Factors Limiting Growth in Astragalus Michauxii (Sandhills milk-vetch)

Abstract

The seed germination-inducing mechanism of Astragalus michauxii and effects of heat and smoke on germination were explored. Seed germination is limited by a hard outer coat, a tough inner coat and possible germination-inhibitory substances in the embryo. Ninety-five percent seed germination was accomplished artificially by scarification in sulfuric acid, followed by piercing of the inner coat, and then leaching. Heat treatment between 80&#61616;C and 86&#61616;C resulted in 20% imbibition (p<.0001), 2% germination (p<.0031). All ungerminated, imbibed seeds were killed. Smoke did not promote germination. Effects of height, age, stem number and inter-specific competition on over-winter survival were studied A younger and an older group of plants were raised under controlled conditions and distributed among two 20x10-meter plots at Fort Bragg Military Reservation, NC, in the summer of 2002. Diameter and distances of each A. michauxii plant from the nearest oak sapling, pine tree, pine seedling, and wiregrass cluster were measured and a competition index (CI) calculated as &#8721; [diameter of each competitor / (distance)2]. Fifty-nine percent of plants survived over-winter to the following spring. Logistic regression predicted height (p<.0001) and stem number (p=.0004) to increase over-winter survival. Effects of fire history and competition on vegetative and reproductive efforts and population persistence in A. michauxii were investigated among 48 sub-populations. Surveys of ground and canopy covers, growth and reproductive effort, along with fire history and population persistence data indicated that sub-populations with lower percentages of understory species covers other than pine, oak, and wiregrass were more likely to persist. Low regression coefficients for effects of competition and fire history indicate that a number of factors work together to impact growth and reproduction. A population demographic model determined the population to be declining. Management conservation efforts can best be directed by 1) collection and artificial seed germination and transplanting of seedlings into existing sub- populations, and 2) experimental testing of the impacts of varying fire regimes on habitat quality, plant growth and reproduction, and population growth and persistence.