Dispersal Constraints Research Articles

Beech Range Extension and Vegetation History: Pollen Stratigraphy of Two Wisconsin Lakes.

The pollen stratigraphy of two small lakes in eastern Wisconsin (Radtke Lake, Washington county, and Gass Lake, Manitowoc County) records the Holocene (past 10 000 yr) spread of beech (Fagus grandifolia: Fagaceae). Radiocarbon dates were obtained for the oldest stratigraphic levels at which beech pollen appeared consistently in amounts > 0.5% of terrestrial pollen. A spatially continuous pattern of beech expansion from the north was ruled out, because beech trees grew in Wisconsin by 6000 BP, 2000 yr before adjacent populations were established to the north. Alternative geographic patterns of speed (from the south or east) were spatially discontinuous, requiring seed dispersal distances of perhaps 25-130 km. That beechnuts could be dispersed across such distances suggests (1) the involvement of Blue Jays, Passenger Pigeons, or other vertebrates, and (2) a capacity for reaching climatically controlled range limits, given sufficient time despite such discontinuities in habitat. A lag 1000-2000 yr between the establishment of source populations in Michigan and Indiana and the appearance of beech in Wisconsin suggests that low-probability dispersal events were involved and that dispersal constraints limited the range of beech during this time, although climatic and edaphic explanations for the lag cannot be ruled out. Pollen data from the two sites reveal other features of vegetation history in eastern Wisconsin: an open Picea-Fraxinus woodland prior to 11 000 BP; sequence of Picea, Abies, Betula, and then Pinus forests between 11 000 and 7500 BP; the establishment of a coniferous/deciduous forest ecotone ("tension zone") ° 7000 BP in this region; and the presence of Quercus-dominanted deciduous forests from 7000 BP until 110 BP (time of Euro-American settlement), a period punctuated by a gradual decrease in Ulmus populations (° 4500 and 5700 BP at the two sites) and by an increase in mesophytic tree abundance at the expense of Quercus after 3500 BP.

NETWORK EQUILIBRIUM MODELS OF URBAN LOCATION AND TRAVEL CHOICES: ALTERNATIVE FORMULATIONS FOR THE STOCKHOLM REGION

ABSTRACT Nonlinear programming models of the network equilibrium problem with dispersion constraints pro vide a basis for estimating and evaluating alternative probabilistic choice functions. Four alternative models for the Stockholm region are evaluated for two assumptions about public transit fares. Criteria for choosing among the alternative formulations are discussed.

FACTORS LIMITING FRUIT AND SEED PRODUCTION OF A TEMPERATE SHRUB, STAPHYLEA TRIFOLIA L. (STAPHYLEACEAE)

Staphylea trifolia L., the bladdernut, is a self‐incompatible temperate woodland shrub that flowers in May in Illinois. Factors limiting reproduction were studied at four levels: 1) Seeds/fruit. Seed production in open‐pollinated fruit was frequently limited by too few fertilized ovules. Seed production in hand‐cross‐pollinated fruit was limited by resources or dispersal constraints: seed abortion rates were higher in hand‐cross‐pollinated fruits than in open‐pollinated fruits. 2) Fruits/flower and 3) fruits/inflorescence. The number of fruits set and matured per flower and per infloresence in the open‐pollinated treatment was limited by the number of flowers naturally cross‐pollinated. In hand‐cross‐pollinated inflorescences, fruit set was not limited by resources even though fruit set was ten times greater than in the open‐pollinated treatment. Evidence that resources limited fruit maturation in the hand‐cross‐pollinated inflorescences was equivocal. In hand‐cross‐pollinated flowers, fruit set was lowest when cold nights followed pollination, suggesting that cool temperatures limited postpollination physiological processes. 4) Fruits/individual. Early‐flowering individuals matured fewer fruits than later‐flowering individuals. Within the latter group, fruit production increased with plant size, although a relatively small individual matured the maximum number of fruits. Flowering phenologies and size of individuals varied among patches, resulting in differential reproductive success of patches.

Mantle structure beneath the Japan Sea—A re‐examination

We present a model for the mantle structure beneath the Japan Sea, satisfying both Rayleigh and Love wave dispersion characteristics and constraints from crustal refraction studies. The principal distinguishing feature of model JST‐14 is its lithospheric thickness of only 40 km, overlying an asthenosphere of normal oceanic velocity.