Opinion
Gardening and urban landscaping:
significant players in global change
Ülo Niinemets1 and Josep Peñuelas2
1
Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, Tartu 51014, Estonia
Ecophysiology and Global Change Unit CREAF-CEAB-CSIC, Center for Ecological Research and Forestry Applications,
Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
2
Global warming leads to shifts in vegetation types in
given temperate environments. The fastest species
movement is due to the globalized supply and use of
exotic plants in gardening and urban landscaping. These
standard practices circumvent dispersal limitations and
biological and environmental stresses; they have three
major global impacts: (i) the enhancement of biological
invasions, (ii) the elevation of volatile organic compound
emissions and the resulting increase in photochemical
smog formation, and (iii) the enhancement of CO2 fixation and water use by gardened plants. These global
effects, none of which are currently considered in globalchange scenarios, are increasingly amplified with further
warming and urbanization. We urge for quantitative
assessment of the global effects of gardening and urban
landscaping.
Gardening accompanying urbanization: neglected
global relevance
Changes in atmospheric composition, climate, land use
and biodiversity are well-established components of current global change. However, there is another major component of global change that has not received the attention
it deserves. Rapid increases in human population and
economic development have led to tremendous urbanization and road construction, implying a mosaic of landscapes with most vegetation selected for their ornamental
characteristics and resistance to urban conditions. Rapid
urbanization has become an area of crucial concern in
conservation biology owing to the radical changes in
habitat structure and the loss of species as a result of
urban and suburban development [1]. However, the accompanying phenomena of gardening and urban landscaping
and their global relevance have not yet been given due
consideration. Here, we highlight three major global effects
of expanded gardening and urban landscaping activities.
These activities lead to (i) increased spread of invasive
species, (ii) enhanced emission of biogenic volatile organic
compounds (BVOCs; see Glossary), particularly enhanced
emissions accompanied by elevated formation of photochemical smog (see Glossary) in the winter months and (iii)
increased uptake of CO2 and augmented water use by
urbanized areas. We also emphasize that both gardening
and urban landscaping itself and most of these effects are
favored by global warming [2,3]. Finally, we conclude that
Corresponding author: Peñuelas, J. ([email protected]).
60
an effort by the scientific community to quantify all these
global effects of gardening and urban landscaping is warranted to discern their actual role in global and urban
ecology.
Enhancement of plant invasions and spread of
invasive species
Urban and suburban areas are important foci for the
spread of introduced plant species [4–6]. Gardening practices favor the dispersal and establishment of alien species
in many ways. In particular, the dispersal of gardened
plants is neither limited by climatic requirements for
propagule formation and maturation nor by propagule
natural dispersal. Furthermore, standard gardening practices result in the reduction of abiotic and biotic stresses
that the plants must tolerate (Box 1). As a result, gardening brings together a rich mixture of species of widely
varying geographic origin. The floras of public and
domestic urban gardens form the greatest source of potentially invasive alien plants [7], and many alien plant
species grown in gardens have escaped cultivation and
become invasive [6,8–10]. In addition, hybridization with
other alien species or local species can increase the stress
tolerance and invasibility of gardened species [11].
Human-facilitated introduction of species has contributed
to exotic species’ colonization in locations where they would
never have naturally dispersed because of large geographic
Glossary
Biogenic volatile organic compounds (BVOC): volatile organic compounds
synthesized by organisms, mainly by plants. Volatile isoprenoids (isoprene,
monoterpenes) form the most significant class of BVOC. BVOC contribution
exceeds anthropogenic volatile compound production by more than an order
of magnitude.
Frost hardiness: species potential to tolerate sustained periods with low
temperature. Plants not hardy in given climates cannot be cultivated or require
special care (winter covers etc.).
Hydrologic drought: limited water availability resulting from lowered water
tables that is common in urbanized areas. Urban drought typically results from
engineered deep stream channels, excessive consumption of ground water
and impervious surface.
Laurophyllization: invasion of deciduous temperate communities by broadleaved evergreen vegetation. Currently typically occurring in Northern Italy and
Southern Switzerland as well as at higher altitudes in mountains of
Mediterranean countries.
