Table S2. Summary of the main effects of drought on riparian plant species richness and composition, organized according to the main trends in species declines.
Authors
Main trend
Main observations
Mechanisms
Stromberg et al., 2005
Decline of
herbaceous
species.
During early summer drought,
streamside herbaceous cover and
species richness declined across spatial
gradients of decreased flow
permanence. Composition shifted from
hydric to mesic species at sites with
more intermittent flow.
Depth to groundwater table,
perennial stream flows and high
soil moisture are important to
sustain hydric species.
Westwood et al., 2006
Decline of
herbaceous
species.
Prolonged periods of channel
desiccation and steep channel gradients
lead to a decline in species diversity and
an increase in non-aquatic grasses and
herbs. High local water stages and wide
channels lead to an increase in species
diversity.
Steep channel gradients (≥ 4 m
km-1) and narrow channels (≤2 m
width) have a negative effect on
species richness (correlation
diversity with local slope is 0,302).
Increase of
short-lived and
exotic species
(pioneers).
Overall, richness and density of exotic
species were greatly increased following
warm or dry periods. Plots adjacent to
active river channels showed greatest
sensitivity to drought conditions. Here,
the density of short-lived and exotic
species increased.
There is a high variation in mean
daily discharge between a
normal year, a wet year, and a
dry year.
Jansson et al., 2007
Decline of both
woody and
herbaceous
species.
Groundwater discharge areas in riparian
zones were 36 - 209% more species
rich than non-discharge areas.
Liu et al., 2005
Decline of both
woody and
herbaceous
species.
Species diversity and richness of both
herbs and woody plants declined during
drought. Herbs with shallow roots first
degenerated with declining groundwater
table, then deep-rooted herbs, finally
trees and shrubs with strong tolerance
to drought.
Tabacchi, 1995
Biogeographi
cal region
Research set-up
Thresholds or indicator
species
Desert
Comparison between
sites with perennial
and ephemeral stream
flow. May - August
2000 - 2003.
Hydrologic threshold values
evident for one plant functional
group: Schoenoplectus acutus.
Juncus torreyi, and other hydric
riparian plants declined sharply in
cover with loss of perennial
stream flow.
Atlantic
Post-drought
monitoring (1992-1995)
and recurrence of
drought conditions
(1996-1998).
The macrophyte Ranunculus
peltatus has an important
indicator value for its ability to
withstand drought, compared to
R. penicillatus.
Atlantic
Pioneer vegetation,
changes in space and
time measured. Normal
(1987), wet (1988) and
a dry year (1989).
Plots adjacent to active river
channels showed greater
sensitivity to drought conditions.
Drought stress and low nitrogen
availability are suggested as the
major causes for a decrease in
species diversity.
Atlantic
Groundwater discharge
versus non-discharge
areas.
Riparian species growing in high
elevations more often subject to
drought stress during low-river
stages than low-elevation aquatic
species.
The lower the groundwater, the
lower the species density.
Especially rooting depth played a
role. Groundwater depth ranged
from 5.5 to 11.4 meters between
sites.
Desert
Relationship between
degradation of the
plant community and
groundwater levels
investigated after 30
years of drought.
Herbs with shallow roots were
most sensitive to a declining
groundwater table.
Salinas & Casas, 2007
Decline of both
woody and
herbaceous
species.
At intermittent and highly drought
impacted sites, hydric or mesic species
declined or disappeared, leaving
communities dominated by droughttolerant / xeric species. Decline of
woody sp. richness with shorter
hydroperiod.
Decline in rainfall coupled with
higher temperatures and
evaporative stress at lower
elevation open-canopy sites may
have constrained herbaceous
species. Surface flow
permanence was a limiting factor
for woody riparian vegetation.
Mediter.
Responses of
vegetation to main
environmental gradient
investigated during 2
years.
Increase in abundance of Tamarix
africana accompanied by
halotolerant shrubs or emergent
macrophytes in sites that were
affected by drought.
Stromberg et al., 2007
Decline of both
woody and
herbaceous
species.
As stream flow becomes intermittent
and groundwater deepens, diversity and
cover of herbaceous species along the
low-flow channel decline. Species
composition shifts from wetland
pioneers (Populus, Salix spp.) to more
drought-tolerant shrub species (Tamarix,
Bebbia spp.).
