Abiotic constraints on the establishment of Quercus seedlings in grassland

High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first‐year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland‐forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long‐term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late‐season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present‐day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue.     

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.     

Effects of competition and indirect facilitation by shrubs on <Emphasis Type="Italic">Quercus robur</Emphasis> saplings

Indirect facilitation by shrubs has been suggested as a cost-effective way of regenerating oaks in forests of conservation interest. In this study, we tested whether shrubs can enhance growth in pedunculate oak (Quercus robur) by suppressing herbaceous competitors. We studied interactions between young oaks, shrubs, and/or herbaceous vegetation in an open-field experiment, in southern Sweden, over the first 3 years after planting. Oak saplings were grown in four competition treatments: no competing vegetation; with herbaceous vegetation; with shrubs; and with both herbaceous vegetation and shrubs. Competition from shrubs and herbaceous vegetation both reduced stem diameter and biomass accumulation, but they affected biomass partitioning differently. Saplings grown with competition from shrubs partitioned biomass primarily into height growth, while those saplings exposed to competition from herbaceous vegetation invested a relatively higher proportion in root growth. Competition between shrubs and herbaceous vegetation reduced the above-ground biomass of the herbaceous vegetation, resulting in an indirect facilitative effect for the oaks during the first 2 years after planting. However, during the third year, shrubs had a negative effect on biomass accumulation. In summary, results from this study suggest that shrubs indirectly facilitate biomass accumulation of oak saplings by suppressing herbaceous vegetation, possibly by reducing competition for below-ground resources. However, owing to the relatively short duration of positive net outcome for the oak, we recommend that a longer-term assessment of the interaction between oak regeneration and neighboring shrubs be made before the outcome of this study is applied to practical forestry.     

Ecosystem engineering varies spatially: a test of the vegetation modification paradigm for prairie dogs

Colonial, burrowing herbivores can be engineers of grassland and shrubland ecosystems worldwide. Spatial variation in landscapes suggests caution when extrapolating single‐place studies of single species, but lack of data and the need to generalize often leads to ‘model system’ thinking and application of results beyond appropriate statistical inference. Generalizations about the engineering effects of prairie dogs (Cynomys sp.) developed largely from intensive study at a single complex of black‐tailed prairie dogs C. ludovicianus in northern mixed prairie, but have been extrapolated to other ecoregions and prairie dog species in North America, and other colonial, burrowing herbivores. We tested the paradigm that prairie dogs decrease vegetation volume and the cover of grasses and tall shrubs, and increase bare ground and forb cover. We sampled vegetation on and off 279 colonies at 13 complexes of 3 prairie dog species widely distributed across 5 ecoregions in North America. The paradigm was generally supported at 7 black‐tailed prairie dog complexes in northern mixed prairie, where vegetation volume, grass cover, and tall shrub cover were lower, and bare ground and forb cover were higher, on colonies than at paired off‐colony sites. Outside the northern mixed prairie, all 3 prairie dog species consistently reduced vegetation volume, but their effects on cover of plant functional groups varied with prairie dog species and the grazing tolerance of dominant perennial grasses. White‐tailed prairie dogs C. leucurus in sagebrush steppe did not reduce shrub cover, whereas black‐tailed prairie dogs suppressed shrub cover at all complexes with tall shrubs in the surrounding habitat matrix. Black‐tailed prairie dogs in shortgrass steppe and Gunnison's prairie dogs C. gunnisoni in Colorado Plateau grassland both had relatively minor effects on grass cover, which may reflect the dominance of grazing‐tolerant shortgrasses at both complexes. Variation in modification of vegetation structure may be understood in terms of the responses of different dominant perennial grasses to intense defoliation and differences in foraging behavior among prairie dog species. Spatial variation in the engineering role of prairie dogs suggests spatial variation in their keystone role, and spatial variation in the roles of other ecosystem engineers. Thus, ecosystem engineering can have a spatial component not evident from single‐place studies.     

