Mammalian Faunal Succession in the Cretaceous of the Kyzylkum Desert

Both metatherians and eutherians are known from the Early Cretaceous (Barremian, 125 mya; million years ago) of China, while eutherian-dominated mammalian faunas appeared in Asia at least by the earliest Late Cretaceous (Cenomanian, 95 mya). The approximately 99–93 my old (Cenomanian) Sheikhdzheili l.f. from western Uzbekistan is a small sample of only eutherians, including three zhelestids and a possible zalambdalestoid. The much better-known 90 my old (Turonian) Bissekty l.f. at Dzharakuduk in the central Uzbekistan includes 15 named and unnamed species, based on ongoing analyses. Of these, 12 are eutherians represented by at least the three groups—asioryctitheres, zalambdalestids, and zhelestids—plus an eutherian of uncertain position—Paranyctoides. Zalambdalestids and zhelestids have been argued to be related to the origin of the placental gliriforms (Euarchontoglires) and ferungulates (Laurasiatheria), respectively. Although there are four previously recognized metatherians, we believe three are referable to the deltatheroid Sulestes karakshi and the fourth, Sailestes quadrans, may belong to Paranyctoides. There is one multituberculate and one symmetrodont in the Bissekty l.f. While comparably aged (Turonian) localities in North America have somewhat similar non-therians, they have more metatherians and no eutherians. The next younger localities (early Campanian, ∼80 mya) in North America have both a zhelestid and Paranyctoides, suggesting dispersal of eutherians from Asia. At Dzharakuduk, the approximately 85 my old (late Turonian/Coniacian) Aitym l.f. is much less well known than the Bissekty l.f., but yields nearly identical taxa, with two non-therians, one metatherian, and six eutherians.     

Island life in the Cretaceous - faunal composition,biogeography, evolution,and extinction of land-living vertebrates on the Late Cretaceous European archipelago

The Late Cretaceous was a time of tremendous global change, as the final stages of the Age of Dinosaurs were shaped by climate and sea level fluctuations and witness to marked paleogeographic and faunal changes, before the end-Cretaceous bolide impact. The terrestrial fossil record of Late Cretaceous Europe is becoming increasingly better understood, based largely on intensive fieldwork over the past two decades, promising new insights into latest Cretaceous faunal evolution. We review the terrestrial Late Cretaceous record from Europe and discuss its importance for understanding the paleogeography, ecology, evolution, and extinction of land-dwelling vertebrates. We review the major Late Cretaceous faunas from Austria, Hungary, France, Spain, Portugal, and Romania, as well as more fragmentary records from elsewhere in Europe. We discuss the paleogeographic background and history of assembly of these faunas, and argue that they are comprised of an endemic ‘core’ supplemented with various immigration waves. These faunas lived on an island archipelago, and we describe how this insular setting led to ecological peculiarities such as low diversity, a preponderance of primitive taxa, and marked changes in morphology (particularly body size dwarfing). We conclude by discussing the importance of the European record in understanding the end-Cretaceous extinction and show that there is no clear evidence that dinosaurs or other groups were undergoing long-term declines in Europe prior to the bolide impact.     

Growing and shrinking in the smallest tortoise, <Emphasis Type="Italic">Homopus signatus signatus</Emphasis>: the importance of rain

Climate change models predict that the range of the world’s smallest tortoise, Homopus signatus signatus, will aridify and contract in the next decades. To evaluate the effects of annual variation in rainfall on the growth of H. s. signatus, we recorded annual growth rates of wild individuals from spring 2000 to spring 2004. Juveniles grew faster than did adults, and females grew faster than did males. Growth correlated strongly with the amount of rain that fell during the time just before and within the growth periods. Growth rates were lowest in 2002–2003, when almost no rain fell between September 2002 and August 2003. In this period, more than 54% of the tortoises had negative growth rates for their straight carapace length (SCL), shell height (SH), and shell volume (SV); maximum shrinking for SCL, SH, and SV was 4, 11, and 12%, respectively. The shell of H. s. signatus has some flexibility dorso-ventrally, so a reduction in internal matter due to starvation or dehydration may have caused SH to shrink. Because the length and width of the shell seem more rigid, reversible bone resorption may have contributed to shrinkage, particularly of the shell width and plastron length. Based on growth rates for all years, female H. s. signatus need 11–12 years to mature, approximately twice as long as would be expected allometrically for such a small species. However, if aridification lowers average growth rates to the level of 2002–2003, females would require 30 years to mature. Additionally, aridification would lower average and maximum female size, resulting in smaller eggs and hatchlings. These projected life history responses to aridification heighten the threat posed by the predicted range contraction of this red-listed species. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Durophagous predation on scaphitid ammonoids in the Late Cretaceous Western Interior Seaway of North America