Depth to groundwater, intra- and
inter-annual fluctuations in water
table level and groundwater
decline rate influence the riparian
vegetation. A stream flow
permanence of <25%
corresponds with the lowest
species diversity. Groundwater
depth during drought can be >4m
at intermittent sites, with intraannual variation of >1 m,
compared with mean depths of
<2 m and intra-annual
fluctuation of <0.3 m at some
perennial sites.
Desert
Year-round survey of
several reference
rivers in Arizona's
Sonoran Desert region.
As groundwater deepens species
composition shifts from wetland
pioneers (Populus, Salix) to more
drought-tolerant shrub species
(Tamarix, Bebbia). Shallow-rooted
wetland herbaceous plants
respond fast to declines in stream
flow and groundwater depth.
Decline of
(pioneer) tree
and shrub
species.
Tamarix ramosissima can tolerate a
wider range of ground-water fluctuations
and depths than Populus fremontii and
Salix gooddingii.
Surface flow permanence and
groundwater depth, in relation to
plant rooting depth and water use
efficiency. Stream flow
permanence ranged from 29100%, max. depth to
groundwater from 5.3 to 1.3m,
and from 1.5 to 0.05m in max.
groundwater fluctuation (JanJun).
Desert
Dry sites, as
representative of
future dewatered
conditions, were
compared to sites less
affected by drought.
Two year survey.
Populus fremontii and Salix
gooddingii were dominant over
Tamarix ramosissima at sites
where surface flow was present
more than 76% of the time, interannual ground-water fluctuation
was <0.5 m, and average
maximum depth to ground water
was <2.6 m.
Decline of
(pioneer) tree
and shrub
species.
Species richness in the riparian zone
with ephemeral-flow was lower during a
drought season compared to the more
permanent water sources of the
perennial-flow sites. Mesic, annual and
herbaceous perennial species more
diverse at perennial-flow sites than
ephemeral-sites during drought.
Diversity in dry land regions is
frequently limited by surface and
groundwater. At ephemeral-flow
study sites surface flow present
only after major storm events,
depth to groundwater about 20 m
below ground surface. At
perennial-flow study sites depth
to groundwater from about 1-5 m
below ground surface.
Desert
Comparison between
sites with perennial
stream flow and
ephemeral stream flow
over a 2 year period
spanning drought and
wetter conditions.
Higher relative abundance of xeric
species during drought.
Lite & Stromberg, 2005
Stromberg et al., 2009
Stromberg et al., 2010
Ström et al., 2011
Decline of
(pioneer) tree
and shrub
species.
Field data indicate that increased
drought conditions cause the floodplain
plant community to shift from hydric
pioneer trees and shrubs towards mesic
species.
Stream flow permanence was a
determinant factor and ranged
from 29-71% at the dry sites
compared to the wet sites, where
flow permanence ranged from
79-100%. Mean depth to ground
water table ranged from about 5 1 m.
Desert
Comparison between
sites differing in degree
of stream intermittency
and flood intensity.
Experiment was
conducted for 2 years.
Tamarix species are very tolerant
to drought. Plant traits and
adaptations to drought are
important, especially long rooting
depth of the mesic species.
Increase in
species
richness, shift in
species
composition.
Riparian vegetation from the lowest end
transplanted to higher elevation
decreased in biomass and increased in
species richness. Climate-driven
hydrologic changes will stimulate the
replacement of riparian plant
communities by terrestrial comm. at the
top, resulting in narrower riparian zones.
Flood duration played a strong
role for structuring riparian plant
community organisation. The low
elevation in the riparian zone was
flooded on average 37.4 days
per year, while the mid elevation
was flooded on average 6.9
days, and the high elevation 1.7
days per year.
Boreal
Transplantation
experiment with turfs
transplanted higher up
the floodplain to
simulate drought
conditions, monitored
over 6 years.
Carex canescens and C. acuta
remained high in frequency at
high elevations, but decreased in
biomass production, potentially as
a result of drought stress.
Biomass response to new
hydrological regimes is rather
quick (one season to the next),
while overall community
adjustments in species
composition take much longer.