Thomas W. Jurik

Differential costs of sexual and vegetative reproduction in wild strawberry populations

SummaryThe CO2 costs of producing sexual and vegetative reproductive propagules were calculated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, New York, USA, were studied for two or three years each. Field studies of phenology, biomass, demography, and environment and laboratory studies of CO2 exchange were integrated using a computerbased model of CO2 dynamics to estimate costs of propagules.The percentage of plants flowering and the number of flower buds produced were highest in an open, recently disturbed habitat and lowest in a forest habitat. The openhabitat plants had the greatest success in converting flower buds into ripe fruits and also produced the highest numbers of runners and runner plantlets. On the basis of total investments in structure and respiration minus any photosynthetic gain of all reproductive structures, the cost per seed was lowest in the most open habitats and highest and increasingly variable in the more closed habitats. The cost of plantlets also was lowest in the most open habitat. The differences among habitats in cost of plantlets alive after one or two growing seasons increased due to differential survivorship of plantlets, with the open habitat continuing to have the lowest cost per plantlet. Theoretical treatments of life history characteristics such as reproductive effort should recognize that costs of equivalent type and size of propagule may vary among environments.

Leaf dynamics and profitability in wild strawberries

Leaf dynamics and carbon gain were evaluated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, N.Y., U.S.A., were studied for two or three years each. A computer-based model of plant growth and CO2 exchange combined field studies of leaf biomass dynamics with previously-determined gas exchange rates to estimate carbon balances of leaves and whole plants in different environments. Leaves were produced throughout the growing season, although there was usually a decline in rate of leaf-production in mid-summer. Leaves produced in late spring had the largest area and longest lifespan (except for overwintering leaves produced in the fall). Specific Leaf Weight (SLW) varied little with time of leaf production, but differed greatly among populations; SLW increased with amount of light received in each habitat. The population in the most open habitat had the least seasonal variation in all leaf characters. F. vesca produced lighter, longer-lived leaves than F. virginiana. Simulations showed that age had the largest effect on leaf carbon gain in high-light environments; water stress and temperature had lesser effects. Leaf carbon gain in lowlight environments was relatively unaffected by age and environmental factors other than light. Leaves in high-light environments had the greatest lifetime profit and the greatest ratio of profit to cost. Increasing lifespan by 1/3 increased profit by 80% in low-light leaves and 50% in high-light leaves. Increasing the number of days during which the leaf had the potential to exhibit high photosynthetic rate in response to high light led to little change in profit of low-light leaves while increasing profit of high-light leaves by 49%.

Canopy Architecture, Light Extinction and Self-shading of a Prairie Grass, Andropogon gerardii

Abstract The three-dimensional characteristics of canopy structure and light environment of clones of a common tallgrass prairie species, Andropogon gerardii (big bluestem), were analyzed in native and reconstructed prairies near Ames, Iowa to determine if clones are limited in size by the effects of self-shading. Clones tended to be nearly circular, with a mean circumference of 158 cm and a mean cross-sectional area of 2060 cm2; clone height was consistently near 60 cm in early summer. Irradiance at a given height in the canopy, calculated as a fraction of radiation above the canopy, decreased rapidly during May and June as plants completed most of their seasonal growth, but was relatively constant in July and August. A vertical gradient in light dominated the spatial variation in light within clones. Horizontally through a clone, light levels were higher around the southern (sun-side) periphery. Spatial variation in light, in terms of the range of values found, was least at the bottom and upper parts of the canopy and greatest in the middle of the canopy. Leaf mass/area and total leaf area per increment of canopy height were highest at 30 cm below the top of the canopy and were lower at 15 cm and 45 cm below the top of the canopy. Mean leaf area index of clones was 6.58. Diameter of clones had a weak effect, if any, on the light environment within a clone. Only clones with diameters of <20–30 cm are likely to have substantially less self-shading than larger clones. Competition with other plants or limitation by other morphological constraints, rather than limitation by self-shading, may control the upper size limit of clones.