This study is the first to report a trend of predation intensity on scaphitid ammonoids from the Turonian to the Maastrichtian (Late Cretaceous) on the basis of analysis of ventral shell breakage in large samples from the US Western Interior Province. Analysis of 835 adult specimens revealed ventral shell breakage in 50 specimens. In most of the damaged specimens, the breakage occurred in a preferred position at the rear part of the body chamber. Ventral breakage is rare in the Turonian specimens, whereas it is common in the Campanian and Maastrichtian specimens. The shell diameter of adult scaphitid ammonoids tends to increase with time. The position of the breakage and the absence of repairs indicate that the ventral breakage resulted from lethal predation. Based on the incidence of breakage and the size and shape of the breaks, possible predators include fish, reptiles and cephalopods such as Placenticeras, Eutrephoceras and coleoids. Our statistical analysis of ventral shell breakage indicates that the incidence of lethal predation increased in conjunction with an increase in adult shell size, suggesting that the body size of the prey was an important factor in predator–prey interactions. In addition, the predatory damage is more extensive in larger adults.     

Remodelling of skeletal tissues bone and structural specialisations in an elasmosaurid (Sauropterygia: Plesiosauroidea) from the Upper Cretaceous of Patagonia,Argentina

Elasmosauridae were cosmopolitan Late Cretaceous plesiosaurs with conspicuous morphological diversity. Within this group, vertebral morphology is a criterion for estimating relative age in plesiosaur. On the other hand, the microstructure of plesiosaur bone is considered as indicative of ontogenetic stage. However, knowledge about ontogenetic tissue transformation in different elements of the skeleton is poorly known. Resorption and remodelling of skeletal tissues are required for development and growth, mechanical adaptation, repair and mineral homeostasis of the vertebrate skeleton. This contribution analyses different postcranial elements of a Late Cretaceous elasmosaurid from Patagonia. Characterisation of bone microstructure indicates the presence of compact bone inner organisation in an adult derived plesiosaur from the Cretaceous and that the distribution of bone specialisations depicts conspicuous variations within a single skeleton depending on the skeletal element considered. Bone compactness or degree of remodelling in elasmosaurids is not necessarily correlated with the ontogenetic age of the animal or to costal versus pelagic lifestyles. The available data are still scarce, but we propose a topic of discussion: perhaps the degree of remodelling and compactness also may be related to the activity level and increased mechanical load in different skeletal elements.     

On the Evolution of the Palynoflora and Climatic Environment During Late Pleistocence in Tengger Desert,China

Palynoflora studies on the Duantouliang section, located at 39º40'N, 103º55'E in Northwestern Tengger Desert, China showed that, based on the spore-pollen assemblages, the major vegetation and climatic environment between 42 000 to 23 000 a BP could be divided into the following different periods: Ⅰ. From 42 000 to 38 000 a BP, the spore-pollen assemblages displayed that the mixed conifer/deciduous broad-leaved forests developed on the mountain and its foothill regions where the bare Gobi-desert are at present; At that time, Populus and Salix forests and grassland surrounded the Paleaolake, the climate condition was much warmer and humid than today; Ⅱ. From 38 000 to 37 000 a BP, the climate was warm and moist, it was the most suitable period for the plant growth, the studied area was dominated by the temperate and warm-temperate mixed broad-leaf deciduous and needleleaf forest, there was meadow spreaded on the river sides and lake beaches; Ⅲ. From 31 000 to 30000 a BP, the needleleaf forests and cold-temperate Salix oritrepha shrubs were flourishing, and the climate at that time was relatively cold;Ⅳ. From 30000 to 28000 a BP, the temperatures began increasing, the high lake levels was formed during this time, and the vegetations were meadows and swamps; V. From 28 000 to 23 000 a BP, temperate Cupressaceae and Betula mixed conifer/deciduous forests grew on mountain and foothill region, grassland developed on plain areas, Salix was on lake and river sides. This indicates a warm and moist climate condition but it was drier than the earliest period.     