Plant–community responses to shrub cover in a páramo grassland released from grazing and burning

Shrub encroachment can follow grazing or burning release in páramo grasslands. While encroachment decreases herbaceous species richness in some grassland systems, the effects of this process on the herbaceous community in páramo grasslands are currently unknown. We collected data on shrub cover, herbaceous‐species cover and species composition in a páramo grassland 12 years after release from burning and cattle grazing near Zuleta, Ecuador. Topographic and soil measures were also included as predictor variables of differences in community composition. Contrary to studies in other systems, shrub cover did not have a significant effect on herbaceous‐species richness, whereas shrub‐species richness significantly increased with shrub cover. However, shrub cover was associated with significant shifts in herbaceous–community composition. Most notably, there was an increase in some shade‐tolerant forbs and tall‐statured wetland grasses with increasing shrub cover, and a corresponding decrease in some short‐statured grasses and early successional forbs. These results could indicate that the ameliorative effects of shrubs (e.g. frost and wind protection) in harsh alpine environments may partially compensate for the expected competitive effect of shrubs due to shading.     

CO2‐caused change in plant species composition rivals the shift in vegetation between mid‐grass and tallgrass prairies

Atmospheric CO₂ enrichment usually changes the relative contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which change is mediated differ among ecosystems. We measured changes in the contributions of C₃ perennial forbs and C₄ grasses to aboveground biomass production of tallgrass prairie assemblages grown along a field CO₂ gradient (250–500 μmol mol^?1) in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season mean of precipitation for the area. We predicted that CO₂ enrichment would increase the forb contribution to community production, and favor tall‐grasses over mid‐grasses by increasing soil water content and reducing the frequency with which soil water fell below a limitation threshold. CO₂ enrichment favored forbs over grasses on only one of three soil types, a Mollisol. The grass fraction of production increased dramatically across the CO₂ gradient on all soils. Contribution of the tall‐grass Sorghastrum nutans to production increased at elevated CO₂ on the two most coarse‐textured of the soils studied, a clay Mollisol and sandy Alfisol. The CO₂‐caused increase in Sorghastrum was accompanied by an offsetting decline in production of the mid‐grass Bouteloua curtipendula. Increased CO₂ favored the tall‐grass over mid‐grass by increasing soil water content and apparently intensifying competition for light or other resources (Mollisol) or reducing the frequency with which soil water dipped below threshold levels (Alfisol). An increase in CO₂ of 250 μmol mol^?1 above the pre‐industrial level thus led to a shift in the relative production of established species that is similar in magnitude to differences observed between mid‐grass and tallgrass prairies along a precipitation gradient in the central USA. By reducing water limitation to plants, atmospheric CO₂ enrichment may alter the composition and even structure of grassland vegetation.     

Using consecutive prescribed fires to reduce shrub encroachment in grassland by increasing shrub mortality

Woody plant encroachment into open grasslands occurs worldwide and causes multiple ecological and management impacts. Prescribed fire could be used to conserve grassland habitat but often has limited efficacy because many woody plants resprout after fire and rapidly reestablish abundance. If fire‐induced mortality could be increased, prescribed fire would be a more effective management tool. In California's central coast, shrub encroachment, especially of Baccharis pilularis (coyote brush), is converting coastal prairie into shrub‐dominated communities, with a consequent loss of native herbaceous species and open grassland habitat. B. pilularis has not been successfully controlled with single prescribed fire events because the shrub resprouts and reestablishes cover within a few years. We investigated whether two consecutive annual burns would control B. pilularis by killing resprouting shrubs, without reducing native herbaceous species or encouraging invasive plants. As expected, resprouting did occur; however, 2 years after the second burn, B. pilularis cover on burned plots was only 41% of the cover on unburned plots. Mortality of B. pilularis more than doubled following the second burn, likely maintaining a reduction in B. pilularis cover for longer than a single burn would have. Three native coastal prairie perennial grasses did not appear to be adversely affected by the two burns, nor did the burns result in increased cover of invasive species. Managers wanting to restore coastal prairie following B. pilularis encroachment should consider two consecutive annual burns, especially if moderate fire intensity is achievable.     

