**7. Responses of urban forest on the changed mesoclimate and soil properties**

Mongolian oak (*Quercus mongolica*) forests are the most widely distributed and dominant forest of the late successional stage in Korea [85]. The DCA ordination (**Figure 6**) showed that stands in the urban area (Mts. Nam, Acha, Daemo, Bulam,

**Figure 6.**

*Stand ordination of the Mongolian oak forest established in urban and suburban areas around Seoul.*

Bukhan, Cheonggye, and Surak) were clustered in the left corner of the graph, with stands in the natural area (Mt. Jeombong) on the opposite end of axis I. This result shows that species composition in the urban forest was differed from that in the natural forest.

Species richness was usually lower in the urban areas than that in natural areas although a few exceptional areas exist such as Mts. Surak, Bukhan, Bulam, and Cheonggye (see x-axis in **Figure 7**). The slope of species rank-dominance curve was steeper in sites with low species richness than that in sites with high species richness and thereby showed lower evenness (**Figure 7**).

Mongolian oak stands established in urban area showed a difference even in successional trend from those in natural area. In mountains located on urban area, the diameter class distribution of major trees in these Mongolian oak stands revealed that oaks dominated the larger diameter classes, while *Sorbus alnifolia* dominated the smaller diameter classes. On the other hand, Mongolian oak dominated all diameter classes in natural areas (**Figure 8**).

Size distributions of trees are useful indicators for understanding the structure of tree populations and for predicting dynamics of them [86–88]. The diameter class distribution of plant populations has generally been computed as frequency histograms [89]. Frequency distribution patterns of each diameter class indicate the potential change of the population in a plant community. A plant population, where young individuals are numerous and mature ones are fewer, is recognized as having a reverse J-shaped diameter distribution pattern [90, 91]. It is recognized that the population that shows a reverse J-shaped distribution pattern can persist continuously [90–93]. On the other hand, the normal population pattern with fewer

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**Figure 8.**

periodic disturbance [94, 95].

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea*

juveniles relative to adults is typically replaced by another population in the future [92, 93], but a bimodal pattern is shown in a population that is regenerated with

*Frequency distribution of diameter classes of major tree species composed of the Mongolian oak forests* 

*established in several mountains of Seoul and in Mt. Jeombong as a natural area.*

Based on this principle, it is expected that Mongolian oak stands in the natural area could be maintained continuously, whereas urban oak stands would be replaced by Korean mountain ash. Considered that Mongolian oak stands are the representative vegetation of the late successional stage in the Korean peninsula [85],

As was mentioned above, the Mongolian oak forests in urban area of Seoul had different species composition, lower diversity, and retrogressive successional

this successional trend could be interpreted as being retrogressive [95].

*DOI: http://dx.doi.org/10.5772/intechopen.86248*

**Figure 7.** *Rank-abundance curves of the Mongolian oak forests established in 10 study areas.*

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea DOI: http://dx.doi.org/10.5772/intechopen.86248*

**Figure 8.**

*Forest Degradation Around the World*

and thereby showed lower evenness (**Figure 7**).

diameter classes in natural areas (**Figure 8**).

natural forest.

Bukhan, Cheonggye, and Surak) were clustered in the left corner of the graph, with stands in the natural area (Mt. Jeombong) on the opposite end of axis I. This result shows that species composition in the urban forest was differed from that in the

Species richness was usually lower in the urban areas than that in natural areas although a few exceptional areas exist such as Mts. Surak, Bukhan, Bulam, and Cheonggye (see x-axis in **Figure 7**). The slope of species rank-dominance curve was steeper in sites with low species richness than that in sites with high species richness

Mongolian oak stands established in urban area showed a difference even in successional trend from those in natural area. In mountains located on urban area, the diameter class distribution of major trees in these Mongolian oak stands revealed that oaks dominated the larger diameter classes, while *Sorbus alnifolia* dominated the smaller diameter classes. On the other hand, Mongolian oak dominated all

Size distributions of trees are useful indicators for understanding the structure of tree populations and for predicting dynamics of them [86–88]. The diameter class distribution of plant populations has generally been computed as frequency histograms [89]. Frequency distribution patterns of each diameter class indicate the potential change of the population in a plant community. A plant population, where young individuals are numerous and mature ones are fewer, is recognized as having a reverse J-shaped diameter distribution pattern [90, 91]. It is recognized that the population that shows a reverse J-shaped distribution pattern can persist continuously [90–93]. On the other hand, the normal population pattern with fewer

**80**

**Figure 7.**

*Rank-abundance curves of the Mongolian oak forests established in 10 study areas.*

*Frequency distribution of diameter classes of major tree species composed of the Mongolian oak forests established in several mountains of Seoul and in Mt. Jeombong as a natural area.*

juveniles relative to adults is typically replaced by another population in the future [92, 93], but a bimodal pattern is shown in a population that is regenerated with periodic disturbance [94, 95].