Specialization of bone structure in Pachyvaranus crassispondylus Arambourg, 1952, an aquatic squamate from the Late Cretaceous of the southern Tethyan margin

The histological organization of the vertebrae of the Maastrichtian squamate Pachyvaranus crassispondylus Arambourg, 1952 , was compared to that of various extant squamates, in order to further document the causes and functional consequences of the so-called 'pachyostosis', frequently observed in Late Cretaceous squamates. The vertebrae of Pachyvaranus are composed of the same basic bone tissue types as those of extant lizards and snakes. In particular, periosteal cortices are made of a pseudolamellar (or 'parallel-fibred') tissue, with radial vascular canals, Sharpey's fibres and conspicuous cyclic growth marks that are strictly identical to that found in extant varanids. Conversely, the vertebrae of Pachyvaranus are extremely compact, whereas those of extant squamates are very cancellous and lightly built. This difference is due to the absence in Pachyvaranus of a broad internal resorption field that, in extant lizards and snakes, transforms compact cortices into loose spongy formations. This absence of inner bone resorption typically corresponds to an osteosclerotic process. In Pachyvaranus , cortical hyperplasy, or pachyostosis stricto sensu , was restricted to the walls of the neural spine. Extreme vertebral porosity is likely to be a primitive condition in squamates, because all lizards and snakes examined in this study display this feature. Therefore, the high vertebral compactness observed in Pachyvaranus would be a derived condition arising from the loss (or de-differentiation) of a morphogenetic process: the broad internal resorption of the vertebrae. Possible palaeoecological bearings of these results are discussed.     

New remains of <Emphasis Type="Italic">Hungarosaurus tormai</Emphasis> (Ankylosauria,Dinosauria) from the Upper Cretaceous of Hungary: skeletal reconstruction and body mass estimation

Hungarosaurus tormai is a medium-sized nodosaurid ankylosaur that was described on the basis of four partial skeletons from the Santonian (Upper Cretaceous) of Hungary. In this paper, a new fifth skeleton and several isolated remains are described which greatly improve our knowledge of this primitive nodosaurid ankylosaur. Isolated cranial remains referred to juvenile individuals provide new information on the development of cranial ornamentation in nodosaurid ankylosaurs. Apart from both preserved mandibles with in situ dentition, the fifth partial skeleton contains several previously unknown limb elements (humerus, ulnae, radius) that indicate unusual limb proportions for Hungarosaurus compared with other ankylosaurs. On the basis of the five partial skeletons and the isolated remains, a skeletal and dermal armor reconstruction is attempted. Body mass calculations using three different methods yield an estimate of 650 kg for H. tormai.      

Formation binning: a new method for increased temporal resolution in regional studies,applied to the Late Cretaceous dinosaur fossil record of North America

The advent of palaeontological occurrence databases has allowed for detailed reconstruction and analyses of species richness through deep time. While a substantial literature has evolved ensuring that taxa are fairly counted within and between different time periods, how time itself is divided has received less attention. Stage-level or equal-interval age bins have frequently been used for regional and global studies in vertebrate palaeontology. However, when assessing diversity at a regional scale, these resolutions can prove inappropriate with the available data. Herein, we propose a new method of binning geological time for regional studies that intrinsically incorporates the chronostratigraphic heterogeneity of different rock formations to generate unique stratigraphic bins. We use this method to investigate the diversity dynamics of dinosaurs from the Late Cretaceous of the Western Interior of North America prior to the Cretaceous–Palaeogene mass extinction. Increased resolution through formation binning pinpoints the Maastrichtian diversity decline to between 68 and 66 Ma, coinciding with the retreat of the Western Interior Seaway. Diversity curves are shown to exhibit volatile patterns using different binning methods, supporting claims that heterogeneous biases in this time-frame affect the pre-extinction palaeobiological record. We also show that the apparent high endemicity of dinosaurs in the Campanian is a result of non-contemporaneous geological units within large time bins. This study helps to illustrate the utility of high-resolution, regional studies to supplement our understanding of factors governing global diversity in deep time and ultimately how geology is inherently tied to our understanding of past changes in species richness.     

