Defenses to these charges will likely focus on two potential targets.
1) Actus Reus (the action of the crime): I did not do these acts. This may be hard to argue if the evidence is clear enough. What will be hard for the prosecution to prove (if it goes to trial) is that the same person on the video footage is the same person responsible for any other individual acts of sabotage that actually posed threats to public safety and comprised the said crime. The person may choose to challenge the accuracy of the evidence if it does not provide a positive identification. However, the onsite trailhead arrest makes this approach less likely.
2) Mens Rea (the mental intent of the crime): I did not have the intention to do this crime. Generally, ignorance to an offence is not an excuse (unless one is insane). One may questions the value of a defense that is based on a person with a mistaken belief that what they were doing was legal, and that they were actually attempting to perform a community service by protecting sensitive habitat, or protecting public safety in the same means as a contractor may install speed-bumps on a road. The challenge for the prosecution is to demonstrate the intent of the accused was in fact to cause disruption, damage, or indeed physical harm to people or at least to demonstrate that a reasonable person would know that such actions are likely to result in such outcomes. Here, the defense may plead ignorance to the potential consequences of their actions, and argue that they had no idea that they were endangering people, and that they thought they might just be slowing them down a little. Here is where the innocent little old lady act comes into play, and a good lawyer does his or her best coaching. You must again become the innocent victim in all this. At first you were just defending the forests, now you must defend yourself as a victim of a punitive and overly harsh justice system that has misinterpreted the actions of a concerned citizen as those of a criminal. The defense (again assuming trial) may likely seek to uphold exactly that image I suggested before (the nice old lady on the bus) in the courtroom. The lawyer may even instruct her to wear a hairnet, and walk with a cane. Dont be afraid to whisper or stutter Dearie. Show the court that youre not a threat.
Its cases like these that make you wish the best lawyers were prosecutors. Unfortunately, you generally have to work defense to make enough scratch to buy new bike.