Impact of the Urban Heat Island Effect on Plant PhenologyElizabeth Wesley

Urban Heat Islands

Urban areas tend to have higher temperatures than the surrounding rural areas, a

phenomenon known as the Urban Heat Island (UHI) effect. The UHI represents a convex

dome of relatively warmer air over a city, and is the result of anthropogenic activities and

land-use and land-cover changes within the urban center (Mallick et al., 2013). Land

surface temperature (LST) increases when pervious land-cover types such as vegetative

surfaces are transformed to impervious land-cover types such as asphalt and concrete

(Fig. 1). These surfaces have low albedo and a high thermal capacity. Combined with

the complicated surface geometry of cities and anthropogenic heat release (Mallick et al.,

2013), these surface qualities result in temperatures in the urban center frequently

reaching 1-3°C higher than in the surrounding rural areas (NASA, 2009). Trees and

vegetation not only have lower LSTs (Fig. 1) but also help maintain soil moisture,

provide shade, and perform evapotranspiration, all of which have cooling effects on the

landscape (Mallick et al., 2013). Impervious surfaces also create more runoff, which

reduces the cooling effects of water on the landscape (NASA, 2009). The effects of

UHIs are numerous and include an elevation in ground level ozone, an increase in energy

consumption, and the deterioration of ecosystems (Dennis et al., 2008).

Elizabeth Wesley

Fig. 1 Different land-use and land-cover types exhibit different heat signatures. Impervious surfaces found

in urban areas have high surface temperatures (Mallick et al., 2013).

With global temperatures rising and 51% of the world’s population now living in cities it

is of increasing importance to understand the effects of UHIs. The footprint of an urban

area is 2.4x the actual size of the area, with the effects of urbanization on ecosystems

extending to 10 km beyond the urban land cover (Zhang et al., 2004b). The effects of

UHIs and those of global warming are similar; by studying the effects of UHIs we can

make predictions concerning the effects of human-induced climate change on regional

and global levels (Luo et al., 2007). “Urbanization strongly affects vegetated ecosystems,

with significant and detectable effects (Zhang et al., 2004b, 3)” on plant species,

therefore studying urban environments can help answer the question: How do Urban Heat

Islands impact plant phenology?

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Plant Phenology

Phenology is the study of the biological timing of life cycle events, and in plants concerns

such events as germination, flowering, fruiting, and dormancy (Rathcke and Lacey,

1985). These phenological events are determined by local climate, which in turn is

controlled by insolation, temperature, and precipitation. Of these controls, temperature

has the greatest effect on plant phenology. Most temperate woody species flower in

response to temperature (Rathcke and Lacey, 1985), and plants will end dormancy and

resume active growth when exposed to warmth after a period of low temperatures (The

University of Arizona, 1998).

Effects of UHIs on Plant Phenology

Studies using both remote-sensing technology and ground-based data collection and

observation consistently reveal that green-up onset and bud burst (initial flowering) occur

earlier, and dormancy later, in urban areas compared to surrounding rural environments

(Zhang et al., 2004b). One study of MODIS data from about 70 urban areas in the

Northeast United States showed that green-up advanced by approximately 7 days, while

dormancy was delayed by approximately 8 days (Zhang et al., 2004b). In general,

flowering dates advance 4 days per °C (Zhang et al., 2004a) and vegetative green-up

advances 3 days per °C (Zhang et al., 2004b). A comparative study between New York

City and Ithaca, NY found that the timing of spring phenological events was strongly tied

to the local environment. Bud burst in London Plane trees occurred earlier in New York

City than in Ithaca as a result of a larger UHI in New York City (Dhami et al., 2011). A

study of ground-based observations in Rennes, France showed that not only was an

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advance in bud burst for Platanus acerifolia and Prunus cerasus correlated with

increased temperature, but that urbanization and consequent land-use changes led to a

more random and less clear phenological sequence in the urban center (Fig. 2) (Mimet et

al., 2009). The possibility is distinct that these changes in the timing of phenological

events will impact plant reproduction (Mimet et al., 2009) as well as the distribution,

composition, and life cycle of species (Dhami et al., 2011).

Fig. 2 Percentage range of flowered buds of Platanus acerifolia in Rennes, France, 8 April 2005. Bud

burst is advanced in the urban center, and also complicated by city geometry and land-use land-cover

(Mimet et al., 2009).

