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We estimate maintenance respiration for boles of four temperate conifers (ponderosa pine, western hemlock, red pine, and slash pine) from CO 2 efflux measurements in autumn, when construction respiration is low or negligible. Maintenance respiration of stems was linearly related to sapwood volume for all species; at 10°C, respiration per unit sapwood volume ranged from 4.8 to 8.3 μmol CO 2 m -3 s -1 . For all sites combined, respiration increased exponentially with temperature (Q 10 =1.7, r 2 =0.78). We estimate that maintenance respiration of aboveground woody tissues of these conifers consumes 52-162 g C m -2 y -1 , or 5-13% of net daytime carbon assimilation annually. The fraction of annual net daytime carbon fixation used for stem maintenance respiration increased linearly with the average annual temperature of the site.

A new version of the ecosystem process model FOREST-BGC is presented that uses stand water and nitrogen limitations to alter the leaf/root/stem carbon allocation fraction dynamically at each annual iteration. Water deficit is defined by integrating a daily soil water deficit fraction annually. Current nitrogen limitation is defined relative to a hypothetical optimum foliar N pool, computed as maximum leaf area index multiplied by maximum leaf nitrogen concentration. Decreasing availability of water or nitrogen, or both, reduces the leaf/root carbon partitioning ratio. Leaf and root N concentrations, and maximum leaf photosynthetic capacity are also redefined annually as functions of nitrogen availability. Test simulations for hypothetical coniferous forests were performed for Madison, WI and Missoula, MT, and showed simulated leaf area index ranging from 4.5 for a control stand at Missoula, to 11 for a fertilized stand at Madison, with Year 50 stem carbon biomasses of 31 and 128 Mg ha(-1), respectively. Total nitrogen incorporated into new tissue ranged from 34 kg ha(-1) year(-1) for the unfertilized Missoula stand, to 109 kg ha(-1) year(-1) for the fertilized Madison stand. The model successfully showed dynamic annual carbon partitioning controlled by water and nitrogen limitations.

Simulation models of ecosystem processes may be necessary to separate the long-term effects of climate change on forest productivity from the effects of year-to-year variations in climate. The objective of this study was to compare simulated annual stem growth with measured annual stem growth from 1930 to 1982 for a uniform stand of ponderosa pine (Pinus ponderosa Dougl.) in Montana, USA. The model, FOREST-BGC, was used to simulate growth assuming leaf area index (LAI) was either constant or increasing. The measured stem annual growth increased exponentially over time; the differences between the simulated and measured stem carbon accumulations were not large. Growth trends were removed from both the measured and simulated annual increments of stem carbon to enhance the year-to-year variations in growth resulting from climate. The detrended increments from the increasing LAI simulation fit the detrended increments of the stand data over time with an R(2) of 0.47; the R(2) increased to 0.65 when the previous year's simulated detrended increment was included with the current year's simulated increment to account for autocorrelation. Stepwise multiple linear regression of the detrended increments of the stand data versus monthly meteorological variables had an R(2) of 0.37, and the R(2) increased to 0.47 when the previous year's meteorological data were included to account for autocorrelation. Thus, FOREST-BGC was more sensitive to the effects of year-to-year climate variation on annual stem growth than were multiple linear regression models.

A water flux model with daily resolution is described which permits one to assess how changes in the rooting volume, amount of sapwood, leaf area and conductance properties interact to affect water uptake, internal storage, and transpiration.A root zone water compartment is defined for a particular tree on the basis of root depth, lateral extension and moisture holding characteristics of the soil. Water is taken up from different subcompartments of the root zone as a function of vertical position, soil water content, and water deficit within the sapwood. Excess water entering the root zone is channeled into runoff or seepage.The sapwood compartment of the model is restricted to the main stem of the tree and does not include sapwood in the branches or roots. The model assumes whatever water deficit is built up in the sapwood during the day will be replenished at night if the root zone water supply/capacity ratio exceeds 20%. A complex exponential equation describes the amount of water extractable from 20% to 0 capacity when no uptake is possible. The maximum change in volume of water in the sapwood of a large Douglas-fir is estimated to represent more than a 10 day supply for transpiration.Water loss through transpiration is predicted as a function of the mean daily absolute humidity deficit, leaf area, leaf conductance and daylength. Leaf conductance is controlled by predawn plant moisture stress which in turn is a function of the rooting zone water supply.The model incorporates two special constraints upon water uptake and transpiration. The first accounts for the effect of cold soil temperatures reducing the possible uptake by Douglas-fir to half at 2°C and to 0 at-2°C. The second represents a critical absolute humidity deficit sufficient to cause stomatal closure which results in leaf conductance being reduced to a minimum.The model is employed to compare trees of different sizes and those with different stomatal behavior. From this experience, it is suggested that future studies include, at a minimum, simultaneous measurements of: absolute humidity deficit, leaf area, sapwood volume and change in water content, predawn stress and leaf conductance.

The influence of low temperature in the root zone on water uptake in lodgepole pine (Pinus contorta Dougl. ex Loud.) was studied under laboratory conditions. To remove soil hydraulic influences, two-year-old seedlings were transferred to solution cultures and maintained in temperature controlled water baths. Short term measurements of leaf conductance, leaf water potential and tritiated water movement were taken at root temperatures from 22 C down to 0 C. Root resistance was calculated to be 67% of total plant resistance at 7 C and 93% at 0 C. In addition an Arrhenius break was found in a plant resistance versus temperature plot, suggesting a significant change with temperature in the membrane pathway in the root water uptake system.