What is the default kernel initializer in tf.layers.conv2d and tf.layers.dense?

Great question! It is quite a trick to find out!

• As you can see, it is not documented in tf.layers.conv2d
• If you look at the definition of the function you see that the function calls variable_scope.get_variable:

In code:

self.kernel = vs.get_variable('kernel',
                                  shape=kernel_shape,
                                  initializer=self.kernel_initializer,
                                  regularizer=self.kernel_regularizer,
                                  trainable=True,
                                  dtype=self.dtype)

Next step: what does the variable scope do when the initializer is None?

Here it says:

If initializer is None (the default), the default initializer passed in
the constructor is used. If that one is None too, we use a new
glorot_uniform_initializer.

So the answer is: it uses the glorot_uniform_initializer

For completeness the definition of this initializer:

The Glorot uniform initializer, also called Xavier uniform initializer.
It draws samples from a uniform distribution within [-limit, limit]
where limit is sqrt(6 / (fan_in + fan_out))
where fan_in is the number of input units in the weight tensor
and fan_out is the number of output units in the weight tensor.
Reference: http://jmlr.org/proceedings/papers/v9/glorot10a/glorot10a.pdf

Edit: this is what I found in the code and documentation. Perhaps you could verify that the initialization looks like this by running eval on the weights!