Spacial distribution of photosynthetic capacity and performance in a mountain spruce forest of Northern Germany : III. The significance of the evergreen habit
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Abstract
Growth and CO2 uptake in the crown of a spruce tree is described and the production
processes of this evergreen conifer are compared with those of a deciduous beech.
Spruce had 60% lower rates of net photosynthesis per dry weight than beech. But, beech
had a 30% shorter growing season and a 84% smaller biomass than spruce. The annual
CO2 gain was 40% lower in beech than it was in spruce.An analysis shows the following
conclusions for this habitat. (1) The effect of a prolonged growing season is small.
The annual CO2 gain of spruce would be reduced only by 9% if the growing season was
the same length as for beech. (2) The annual CO2 gain would increase 14% if all needles
in spruce were deciduous, because the current year needles have a higher average rate
of CO2 uptake than 3-year old and older needles, but a lower average rate than 1-
and 2-year old ones. However, the carbon balance of the tree shows that spruce could
not afford to produce the existing needle biomass (14 t ha-1) each year. (3) If spruce
were to produce the same deciduous foliage biomass during the same growing season
as beech then total production by spruce would be reduced 67%. (4) The annual CO2
uptake by evergreen spruce was higher than deciduous beech not because of a long growing
season, but because of the longevity of its needles, which during their total life
time (an average of 5 years) have a two to three times greater CO2 uptake than a deciduous
leaf in one summer season. The relatively small investment in current year needles
produces an annually low, but long lasting assimilation of CO2.