Donaldson and Forrester both struck in the first half as the Blue Square Bet South high-fliers lost for the first time in eight matches in all competitions.
Donaldson put Brentford ahead after 15 minutes, cutting inside from the right to fire low past Wood goalkeeper James Russell. Jonathan Douglas nearly made it two-nil but his free-kick flew off the crossbar with Russell motionless.
Brentford's Leon Legge almost headed into his own goal with a failed clearance after 27 minutes while Omer Riza rattled the bar soon after as the hosts' hunt for a leveller gathered pace.
But Forrester grabbed the Bees' second after 43 minutes, linking up with Donaldson and Paul Hayes to fire home from inside the box.
After the break Brentford almost made it three but Tony Craig's header was bundled off the line and Russell produced a fine double save to deny Donaldson and Hayes.