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Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a shade intolerant species, and western hemlock (Tsuga heterophylla (Raf.) Sarg.), a shade tolerant species, were compared to learn more about the temporal pattern of release from suppression in both species, whether hydraulic architecture or photosynthetic capacity constrain release and how wood functional properties change after release from suppression. The study was conducted in 10-20 year old Douglas-fir and western hemlocks, either in a site that had been thinned to release suppressed trees or in a site that remained unthinned. Douglas-fir had lower height growth (from 1998-2003) and lower relative height growth (height growth from 1998-2003/height in 1998) than western hemlock. However, the relative height growth of released vs. suppressed trees was much higher in Douglas-fir (130%) than western hemlock (65%), suggesting that although height growth was lower, Douglas-fir did indeed release from suppression.
Release seems to be constrained initially by photosynthetic capacity in Douglas-fir and western hemlock. In Douglas-fir released trees had 14 times the leaf area and 1.5 times the nitrogen per unit leaf area (Narea) as suppressed trees. Needles on released western hemlock trees had approximately twice the maximum assimilation rate (Amax) at ambient [CO2] as suppressed trees and did not exhibit photoinhibition at the highest light levels. Hydraulic architecture appears to constrain further release from suppression in Douglas-fir more so than western hemlock after the increase in leaf area, leaf N content and overall photosynthetic capacity. Released trees had significantly less negative foliar 13C values and there was also a positive relationship between leaf area:sapwood area ratios and 13C suggesting that trees with more leaf area for a given sapwood area experienced a stomatal limitation on carbon gain. Growth of released trees, thus, may have been limited by stomatal constraints on carbon gain despite a doubling of Amax after release. Nonetheless, trees exhibited no significant differences between the leaf-specific conductivities (KL) of suppressed vs. released trees of either species. However, leaf-specific root conductance (kRL) was significantly greater in suppressed Douglas-fir compared to released trees.
Functional wood characteristics were also much different in trees released from suppression and those that remained suppressed. Growth ring widths in released trees increased by 370% for Douglas-fir and 300% for western hemlock, while specific conductivity (Ks) increased by 182% for Douglas-fir and 42% for western hemlock compared to suppressed trees. Earlywood width was approximately four times greater in released than suppressed trees of both species, whereas the relative increase in latewood width between suppressed and released trees was much greater in Douglas-fir than in western hemlock. Latewood proportion decreased by 21% in released Douglas-fir and by 47% released western hemlock compared to suppressed trees. Tracheids were 25% wider and 11% longer in released Douglas-fir saplings than suppressed saplings, whereas in western hemlock released saplings had 19% wider tracheids that were approximately the same length as suppressed saplings. Wood moisture content was 66% higher in released Douglas-fir compared to suppressed Douglas-fir and 41% higher in released western hemlock compared to suppressed western hemlock. Wood density decreased from 0.57 to 0.47 g cm-3 in Douglas-fir trees released from suppression and from 0.50 to 0.45 g cm-3 in western hemlock trees released from suppression. Therefore, it appears that as management patterns switch from even-age systems to uneven-age systems, both Douglas-fir and western hemlock will be able to release from suppression and the wood of released trees will be of good quality for most applications. |
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