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	78	plant	23	environment	52	plant	123	species	198
2	vegetation	55	soil	20	species	51	habitat	79	plant	175
3	structure	55	vegetation	20	change	51	species	59	diversity	174
4	diversity	43	urban	20	vegetation	46	diversity	57	habitat	142
5	community	30	species	18	diversity	46	climate	53	vegetation	128
6	plant	30	area	14	climate	44	environment	42	environment	125
7	soil	24	tourism	12	plant	44	ecosystem	40	climate	101
8	landscape	24	ecosystem	12	ecosystem	41	area	34	ecosystem	97
9	management	21	structure	11	soil	36	vegetation	32	soil	77
10	urban	21	diversity	11	structure	34	healing	29	management	70
11	Korea	18	index	11	index	33	distribution	28	biology	66
12	nature	17	nature	10	management	31	management	27	landscape	64
13	correlation	17	environment	10	area	31	naturalization	27	Korea	63
14	importance	17	habitat	9	conservation	19	value	26	structure	60
15	community	16	Korea	9	system	19	landscape	22	urban	57