Q10 value: rate of reaction at temperature T + 10 8C relative to the rate of
reaction at temperature T.
Photochemical smog: formation of particulate matter (secondary aerosols) and
ground-level ozone as the result of light-driven chemical reactions in the
participation of nitrogen oxides (NOx) and volatile organic compounds.
Secondary aerosols: particulate matter formed in the atmosphere as the result
of oxidation reactions and condensation of vapours of organic compounds.
1360-1385/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2007.11.009 Available online 11 February 2008
Opinion
Trends in Plant Science
Vol.13 No.2
Box 1. Facilitation of plant dispersal and survival by gardening
Common gardening practices facilitate establishment, growth and
survival of plants by diminishing dispersal limitations, amelioration of
environmental and biological stresses and improving winter survival.
Facilitation of species dispersal by gardening
Non-facilitated dispersal of local species or invasive alien species
requires the maturation of viable propagules (seeds, vegetative
propagules) during the growing season and the successful dispersal
of these to potential growth habitats. For plants from warmer climates
transferred to cooler environments, the length of the growing season
is often not long enough for seed maturation. In addition, flower buds
and seeds are significantly less resistant to frost than leaves and
vegetative buds (see Ref. [58] for a detailed review). As a consequence, plants hardy enough to grow and survive in cooler climates
might not form viable offspring. Furthermore, pollination and seed
dispersal of alien species can be curbed by lack of pollinators and
specialized seed dispersing animals. By contrast, all these hurdles are
easily overcome by alien plants cultivated in gardens. Furthermore,
plants are propagated with lower cost in milder climates and
container-grown seedlings or saplings are transferred to cooler
environments (Figure I). Gardening companies producing plant
material in warmer climates offer constant supply, and local
businesses in northern, cool-temperate climates can introduce novel
ornamental species that are not necessarily hardy enough for local
climate (i.e. do not exhibit frost hardiness; see Glossary) or are unable
to cope with abnormally cold winters, commonly occurring about
every ten years [40]. Nevertheless, this constant supply of partly
hardy plant material implies that the number of species growing in
gardens in any given time and location closely reflects the potential
species frost-resistance limits (see Figure 1 in main text). Therefore,
gardening practices respond to global warming essentially immediately [41].
level of biological stress and improve survival and growth of
gardened plants.
Facilitation of winter survival
Limited winter survival is the major obstacle for growing many
ornamental plants in cooler environments. Winter survival is affected
by several factors including tolerance of minimum temperatures (frost
hardiness), resistance of snow loads and tolerance of above-ground
desiccation stress, as well as enhanced photoinhibition in warmer clear
days in spring when the rooting zone is still frozen [58,60,61]. Various
types of protective covers are generally employed to avoid dieback
resulting from excessively low minimum temperatures and damage
due to spring photoinhibition and desiccation stresses. Higher carbohydrate reserves that plants in cultivation with ample supply of water and
nutrients and without biotic competition can build further contribute to
the enhanced survival in harsh winter conditions and to faster and more
complete recovery from winter damage [60]. In cultivation, a given
temperate plant species can typically grow in climates with 5 to 10 8C
lower minimum temperatures (1–2 USDA hardiness zones) than the
same species in wild conditions [62–64].
Diminution of environmental stresses
Survival of plants in any given environment is significantly affected
by various abiotic stresses. For example, plant survival and growth is
often limited by low soil fertility and by episodic low water
availability, limiting the number of alien invaders, as well as local
species that can colonize a given habitat [59]. The additional supply of
resources during the growing season, such as irrigation or fertilization, is part of routine gardening practices, allowing plants that would
not survive in natural habitats to succeed.
