Effects of Timing of Nutrient Addition on Competition Within and Between Two Annual Plant Species

Abstract

(1) The effects of early and late nutrient addition on competition within pure and mixed pairs of two co-occurring species of old-field annuals, Abutilon theophrasti and Datura stramonium, were examined in two experiments. In one of the experiments, relative emergence times of the members of a pair were varied. (2) In the first experiment, fewer fruits and seeds were produced by pure pairs of A. theophrasti that received the late nutrient addition compared with those receiving the early addition. Seeds were heavier under the late addition than the early addition for pure-pair A. theophrasti, but the opposite was true for A. theophrasti in mixed pairs. In pure pairs of D. stramonium, numbers and weights of seeds per plant were similar in the two treatments, but the seeds were packaged in a larger number of smaller fruits under the late nutrient addition compared with the early addition. Seed production by A. theophrastiwas lower in mixed pairs than in pure pairs; that of D. stramonium was higher in mixed than in pure pairs. (3) In the second experiment, A. theophrasti plants in pure pairs that received a nutrient addition produced more fruits and seeds than the controls, but fruit and seed numbers did not differ between earlyand late-addition plants. Seeds were heavier in plants receiving the late addition than in those receiving the early addition. D. stramonium plants in pure pairs produced more seed under the early addition than the late addition. Older plants of uneven-aged pairs had greater seed production than plants in even-aged pairs or younger plants of uneven-aged pairs. (4) In mixed pairs, both A. theophrasti and D. stramonium produced more fruits and seeds under the late nutrient addition than the early addition. Plants with earlier relative emergence times had greater seed production, compared with others of their species, in both pure and mixed pairs. (5) Total seed production of pairs was affected by both relative emergence time and nutrient treatment for pure pairs of A. theophrasti and for mixed pairs, and by nutrient treatment only for pure pairs of D. stramonium. The proportional distribution of seed production between members of a pair was significantly affected by relative emergence time, but not by the timing of the nutrient treatment, in both pure and mixed pairs. However, there was a tendency for the seed production advantage of the older plant of a pair to be greater under the early addition than the late addition. (6) The results for pure pairs in the two experiments show that both species' abilities to use soil nutrients decline over time, that the decline does not occur at the same time for the two species, and that the timing of the decline may be modified by factors such as temperature. (7) The results for mixed pairs were not predictable from the two species' responses in pure pairs.