Fitness Impacts Of Herbivory Through Indirect Effects On Plant-Pollinator Interactions In Oenothera Macrocarpa

Ecology, Jan, 2000 by Kristine Mothershead, Robert J. Marquis

Plants of all treatments received natural leaf damage (on average 9% of leaf area for the entire season, measured in July). However, plants in leaf damage and hand damage treatments (hereafter = increased leaf damage treatment), in addition to natural leaf damage, had [tilde]25% area removed on each leaf (avoiding the primary vein) using a paper punch to mimic natural insect damage. Application of leaf damage was staggered: during 14-16 May, 25% of every other leaf was damaged on plants in the increased leaf damage treatments, and the following week, 21-23 May, the remaining leaves received 25% leaf damage. Late season new leaf growth was damaged 25% on 21 June. Natural and increased leaf damage both occurred after early spring floral bud formation and continued throughout the growth season. Twenty-five percent leaf damage was chosen to maximize the likelihood of treatment effects, by increasing leaf damage levels above natural leaf damage levels, but to fall within the natural range of leaf damage based on plant observations prior to the 1996 field season.

Plants in treatments hand pollination and hand damage (hereafter = hand pollination treatment) received nightly supplemental hand pollination to all flowers during the flowering season. Hand pollination was done by clipping the anthers of 2-3 male donors from flowers of nonstudy plants (2-3 m from recipient plants) and then rubbing the anthers with pollen grains across the stigmas of recipient flowers.

Measurement and analysis of plant leaf area and leaf damage

Total plant leaf area was measured on all study plants three times: at beginning of the flowering season (20-22 May 1996) (initial leaf area), at the end of the flowering season (14-19 June), and at the end of the growing season (16-19 July). At each of these censuses total leaf number was counted and the length and width of every fifth leaf (minimum of five leaves per plant) was measured (to the nearest mm) for each plant. Leaf area missing (natural and experimental) was also measured on every fifth leaf (same leaf as growth measurement) during the June and July censuses using a clear plastic grid (square equal to 0.25 [cm.sup.2]).

Total potential area of each leaf was estimated by regression analysis (potential leaf area = 0.528 0.457 (L X W) [[R.sup.2] = 0.947, P [less than] 0.001, N = 223]). Potential leaf area per plant was estimated for each census by multiplying the mean leaf area by the number of leaves present at that census. Percentage leaf damage was calculated as the sum of leaf area missing/potential leaf area for that census (June or July). Percentage leaf damage was transformed by using arcsine(square root[percentage damage 1]) to improve normality (Sokal and Rohlf 1995). Treatments did not differ in initial plant size (=potential leaf area in May; ANOVA, [F.sub.3,141] = 0.18, P = 0.90).

Measurement and analysis of floral traits

We measured corolla diameter and floral tube length of all flowers produced by study plants. Corolla diameter was estimated as the mean of two measurements across the flower from petal tip to petal tip (to the nearest mm). Measurements were made nightly, starting [tilde]1 h after dusk when flowers were fully open. Floral tube length (to the base of the sepal lobes) was measured on individual flowers the following day (before wilting) to avoid damaging flowers. A colored embroidery thread was then tied around the base of each ovary to follow the fate of individual flowers.