Consequences

Plant species respond at different rates to changing temperatures, and interspecies

phenological differences may affect trophic structures and ecosystem processes (Luo et

al., 2007). The early onset of spring events such as green-up, bud burst, and flowering

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may disrupt temporal relationships between plants and pollinators, seed dispersers, and

herbivores (Dhami et al., 2011). In Northern Japan it was shown that the seed-set success

of bee-pollinated plants was strongly restricted by the presence of pollinators (Luo et al.,

2007). Mutualistic decoupling of plants and pollinators may occur due to artificial shifts

in temperature as a result of UHIs. Such decoupling is especially likely and may be more

pronounced when one species takes its phenological cues from a stable control like

sunlight and another is cued by a changing control like temperature (Hughes, 2000). The

resulting shifts in ecosystem dynamics will cause changes in the associated trophic

communities (Luo et al., 2007), and if species do not change their phenological responses

with respect to each other, increasing temperature may eventually lead to migration,

adaptation, and extinction of species (Dhami et al., 2011).

Conclusions

Urbanization plays a major role in shaping ecosystems, not only within urban areas but

also in their surrounding environments. Urban Heat Islands trap warmer air within urban

areas, thereby altering the phenology of plant species: flowering advances and dormancy

is delayed where temperatures are higher. UHIs provide valuable insight into the

possible consequences of global climate change. Although increased warmth extends the

growing season, it may also lead to phenologic mismatch between species with possible

far-reaching trophic consequences. Many of these consequences are speculative, and

although further study is needed, it is imperative that we attempt to mitigate warming on

local, regional, and global scales.

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Elizabeth Wesley

References

Dennis, Y. C. L., C. Liu, and A. M. Rizwan. (2008). “A review on the generation,determination and mitigation of Urban Heat Island.” Journal of Environmental Sciences 20:120-128. http://www.upv.es/contenidos/CAMUNISO/info/U0564937.pdf

Dhami, I., K. G. Arano, T. A. Warner, R. M. Gazal, and S. Joshi. (2011). “Phenology oftrees and urbanization: a comparative study between New York City and Ithaca, New York.” Geocarto International 26:7 507-526. doi: 10.1080/10106049.2011.607517.

Hughes, L. (2000). “Biological consequences of global warming: is the signal already.” TREEI 15:2 56-61. http://marineecology.wcp.muohio.edu/climate_projects_04/species-distributions/pdfs/BiolConseqGWarm.pdfhttp://marineecology.wcp.muohio.edu/climate_projects_04/species-distributions/pdfs/BiolConseqGWarm.pdf

Luo, Z., O. J. Sun, Q. Ge, W. Xu, and J. Zheng. (2007). “Phenological responses ofplants to climate change in an urban environment.” Ecological Research. 22:3 507-514. doi: 10.1007/s11284-006-0044-6.

Mallick, J., A. Rahman, and C. K. Singh. (2013). “Modeling urban heat islands inheterogeneous land surface and its correlation with impervious surface area by using night-time ASTER satellite data in highly urbanizing city, Delhi-India.” Advances in Space Research 52:4 639-655. doi: 10.1016/j.asr.2013.04.025.

Mimet, A., V. Pellissier, H. Guénol, R. Aguejdad, V. Dubreuil, and F. Rozé. (2009).“Urbanisation induces early flowering: evidence from Platanus acerifolia and Prunus cerasus.” Int J Biometeorol 53:287-298. doi: 10.1007/s00484-009-0214-7.

NASA. (2009). “Ecosystem, vegetation affect intensity of Urban Heat Island Effect.”Accessed November 16, 2013. http://www.nasa.gov/mission_pages/terra/news/heat-islands.html

Rathcke, B. and E. Lacey. (1985) “Phenological patterns of terrestrial plants.” Annual Review of Ecology and Systematics 16: 179-214. http://links.jstor.org/sici?sici=0066-4162%281985%2916%3C179%3APPOTP%3E2.0.CO%3B2-F

The University of Arizona. (1998). “Environmental factors that affect plant growth.”Accessed November 13, 2013. http://ag.arizona.edu/pubs/garden/mg/botany/environmental.html

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Zhang, X., M. A. Friedl, C. B. Schaaf, A. H. Strahler. (2004a). “Climate controls onvegetation phenological patterns in northern mid- and high latitudes inferred from MODIS data.” Global Change Biology 10: 1133-1145. doi: 10.1111/j.1365-2486.2004.00784.x.

Zhang, X., M. A. Friedl, C. B. Schaaf, A. H. Strahler, and A. Schneider. (2004b). “The footprint of urban climates on vegetation phenology.” Geophysical Research Letters 31:L12209. doi: 10.1029/2004gl020137.

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