a decreasing trendp value < 0.001
A decreasing trend of temporal variation was found by using simple linear regression of the production in response to the year during 1985–2012 (slope = − 039; p value < 0.001).
A decreasing trend of temporal variation was found by using simple linear regression of the production in response to the year during 1985–2012 (slope = − 039; p value < 0.001).