Based on this principle, it is expected that Mongolian oak stands in the natural area could be maintained continuously, whereas urban oak stands would be replaced by Korean mountain ash. Considered that Mongolian oak stands are the representative vegetation of the late successional stage in the Korean peninsula [85], this successional trend could be interpreted as being retrogressive [95].

As was mentioned above, the Mongolian oak forests in urban area of Seoul had different species composition, lower diversity, and retrogressive successional trends compared to those in natural area (**Figures 6–8**). These differences are likely due to the development of thin canopy crowns in overstory composed of Mongolian oaks, which have been exposed to severe air pollution stress over long years [39, 47, 48]. By increasing the supply of light and precipitation to the forest floor, thin crowns of canopy trees cause dense growth of subcanopy trees, such as the Korean mountain ash. Therefore, vegetation structure and successional trends change over time [96–99]. Once the subcanopy layer becomes denser, light again decreases, and species richness can be expected to decline, a pattern we observed in our urban forests (**Figure 7**).

Retrogressive succession, signs of which appeared in our urban oak communities, is usually caused by frequent or intense disturbance [100, 101]. Although such situations have been frequently observed in the vicinity of industrial complexes exposed to severe air pollution [102–107], it is a very rare phenomenon in urban areas. Retrogressive succession would be expected where pollution damage to forests is usually intense and acute. However, pollution in most urban areas is less severe than near industrial sites but is chronic [106]. Although we could observe signs of severe air pollution damage from analyzing the vegetation structure in Seoul, severe visible damage on vegetation surface was not found as observed in forests near the industrial areas [50, 108]. Therefore, our results in Seoul could be explained as resulting from synergistic interactions between chronic air pollution and urban climate, rather than resulting solely from severe pollution [109]. Air circulation specific to urban area from interaction of atmospheric temperature inversions and microcurrents occurred due to local temperature differences, and soil acidification due to air pollutants transported along the air circulation interact to cause a change in vegetation structure and consequently change vegetation dynamics. From these results, we can recognize a new type of forest decline in Mongolian oak stands as a general phenomenon occurring on the upper slopes surrounding the Seoul basin [39, 53, 54].

#### **8. Occurrence of drought due to climate change**

Although annual precipitation showed a variation, precipitation when the amount was low, for example, 2014 and 2015, fell short of the threshold that temperate forest can be persisted in this region (**Figure 9**). Considering that annual mean temperature in Seoul is 12.2°C, precipitation more than 100 cm is required to maintain temperate forest [110, 111]. But precipitations in 2014 and 2015, 80.89 cm and 79.21 cm, did not fulfill the level.

Trends of monthly mean precipitation and potential evapotranspiration also showed very dangerous pattern (**Figure 9**). Gaps between precipitation and potential evapotranspiration during spring and fall seasons in 2017 when droughtinduced plant damage was investigated were far bigger than that between mean values of them. In 2018, rainfall during spring season is far more than that of normal year, while that during rainy season, usually July to August, was very short (**Figure 10**). Consequently, rainfall pattern was deviated greatly from the normal pattern. Patterns in 2014 and 2015 when precipitation was very short resembled that in 2018. Water budget in 2014 and 2015 evaluated based on relationship between precipitation and potential evapotranspiration was more severe. Potential evapotranspiration exceeded precipitation.

From those results we can deduce that plants would endure severe water deficiency during growing season particularly. In fact, drought-induced plant damage investigated in urban forest of Seoul reflects those results.

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**Figure 10.**

*from Korea meteorological agency (https://data.kma.go.kr).*

**Figure 9.**

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea*

*Changes of annual precipitation for recent 30 years in Seoul. Considered annual mean temperature is 12.2°C; precipitation more than 100 cm is required to maintain temperate forest. But years, which is not fulfill the level,* 

*Mean monthly trends of precipitation and potential evapotranspiration in 2014, 2015, 2017, and 2018 compared with mean values for recent 30 years from 1981 to 2010 in Seoul. Seoul, which is attributed to Asian monsoon climate zone, usually shows water balances with deficits in the spring season of the year, but the phenomenon was severer in 2017 and showed very different pattern in 2018. Patterns in 2014 and 2015 when precipitation was very short resembled that in 2018. Potential evapotranspiration exceeded precipitation in 2014 and 2015. Potential evapotranspiration was obtained applying a method of Blaney and Criddle [66]. Data were derived* 

*for example 2014 (80.89 cm) and 2015 (79.21 cm) appear in recent years due to climate change.*

*DOI: http://dx.doi.org/10.5772/intechopen.86248*

*Forest Decline Under Progress in the Urban Forest of Seoul, Central Korea DOI: http://dx.doi.org/10.5772/intechopen.86248*

#### **Figure 9.**

*Forest Degradation Around the World*

our urban forests (**Figure 7**).