Problems, Approaches, and Results in Restoration of Dutch Calcareous Grassland During the Last 30 Years

This paper is based on research of the restoration of species‐rich calcareous grasslands in The Netherlands, over the last 30 years. Chalk grassland is a semi‐natural vegetation with a high density of species at a small scale. This type of vegetation was once widespread in Western Europe as common grazing land, mainly for flocks of sheep for which the main function was dung production. In some regions of Central Europe, these grasslands were also used for hay production. The dung was used to maintain arable field production at a reasonable level. In the chalk district in the southernmost part of The Netherlands some 25 sites of this vegetation, varying in area from 0.05–4.5 ha, are still present. Chalk grassland completely lost its significance for modern agricultural production after the wide application of artificial fertilizer following World War II. This grassland has a high conservation value both for plants and animal species, of which a large number of species are exclusively restricted to this biotope. When conservation activities started at a large scale in the early 1960s, three different types of restoration activities could be distinguished: (1) restoration of fertilized sites; (2) restoration of abandoned grasslands; and (3) recreation of chalk grassland on former arable fields. The main aim of the restoration attempt is to create and/or improve sustainable conditions for both plant and animal species characteristic of the chalk grassland ecosystem. In the process of restoration, several phases of different activities can be distinguished: (1) pre‐restoration phase, during which information of the land use history is collected and, based on these data, clear restoration goals are established; (2) initial restoration phase, during which effects of former, non‐conservational land use has to be undone in order to stimulate germination and establishment of target species originating from soil seed bank and species pool; (3) consolidation phase, including the introduction and continuation of a regular management system for sustainable conservation; and (4) long‐term conservation strategy, including measures to prevent disturbance from the outside and genetic erosion and extinction of locally endangered plant populations.     

Integrating palynological and geochemical data in a new approach to palaeoecological studies: Upper Cretaceous of the Banterwick Barn Chalk borehole, Berkshire, UK

The dinoflagellate cyst record from an Upper Cretaceous (uppermost Cenomanian–upper Coniacian) Chalk core, drilled at Banterwick Barn, Berkshire, is described and statistically correlated with elemental and stable isotope bulk sediment geochemical data from the same core. Seventy-two dinocyst species and subspecies are recorded, and stable carbon and oxygen isotopic (δ¹³C, δ¹⁸O) trends are documented. Lithostratigraphy and chemostratigraphic correlation of the δ¹³C curve with an expanded section at Dover, Kent, are used to identify stratigraphically significant marls, and determine the positions of macrofossil zones and stage boundaries in the Banterwick Barn core. These data indicate that >30 m of chalk at Dover are represented by <2 m of Chalk Rock at Banterwick Barn, with much of the succession being absent due to erosion and non-deposition. First and last appearance datums (FAD, LAD), first and last common occurrences, and acmes of key Turonian–Coniacian dinocyst species are documented and compared with other records from the Anglo–Paris Basin. A new subspecies, Senoniasphaera rotundata alveolata is proposed, which has a FAD in the lower Turonian and last appears (LAD) in the lower Coniacian. Senoniasphaera rotundata rotundata [autonym, herein] has its FAD in the middle Turonian, first common occurrence in the uppermost Turonian, and LAD in the upper Coniacian. An extremely impoverished assemblage of dinocysts in the highest Cenomanian to lowest Turonian is considered to be largely a preservational artefact of intraclastic nodular and calcarenitic chalks, and is not related directly to the well-documented global oceanic anoxic event (OAE2) occurring at that time (93.5 Ma). A sharp increase in dinocyst abundance in the lower Turonian corresponds with a change in lithology to more marly chalks. A gradual decrease in the number of species is observed through the middle Turonian to upper Coniacian; δ¹⁸O records show that this was associated with global climatic cooling. Cluster analysis of the dinocyst abundance record with geochemical data indicates four distinct species groups with characteristic geochemical associations, i.e. Groups 1–4. Groups 1 and 2 are associated with phases of increased siliciclastic supply; a positive correlation with higher δ¹³C values differentiates the latter. Group 3 is independent of carbonate and detrital input, and Group 4 is associated with high carbonate flux and low detrital supply. These groupings suggest that cyst-forming dinoflagellates exhibited a range of ecological niches in the Late Cretaceous. Although the relationship between the encystment process and the geochemical associations is unclear, key environmental factors are likely to be sea-level and climate related, including water depth, turbidity, nutrient supply, sea-surface temperature, and environmental stability/predictability. Integrated geochemical and palynological studies have great potential for inter-regional correlation and palaeoenvironmental interpretation.     

