cell_id  x        y      year    percentpp
1   250540  170.75  -83.75  2011    3
2   250541  171.25  -83.75  2011    1
3   250542  171.75  -83.75  2011    0
4   250543  172.25  -83.75  2011    0
5   250540  170.75  -83.75  2012    1
6   250541  171.25  -83.75  2012    5
7   250542  171.75  -83.75  2012    1
8   250543  172.25  -83.75  2012    5
9   250540  170.75  -83.75  2013    2
10  250541  171.25  -83.75  2013    2
11  250542  171.75  -83.75  2013    2
12  250543  172.25  -83.75  2013    0
12  250540  170.75  -83.75  2014    2
13  250541  171.25  -83.75  2014    3
14  250542  171.75  -83.75  2014    3
15  250543  172.25  -83.75  2014    3

cell_id  x        y      year    percentpp
1   250540  170.75  -83.75  2011    3
2   250541  171.25  -83.75  2011    1
3   250542  171.75  -83.75  2011    0
4   250543  172.25  -83.75  2011    0
5   250540  170.75  -83.75  2012    3
6   250541  171.25  -83.75  2012    5
7   250542  171.75  -83.75  2012    1
8   250543  172.25  -83.75  2012    5
9   250540  170.75  -83.75  2013    3
10  250541  171.25  -83.75  2013    5
11  250542  171.75  -83.75  2013    2
12  250543  172.25  -83.75  2013    5
13  250540  170.75  -83.75  2014    3
14  250541  171.25  -83.75  2014    5
15  250542  171.75  -83.75  2014    3
16  250543  172.25  -83.75  2014    5

function(x) {
  if df$percentpp[year+1,] is greater than df$percentpp[year,]
  when cell_id$year = cell_id$year+1
  then df$percentpp[year,] <- df$percentpp[year+1,]
}