What happened to your trees is this, Zoe. Anytime you have a subject backlit by bright light...for example, an indoor subject against a window or a subject standing in the shade with the sunlight behind them...the camera has to choose (unless you do) which area of the picture to expose for. If it exposes the film for a short period of time, the background will come out, but the foreground will be black. Here's an example my mother-in-law took that shows the opposite effect to what happened to you:



In this image the camera chose to expose the film for the lighter background ... if it's got a lot of light, the shutter doesn't need to be open for very long to capture the image. But this meant the shutter didn't stay open long enough to capture the details of the people inside. So they're extremely underexposed and can't be seen.

In your image, the exposure was set for the darker area (the indoor flowers). By leaving the shutter open long enough to capture their details, the well-lit outdoor areas became over-exposed, and in this case that means it washed out entirely. Which just goes to show, over-exposure isn't always bad. Then again, underexposure isn't bad in the right circumstances. This is why it's nice to have control over the aperture or f-stops (which means how wide the shutter opens) and the shutter speeds, which controls how long the shutter stays open.