Flowering phenology, fruiting success and progressive deterioration of pollination in an early-flowering geophyte

Abstract

Spatio-temporal patterns of snowmelt and flowering times affect fruiting success in Erythronium grandiflorum Pursh (Liliaceae) in subalpine western Colorado, USA. From 1990 to 1995, I measured the consistency across years of snowmelt patterns and flowering times along a permanent transect. In most years since 1993, I have monitored fruit set in temporal cohorts (early- to late-flowering groups of plants) at one site. To assess 'pollination limitation', I have also conducted supplemental hand-pollination experiments at various times through the blooming season. The onset of blooming is determined by snowmelt, with the earliest years starting a month before the latest years owing to variation in winter snowpack accumulation. Fruit set is diminished or prevented entirely by killing frosts in some years, most frequently but not exclusively for the earlier cohorts. When frosts do not limit fruit set, pollination limitation is frequent, especially in the earlier cohorts. Pollination limitation is strongest for middle cohorts: it tends to be negated by frost in early cohorts and ameliorated by continuing emergence of bumble-bee queens in later cohorts. This lily appears to be poorly synchronized with its pollinators. Across the years of the study, pollination limitation appears to be increasing, perhaps because the synchronization is getting worse.

Figure 1.

Phenological indicators at the study site during the period studied. Data are missing for some years. (a) The date on which the last winter snowpack disappeared in the property that formed the core of the study area. The property is mostly an open meadow with scattered trees. Snow remained beyond this date in nearby forests. (b) The day on which ice broke up on Lake Irwin, adjacent to the study area. Lines are simple linear regressions; neither indicator shows a significant trend over the years of the study. The slight negative trend for ice breakup disappears if the abnormally snowy year of 1995 is ignored.

Figure 2.

Patterns of snow depth along the permanent transect for 1995. The circles (not to scale) diagrammatically suggest the approximate positions and relative sizes of large trees near the transect. The trees were all to the northwest of the transect, and strongly influenced snowmelt patterns.

Figure 3.

Patterns of fruit set over 11 years of observation of numerous temporal cohorts per year of unmanipulated, open-pollinated, single-flowered plants of E. grandiflorum. (Sample sizes approximate 100 plants for each cohort. The black portions of bars indicate the fraction of flowers that successfully set fruits, and the white portions indicate flowers that failed for any cause other than grazing. A few flowers were typically lost to herbivory (see table 4 for the magnitude of herbivory in different years); these were removed from this tabulation.)

Figure 4.

Temporal patterns of response to supplemental hand-pollination experiments for the 6 years in which three experiments were done in the early, middle and late portions of the flowering period (table 4). (Filled circles represent fractional fruit set of flowers receiving supplemental outcross pollen; open circles represent open-pollinated controls.)

Figure 5.

Temporal trends in the degree of pollination limitation over the course of the study. (a) Indices of pollen limitation for supplementation experiments done in the early, middle or late portions of the flowering period are indicated by squares, circles and triangles, respectively. Symbols that overlapped have been offset horizontally. Simple linear regression lines are presented to indicate trends within each dataset (early, solid; middle, long dash; late, short dash). These graphs include points from 1993 and 1998 experiments with such heavy frost damage that few or no fruits were produced in either treatment. These points are excluded from the analyses of significance (see text and table 4). (b) As above, but the data have been transformed to log-response ratios, the problematic 1993 and 1998 data removed and the early and late datasets rescaled by factors representing the average differences among the three temporal supplementation experiments within years. This allows an overall test of the hypothesis that pollination service has deteriorated over the study period (r = 0.434, n = 23, p = 0.039).