Reintroducing a keystone burrowing rodent to restore an arid North American grassland: challenges and successes

Prairie dogs (Cynomys spp.) are important ecosystem engineers in North America's central grasslands, and are a key prey base for numerous predators. Prairie dogs have declined dramatically across their former range, prompting reintroduction efforts to restore their populations and ecosystem functions, but the success of these reintroductions is rarely monitored rigorously. Here, we reintroduced 2,400 Gunnison's prairie dogs (C. gunnisoni) over a period of 6 years to the Sevilleta National Wildlife Refuge, in central New Mexico, U.S.A., a semi‐arid grassland ecosystem at the southern edge of their range. We evaluated the population dynamics of prairie dogs following their reintroduction, and their consequent effects on grassland vertebrates. We found postrelease survival of prairie dogs stabilized at levels typical for the species (ca. 50%) after approximately 1 month, while average annual recruitment was ca. 0.35 juveniles per female, well below what was required for a self‐sustaining, stable population. Extreme drought conditions during much of the study period may have contributed to low recruitment. However, recruitment increased steadily over time, indicating that the reintroduced colony may simply need more time to establish in this arid system. We also found well‐known associates of prairie dog colonies, such as American badgers (Taxidea taxus) and burrowing owls (Athene cunicularia), were significantly more common on the colonies than off. After 7 years, we have yet to meet our goal of establishing a self‐sustaining population of Gunnison's prairie dogs in this semi‐arid grassland. But despite the uncertainty and challenges, our work shows that reestablishing keystone species can promote ecosystem restoration.     

Hydraulic properties and freezing-induced cavitation in sympatric evergreen and deciduous oaks with contrasting habitats

We investigated the hydraulic properties in relation to soil moisture, leaf habit, and phylogenetic lineage of 17 species of oaks (Quercus) that occur sympatrically in northern central Florida (USA). Leaf area per shoot increased and Huber values (ratio of sapwood area to leaf area) decreased with increasing soil moisture of species’ habitats. As a result, maximum hydraulic conductance and maximum transpiration were positively correlated with mean soil moisture when calculated on a sapwood area basis, but not when calculated on a leaf area basis. This reveals the important role that changes in allometry among closely related species can play in co‐ordinating water transport capacity with soil water availability. There were significant differences in specific conductivity between species, but these differences were not explained by leaf habit or by evolutionary lineage. However, white oaks had significantly smaller average vessel diameters than red oaks or live oaks. Due to their lower Huber values, maximum leaf specific conductivity (K_L) was higher in evergreen species than in deciduous species and higher in live oaks than in red oaks or white oaks. There were large differences between species and between evolutionary lineages in freeze–thaw‐induced embolism. Deciduous species, on average, showed greater vulnerability to freezing than evergreen species. This result is strongly influenced by evolutionary lineage. Specifically, white oaks, which are all deciduous, had significantly higher vulnerability to freezing than live oaks (all evergreen) and red oaks, which include both evergreen and deciduous species. These results highlight the importance of taking evolutionary lineage into account in comparative physiological studies.     

Effects of Initial Site Treatments on Early Growth and Three-Year Survival of Idaho Fescue

Prairies in the Pacific Northwest have been actively restored for over a decade. Competition from non‐native woody and herbaceous species has been presumed to be a major cause for the failure of restoration projects. In this research, plugs of the native prairie bunchgrass, Festuca idahoensis Elmer var. roemeri (Pavlick), were grown from seed in a nursery and transplanted into a grassland site dominated by non‐native pasture grasses. The growth of the plants was followed for three years, and biomass of all volunteer plants was measured. Before planting, five treatments were applied to the plots: removal of vegetation by burning, removal of vegetation by an herbicide‐and‐till procedure, soil impoverishment by removal of organic matter, fertilizer application, and compost mulch application. Initial growth of Idaho fescue plugs was greatest with fertilizer and compost mulch. Plants grown in mulched plots were also able to photosynthesize later into the dry summer season. After the first year, plots initially fertilized or composted had the lowest survival rate of Idaho fescue. Impoverished and herbicide‐and‐till plots had the greatest 3‐year survival. Mulched plots supported the greatest weed growth after three years. Stressful environments give a competitive advantage to Idaho fescue in prairie restoration projects. As weedy species increase, growth and survival of Idaho fescue decreases.     

