Fruits generally contain sugars which are easily digested carbohydrates; there's a Polish bushcrafter/botanist Lukasz Luczaj who has written articles about wild fruit in one of the bushcraft mags (not the glossy one) Wild pears in particular have a lot of calories.
According to some diets such as paleo/primal very minimal amount of carbohydrates are needed. It's calories that you need and they are more concentrated in fats which are even harder to find. Nuts and seeds may be a good source of both of these. Some tree products such as sap and inner bark may also be good calorie providers. You can train your body to accept or prefer different foodstuffs - a diet of fruits, leafy greens and meats before you go out to live on the land would help cut down on the cross over period.
