Ecological Archives E095-143-A2

Susan C. Cook-Patton, Anurag A. Agrawal. 2014. Exotic plants contribute positively to biodiversity functions but reduce native seed production and arthropod richness. Ecology 95:1642–1650. http://dx.doi.org/10.1890/13-0782.1

Appendix B. Trait measurements on natives and exotics.

Using plants within monocultures, we measured ten traits that were related to overall plant morphology (height, branch number, internode length, biomass) and phenology (time to bolt, time to flower, flowering duration). We also quantified leaf-level traits related to herbivory (trichome density, leaf toughness, specific leaf area, leaf damage). Plant height (in units of 0.1 cm) corresponded to the distance between the ground and the tip of the highest leaf when stretched. Branch number records the number of branches on each plant that initiated within the first 10 cm above the ground. We measured internode length (in units of 0.1 cm) by taking the mean of three consecutive internodes. If the plant had less than three internodes we measured the mean of two internodes or a single internode. Biomass corresponded to dried weight of all aboveground biomass (0.1g). These data were collected in 2009.

We began the phenology survey on April 15th, 2009 and continued checking plants once a week until harvest. Time to bolt equaled the number of days between the survey start date and that date when each plant began to bolt. Time to flower was the number of days between survey start date and the day when the first fully-opened flower appears on each plant. Flower duration was the difference between the flowering start date and the flowering stop date. If a plant stopped flowering, but recommenced flowering during subsequent surveys, we used the final stop date.

To measure trichome density, we took a 29.29 mm² hole punch from the tip of the youngest fully expanded leaf, centered on the mid-vein and used a dissecting scope to count trichomes on the top and bottom of each leaf disk. Specific leaf area (SLA, area/dry mass) is a measure of leaf density. To measure SLA(mm²/mg), the leaf discs from the trichome count were dried at 45°C overnight and weighed to determine dry mass. We measured leaf toughness, which affects palatability of the leaves to herbivores with a penetrometer (Type 516, Chatillon Corp. NY) to record the amount of force required to penetrate the youngest fully expanded leaf on a plant. Trichome density, SLA and leaf toughness were measured in 2008. We calculated leaf damage as the mean the damage estimates recorded in June, early July and late July 2009.

We asked whether the exotic species were generally different than the native species in the traits measured. We first calculated a mean trait value per monoculture (to account for the non-independence of individuals within a monoculture) and used the means as data in our model (N = 55-61 depending on the trait). Note that the few Dipsacus laciniatus individuals were excluded from the calculations. Except for time to flower, flower duration and height, the trait data were ln +1 transformed to improve the normality of the residuals. We ran univariate analyses of variance (ANOVA) with origin as the main effect.

For the traits measured, natives and exotics were not significantly different (Table B1). We found some evidence that exotics flowered earlier than natives (F1,54 = 4.6, p = 0.0370). However, because we examined ten traits it may be that this single significant effect was due to chance (binomial expansion test, p = 0.329).

Table B1. Mean trait values ± standard deviation for natives and exotics in monoculture. Only time to flower significantly differed between native and exotic species pools.

Trait

Exotic

Native

p value

Time to bolt (days)

37.3 ± 28.4

27.9 ± 13.0

0.1290

Time to flower (days)

85.6 ± 19.2

100.1 ± 30.4

0.0370

Flower duration (days)

45.3 ± 27.2

37.9 ± 13.9

0.2070

Height (cm)

70.3 ± 49.9

86.0 ± 43.5

0.1951

Number of branches

7.2 ± 8.5

6.9 ± 4.9

0.2262

Iinternode length (cm)

14.1 ± 13.5

14.8 ± 12.1

0.5412

Biomass (g)

45.6 ± 66.3

48.2 ± 40.1

0.2905

Trichomes (hairs/cm)

11.5 ± 21.4

14.4 ± 14.3

0.1100

Toughness

105.4 ± 33.5

121.3 ± 60.1

0.5619

Damage (% leaf tissue consumed)

10.9 ± 8.9

15.9 ± 13.0

0.0565

SLA (mm²/mg)

24.7 ± 8.3

24.8 ± 9.1

0.9943


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