An alien world with a significant amount of tall metal trees allows lightning to occur at lower voltages than usual.
How low could that voltage be pushed and still break apart Nitrogen molecules in the atmosphere?
The current answer by Aric Fowler gives a good summary of voltages required based on air resistance, which would be the major factor in voltage requirement, but the distance is the really important part. You can arc across a smaller distance with a lower voltage.
As noted in the previous answer, cumulonimbus storm clouds can form as low as 150m, however you should also consider the tree height. The tallest trees on Earth (redwood sequoias) can grow to 115m tall. Your metal trees may be able to grow even taller than that making the distance even shorter.
Given low storms at 150m and tall metallic trees (of negligible electrical resistance) at 115m you only need to span 35m of air. Using the 3x10^6 V/m this gives 105 MV. You could of course go closer to the point of the static charges dissipating with even lower voltages. The fun science apparatus electrostatic generators, can make sparks in the low kVolt range depending on air conditions, but usually operate in the 1-5 MV range.
Which brings me to the most important point. Low voltage sparks are not actually what you want you want the high voltage because the high voltage differential between the cloud and the ground is what allows for a large buildup of charge to happen. As is often the case the current is as important (if not more) than the voltage. You want the current to be high so you can get the required energy to form NOx compounds. The formation of nitrogen oxides happens during lightning storms due to the extreme heat of the lightning, it has nothing specific to do with the electricity involved. To form NOx compounds you need to heat air (nitrogen and oxygen mix) to temperatures around 2800° F (1540C). The specific heat of air is ~1 kJ/kg.K. so to raise a kg of air from ~20C to 1540C is going to take ~1.5 MJ. So you need a lot of energy to get the formation of NOx not just some low voltage low power sparks.
The dielectric strength of air is generally accepted as 3x10^6 V/m.
This means that is takes 3x10^6 Volts for every meter of direct distance between the electrodes.
Assuming you want a cloud-to-ground flash, the lightning will have only one electrode and a "ground" which is the metal trees. The distance will be the altitude of your clouds:
Storm clouds called cumulonimbus clouds, can tower 12-15 miles high (Mount Everest is 5.5 miles high.). They form at altitudes of 150 to 4000 metres and typically have peaks 6100m, though in extremes cases they can be as high as 24000m.
At the lowest altitude of 150m, the voltage needed to create lightning would be 4.5x10^8 Volts. A tall thunderstorm cloud can hold over a 100 million volts of potential.
So, in ideal conditions your voltage can be as low as 450MV