Seoul basin [39, 53, 54].

**8. Occurrence of drought due to climate change**

and 79.21 cm, did not fulfill the level.

evapotranspiration exceeded precipitation.

investigated in urban forest of Seoul reflects those results.

Although annual precipitation showed a variation, precipitation when the amount was low, for example, 2014 and 2015, fell short of the threshold that

temperate forest can be persisted in this region (**Figure 9**). Considering that annual mean temperature in Seoul is 12.2°C, precipitation more than 100 cm is required to maintain temperate forest [110, 111]. But precipitations in 2014 and 2015, 80.89 cm

Trends of monthly mean precipitation and potential evapotranspiration also showed very dangerous pattern (**Figure 9**). Gaps between precipitation and potential evapotranspiration during spring and fall seasons in 2017 when droughtinduced plant damage was investigated were far bigger than that between mean values of them. In 2018, rainfall during spring season is far more than that of normal year, while that during rainy season, usually July to August, was very short (**Figure 10**). Consequently, rainfall pattern was deviated greatly from the normal pattern. Patterns in 2014 and 2015 when precipitation was very short resembled that in 2018. Water budget in 2014 and 2015 evaluated based on relationship between precipitation and potential evapotranspiration was more severe. Potential

From those results we can deduce that plants would endure severe water deficiency during growing season particularly. In fact, drought-induced plant damage

trends compared to those in natural area (**Figures 6–8**). These differences are likely due to the development of thin canopy crowns in overstory composed of Mongolian oaks, which have been exposed to severe air pollution stress over long years [39, 47, 48]. By increasing the supply of light and precipitation to the forest floor, thin crowns of canopy trees cause dense growth of subcanopy trees, such as the Korean mountain ash. Therefore, vegetation structure and successional trends change over time [96–99]. Once the subcanopy layer becomes denser, light again decreases, and species richness can be expected to decline, a pattern we observed in

Retrogressive succession, signs of which appeared in our urban oak communities, is usually caused by frequent or intense disturbance [100, 101]. Although such situations have been frequently observed in the vicinity of industrial complexes exposed to severe air pollution [102–107], it is a very rare phenomenon in urban areas. Retrogressive succession would be expected where pollution damage to forests is usually intense and acute. However, pollution in most urban areas is less severe than near industrial sites but is chronic [106]. Although we could observe signs of severe air pollution damage from analyzing the vegetation structure in Seoul, severe visible damage on vegetation surface was not found as observed in forests near the industrial areas [50, 108]. Therefore, our results in Seoul could be explained as resulting from synergistic interactions between chronic air pollution and urban climate, rather than resulting solely from severe pollution [109]. Air circulation specific to urban area from interaction of atmospheric temperature inversions and microcurrents occurred due to local temperature differences, and soil acidification due to air pollutants transported along the air circulation interact to cause a change in vegetation structure and consequently change vegetation dynamics. From these results, we can recognize a new type of forest decline in Mongolian oak stands as a general phenomenon occurring on the upper slopes surrounding the

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*Changes of annual precipitation for recent 30 years in Seoul. Considered annual mean temperature is 12.2°C; precipitation more than 100 cm is required to maintain temperate forest. But years, which is not fulfill the level, for example 2014 (80.89 cm) and 2015 (79.21 cm) appear in recent years due to climate change.*

#### **Figure 10.**

*Mean monthly trends of precipitation and potential evapotranspiration in 2014, 2015, 2017, and 2018 compared with mean values for recent 30 years from 1981 to 2010 in Seoul. Seoul, which is attributed to Asian monsoon climate zone, usually shows water balances with deficits in the spring season of the year, but the phenomenon was severer in 2017 and showed very different pattern in 2018. Patterns in 2014 and 2015 when precipitation was very short resembled that in 2018. Potential evapotranspiration exceeded precipitation in 2014 and 2015. Potential evapotranspiration was obtained applying a method of Blaney and Criddle [66]. Data were derived from Korea meteorological agency (https://data.kma.go.kr).*