Extreme weather change and the dynamics of oviposition behavior in the pipevine swallowtail, <Emphasis Type="Italic">Battus philenor</Emphasis>

Prospects of global increases in extreme weather change provide incentive to examine how such change influences animal behavior, for example, behavior associated with resource use. In this study, we examined how oviposition behavior in a southern Arizona population of pipevine swallowtails (Battus philenor L.) responded to changes in their Aristolochia host resource and vegetative background caused by the North American monsoon system. Summer monsoon rains resulted in a flush of non-host vegetation and a more than doubling in rate of landings by host-searching females on non-host vegetation. Rates of discovery of the host species A. watsoni Woot. Standl. decreased by 50% after monsoon rains. Rains did not alter host density appreciably, but resulted in significant increases in host plant size and new growth, two indicators of host suitability for B. philenor larvae. After the rains, mean clutch size on individual host plants increased by a factor of 2.5; the mean proportion of host plants encountered on which a female laid eggs also increased significantly. Females were discriminating about the host plants on which they laid eggs after alightment; plants accepted for oviposition were larger, bore more new growth, and bore fewer larvae than rejected plants. Contrary to predictions from foraging theory, degree of discrimination did not change seasonally. Finally, the rate at which eggs were laid increased seasonally, suggesting that oviposition rates were limited more before monsoon rains by the relatively low quality of hosts than they were after the rains by the relatively low rate at which hosts were found. This latter result suggests that, while butterflies possess behavioral flexibility to respond to extreme weather change, such flexibility may have limits. In particular, expected increases in the severity and frequency of droughts may result in reduced oviposition rates, reductions that could have adverse demographic consequences.     

Extinction pattern of marine Ostracoda across the Pliensbachian-Toarcian boundary in the Cordillera Ibérica, NE Spain: Causes and consequences

This paper discusses the extinction pattern of the Pliensbachian-Toarcian boundary (PTB) ostracod assemblages at the Almonacid de la Cuba section (Cordillera Ibérica, NE Spain), which has been recently proposed as auxiliary boundary stratotype for the PTB. The ostracod record shows that the main Early Jurassic ostracod extinction event occurred not at the end of the Pliensbachian, but near the top of the Mirabile ammonite Subzone, Tenuicostatum ammonite Zone (Early Toarcian). On the basis of the evaluation of PTB ostracod record, a new causal explanation for the Early Toarcian ostracod turnover is proposed. This paper suggests that a reorganization of surface and deep-water circulations caused by the opening of the Hispanic Corridor could have generated a mild cooling episode, finally affecting the survival of healdioid ostracods.     

Analysis of Late Jurassic to Early Cretaceous algal debris-facies of the Plassen carbonate platform in the Northern Calcareous Alps (Germany,Austria) and in the Kurbnesh area of the Mirdita zone (Albania): a tool to reconstruct tectonics and palaeogeography of eroded platforms

A characteristic microfacies of the Late Jurassic to Early Cretaceous allodapic Barmstein Limestone of the Northern Calcareous Alps are clasts of wackestones with numerous fragments of calcareous algae (“algal debris-facies”). According to dasycladale palaeocoenoses, several subtypes comprising different associations can be distinguished. One association is characterized by the debris of an unknown large dasycladalean alga reported as dasycladalean alga indet. sp. 1 from different localities in the Northern Calcareous Alps, typically forming a monospecific assemblage. Another microfacies type contains star-like calcitic bodies tentatively referred to the morphospecies Coptocampylodon pantici Ljubović-Obradović and Radoičić, originally described as being from the Turonian of NW-Serbia. Other Coptocampylodon-like bodies represent the calcified tufts of the laterals of Selliporella neocomiensis (Radoičić). The occurrence of Coptocampylodon pantici-like microfossils in the Late Tithonian to Early Berriasian, shows that obviously different species of dasycladaleans display identical to similar shaped tufts of laterals in transverse sections when becoming fragmented. Coptocampylodon pantici Ljubović-Obradović and Radoičić was observed only from different occurrences of Barmstein Limestone, but not from the autochthonous platform carbonates of the Plassen carbonate platform. The Coptocampylodon algal debris-facies is also reported from the Late Jurassic of Albania, Mirdita zone. Occurrences of different types of algal debris-facies in components of mass-flow deposits can be used as a tool to reconstruct eroded carbonate platforms and tectonics, as demonstrated in the Northern Calcareous Alps and the Albanides. Finally, the general occurrences of algal debris-facies in both settings—intra-Tethyan mostly isolated platforms (Alps, Albanides) vs. extended epeiric platforms (Middle East)—are compared and discussed.     

