gistfile1.r

txt1 <- "ID  block   plot    SPID    TotHeight
1   1   1   4   44.5
2   1   1   4   51
3   1   1   4   28.7
4   1   1   4   24.5
5   1   1   4   27.3
6   1   1   4   20
17  1   10  1   44.5
19  1   10  1   51
1   1   11  21  28.7
2   1   11  21  24.5
3   1   11  21  27.3
4   1   11  21  20
5   1   11  21  12.88666667
6   1   11  21  7.235238095
7   1   11  21  1.583809524
"

txt2 <- "ID  block   plot    SPID    Species TotHeight
1   1   1   4   BENI    72
2   1   1   4   BENI    55
3   1   1   4   BENI    51
4   1   1   4   BENI    47
5   1   1   4   BENI    49
6   1   1   4   BENI    34
7   1   1   4   BENI    .
8   1   1   4   BENI    51
9   1   1   4   BENI    66
10  1   1   4   BENI    40
11  1   1   4   BENI    24
12  1   1   4   BENI    62
13  1   1   4   BENI    34
14  1   1   4   BENI    49
15  1   1   4   BENI    57
16  1   1   4   BENI    22
17  1   1   4   BENI    76
18  1   1   4   BENI    56
19  1   1   4   BENI    55
20  1   1   4   BENI    29
21  1   1   4   BENI    24
22  1   1   4   BENI    18
23  1   1   4   BENI    65
24  1   1   4   BENI    55
25  1   1   4   BENI    63
26  1   1   4   BENI    57
27  1   1   4   BENI    57
28  1   1   4   BENI    57
29  1   1   4   BENI    45
30  1   1   4   BENI    83
31  1   1   4   BENI    37
32  1   1   4   BENI    56
33  1   1   4   BENI    65
34  1   1   4   BENI    75
35  1   1   4   BENI    51
36  1   1   4   BENI    .
1   1   2   16  PRSE    141
2   1   2   16  PRSE    192
3   1   2   16  PRSE    .
4   1   2   16  PRSE    197
5   1   2   16  PRSE    172
6   1   2   16  PRSE    143
7   1   2   16  PRSE    141
8   1   2   16  PRSE    155
9   1   2   16  PRSE    167
10  1   2   16  PRSE    155
11  1   2   16  PRSE    175
12  1   2   16  PRSE    190
13  1   2   16  PRSE    148
14  1   2   16  PRSE    180
15  1   2   16  PRSE    .
"


dat1 <- read.table(textConnection(txt1), header = TRUE)
dat2 <- read.table(textConnection(txt2), header = TRUE)

merge(dat1, dat2, by = c("ID", "block", "plot"))