Table 4. Frequency of co-occurring words for each period(top 15 words).

Rank	The 1st period (2001∼2005)		The 2nd period (2006∼2010)		The 3rd period (2011∼2015)		The 4th period (2016∼2020)		Total
	Word	Freq.*	Word	Freq.	Word	Freq.	Word	Freq.	Word	Freq.
1	species	35	restoration	12	management	33	vegetation	32	vegetation	81
2	diversity	17	ecology	11	vegetation	32	mountain	32	species	73
3	section	17	vegetation	8	species	28	environment	19	mountain	60
4	hiking	12	maruguem	7	section	27	diversity	13	diversity	56
5	vegetation	9	environment	6	diversity	26	hiking	11	section	49
6	structure	8	Baekdusan	5	forest	24	forest	10	management	45
7	management	7	geography	5	mountain	22	species	10	forest	43
8	correlation	7	sangyungpyo	5	Gangwondo	20	flora	10	environment	36
9	area	6	mountain	4	conservation	19	temple	9	flora	35
10	forest	6	tour	4	flora	17	ecology	9	ecology	32
11	Jirisan	6	Gaya	4	Quercus mongolica	16	climate	9	restoration	29
12	flora	6	Baekje	4	structure	15	index	9	Gangwondo	28
13	river	6	development	4	protected area	15	trail	8	maruguem	28
14	ridge	5	landscape	4	distribution	14	development	8	conservation	25
15	rare	5	climate	3	community	12	maruguem	8	structure	25

Freq. indicates frequency