青海布哈河上游地区同域分布的普氏原羚与藏原羚草青期的集群比较

2005年6—7月和9月,运用样线法研究了青海省天峻县布哈河上游同域分布的普氏原羚（Procapra przewalskii）和藏原羚（Procapra picticaudata）的集群行为。该地区的普氏原羚和藏原羚集群类型分为雌性群、雄性群、混合群及母子群。四种集群类型中,雄性群出现的频次均为最高（普氏原羚和藏原羚分别为4l％和50％）;其次为雌性群（普氏原羚和藏原羚分别为34％和30％）;再次为母子群和混合群。比较集群规模发现,除藏原羚母子群显著大于普氏原羚（P=0．015）外,两个种其余集群类型规模差异不显著。集群规模出现频次：单只出现的频次最高（普氏原羚和藏原羚分别为21．9％和18．7％）;2—8只,普氏原羚和藏原羚分别为62．6％和67．6％;大于9只,普氏原羚和藏原羚分别仅占15．5％和13．7％。另外,普氏原羚和藏原羚产羔后平均集群规模显著减小（普氏原羚,P=0．014;藏原羚,P=0．008）;在雄性群、混合群规模均较产羔前略有增大的情况下,雌性群的规模却显著减小（普氏原羚,P〈0．001;藏原羚,P=0．002）,成为普氏原羚和藏原羚产羔后集群规模变小的主要原因。布哈河上游同域分布的普氏原羚和藏原羚草青期的集群类型、集群规模相差不大。     

Decoupled responses of soil bacteria and their invertebrate consumer to warming,but not freeze–thaw cycles,in the Antarctic Dry Valleys

Altered temperature profiles resulting in increased warming and freeze–thaw cycle (FTC) frequency pose great ecological challenges to organisms in alpine and polar ecosystems. We performed a laboratory microcosm experiment to investigate how temperature variability affects soil bacterial cell numbers, and abundance and traits of soil microfauna (the microbivorous nematode Scottnema lindsayae) from McMurdo Dry Valleys, Antarctica. FTCs and constant freezing shifted nematode body size distribution towards large individuals, driven by higher mortality among smaller individuals. FTCs reduced both bacterial and nematode abundance, but bacterial cell numbers also declined under warming, demonstrating decoupled consumer–prey responses. We predict that higher occurrence of FTCs in cold ecosystems will select for large body size within soil microinvertebrates and overall reduce their abundance. In contrast, warm temperatures without FTCs could lead to divergent responses in soil bacteria and their microinvertebrate consumers, potentially affecting energy and nutrient transfer rates in soil food webs of cold ecosystems.     

Effects of environment and grazing disturbance on tree establishment in meadows of the central Cascade Range,Oregon, USA

Abstract. Within the last century there has been widespread establishment of trees in mountain meadows of the Pacific Northwest. We reconstructed patterns of tree invasion at 17 meadow sites in the central Cascade Range of Oregon, USA -sites representing diverse physical environments and vegetation types and experiencing different histories of recent anthropogenic disturbance (sheep grazing). Spatial distributions and age structures of invasive tree populations were analysed with respect to climatic records and grazing history. Patterns of establishment varied considerably among meadows, reflecting strong differences in environment and grazing history. In montane hydric meadows, tree establishment was spatially clumped beneath large old trees and on elevated microsites; however the timing of invasion differed between sites with stable versus fluctuating water tables. In upland mesic/dry montane meadows, timing of invasion corresponded with cessation of sheep grazing (early 1940s) and the onset of wetter summers (mid 1940s). In the subalpine zone, climate and aspect interacted to produce contrasting histories of invasion on north- and south-facing slopes. Establishment on north-facing slopes, concentrated in heath-shrub communities, coincided with regional warming (ca. 1920–1945) when snowpacks were lighter and melted earlier. Recruitment of trees onto south-facing slopes occurred later, when conditions were wetter (1945–1985). In many environments, the spatial distribution of recruitment suggests that once trees have established, autogenic factors become increasingly important as individual trees or groups of trees alter the physical or biotic conditions that once inhibited establishment. Knowledge of the factors that influence invasion, and of their varying importance across gradients in environment and vegetation, is critical to predicting future changes in these dynamic systems.