The role of seed predation in the maintenance of the Cross Timbers ecotone of Oklahoma,USA

I investigated how seed predation differed among tree species and among microhabitats across the Cross Timbers and what that variation may tell us about how this ecotone is maintained. The ecotone is located in Oklahoma, USA, between the eastern deciduous forest and tallgrass prairie where seeds of eight common tree species were placed in three microhabitats (oak forest, tallgrass prairie, and sumac shrub/small-tree/grass mix). After nine days in the field, percent seeds remaining were scored for each of the 120 (8 species×3 microhabitats×5 replicates) dishes. I found for both wind-dispersed tree species, (ash, elm) there was significantly more predation in the prairie microsite, with similar small predation levels in the shrub and forest. For two of the three bird-dispersed species (dogwood, hackberry), there was significantly more predation in the prairie and shrub microsites compared to the forest. Red cedar, however, was not taken by predators very much anywhere. Finally, all three mammal-dispersed tree species (two oaks, pecan) showed significantly more predation in the shrub and forest microsites compared to the prairie. Whereas wind- and bird-dispersed species suffered less predation as microsites became more woody and dark, the dominant oaks showed the opposite trend. Consequently, seed predators are not preventing oaks from advancing across this ecotone, but yearly fluctuations in predator population density, especially in the shrub transitional zone, could be helping to maintain it.     

Low allelopathic potential of an invasive forage grass on native grassland plants: a cause for encouragement?

Tall fescue (Festuca arundinacea Schreb.), a highly competitive European grass that invades US grasslands, is reportedly allelopathic to many agronomic plants, but its ability to inhibit the germination or growth of native grassland plants is unknown. In three factorial glasshouse experiments, we tested the potential allelopathic effects of endophyte-infected (E+) and uninfected (E−) tall fescue on native grasses and forbs from Midwestern tallgrass prairies. Relative to a water control, at least one extract made from ground seed, or ground whole plant tissue of E+ or E− tall fescue reduced the germination of 10 of 11 species in petri dishes. In addition, the emergence of two native grasses in potting soil was lower when sown with E+ and E− tall fescue seedlings than when sown with seeds of conspecifics or tall fescue. However, when seeds of 13 prairie species were sown in sterilized, field-collected soil and given water or one of the four tall fescue extracts daily, seedling emergence was lower in one extract relative to water for only one species, and subsequent height growth did not differ among treatments for any species. We conclude that if tall fescue is allelopathic, its inhibitory effects on the germination and seedling growth of native prairie plants are limited, irrespective of endophyte infection. On the other hand, the apparent inability of these plants to detect tall fescue in field soil could hinder prairie restoration efforts if germination near this strong competitor confers fitness consequences. We propose that lack of chemical recognition may be common among resident and recently introduced non-indigenous plants because of temporally limited ecological interactions, and offer a view that challenges the existing allelopathy paradigm. Lastly, we suggest that tall fescue removal will have immediate benefits to the establishment of native grassland plants.     

Effects of Quercus emoryi on herbaceous vegetation in a semi-arid savanna

Ten trees (5–70 m² canopy area) were selected to determine effects of tree size (crown area) on herbaceous species composition and biomass in a Quercus emoryi savanna in southeastern Arizona. Consistent with most studies in temperate savannas, herbaceous biomass was reduced beneath the canopy relative to grassland areas. However, tree size appeared to exert no influence over herbaceous biomass. In contrast to most temperate savannas, Q. emoryi trees did not affect distribution of herbaceous species.     

