Document: JEN-3-37-9

Local scale effects of growing season length on the population structure of scarlet monkeyflower, Mimulus cardinalis.

WILLIAMS, J.L.* and J.M.LEVINE

University of California, Berkeley, CA 94720 ¹

Abstract:
The ability of growing season length to control population structure over altitudinal and biogeographic scales is well appreciated, but its effect at the scale of meters is largely unexplored. Along the South Fork Eel River in northern California, the scarlet monkeyflower, Mimulus cardinalis, grows significantly larger at sites high above the channel as compared to sites low in the channel. It grows even larger at tributary confluences with the main stem of the river. We explored the hypothesis that this size variation is controlled by vastly different growing season lengths at these different positions in the channel. Due to the very gradual retreat of the water level following winter flooding, the period of emergence time ranges from four months for plants growing low in the channel to seven months for plants growing at tributary confluences. To distinguish the effects of growing season length from the possibility that the channel positions differ in habitat quality, we transplanted individuals into each site. If, rather than emergence times, the size differences reflected differences in nutrient or water availability, one would expect growth differences consistent with the natural size patterns. Instead, we found no differences in transplant growth between high, low, and tributary sites. A controlled growth experiment showed that plants emerging early in the year gain a distinct size advantage over those emerging later, even though growing conditions are less ideal. These results suggest that emergence time may be a key determinant of plant size along rivers and that changing climate may alter local scale population structure.

Keywords: population structure, growing season, Mimulus cardinalis

This abstract is being presented at: 3:15 PM in session:
Oral Session #33: Plant Demography.