No abstract

Leaves of many desert and agricultural species have the ability to move diurnally, orienting perpendicular or parallel to the sun's direct rays. This phenomenon is widespread in many plant families and occurs in both C(3) and C(4) photosynthetic pathway plants. In the annual flora of desert communities, solar tracking becomes more frequent as the length of the growing season decreases. Leaves that are perpendicular to the sun's direct rays for tracking appear to have high photosynthetic rates throughout the day, whereas leaves parallel to the sun's rays have reduced leaf temperatures and transpirational water losses.

Detrimental effects of vines on tree growth in successional environments have been frequently reported. Little is known, however, about the relative importance of below and aboveground competition from vines on tree growth. The objective of this study was to quantify and compare the growth responses of Liquidambar styraciflua saplings to below and/or aboveground competition with the exotic evergreen vine, Lonicera japonica (Japanese honeysuckle), and the native deciduous vine, Parthenocissus quinquefolia (Virginia creeper). Soil trenching and/or vine-trellising were used to control the type of vine competition experienced by trees. Comparisons among untrenched treatments tested for effects of belowground competition. Comparisons among trenched treatments tested for effects of aboveground competition. After two growing seasons, Lonicera japonica had a greater effect on the growth of L. styraciflua than did P. quinquefolia. This effect was largely due to root competition, as canopy competition only had a negative effect on tree growth when it occurred in combination with root competition. Leaf expansion was consistently and similarly affected by all treatments which involved belowground competition.

Nonlinear response curves are often used to model the physiological responses of plants. These models are preferable to polynomials because the coefficients fit to the curves have biological meaning. The response curves are often generated by repeated measurements on one subject, over a range of values for the environmental variable of interest. However, the typical analysis of differences in coefficients between experimental groups does not include a repeated measures approach. This may lead to inappropriate estimation of error terms. Here, we show how to combine mixed model analysis, available in SAS, that allows for repeated observations on the same experimental unit, with nonlinear response curves. We illustrate the use of this nonlinear mixed model with a study in which two plant species were grown under contrasting light environments. We recorded light levels and net photosynthetic response on anywhere from 8 to 10 points per plant and fit a Mitscherlich model in which each plant has its own coefficients. The coefficients for the photosynthetic light-response curve for each plant were assumed to follow a multivariate normal distribution in which the mean was determined by the treatment. The approach yielded biologically relevant coefficients and unbiased standard error estimates for multiple treatment comparisons.

Summary1 A 2-year field study examined the demographic consequences of association with shrubs in an herbaceous perennial, Cryptantha flava . Physiological data were collected to evaluate whether shrub effects were mediated primarily through water, nutrient or light availability. 2 Microclimatic conditions under the north side of shrubs differed from open microhabitats, primarily in light availability. Due to little photosynthetic acclimation to light, daily photosynthesis for plants under shrubs was reduced proportionally to the light regime. 3 Shading did not reduce stomatal conductance proportionally to photosynthesis, which led to decreased water use efficiency for plants under shrubs. Few differences were found in leaf water potential between microhabitats, indicating that little competition for water was occurring. 4 There was little evidence for shrub-induced nutrient island effects. Soil nitrogen, phosphorus and organic content did not differ between open and shrub microhabitats. Leaf nitrogen content also differed little between plants in the two microhabitats. 5 Growth and flowering responses of individuals under shrubs were reduced relative to those in the open, even for plants located on the south side of shrubs. 6 Over this 2-year period of average to above-average rainfall, association of C. flava with shrubs was dominated by competition for light, rather than for water or nutrients. Future investigations will address whether this asymmetric competitive interaction changes during years with below-average rainfall to a facilitative interaction, or one of increased competition for water.