Protection from and amelioration of biological stresses
A variety of biological stresses, such as pathogen attack, herbivory
and competition with neighboring plants, further affects plant
survival and growth. The mechanical and chemical control of weeds,
the use of a variety of pesticides and fungicides and mechanical
shields, such as bark protections against rodents, belong to standard
gardening practices. Such measures significantly curb the overall
distances. For instance, in isolated New Zealand, characterized by very low rates of species invasions in the geological past, more than 25 000 exotic species have been
introduced during the past 150 years, many of which have
become invasive weeds [12]. Recent studies demonstrate
that several components of global change, such as
enhanced nitrogen depositions and elevated CO2 concentration, can favor the competitive ability of alien species,
and that these drivers have enormously enhanced species
invasions globally [13]. Gardening worldwide has contributed—and keeps contributing significantly—to this
enhanced invasion, even though the contribution differs
between regions. For instance, the contribution of gardening to species invasions is slightly less important in
Southern Europe than in Central and Northern Europe
owing to water limitations in Southern Europe that con-
Figure I. Plants are propagated with lower cost in milder climates and containergrown seedlings or saplings are shipped to cooler environments. In Europe,
there is a major export (indicated by red arrows) of gardening plant material
from countries with a mild temperate oceanic climate, such as Denmark and the
Netherlands, to Scandinavian and Baltic countries with a more continental
temperate climate, as well as from countries with a Mediterranean climate, such
as Italy and France, to countries with a mild temperate climate, such as Germany,
and from semi-arid Northern Africa to Mediterranean Southern Europe.
strain the number of species that can potentially grow and
become invasive in areas where water is not supplied
artificially.
Global warming further amplifies species invasions
Many of the species cultivated in gardens are potentially
not invasive at the time of introduction because of lack of
natural dispersal mechanisms and because they rely on
protective measures for winter survival (Box 1). However,
as temperatures are rising globally, many species can
escape from the rich species pool present in the gardens.
Global warming-driven shifts in vegetation boundaries
accompanied by enhanced species richness have occurred
many times in the Earth’s geological past [14–16]. Because
of the moderate rate of temperature increase and the long
life-span of woody vegetation, such invasions of species
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Opinion
Figure 1. Tolerance of minimum temperatures by vegetative buds of temperate,
warm temperate and subtropical species (based on the summary of Ref. [55],
n = 121 for conifers and n = 226 for broad-leaved species). Data were fitted by
second-order polynomial regressions (solid lines). The dashed line depicts the
cumulative percentage of species able to survive in any given location if winter
minimum temperature rises by 5 8C. Recent predictions suggest that winter
minimum temperatures rise by 5–8 8C and more so in temperate and boreal
environments by 2099 [3,39]. Predicted enhancement of potential species richness
with warming agrees with trends in species richness at a global scale, that is,
increase of species richness with decreasing latitude at given water availability
(see, for example, Refs [56,57]). Owing to profit-driven supply of both hardy and
semi-hardy species grown in milder temperate climates, then transferred and sold
in northern cooler temperate climates, it is expected that the actual species
number grown in gardens at any time closely matches the maximum species
number expected to grow in a given habitat under given conditions (Box 1).
from warmer habitats have progressed at a relatively slow
rate. In recent decades, warming has occurred significantly
faster, probably faster than any other time during past
1300 years [2]. The average land surface temperature in
the Northern hemisphere has increased by a rate of 0.29–
0.34 8C per decade between 1979 and 2005 [2], and the
average land surface temperature is predicted to increase
by 2.5–5 8C by 2099 [3]. Furthermore, winter temperatures
have risen much faster, particularly in Northern Europe
and the northern part of North America (0.7–1.3 8C per
decade); this fast change is visible in altered species distributions [17,18]. Moreover, the winter minimum land surface temperature is predicted to increase further by 5–8 8C
and by more in Boreal Europe and North America by 2099
[3]. Besides, the urban-heat-island effect increases the
temperature in urbanized areas even more than in natural
areas and agricultural plantations (for example, see
Ref. [19]).
In temperate environments exhibiting winters with
freezing temperatures, an increase in minimum winter
temperature by 5 8C is expected to increase the number
of species able to grow in any given location by 7–20%
(Figure 1). A 10 8C increase in minimum winter temperature is even predicted to increase the potential species pool
by 10–40% (based on the data and fits in Figure 1). Detailed
simulations of the responses of potential plant species
richness to climate change in Fennoscandian ecosystems
predict on average a 26% increase in species richness by
the end of this century [20].
