Abstract:
Xylem conducting tissue or sapwood is an important storage organ
for water, carbohydrates, and nutrients, and this storage helps trees
accommodate environmental change. However, the living ray parenchyma
cells in sapwood, which store the carbohydrates and nutrients,
require energy for maintenance. I examined how sapwood maintenance
costs vary among species, and how these costs change with tree growth
and stand development.
First, I explored how sapwood volume and leaf area, indices of
maintenance costs and net assimilation, varied with tree size among
three subalpine conifers (Engelmann spruce, subalpine fir, and
lodgepole pine). As trees grew in size and leaf area, sapwood volume
increased exponentially. The ratio of sapwood volume to leaf area
increased with tree size, indicating that maintenance costs were
greater in large trees, relative to leaf area. Lodgepole pine showed the most rapid increase in sapwood volume with increasing leaf area,
suggesting storage capacity and sapwood maintenance are important
processes for this species.
Next, I measured stem growth and maintenance respiration for
Engelmann spruce and lodgepole pine. Stem maintenance respiration
was linearly related to sapwood volume for lodgepole pine from 4 to
40 cm dbh and for Engelmann spruce from 0 to 20 cm dbh. Maintenance
respiration was not correlated with annual stemwood production or
phloem volume for either species, but stem respiration during the
growing season, corrected for maintenance, correlated well with
annual stemwood growth. Annual stem maintenance respiration for
trees and stands can be estimated using sapwood volume, sapwood
temperature and knowledge of respiratory behavior.
Finally, I examined a chronosequence of subalpine lodgepole pine
stands to test the hypothesis that the balance between photosynthetic
and respiring tissue changes with stand development. I predicted
that woody tissue respiration (particularly stem sapwood) would
increase with stand development and account for the majority of
observed decreases in stem growth. Leaf area and nutrient
availability indicated that carbon assimilation was similar for the
chronosequence, but stem growth decreased from 0.14 to 0.04 kg C ha⁻¹
y⁻¹. However, maintenance respiration of woody tissues in stems, branches and coarse roots accounted for less than 40% of the decrease
in stem growth.
