Reproductive Biology of Larrea tridentata: A Comparison of Core and Isolated Shrubs
Karen
1
Wetherill ,
Rosemary
2
Pendleton
and Burton
2
Pendleton
1Sevilleta
LTER, University of New Mexico, Albuquerque, NM, and
2Rocky Mountain Research Station, USDA Forest Service, Albuquerque, NM
% Fruit set
140
INTRODUCTION
METHODS
120
RESULTS
100
% Seed fill
20
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 welldispersed into the grassland. This study
investigates the effects of isolation on Larrea
pollination and seed production.
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.
Bee pollinators of Larrea tridentata – specialists in red
Family
Andrenidae
Apidae
Colletidae
Centris caesalpiniae
(photo by K. Wetherill)
Larrea expansion into McKenzie Flats grassland
(photo by R. Pendleton)
Flowers and developing
fruits (photo by K. Wetherill)
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.
Halictidae
Scientific name
Andrena prunorum
Perdita semicaerula
Anthophora sp. 1
0
18
0
Apis mellifera
Bombus morrisoni
Centris caesalpiniae
C.cockerelli
C. ferrisi
Diadasia rinconis
Epeolus mesillae
Melissodes sp. 1
1
2
4
2
0
0
0
10
3
0
3
0
1
1
2
2
Triepeolus sp. 1
Xylocopa californica
Colletes clypeonitens
C. covillae
C. hyalinus
C. louisae
C salicicola
C. sphaeralceae
Agapostemon angelicus
0
0
6
0
4
4
66
1
4
1
2
0
8
0
4
18
1
2
Halictus ligatus
H. tripartitus
Lasioglossum sp. 3
1
197
6
0
5
2
L. sp. 5
L. morrilli
L. pruinosiformis
L.sisymbrii
2
7
72
2
1
1
42
3
0
0
0
4
1
1
2
0
1
2
0
0
0
0
7
1
400
23
1
6
10
132
25
5
36
12
M. sp. 2
M. sp. 3
Trachusa larreae
Hesperapis sp. 1
OBJECTIVES
Mellitidae
•Compare bee pollinator guilds at core shrubland
and grassland sites
Total number of individuals
Total number of species
Total number of specialist species
Total number of individual specialists
Number of site specific species
•Compare shrub potential for self-pollination at
both locations
Larrea branches bagged in
nylon mesh (photo by B.
Pendleton)
# on isolated # on core
bushes
bushes
1
0
1
Megachilidae Anthidium cockerelli
Ashmeadiella bigeloveae
Hoplitis biscutellae
Megachile sp. 1
Isolated
Core
Isolated
Core
80
15
60
40
10
20
0
Selfed
5
Outcrossed
Seeds per flower
1.0
Isolated
Core
0
Selfed
Outcrossed
0.8
0.6
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
0.4
0.2
0.0
Selfed
Outcrossed
CONCLUSIONS
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.
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.
Acknowledgements
We thank Julieta Betinelli, Steve Bodio, Justyn Bell,
Joanna Redfern, and Carol Jones for assistance in
bagging, seed processing, bee collection and
pinning of specimens.