Any folliage (trees, leaves, etc.) has a tendancy to absorb radio signals, particularly in the UHF (and higher) spectrum. Locally, I notice that reception of my 800 mHz trunked system is much clearer in the winter, when the leaves are off the trees. In the summer, there is a noticiable attenuation of signal levels with more swishing noise and hiss. This is particularly noticeable the farther away you are from the transmitter. I live on the outskirts of town where the signals are not that strong to begin with. So, I am fairly confident that your pine cones could have some effect on the signals you receive. However, if you live in a strong signal area, you may not notice a weakening of signals.