Previous studies have also shown that the concentration of microorganisms increases with increasing temperature in the range of 5.3–22.2 °C (Li et al., 2011), which indicates that temperature is important to microorganisms. Some catkins were observed floating in the air during sampling, most clearly on May 15th, which indicates that the growth of plants may also contribute to the increase of abundance of microbial activity because they are known to be sources of bioaerosols as well as media for the production and growth of microorganisms (Christner et al., 2008 and Kurkela, 1997).