysis is an extension of co‐inertia analysis and is based on neighbourhood graph theory and classical RLQ analysis. This analysis finds linear combinations of variables of the two data sets which maximize the spatial cross‐covariance. This provides a co‐ordination of the two data sets according to their spatial relationships. A vegetation study concerning the forest of Chizé (western France) is presented to illustrate the method.     

Carbon and water relations of juvenile Quercus species in tall‐grass prairie

Abstract. In ecosystems where environments are extreme, such as deserts, adult plant species may facilitate the establishment and growth of seedlings and juveniles. Because high temperatures and evaporative demand characterize tall‐grass prairies of the central United States (relative to forests), we predicted that the grassland‐forest ecotone, by minimizing temperature extremes and moderating water stress, may function to facilitate the expansion of Quercus species into undisturbed tall‐grass prairie. We assessed the carbon and water relations of juvenile Quercus macrocarpa and Q. muhlenbergii, the dominant tree species in gallery forests of northeast Kansas, in ecotone and prairie sites. To evaluate the potentially competitive effects of neighboring herbaceous biomass on these oaks, juveniles (< 0.5 m tall) of both species also were subjected to either: (1) removal of surrounding above‐ground herbaceous biomass, or (2) control (prairie community intact) treatments. Herbaceous biomass removal had no significant effect on gas exchange or water relations in these oak species in either the prairie or the ecotone environment. Although the ecotone did alleviate some environmental extremes, photosynthetic rates and stomatal conductance were ca. 20 % higher (p < 0.05) in both oaks in prairie sites vs. the ecotone. Moreover, although leaf temperatures on average were higher in oaks in the prairie, high leaf temperatures in the ecotone had a greater negative effect on photosynthesis. These data suggest that the grassland‐forest ecotone did not facilitate the growth of Quercus juveniles expanding into this grassland. Moreover, the carbon and water relations of juvenile oaks in the prairie appeared to be unaffected by the presence of the dominant C₄ grasses.