Gardening companies operating in a globalized world
generally produce at lower cost in countries with a warmer
climate planting material that will be sold and planted in
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Trends in Plant Science Vol.13 No.2
gardens of cooler climates. Although the worldwide distribution of gardened species is also somewhat limited by
biogeographically defined anthropogenic and historical
factors [21], there is more and more evidence of gardened
species transfer across the continents [21,22], further
increasing the potential gardened species pool.
In addition to plants that are fully hardy in cooler
climates, barely hardy plants are often offered and planted
in cooler locations. The consistent supply of semi-hardy
plants implies that the actual increase in species number
in urban habitats keeps close track with the global warming-driven potential species number (Box 1). There is
ample evidence demonstrating that the species number
in urban vegetation has followed the climatic warming in
the past [23]. Thus, gardening in a warming world strongly
facilitates species movement to higher latitudes, and cultivated species numbers respond to global warming almost
instantly. This is in contrast to the long-term changes in
natural vegetation boundaries, the speed of which is limited by the lifespan of dominating woody species and
competition by established and invading species. The current alien flora of cities in temperate climates is very
species-rich compared with surrounding native vegetation
[7,24], which confirms that gardening is an effective way to
enhance species richness and to take advantage rapidly of
warmer climate.
The enhanced pool of exotic species further increases the
number of species that can potentially escape cultivation
and become invasive [6]. For instance, a series of Mediterranean broad-leaved evergreen species has escaped
gardens and become an important forest component in
several Central European and Western European
countries, leading to a ‘laurophyllization’ (see Glossary)
of typical evergreen needle-leaved or broad-leaved deciduous forests [11,25].
Amplification of volatile organic compound emissions
by altered species composition and planting design of
urban habitats
An important implication of gardening practices from a
global change perspective is that many ornamental plants
are strong emitters of BVOCs, such as isoprene and monoterpenes [26]. For instance, most broad-leaved species from
genera Eucalyptus, Liquidambar, Liriodendron, Populus,
Quercus and, essentially, all conifers are important emitters of volatile isoprenoids; a list that is being constantly
complemented with new entries (see, for example, Refs
[27,28] and Centritto et al., unpublished*). Such plantgenerated emissions strongly contribute towards ozone
formation in the atmosphere, as well as to the formation
of secondary aerosols (see Glossary), collectively leading to
the creation of photochemical smog in urban atmospheres
[29–31].
Most of the current emission inventories are for local
native species, and the emission scenarios for urban atmospheres have been developed on the basis of these native
species checklists [32,33], which are not necessarily valid
for urbanized environments. A completely different set of
*
Centritto, M. et al., abstract P1994, XVII International Botanical Congress,
Vienna, Austria, July 2005.
Opinion
species generally dominates urban environments [7], and
many of these species have significantly higher BVOC
emission potentials than local native species. For instance,
many species introduced from Mediterranean Europe to
Western and Central Europe have strongly fragrant foliage
with large monoterpene reservoirs in the leaves and large
constitutive monoterpene emission potentials, whereas the
typical dominants of late-successional broad-leaved deciduous forests in Europe have low-to-moderate isoprene
emission potentials [34].
Furthermore, plants growing in urban habitats are
generally spaced far apart, thus most of the plants’ foliage
is exposed to high light. This is relevant as the emission of
many plant volatiles is much larger in leaves intercepting
high-light intensities than in shaded leaves [35]. Although
the total area of urban vegetation can be relatively small,
the contribution to BVOC emissions can be disproportionately large. Volatile-isoprenoid emissions also respond
very strongly to temperature with reaction rate increasing
3–6-fold for a 10 8C increase in temperature (Q10 value; see
Glossary). This temperature response is significantly
stronger than that of typical biochemical reactions such
as photosynthesis (Q10 = 2–3) [36]. Thus, plant-generated
BVOC emissions, in particular emissions from gardened
plants, are expected to increase strongly with globally
rising temperatures [37]. Furthermore, ozone is also a
strong greenhouse gas, and plant-mediated ozone production might be partly responsible for the warming in
boreal to arctic latitudes [38].
Enhancement of winter emissions
As evergreen vegetation stays ornamental throughout the
year, there is particular interest in planting evergreens in
cool-temperate and temperate gardens and roadsides.
