Here is the definition valve gave us.

Time spent determining what will be rendered that frame, and time spent sorting translucent objects (rendered back to front). The number of times the world is rendered per frame will directly increase the time spent here. Reduce the number of monitors, reflective water surfaces, and any other special effects that render extra views of the world. Large amounts of detail props will also contribute. Turning off detail props entirely may produce extra performance benefits when the engine does not have to consider sorting any detail props.

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So, your trees and plants are probably the main issue. Theres probably several ways to get around this.

I for one, have made a test forest map with the JT woods model pack on this site, and havn't had much of a problem. I was using prop_statics. Perhaps you could get around this problem by turning some of them into prop_dynamic_override, or prop_static, or prop_detail (does detail actually work?)

Unfortunately, HL2 does not = Farcry, so you won't get as much vegetation as you hope to get. But you can make very lush areas, and have vis blockers, such as mountains, or you could turn on fog and set the maxfade distance in the map properties. Theres a lot of ways you can optimize it.

Show us a screenshot of your largest area, showing the most vegetation.