Yes trees obstruct radio reception. In general, FM waves travel lines of sight, very similar to the transmission of light waves. Open terrain has different reception than wooded terrain. This is due to the conductivity of leaves and branches of different trees as well as conductivity of their sap. This paper on radio wave propagation in potato fields addresses the propagation loss to foliage. Yeah it's z sleeper, but interesting to us radio geeks.
Thankfully there are solutions that mitigate if not eliminate the problem. But the frequency matters. If a vertical 28 MHz antenna is surrounded by trees that are taller than the top of the antenna, it's signals will be attenuated by an average of 2 to 3 dB. Tree attenuation with horizontal polarization under the same conditions usually will be negligible. But at 144 MHz the same tree scenario will cause 6 to 12 dB of attenuation on that vertically polarized signal and 3 to 4 dB with horizontal polarization.
Interestingly enough, cell phones get the worst of the foliage attenuation. Pine needles are nearly the same length as 800 MHz antennas. Because of this twist of fate, 800 MHz systems are rarely constructed in forested areas.
The cell phone picture is stolen from Howstuffworks.com. I will admit I've used that site. there are infact people who know more than me about certain topics.