Strong isoprenoid-emitting shade-intolerant ornamental
conifers and emitting broad-leaved evergreens are currently relatively rarely planted in Boreal and temperate
European cities because there are only 3–5 native species
with few ornamental cultivars. Among introduced North
American conifers, there are several cold-tolerant species
from the genera Picea and Thuja planted in gardens;
however, owing to dense, strongly shading conical crowns,
these species are not favored in strongly urbanized spacelimited areas. Currently, major ornamental, urban conifer
genera, such as sparse-foliated Araucaria and Cedrus or
narrow-crowned Cupressus, which dominate maritime
temperate and Mediterranean cities, do not tolerate cool
temperate climates.
Global warming in temperate and boreal environments
not only means warmer average and warmer winter temperatures, but also implies wetter winters with enhanced
snow depth during the frost period and extended growing
season [39]. The length of the growing season currently
increases ca. two days per decade, and this increase is
expected to accelerate in future climates [2,3]. Enhanced
snow depth during periods with freezing temperatures
improves the winter survival of evergreen broad-leaved
shrubs and young trees, whereas extended growing season
length generally favors warm-temperate and Mediterranean evergreen broad-leaved vegetation [25]. Furthermore, gardening of evergreen exotic vegetation can be
Trends in Plant Science
Vol.13 No.2
limited by exceptionally cold winters. As more than 300
years of gardening experience in Russia demonstrates,
such severe winters typically occur every 10 years
[40,41]. With global warming, it is predicted that the
frequency of exceptional cold air outbreaks in the northern
hemisphere will decrease by 50–100% [3,42]. All these
modified climatic factors potentially increase the contribution of evergreen vegetation in temperate to cooltemperate gardens.
Apart from temperature, the variety of frost-tolerant
species along major roads in temperate and boreal climates is further significantly constrained by the use of
high amounts of chloride salts for de-icing of roads in
winter. None of the native conifers growing in temperate
and boreal forests have foliage and roots that are salt
stress tolerant, and many strong deciduous BVOC emitters, such as Populus and Quercus, do not tolerate rootzone salt stress either [43]. Therefore, current roadside
vegetation in temperate and boreal climates primarily
consists of deciduous species with low emitting potential,
such as Acer and Tilia. As the need for de-icing salts is
expected to decrease owing to enhanced periods with
non-freezing temperatures [44,45], the potential pool of
strong BVOC-emitting species, such as Populus and
Quercus, is further enhanced by global warming. Moreover, extensive research is currently being carried out to
find de-icing chemicals with lower environmental
impact, (de-icing chemicals that decay faster, chemicals
with lower ecotoxicity [46,47]). The use of environmentally ‘friendly’ de-icing chemicals would also allow roadside cultivation of evergreens that are already hardy in
the current climates.
Because they emit volatiles throughout the year, the
increased planting of ornamental coniferous evergreen
species and emitting broad-leaved evergreen species in
urban areas can strongly enhance BVOC emissions in
cities. Although the biogenic emissions strongly depend
on temperature and are generally at low levels during
winter, emission bursts can be observed on sunny days
in which the temperature of evergreen foliage rises 8–10 8C
above the ambient temperature. Such emission bursts
currently play a decisive role in the formation of winter
smog in Mediterranean and mid-latitude temperate cities
and are projected to become more and more significant in
northern temperate and boreal cities as well. Currently,
there is only a limited formation of spring or winter smog in
the northern habitats, but the conditions might be right to
produce them in the future if gardening continues its
current rapid growth.
