So, I used MS Excel to find this third degree polynomial that matches these numbers

Year Total Forest 2003 31.1 Area (% of Land Area) 1990 31.6 2004 31 1991 31.6 2005 31 1992 31.6 2006 31 1993 31.6 2007 30.9 1994 31.5 2008 30.9 1995 31.5 2009 30.9 1996 31.4 2010 30.9 1997 31.3 2011 30.8 1998 31.3 2012 30.8 1999 31.2 2013 30.8 2000 2014 31.2 30.8 2001 31.1 2015 30.7 2002 31.1 2016 30.7\fD E F G H M N P Q R S W X Chart Total Data Format Forest Area as Percent of Land over time Forest Area (% of 31.8 Series "" Land Area) Series Name Year 31.6 Outline 31.6 31.6 31.6 1AWNHO Marker Options 31.6 31.5 Marker Style Circle V 31.5 31.4 31.4 Marker Size 31.3 y - 26-05x" - SE-05x3 - 0.0481x + 31.67 O 5 31.3 R3 = 0.9837 31.2 da Fill ~ Outline 31.2 31.2 31.1 > Data Labels 12 31.1 13 31.1 > Horizontal Error Bar 6090 14 31 > Vertical Error Bar 15 31 31 16 31 v Trendline "" 17 30.9 18 30. Trendline Name 0.9 30.8 Trend Type Polynomial v 0.9 0.8 Order 3 30. Display Equation on chart 0.8 30.8 30.6 Display R-squared value on chart 30.7 10 15 30 25 20 25 26 30. Set Intercept 0