Degree of leaf surface damage, the number of damaged plants, and the number of dead plants due to drought in an urban forest (Mt. Bulam) of Seoul, central Korea.
Vegetation in the urban area showed not only a difference
Degree of leaf surface damage, the number of damaged plants, and the number of dead plants due to drought in an urban forest (Mt. Bulam) of Seoul, central Korea.
Vegetation in the urban area showed not only a difference in species composition but also lower diversity compared with that of the natural area. Successional trend was normal in natural area, but that in urban areas showed a retrogressive pattern. Korean mountain ash (Sorbus alnifolia (Siebold & Zucc.) K.Koch), a shade intolerant species, dominated such a retrogressive succession. The vegetation decline is due to changes of mesoclimate and soil properties that imbalanced distribution of green space induced as the result of urbanization. In recent years, new environmental stress due to climate change is imposed additively to this forest decline. Drought is the very environmental stress. Drought-induced plant damage started from withering of leaves of plants introduced for landscaping in the urban area. Over time, branches died and death of the whole plant body followed. In particular, damage of Korean mountain ash, the product of retrogressive succession, was remarkable. As retrogressive succession has already progressed much, thus such phenomenon could be recognized as crisis of urban forest.
- forest decline
- retrogressive succession
- urban forest
Urbanization expanding globally is recognized as a major causing environmental change. Reduction of habitat size, fragmentation, and imbalanced distribution of green space due to urbanization led to influences on dynamics of vegetation remaining in urban area [2, 3]. Increases of temperature, precipitation, and nitrogen deposition due to urbanization also altered abiotic conditions of habitat patches remaining in urban area [1, 4]. These changes influence habitat quality, and, consequently, the species composition, species diversity, and functional diversity of vegetation remaining there [3, 5, 6], which in turn affect the ecosystem functions.
Forests are the typical types of urban green space. Urban forests function as habitat of native species as well as recreation site for citizens [9, 10]. Urban forests can play a role of refugia of rare and threatened species and thus can display high conservation value [11, 12]. Among urban landscape elements, forest has substantially different site history, intensity of management and disturbance, and consequently different species composition from other landscape elements [3, 12, 13]. Urban forests are remnants of former continuous forests, a result of succession or artificial plantation. They can also include urban orchards, urban park, cemeteries overgrown by trees, or residential garden [12, 13].
Land transformation and increase of impervious surface cover affect forests throughout the landscape through increased local temperatures and altered ecosystem processes. Anthropogenic drivers of global change, i.e., land-use change, introduction of exotic species, pollution, and climate change, affect forest composition and function across the landscape. In particular, change of land-use pattern including urbanization and their effects on remaining vegetation constitute one of the major factors influencing on natural ecosystems [16, 17]. In the case of forests, about 70% of remaining forest around the world is within 1 km from the forest’s edge. Therefore, it is very difficult that they maintain integrate structure and healthy function. As land is transformed into urbanized area, the effects of the transformation on the remaining vegetation are getting more apparent. Increased local temperature and altered hydrologic and nutrient cycle, land transformation, and increased impervious surface cover have been recognized as elements affecting forest health and resilience to other stress factors [19, 20, 21]. Those remaining natural forest patches are still critical in terms of air quality improvement, flooding reduction, urban heat island effect mitigation, and supply of other ecosystem services that are important to both human societies and natural environment [22, 23, 24]. Therefore, understanding how urbanization affects structure and function of remnant forest ecosystems is critical to both conservation and management of this ecological resource.
One of the principal changes that urbanization induced is the increase of land covered by impervious surfaces such as asphalt and concrete pavement, concrete buildings, and tightly compacted soils. Percentage-paved land surface is an appropriate proxy for urban heat island effects as a main factor increasing the land surface temperature [25, 26]. Urban heat islands develop around areas with high heat absorptive capacity such as asphalt, c