Increased CO2 uptake and water use
A further important effect of gardening is that species
introduction and landscape disassembly to mosaic also
affect the carbon balance of the Earth. In particular, the
species introduced in the framework of common gardening
practices generally grow faster at the beginning of their
introduction owing to the lack of natural parasites and
enemies, as well as human-mediated alleviation of
environmental stresses (Box 1). Furthermore, plants in
gardens and roadsides are generally widely spaced and,
consequently, are exposed to high light. Together with
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Opinion
irrigation and fertilization practices common in gardening,
plants grown in gardens achieve much higher rates of
photosynthesis than non-gardened native plants grown
under a plethora of biological and environmental stresses
in natural environments. Furthermore, plants in urban
ecosystems are exposed to higher temperatures, CO2 and
nitrogen deposition than plants in rural areas. These
factors usually have a beneficial influence on plant photosynthesis and growth. In certain situations, these urban
plants can even be submitted to lower cumulative urban
ozone (O3) exposures and adverse effects, amidst a background of higher regional exposures beyond the urban core
[48]. In fact, the managed green spaces (e.g. parks, gardens
and golf courses) often have elevated local plant productivity relative to the surrounding wild land, and the
productivity of such green spaces can reach the higher end
of the maximum productivities observed in given environments [49,50].
This reasoning and the evidence outlined suggest that
although the total area of urban vegetation can be relatively limited, its contribution to the Earth’s carbon balance might be disproportionately large. Altered rates of net
primary productivity (NPP) in urban green spaces can
partially mitigate losses in productivity caused by extensive impervious urban surface cover [49,50]. An analysis of
NPP for the USA found elevated total NPP in urban areas
relative to nearby wild land, despite increased impervious
surface cover, in two general cases: (i) cities in arid environments and (ii) cities with lower-density development [49].
Direct measures of NPP in the arid grasslands of Colorado
corroborate this finding. Urban lawns exhibited four to five
times the aboveground NPP relative to native grasslands
[50]. Analogously, in three Korean cities, although the
mean annual carbon uptake by woody plants ranged from
1.60 to 3.91 t ha 1 yr 1 for natural forest patches within
the cities and 0.53–0.80 t ha 1 yr 1 for urbanized area,
woody plant coverage was almost an order or magnitude
less in urbanized locations, demonstrating considerably
higher carbon fixation at given woody species coverage
in urban areas [51].
Urban and road gardening increases NPP, but because
it increases water consumption, it can also generate ‘hydrologic drought’ (see Glossary) by lowering water tables,
which can be particularly significant in drier climates by
altering soil, vegetation and microbial processes [52]. As
drinking water is often used for irrigation of plants in the
gardens, gardening can globally contribute to the overall
shortage of drinking water [53]. For instance, studies
demonstrate that the use of drinking water for irrigation
of lawns and gardened woody vegetation has increased by
more than 400% between the mid-1970s to the mid-1990s
in some European countries [54].
Conclusions and perspectives
With globally changing temperatures, the number of plant
species that can potentially grow in northern temperate
and boreal environments is rapidly increasing. This
increase is particularly enhanced if the limitations on
species dispersal are removed and if abiotic and biotic
stresses are alleviated by gardening practices. Many of
the species facilitated by gardening practices are invasive
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Trends in Plant Science Vol.13 No.2
species or can become invasive in a globally changing
environment. Many of these species are also strong emitters of volatile organic compounds. This is particularly
significant for emissions during the winter months because
future gardening practices will probably favor evergreens
in current temperate and boreal climates; currently, the
urban vegetation under these climates is commonly deciduous. Typical gardening practices can further lead to
higher photosynthetic rates (and water use) in gardened
plants (in particular in semi-arid and arid lands). We
advocate that the modifications of floras, amplification of
world-scale biogenic volatile compound emissions and
enhanced CO2 uptake that result from changes in species
distribution and diversity as a result of gardening practices
constitute a significant component of global change.
Until now, urban ecology has mostly focused on the
effects of urban greening on energy budget and carbon
uptake of urban environments. We suggest that the outlined potential effects of increasing gardening and urban
landscaping call for their introduction in the agenda of
urban ecology, and altogether warrant an effort by the
scientific community to assess those effects quantitatively.
Acknowledgements
We acknowledge financial support by the Estonian Ministry of Education
and Science (grant SF1090065s07), the Estonian Academy of Sciences,
the Spanish Ministry of Education and Science (grants CGL2004–01402/
BOS and CGL-2006–04025/BOS), the European Commission RTN
‘ISONET’ contract MC-RTN-CT-2003–504720, the European Science
Foundation ‘VOCBAS’ program, the Fundació Abertis (2007 grant) and
the Catalan government (grant SGR2005–00312).
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