Showing papers by "James B. Grace published in 1992"

Size bias in traditional analyses of substitutive competition experiments.

Abstract: The objective of this study was to examine the claim that traditional measures of competitive performance in substitutive experiments are biased towards larger plants. Results from a three-year diallele experiment of 6 marsh plant species were analyzed using both Relative Yields (a traditional analysis) and the Relative Efficiency Index (a recently proposed analysis presumed to be size-independent). In adddition, a mechanistic model of competition was used to explore the behavior of both methods of estimating competitive performance. Results from the three-year experiment showed that Relative Yields (RYs) were correlated with the initial sizes of plants for the first two years but not the third. By the third year, RYs were highly correlated with Relative Efficiency Index values (REIs) suggesting that the effects of initial size were eventually overcome. Model results showed that RYs are inherently biased in favor of larger plants during the early phases of competition while REIs are not. Further, model analysis confirmed that the size bias associated with RYs declines with increasing duration of the experiment. It is concluded that current generalizations about the relationship between plant size and competitive ability may be biased by the procedures used to analyze competition experiments.

High carbon dioxide concentrations in aerenchyma of typha latifolia

Abstract: Diurnal and seasonal patterns of CO2 concentration ([CO]) in leaf gas spaces were measured to better understand the relationship of sediment-derived CO2 to photosynthesis in the emergent wetland species, Typha latifolia L. (cattail). Leaf [C02] was above 2,000 Al/liter at dawn on all but the first sampling date. At all sampling dates, leaf [CO2] declined to near atmospheric [C02] at midday and rose to well above atmospheric [C02] in the late afternoon. The maximum leaf [C02] varied with sampling date and was over 18 times atmospheric levels (over 6,300 ,ul/liter) in August. Based on measurement ofphoton flux density and temperature, the diurnal pattern in leaf [C02] may be generally controlled by expected photosynthetic rates. It is hypothesized that seasonal variation in leaf [C02] may be a function of variation in microbial (soil) respiration. Using dye and slight pressurization, it was confirmed that gas spaces in rhizomes were interconnected with the gas spaces in leaves through the rhizome-shoot transition. From anatomical measurements, it was also estimated that over 50% of total leaf volume was occupied by gas spaces and that most of the total gas-space volume in plants was in the shoot. Photosynthetic rate in C3 plants, like cattail, can increase with increasing [C02] under natural conditions. For this reason, cattail and other emergent wetland plants possessing continuous gas-space pathways appear to have a significant carbon supplement as compared to other C3 plants growing in well-aerated soils. The purpose of this study was to determine the diurnal and seasonal patterns of CO2 concentration ([C02]) in leaf gas spaces for the common wetland emergent, Typha la