Field experiments on carbon and water relations at the leaf and at the whole plant level were carried out near Bayreuth (W.-Germany) on 33 year old specimens of deciduous Larix decidua, Larix leptolepis, the hybrid of both species and evergreen Picea abies. CO₂ assimilation rates per leaf area were similar in all larches and were 40% higher than those measured in spruce. However, water use efficiency was higher in spruce than in larches. Stem respiration in hybrid larch reached a maximum in July, when stem increment rate declined.
Increased needle density in hybrid crown was achieved by a combination of having the branching pattern of Japanese larch with the needle density of European larch. For all three larches the annual carbon balances showed similar relative costs for stem and root systems. Heterosis in the larch hybrid was mainly based on a more effective needle arrangement and a more spacious crown compared with the parent species (complementation principle).
Spruce invested less carbon into non-green crown organs and current-year needles than larches. However, the carbon balance costs for the root system was greater than in the larches. From this study, the evergreen life form exhibited less water and nitrogen uptake than the deciduous species, but also required greater longevity of well-functioning needles.