Aboveground production and N and P use by Larix occidentalis and Pinus contorta in the Washington Cascades, USA.

Aboveground net primary production (ANPP) and N and P use patterns were determined for western larch (Larix occidentalis Nutt.), a deciduous conifer, and lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.), an evergreen conifer, in the Cascade Mountains of Washington, USA. Western larch and lodgepole pine retranslocated 87 and 66% of foliage N and 66 and 78% of foliage P, respectively. At the stand level, N use efficiency of western larch was greater than that of lodgepole pine, whereas P use efficiency of lodgepole pine was greater than that of western larch. Western larch and lodgepole pine were comparable in ANPP and production efficiency (ANPP/foliage mass) if needle longevity is considered. The similarity in ANPP of the evergreen lodgepole pine and the deciduous western larch may be related in part to the lower initial construction cost of the foliage, and the efficient use of nitrogen by western larch.

Temperature-Induced Change in the Water Relations of Abies amabilis (Dougl.) Forbes.

Water conductance through Abies amabilis seedlings was measured while the roots were exposed to temperatures from 15 to 0.25 degrees C. Before conductance was measured, the seedlings were preconditioned for 3 months at either a high temperature (23 degrees C) or a low temperature (3 degrees C). For both groups of seedlings, conductance decreased as root temperature decreased. Conductance was lowest at 0.25 degrees C. In addition, preconditioning at 3 degrees C for 3 months significantly lowered conductance to water at all root temperatures. Under the same environmental conditions, seedlings preconditioned at 3 degrees C had less than 25% of the transpirational water loss of seedlings preconditioned at high temperature. A decrease in leaf osmotic potential also resulted from low temperature preconditioning. In trees growing in the subalpine forest, which is the natural habitat of Abies amabilis, both decreased leaf conductance to water vapor and lower osmotic potentials were evident in winter. Since in winter the temperature of the soil in the subalpine zone remains less than 1 degrees C for many months, lowered leaf conductance and decreased osmotic potentials appear to be mechanisms which aid in preventing desiccation damage.

Nitrate losses from disturbed ecosystems.

A systematic examination of nitrogen cycling in disturbed forest ecosystems demonstrates that eight processes, operating at three stages in the nitrogen cycle, could delay or prevent solution losses of nitrate from disturbed forests. An experimental and comparative study of nitrate losses from trenched plots in 19 forest sites throughout the United States suggests that four of these processes (nitrogen uptake by regrowing vegetation, nitrogen immobilization, lags in nitrification, and a lack of water for nitrate transport) are the most important in practice. The net effect of all of these processes except uptake by regrowing vegetation is insufficient to prevent or delay losses from relatively fertile sites, and hence such sites have the potential for very high nitrate losses following disturbance.