Climate control of annual plantshrub coexistence

1
GLOWA Jordan River
Climate control of annual plant-shrub coexistence
Marcelo Sternberg1, Claus Holzapfel1, Hadas A.
Parag2, Katja Tielbörger2 Jaime Kigel3
Abstract Annual plants of semi-arid and arid
areas are often closely associated with shrubs.
The degree of association largely depends on the
balance of positive and negative effects between
these different plant life-forms ranging from
interference to facilitation. This interaction
balance is expected to shift with changing
climatic conditions from positive interactions in
arid areas to negative interactions in more humid
regions. Here we describe such changes for soil
water availability and performance of annual
plants in four contrasting sites along a steep
gradient ranging from the desert to the
Mediterranean regions in Israel. All measured
parameters indicate that shrubs are facilitating
annuals in the desert while competitive effects
were noted in the mesic part of the gradient.
This study is part of a large-scale climate
change field experiment. We are currently
experimentally testing whether climate change
(e.g., increase and decrease in rainfall amounts)
will shift shrub-annual associations in a similar
pattern as observed in the present gradient
study. Such information is essential to model
impending effects of global climate change on
complex species interactions. Yet such
information is currently still very scarce. The
future dynamics of coexisting species can be
predicted only based on such experimental data.
Introduction It has been noted that positive net
interactions (facilitation) among plants are
stronger in stressful sites, while negative net
interactions (interference) predominate in less
stressful environments (Bertness and Callaway
1994). These predictions were developed for
resource gradients in space and have been
empirically confirmed for a number of different
plant communities (e.g., Walker and Chapin III
1986, Bertness and Hacker 1994, Callaway et al.
1996, Pugnaire and Luque 2001). The coexistence
of annual plants with shrubs is determined by the
balance between positive and negative
interaction. In this context we analyze the
association of annuals with dominant shrubs along
an extremely steep environmental gradient in
Israel.
THE GRADIENT Four permanent sites have been
chosen ranging from Humid Mediterranean open
forest communities to the arid desert. In each
site we investigate the annual community under
the canopy of one shrub species (Sarcopoterium
spinosum in the three moister sites and Artemisia
sieberi in the desert site) and in open areas
between shrubs.
How is soil water availability affected by
shrubs? Method TDR/FDR ThetaProbe (Delta T) at
3 cm soil depth under shrub and in open areas
between shrubs
Soil moisture under shrubs and between shrubs
(open) from March 15 to April 14, 2002

• Results
• Generally higher soil moisture under shrubs than
  in the open in arid and semi-arid sites
• lower soil moisture under shrubs in Mediterranean
  and Humid Mediterranean.

How do shrubs affect annual biomass
production? Method Estimated above-ground
biomass (based on correlation between
non-destructive measures and biomass of nearby
harvested plots) from shrub understorey and open
areas.

• Results
• Highest density in humid Mediterranean
• Higher density under shrubs compared to open
  areas in arid site
• Lower density under shrubs in other parts of
  gradient
• Difference between habitat increases with average
  rainfall

Contrasting association pattern in Mediterranean
shrubland (left) and desert
2
How do shrubs affect the density of seeds in the
seed bank? Method Soil samples from shrub
understorey and open habitats were germinated in
a shadehouse.
How do shrubs affect the seedling density of
annual plants? Method Seedling counts on
permanent quadrats in shrub understorey and
between-shrub habitats.

• Results
• Highest density in Mediterranean
• Higher density under shrubs compared to open
  areas in arid site
• Lower density under shrubs in other part of
  gradient difference between habitat increases
  with average rainfall

• Results
• Highest density in Mediterranean
• Higher density under shrubs compared to open
  areas in arid part of gradient
• Lower density under shrubs in humid part of
  gradient

