Karen Wetherill1, Rosemary Pendleton2 and Burton Pendleton2

1Sevilleta LTER, University of New Mexico, Albuquerque, NM, and 2Rocky Mountain Research Station, USDA Forest Service, Albuquerque, NM

Reproductive Biology of Larrea tridentata: A Comparison of Core and Isolated Shrubs

INTRODUCTION

Chihuahuan Desert shrubland is expanding into semiarid grasslands of the Southwest. Establishment of new creosote (Larrea tridentata)seedlings in grassland sites is key to this conversion. Larrea establishment occurs exclusively through the production of seed. Diploid Larrea plants of the Chihuahuan Desert are not clonal as has been reported for hexaploid Mojave populations. At McKenzie Flats in the Sevilleta National Wildlife Refuge, there exists a gradient in Larrea density stretching from core Larrea shrubland to a few isolated shrubs well-dispersed into the grassland. This study investigates the effects of isolation on Larrea pollination and seed production.

•Compare bee pollinator guilds at core shrubland and grassland sites

•Compare shrub potential for self-pollination at both locations

OBJECTIVES

METHODS RESULTS

CONCLUSIONS

Bee pollinator guilds at core shrubland and grassland sites were sampled twice a week throughout the flowering season; once in the morning and once in the afternoon. Sweeps were conducted on five shrubs at each site, with each bush being sampled for 1/4 hour per visit. Bees were collected, pinned, identified to species, and classified as generalist or specialist pollinators.

We used mesh bags to exclude pollinators from 4 branches on six isolated and 6 core shrubs. After flowering was completed, we bagged 4 additional branches per shrub with mesh bags to prevent insect damage and facilitate collection of all flowers and fruits. All bags were collected after the seed had ripened and returned to the lab for processing. For each sampled branch, we counted total numbers of flowers, developed fruits, and filled seeds to determine % fruit set, % seed fill, and return on investment, calculated as the number of filled seed per 100 flowers.

AcknowledgementsWe thank Julieta Betinelli, Steve Bodio, Justyn Bell, Joanna Redfern, and Carol Jones for assistance in bagging, seed processing, bee collection and pinning of specimens.

Isolated grassland shrubs varied greatly in the number of seeds produced in pollinator-exclusion bags, while the number of self-pollinated seeds produced by core site shrubs was more uniform. Overall, however, the difference in seed produced by bagged and unbagged branches of isolated shrubs was much less than that of core shrubs. These observed trends will be explored further in the coming year.

Results from sampling of the bee guild suggest that there are adequate numbers of pollinators at both locations, however, the community composition differs between core and grassland sites. Five of the six Larrea specialist bee species were found only at the core shrubland site. Large numbers of bees were found on isolated bushes, but their efficiency in pollinating is currently unknown. The higher seed fill of open-pollinated shrubs at the core site as compared with isolated shrubs is somewhat suggestive. Pollinator efficiency will be further explored in 2006.

Family Scientific name

# on isolated bushes

# on core bushes

Andrenidae Andrena prunorum 1 0

Perdita semicaerula 0 18

Apidae Anthophora sp. 1 1 0

Apis mellifera 1 3

Bombus morrisoni 2 0

Centris caesalpiniae 4 3

C.cockerelli 2 0

C. ferrisi 0 1

Diadasia rinconis 0 1

Epeolus mesillae 0 2

Melissodes sp. 1 10 2

Triepeolus sp. 1 0 1

Xylocopa californica 0 2

Colletidae Colletes clypeonitens 6 0

C. covillae 0 8

C. hyalinus 4 0

C. louisae 4 4

C salicicola 66 18

C. sphaeralceae 1 1

Halictidae Agapostemon angelicus 4 2

Halictus ligatus 1 0

H. tripartitus 197 5

Lasioglossum sp. 3 6 2

L. sp. 5 2 1L. morrilli 7 1

L. pruinosiformis 72 42

L.sisymbrii 2 3

Megachilidae Anthidium cockerelli 0 1

Ashmeadiella bigeloveae 0 1

Hoplitis biscutellae 0 2

Megachile sp. 1 4 0

M. sp. 2 1 0

M. sp. 3 2 0

Trachusa larreae 0 7

Mellitidae Hesperapis sp. 1 0 1

Total number of individuals 400 132

Total number of species 23 25

Total number of specialist species 1 5

Total number of individual specialists 6 36

Number of site specific species 10 12

Bee pollinators of Larrea tridentata – specialists in red

Selfed Outcrossed

0.0

0.2

0.4

0.6

0.8

1.0

Isolated Core

Selfed Outcrossed

0

5

10

15

20

Isolated Core

Selfed Outcrossed

0

20

40

60

80

100

120

140

Isolated Core

% Fruit set

% Seed fill

Seeds per flower

Means and standard errors for fruit set, seed fill, and number of seeds per flower of selfed and outcrossed branches of core and isolated shrubs

Larrea branches bagged in nylon mesh (photo by B. Pendleton)

Larrea expansion into McKenzie Flats grassland (photo by R. Pendleton)

Flowers and developing fruits (photo by K. Wetherill)

Centris caesalpiniae (photo by K. Wetherill)