Those are not Spring Engine trees, though some might have been ported to Spring Features package. Apparently, they're for a whole separate attempt to remake OTA in 2010 called ta3d.
You can find the models
here, apparently the format is
Rhino3d, a proprietary format . Conversion will be required, and i'm not sure that, for example, Blender can do it. Unless someone did the work already, the feature definition files would have to be redone. And i'm not even sure where they put the textures.
Maybe their own bundled modelling utility can do something if it hasn't bitrotten into nothing since then.
Using features for trees in Spring is a rather unpleasant experience, since they will
simply disappear when crushed, so you get to choose between tolerating that or making trees uncrushable. When shot, they will also just disappear, no fire, no animation. But of course it still looks better.
(!invok knorke to explain that there are workarounds and their cost)