How do shrubs affect the fitness of annual plant
populations? Method Monitoring survival and
seed production of selected dominant annual plant
species on permanent quadrats in shrub
understorey and open habitats. The graph shows
the average of three species which are different
in each site. Due to specific differences of per
capita seed production, only habitats are
comparable.
Conclusions What is the general effect of shrubs
on annuals along the gradient?
Differences between shrub and open habitats in
soil moisture and annual plant performance
indicate the type of net effect shrubs have on
annuals. Expressed as all measured parameters
follow the same trend when expressed as Relative
Neighbor Effect (RNE)
0
Humid Medit.
Arid
Semi-arid
Medit.
Facilitation by shrubs in the arid part of the
gradient (RNE gt 0) and interference in the humid
part of the gradient (RNE lt 0).
Medit.
Humid Medi.
Semi-arid
Arid
RNE((response variable open - resp. var. under
shrub/ higher value) x-1)

• Results
• Higher seed yield in shrub understorey in arid
  site
• No differences in semi-arid and Mediterranean
  sites
• Higher seed yield in the open areas in Humid
  Mediterranean site

Current global climate change models predict
drastic changes of precipitation in semi-arid and
arid areas. We argue that "Space-for-Time"
approaches (using existing environments as
proxies for environments under future changed
climate) are not fully sufficient. This because
they neglect the effect of the current fast pace
of climate change on long-term evolved ecosystem
and community functions. To fill this gap we are
currently also testing climate change scenarios
experimentally by manipulations of rainfall
amount with large-scale rainout shelters and
irrigation.
We provide evidence that shrubs had generally
positive net effects in stressful environments
and negative net effects in mesic environments,
thus following the "Bertness Callaway Rule".
However, an extension of this general pattern to
temporal variation proved to be problematic (see
for instance Casper 1996, Tielbörger Kadmon
2000) and predictions of changes in species
interaction based on temporal variation of
resource availability are ambiguous. Annuals and
shrubs are the dominant biotic structures of
semi-arid and arid environments and we are
interested in the impact of impending global
climate change on their coexistence.
Outlook on Climate Change
Descriptive gradient approaches (as the one
described in this poster) will be compared to
experimental approaches as shown in the outline.
Field Experiments Retractable rain-out shelters
-30 of rainfall (shown in photo) and sprinkler
irrigated plots 30 (both 25x10 m)
Literature Bertness, M. D. and R. Callaway
(1994) Positive interactions in communities.
Trends in Ecology Evolution 9
191-193 Bertness, M. D. and S. D. Hacker (1994)
Physical stress and positive associations among
marsh plants. American Naturalist 144
363-372 Callaway, R. M., E. H. DeLucia, D. Moore,
R. Nowak and W.H. Schlesinger (1996) Competition
and facilitation contrasting effects of
Artemisia tridentata on desert vs. montane pines.
Ecology 77 2130-2141. Casper, B. B. (1996)
Demographic consequences of drought in the
herbaceous perennial Cryptantha flava effects of
density, association with shrubs, and plant
size. Oecologia 106 144-152 Pugnaire, F. I. and
M. T. Luque (2001) Changes in plant interactions
along a gradient of environmental stress. Oikos
93(1) 42-49 Tielbörger, K. and R. Kadmon (2000)
Temporal environmental variation tips the
balance between faciliation and interference in a
desert plant community. Ecology 81
1544-1553 Walker, L. R. and F. S. Chapin III
(1986) Physiological controls over seedling
growth in primary succession on an Alaskan
floodplain. Ecology 67 1508-1523
Quite a number of people made our sometimes
challenging work easier, some of those were Eyal
Sachs, Martina Petru, Katja Schiffers, Sarah
Pariente, Muriel Shamla, Ralfi Bali, Azaria Lupo
and Tal Israel. Thank you all!!
1Tel Aviv University, Dept. of Plant Sciences,
Israel. 2University of Potsdam, Dept. of
Biochemistry and Biology, Section for Vegetation
Ecology and Nature Conservation, Germany. 3The
Hebrew University of Jerusalem, Faculty of
Agriculture, Dept. of Agricultural Botany,
Faculty of Agriculture, Israel
Global Change in the Hydrological Cycle - Project
funded by
Contact address Dr. Claus Holzapfel,
claush_at_post.tau.ac.il
German Federal Ministry of Education and Research
Israeli Ministry of Science, Culture and Sports
Glowa Jordan River 02GJR-T6