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Local Distributions of Old-Field and Woodland Annual Plant Species: Demography, Physiological Tolerances and Allocation of Biomass of Five Species Grown in Experimental Light and Soil-Moisture Gradients

Carmen R. Cid-Benevento and Patricia A. Werner
Journal of Ecology
Vol. 74, No. 3 (Sep., 1986), pp. 857-880
DOI: 10.2307/2260403
Stable URL: http://www.jstor.org/stable/2260403
Page Count: 24
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Local Distributions of Old-Field and Woodland Annual Plant Species: Demography, Physiological Tolerances and Allocation of Biomass of Five Species Grown in Experimental Light and Soil-Moisture Gradients
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Abstract

(1) This study combined the use of demographic variables and physiological criteria to explain species distribution patterns of summer-flowering old-field and woodland annuals. We determined net replacement rates for populations of several species in light and soil moisture gradients, ascertained the role of physiological tolerances in limiting the distribution of these species in the gradients, and determined whether differences in the allocation of biomass could explain differences among species in survivorship and seed production in the gradients. (2) In a glasshouse experiment, we monitored emergence, survivorship and reproduction for two old-field annuals, Chenopodium album, Polygonum pensylvanicum, and three woodland annuals, Acalypha rhomboidea, Pilea pumila and Impatiens capensis, at all treatment combinations of six light levels (< 50-> 1000 μ Einstein m-2s-1) and three soil mixtures of low, medium and high water-holding capacity. We also measured allocation of biomass to aerial vegetative tissue, roots and reproductive tissue for seed-producing plants of the five species at all possible light × soil-moisture treatment combinations. (3) Old-field and woodland annuals did not differ consistently in seedling emergence, survivorship or time of first flowering over either light or soil-moisture gradient. The only consistent difference between them was at the reproductive phase. Seedling emergence was significantly affected by light and soil moisture for individual species, without regard to their habitat. For all five species, survivorship was highest at intermediate light levels, regardless of the soil's water-holding capacity. For both groups of annuals, flowering started much later at low light levels than at intermediate or high light levels. Old-field annuals produced few or no seeds at light levels <250 μ Einstein m-2s-1, unlike woodland annuals which produced seeds both at low and at high light treatments. In all but one case, soil moisture did not have a significant effect on seed production. (4) At low light levels, there was a low net replacement rate (R0) of old-field annuals because of delay in the start of flowering and low seed production, whereas at the highest light levels, low R0 values were largely due to low seedling survival. In woodland annuals, low R0 values at both extremes of the light gradient resulted from either low survivorship or low seed production or both. (5) Averaged over all treatment combinations, old-field annuals had significantly higher percentage allocation of biomass to vegetative shoot tissue and significantly lower allocation to roots, and to reproductive structures than did woodland annuals. (6) Large-seeded species had relatively more vegetative shoot tissue than reproductive tissue than did small-seeded species. (7) At light levels <250 μ Einstein m-2s-1, reproductive effort of old-field annuals was lower than that for woodland annuals, but at higher light levels there were no consistent differences between groups of annuals. For old-field annuals, reproductive effort increased with increasing size of reproductive and vegetative structures. In contrast for woodland annuals, an increase in reproductive effort (%) was not correlated with an increase in overall plant size; it increased with absolute reproductive biomass or with a decrease in relative allocation of biomass to vegetative parts or both. (9) We conclude that summer-flowering old-field annuals do not extend into the often adjacent disturbed woodland areas because the old-field annuals do not allocate enough biomass to reproduction at the low light levels typical of the disturbed woodland environment, to produce enough seeds to allow for population replacement. In contrast, summer-flowering woodland annuals do not colonize old fields due to physiological intolerance of light and soil-moisture conditions in some areas in the old field, as well as other unexamined factors (but not pattern of biomass allocation). Differences between old-field and woodland annuals in shoot and root architecture, rather than percentage biomass allocation, appear more likely to explain the restricted distribution of these woodland annuals.

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