Interactive effects of lateral shade and wind on stem allometry, biomass allocation, and mechanical stability in Abutilon theophrasti (Malvaceae)

The effects of lateral shade and wind on stem allometry, whole-plant biomass allocation, and mechanical stability were examined for Abutilon theophrasti in a fully factorial glasshouse experiment. Lateral shade from neighboring plants increased stem height by 33% relative to control plants grown individually, despite a decrease in plant dry mass. Intermittent wind decreased stem height by 18% in unshaded plants, but by only 3% in shaded plants. Surprisingly, both lateral shade and wind caused decreases in stem diameter, even with diameter controlled for height, resulting in low diameter?:?height ratios in wind-treated plants relative to untreated plants. Under shade, wind-treated plants had higher root allocation than untreated plants, which allowed wind-treated shade plants to compensate for a low diameter?:?height ratio. This did not occur in the absence of shade, where stem tissue density and root allocation of wind-treated plants did not exceed that of untreated plants. Nevertheless, wind-treated plants experienced low drag relative to untreated plants due to a lower leaf area. Consequently, stem deflections of wind-treated plants did not exceed those of untreated plants at any given windspeed. Our results document a complex interaction between shade and wind on plant morphology and suggest that the nature of this interaction is generally that lateral shade acts to reduce or eliminate thigmomorphogenic responses.