The role of hemiparasitic plants: influencing tallgrass prairie quality,diversity, and structure

Wood betony, Orobanchaceae (Pedicularis canadensis) and bastard toadflax, Santalaceae (Comandra umbellata) are two root‐hemiparasitic plant species found in tallgrass prairie communities. Natural resource managers are interested in utilizing these species as “pseudograzers” in grasslands to reduce competitively dominant grasses and thereby increase ecological diversity and quality in prairie restorations and urban plantings. We performed an observational field study at 5 tallgrass prairie sites to investigate the association of hemiparasite abundance with metrics of phylogenetic and ecological diversity, as well as floristic quality. Although no reduction in C₄ grasses was detected, there was a significant association between hemiparasite abundance and increased floristic quality at all 5 sites. Hemiparasite abundance and species richness were positively correlated at one restoration site. In a greenhouse mesocosm experiment, we investigated response to parasitism by P. canadensis in 6 species representing different plant functional groups of the tallgrass prairie. The annual legume partridge pea, Fabaceae (Chamaecrista fasciculata) had the greatest significant dry biomass reduction among 6 host species, but the C₄ grass big bluestem, Poaceae (Andropogon gerardii) had significantly greater aboveground biomass when grown with the hemiparasite. Overall, host species biomass as a total community was significantly reduced in mesocosms, consistent with other investigations that demonstrate influence on community structure by hemiparasitic plant species. Although hemiparasites were not acting as pseudograzers, they have the potential to influence community structure in grassland restorations and remnants.     

Combined effects between temporal heterogeneity of water supply,nutrient level,and population density on biomass of four broadly distributed herbaceous species

Temporal heterogeneity of water supply affects grassland community productivity and it can interact with nutrient level and intraspecific competition. To understand community responses, the responses of individual species to water heterogeneity must be evaluated while considering the interactions of this heterogeneity with nutrient levels and population density. We compared responses of four herbaceous species grown in monocultures to various combinations of water heterogeneity, nutrient level, and population density: two grasses (Cynodon dactylon and Lolium perenne), a forb (Artemisia princeps), and a legume (Trifolium repens). Treatment effects on shoot and root biomass were analyzed. In all four species, shoot biomass was larger under homogeneous than under heterogeneous water supply. Shoot responses of L. perenne tended to be greater at high nutrient levels. Although root biomass was also larger under homogeneous water supply, effects of water heterogeneity on root biomass were not significant in the grasses. Trifolium repens showed marked root responses, particularly at high population density. Although greater shoot and root growth under homogeneous water supply appears to be a general trend among herbaceous species, our results suggested differences among species could be found in the degree of response to water heterogeneity and its interactions with nutrient level and intraspecific competition.     

The influence of savanna trees on nutrient,water and light availability and the understorey vegetation

In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.     

Community changes following shrub invasion of grassland

Summary We studied the development of the shrub Baccharis pilularis ssp consanguinea and its effects on herbs of the annual grassland in Northern California. A series of shands of Baccharis was sampled of ages ranging from 1 yr to>9 yr, representing most of the life cycle of the shrub. In each stand we examined shrub biomass, structure and litterfall. We also determined cover and biomass of all herbaceous species and estimated seed production, seed rain and storage of seed in the soil. Abundances of all herbaceous species declined greatly after Baccharis formed a closed canopy at 2–3 yr, and little seed of herbaceous species was either dispersed into shrub stands or stored in the soil. Exclosures suggested that herbivory by small mammals in the closed shrub stands may be important in reducing the abundance of herbaceous species following shrub invasion of grassland.     

