**2. Study area**

*Forest Degradation Around the World*

from embolism formation [32].

in those ecosystems [39, 40].

extreme weather events like drought [35, 36].

ies overgrown by trees, or residential garden [12, 13].

Urban forests are remnants of former continuous forests, a result of succession or artificial plantation [14]. They can also include urban orchards, urban park, cemeter-

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 [15]. 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 [18]. 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 [19–21]. 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 [22–24]. 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, concrete buildings, bare ground, and other developed lands, which heat up rapidly and increase local temperature greatly compared with surrounding natural areas [27, 28]. In an area that natural forest is conserved to urbanized area, land surface temperature increased more than 70% and soil moisture decreased about 15%. These changes of the microclimate in urbanized areas can affect vegetation remaining there [29]. Trees growing in areas covered by impervious surface densely represented low drought resilience compared with trees in forested areas [30] and experienced severer moisture stress and insect damage compared with trees in intact forests [31]. In general, increased impervious surface cover increases water stress and vulnerability to drought and thus makes trees in intensively urbanized landscapes more sensitive to cavitation and lower protection

Increasing urbanization could aggravate the impact of climate change on forest.

Increasing temperature accompanies severer and more frequent droughts that could increase tree mortality [33, 34]. Even though most forest species can tolerate changes in mean climatic conditions, it is not clear that they could withstand the

Thus, the synergistic effects of extreme weather events, like drought and temperature increase in relation to urbanization, could influence severely on the health and resilience of forests remaining in urban areas [37, 38]. A decline of forest health and the following changes in species composition and vegetation structure would lead to change of ecosystem function and ultimately alter ecosystem services

In Korea, forest began to show decline symptoms around the industrial complexes and large cities [41, 42]. Further, change of mesoclimate due to excessive land use in urban area led to changes of vegetation structure and dynamics as well

**72**

Seoul, the capital of South Korea, is located in the Central Korean Peninsula and covers 605 km2 of land (126°46′15″ to 127°11′15″ E longitude, 37°25′50″ to 37°41′45″ N latitude; **Figure 1**). Topography of Seoul is the typical basin that Han River runs through the center and is backed by mountains. The elevation of the study area ranges from 20 to 800 m above sea level. The parent rock of the mountainous areas around Seoul is usually composed of granite and gneiss, and the flat land beside rivers and streams is consisted of alluvium. Soil in these areas was classified into the Suam, Osan, Asan, and Anryong series, which developed on gneiss and granite bedrock [39, 50]. The climate of Seoul is continental, with warm and moist summers and cold and dry winters. From 1981 to 2010, the mean annual temperature was 12.5°C and the mean annual precipitation was 145.1 cm [51].

The mountainous vegetation of Seoul is consisted of four major plant communities distributed along an elevation gradient: the Korean red pine (*Pinus densiflora*

#### **Figure 1.**

*A map showing the study area, Seoul, the capital of South Korea. (1) Mt. Nam, (2) Mt. Bukhan, (3) Mt. Surak, (4) Mt. Bulam, (5) Mt. Acha, (6) Mt. Daemo, (7) Mt. Cheonggye, (8) Mt. Jeombong.*

Siebold & Zucc.) community in the mountain peaks and around the residential area, the Mongolian oak (*Quercus mongolica* Fisch. ex Ledeb.) community in the upper slopes, the hornbeam (*Carpinus laxiflora* (Siebold & Zucc.) Blume) community in the lower slopes, and the sawleaf zelkova (*Zelkova serrata* (Thunb.) Makino) community in the mountain valleys [52]. East Asian alder (*Alnus japonica* (Thunb.) Steud.) stands remained in the plains and valleys of lowlands that escaped from urbanization [53–55]. Much of the natural forest in the Seoul metropolitan area disappeared due to extensive deforestation for fuel, building material, and other purposes during the twentieth century [56]. The human population of Seoul has increased from 2.4 million in 1960 to 9.8 million as of 2010 [57]. During this period, the percentage of green space decreased from 70% in 1960 to 29% in 2015, mostly to accommodate residential area [54, 56, 58]. Korean government designated most of the forested mountains in suburban areas of Seoul as greenbelt zones in order to prevent further loss of green space. Under the current greenbelt ordinance, no commercial, industrial, or urban development is permitted in those forests [58].
