Ontogenetic Niche Shifts in Old‐Field Annuals

To learn whether niche breadths change during the life of a plant, we tested the responses of three age—classes of six annual plants on four environmental gradients. Several ontogenetic niche shifts were observed. The differences in response among age—classes of a species were larger than the differences in response among other species of similar ages. The species were very similar in their germination responses to the moisture and nutrient gradients, but were quite different from each other on the temperature gradient. The biomass response of seedlings was the broadest of the three age—classes only on the nutrient gradient. The responses of the species were very similar. On the temperature gradient, responses of the species were also similar to each other, but response breadths were low. On the light gradient there were more differences among the species as seedlings than on the other gradients. The reproductive responses of mature plants were most sensitive to changes in nutrient concentration, resulting in narrower responses than for the other two age—classes. Responses of mature plants were relatively broad on the moisture, temperature, and light gradients. The co—occurrence of different ages of plants in a given locality makes these ontogenetic shifts in response to resource gradients important in the outcome of interactions between neighbors.