Bird community composition across an Andean tree‐line ecotone

Few studies have found strong evidence to suggest that ecotones promote species richness and diversity. In this study we examine the responses of a high‐Andean bird community to changes in vegetation and topographical characteristics across an Andean tree‐line ecotone and adjacent cloud forest and puna grassland vegetation in southern Peru. Over a 6‐month period, birds and vegetation were surveyed using a 100 m fixed‐width Distance Sampling point count method. Vegetation analyses revealed that the tree‐line ecotone represented a distinctive high‐Andean vegetation community that was easily differentiated from the adjacent cloud forest and puna grassland based on changes in tree‐size characteristics and vegetation cover. Bird community composition was strongly seasonal and influenced by a pool of bird species from a wider elevational gradient. There were also clear differences in bird community measures between tree‐line vegetation, cloud forest and puna grassland with species turnover (β‐diversity) most pronounced at the tree‐line. Canonical Correspondence Analysis revealed that the majority of the 81 bird species were associated with tree‐line vegetation. Categorizing patterns of relative abundance of the 42 most common species revealed that the tree‐line ecotone was composed primarily of cloud forest specialists and habitat generalists, with very few species from the puna grassland. Only two species, Thlypopsis ruficeps and Anairetes parulus, both widespread Andean species more typical of montane woodland vegetation edges, were categorized as ecotone specialists. However, our findings were influenced by significant differences in species detectability between all three vegetation communities. Our study highlights the importance of examining ecotones at an appropriate spatial and temporal scale. Selecting a suitable distance between sampling points based on the detection probabilities of the target bird species is essential to obtain an unbiased picture of how ecotones influence avian richness and diversity.     

Warm–cold colonization: response of oaks to uplift of the Himalaya–Hengduan Mountains

Clarifying the relationship between distribution patterns of organisms and geological events is critical to understanding the impact of environmental changes on organismal evolution. Quercus sect. Heterobalanus is now distributed across the Himalaya–Hengduan Mountains (HHM) and warm lowland in East China, yet how the distribution patterns of this group changed in response to the HHM uplift remains largely unknown. This study examines the effect of tectonic events in the HHM region on the oaks, providing a biological perspective on the geological history of this region. Fifty‐six populations of Quercus sect. Heterobalanus were genotyped using four chloroplast DNA regions and nine nuclear simple sequence repeat loci to assess population structure and diversity, supplemented by molecular dating and ancestral area reconstructions. The underlying demographic dynamics were compared using ecological niche models of the species distributions during the last glacial maximum and the present. These analyses illustrate that Quercus sect. Heterobalanus diversified as the HHM uplifted and climatic cooling during the mid‐Miocene, colonizing the cold habitats from warm broadleaf mixed forests. Lineages in cold highlands and warm lowlands have diverged as a consequence of local adaptation to diverging climates since the late Miocene. Our results suggest that continuous uplift of the HHM in the late Miocene to early Pliocene accompanied by simultaneous cooling triggered the differentiation of oaks. The biogeography of Quercus sect. Heterobalanus illuminates the geological events responsible for the modern‐day HHM.     

Physiological and growth responses of switchgrass (Panicum virgatum L.) in native stands under passive air temperature manipulation

In the central Great Plains of North America, climate change predictions include increases in mean annual temperature of 1.5–5.5 °C by 2100. Ecosystem responses to increased temperatures are likely to be regulated by dominant plant species, such as the potential biofuel species Panicum virgatum (switchgrass) in the tallgrass prairie. To describe the potential physiological and whole‐plant responses of this species to future changes in air temperatures, we used louvered open‐sided chambers (louvered OSC; 1 × 1 m, adjustable height) to passively alter canopy temperature in native stands of P. virgatum growing in tallgrass prairie at varying topographic positions (upland/lowland). The altered temperature treatment decreased daily mean temperatures by 1 °C and maximum temperatures by 4 °C in May and June, lowered daytime stomatal conductance and transpiration, decreased tiller density, increased specific leaf area, and delayed flowering. Among topographic contrasts, aboveground biomass, flowering tiller density, and tiller weight were greater in lowland sites compared to upland sites, with no temperature treatment interactions. Differences in biomass production responded more to topography than the altered temperature treatment, as soil water status varied considerably between topographic positions. These results indicate that while water availability as a function of topography was a strong driver of plant biomass, many leaf‐level physiological processes were responsive to the small decreases in daily mean and maximum temperature, irrespective of landscape position. The varying responses of leaf‐level gas exchange and whole‐plant growth of P. virgatum in native stands to altered air temperature or topographic position illustrate that accurately forecasting yields for P. virgatum in mixed communities will require greater integration of physiological responses to simulated climate change (increased temperature) and resource availability over natural environmental gradients (soil moisture).