Allometric gender allocation in Ambrosia artemisiifolia (Asteraceae) has adaptive plasticity

Evidence is reported for size-dependent (allometric) gender allocation in the monoecious, wind-pollinated annual Ambrosia artemissifolia. Consistent with established theory, the pattern of allometry displayed adaptive plasticity, depending on the environmental cause of variation in plant size. Plant size gradients were generated in both field and greenhouse experiments using separate and combined gradients of shading, soil nutrient levels, and neighbor proximity. When plant size constraints involved light limitation from shading (e.g., because of close neighbor proximity), decreasing plant size was generally associated with decreasing maleness and increasing femaleness (based on relative male and female flower production, respectively). This is consistent with the pollen-dispersal hypothesis in which the consequences of relatively small plant size (among larger neighbors) imposes less severe limitation for female reproductive success than for male reproductive success (because success as an outcrossing donor of wind-dispersed pollen increases with increasing plant height, especially when neighbors are present). However, when size was constrained by soil nutrient limitation alone (i.e., without shading effects), the results had the converse allometric relationship; i.e., decreasing plant size was generally associated with increasing maleness and decreasing femaleness. This is consistent with the size-advantage and time-limitation hypotheses in which energetic and time limitations (respectively) associated with relatively small plant size impose a less severe limitation for male reproductive success than for female reproductive success.