  We have monitored nearly a square degree in IC 1396A/Tr 37 over 21 epochs
extending over 2014 - 2016 for sources variable in the JHK bands. In our data,
65 +\- 8 % of previously identified cluster members show variations, compared
with < 0.3% of field stars. We identify 119 members of Tr 37 on the basis of
variability, forming an unbiased sample down to the brown dwarf regime. The
K-band luminosity function in Tr 37 is similar to that of IC 348 but shifted to
somewhat brighter values, implying that the K- and M-type members of Tr 37 are
younger than those in IC 348. We introduce methods to classify the causes of
variability, based on behavior in the color-color and color-magnitude diagrams.
Accretion hot spots cause larger variations at J than at K with substantial
scatter in the diagrams; there are at least a dozen, with the most active
resembling EXors. Eleven sources are probably dominated by intervention of dust
clumps in their circumstellar disks with color behavior indicating the presence
of grains larger than for interstellar dust, presumably due to grain growth in
their disks. Thirteen sources have larger variations at K than at J or H. For
11 of them, the temperature fitted to the variable component is very close to
2000K, suggesting that the changes in output are caused by turbulence at the
inner rim of the circumstellar disk exposing previously protected populations
of grains.
