How often do you water the ferns?
Do you have a misting system that runs daily?
Do you keep tabs on the humidity in your backyard?
Sycamores can drink alot of water and are found in riparian areas in California.
I would think that you would need to drench the area under the tree heavily at least once a week-soak for 3-4 hours. This would get water deeper into the soil base.
Do you have a mulch around the plants? This would help preserve soil moisture and provide for some incidental localized humidity near the plants provided you do not have heavy winds in the area.
If wind is a daily or regular feature of your weather pattern then be sure to offset the dessication associated with such by adequate irrigation of your plants.