ALBERT

Description: The pipe model theory presents the idea that a unit weight of tree foliage is serviced by a specific cross-sectional area of conducting sapwood in the crown. Below the crown, a large fraction of the tree bole may be nonconducting tissue, so the sapwood area would have to be known to estimate foliage. We applied the pipe model theory to the analysis of several western coniferous species to learn whether the distribution of canopy leaf area could be accurately estimated from knowledge of the sapwood cross-sectional area at various heights, including breast height (1.37 m). Results are excellent, but taper in the conducting area must be considered when sapwood area is measured below the crown.

Description: Stomatal conductance was measured with porometers in two plots of Pinussylvestris L. with markedly different tree spacings (plot 1, 608 stems ha−1; plot 2, 3281 stems ha−1), and hourly rates of transpiration were calculated using the Penman–Monteith equation at intervals throughout one growing season. Stomatal conductance varied little in relation to height or age of foliage. There was a linear decrease in canopy conductance with increasing water vapour pressure deficit of the air. Transpiration rates on both plots increased during the summer (maximum 0.3 mm h−1); rates on plot 1 were always lower (ca. 0.7 times) than on plot 2. Needle water potentials were similar throughout the season and only slightly lower on plot 1 than on plot 2. The mean hydraulic resistance of the trees on plot 1 was 2.4 times that on plot 2. The results support a hypothesis that considers the changes in transpiration rate, conducting cross-sectional area, canopy leaf area, water potential, and hydraulic resistance following thinning as a set of homeostatic relationships.

Description: Total leaf area varied from 20 to 42 m2/m2 in 250- to 450-year-old forest communities developed under different temperature and moisture conditions. The largest values were in communities at midelevations where winter snowpack accumulated and growing-season temperatures were cool. Shrub and herb leaf area varied from 3% to 14% of the total. Equations for converting from foliage biomass to surface area are included for most species encountered.

Description: The Penman–Monteith equation was used with measured values of stomatal conductance and leaf area, in conjunction with weather station measurements of net radiation, vapor pressure deficit, and wind speed to calculate the transpiration rates of two Scots pine (Pinussylvestris L.) stands of widely different densities. Transpiration was compared with water uptake estimated from 32P radioisotope movement through trees of known conducting area and water content. Uptake consistently lagged behind transpiration throughout the day. From dawn to dawn, however, uptake and transpiration were in close agreement as a result of recharge during the night.

Description: October-fertilized and unfertilized 2-0 Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings were outplanted the following February. Half of each planting block was seeded with grass to induce water stress during the typical summer drought. Sucrose was applied to soil around each seedling to limit availability of nitrogen to tree roots. Fertilized seedlings broke bud 9–10 days earlier, produced more shoot growth, and, as shown in later harvests, had higher relative growth rates than unfertilized seedlings. However, initial differences in growth response were due primarily to the earlier budbreak. Seedlings growing with grass had predawn water potentials of −1.5 MPa by early August; by September 3, unfertilized seedlings growing with grass were significantly more stressed than any others. Although free amino acid and total nitrogen concentrations were higher in fertilized than unfertilized seedlings when planted, they became equal by the end of one growing season. However, fertilized seedlings contained more free amino acids and nitrogen because of their greater size. Grass competition affected both seedling nitrogen and carbohydrate chemistry. After one growing season, fertilized seedlings had greater height increment, shoot growth, leaf area, relative growth rate, and production per unit nitrogen. Although autumn fertilization benefited these Douglas-fir seedlings, negative effects could result from carbohydrate depletion because of increased respiration or from frost damage because of earlier budbreak.

Description: To determine how the longevity of fine roots (those without secondary thickening) is controlled, shoots of Douglas-fir (Pseudotsugamenziesii Mirb. (Franco)) seedlings were exposed to light or maintained in darkness while roots were maintained at 10, 20, or 30 °C. Fine root maintenance respiration rates, estimated from rates of starch and sugar depletion in the seedlings maintained in darkness, ranged from 0.83 to 3.25 mg starch g dry weight−1 day−1. At 20 and 30 °C, starch deposition was curtailed and previously deposited starch was used to maintain the older roots, whether current photosynthate was entering the root system or not. On the other hand, at 10 °C starch was deposited in the roots whenever the root systems grew. Based on these results, we suggest that starch deposition in a fine root occurs only when the root is being formed and the root carbon balance is positive. Starch is subsequently respired to meet maintenance requirements exclusively. A simple means of estimating root biomass production and turnover based on root starch and soil temperature is described and compared with field estimates.

Description: To evaluate whether changes in leaf weight might affect the interpretation of foliar analyses, seasonal samples of current and older Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) needles were compared from contrasting environments in the Coast Range and Cascade Mountains of western Oregon, U.S.A. Foliage from trees grown in the climatically mild and moist Coast Range maintained nearly constant weight and nitrogen content per unit of leaf area throughout the year, regardless of whether stands were thinned or left unthinned. In contrast, foliage from trees growing in the harsher environment of the Cascade Mountains averaged only half the weight per unit of leaf area of samples from the Coast Range during the dormant season. During the summer, foliage weight and nitrogen content per unit of leaf area increased by more than 50% in the Cascades. These values were 20–40% less, respectively, than those observed in the foliage of Coast Range trees. In the Cascades, thinning significantly increased both foliage weight and nitrogen content. Implications of these findings are discussed in regard to interpreting foliar analyses.

Description: Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings at a western Oregon nursery were fertilized in October 1983 with ammonium nitrate and harvested for biochemical analyses on four dates over autumn and winter 1983–1984. Free amino acid and total nitrogen concentrations in the needles of fertilized seedlings showed a pronounced increase 1 month after fertilization. Free amino acid concentrations of fertilized seedlings decreased in needles during winter but remained stable in stems and fine roots. Just before budbreak in mid-March, free amino acid concentrations increased significantly in stems and fine roots. Total nitrogen concentrations increased 1 month after fertilization, remained stable throughout winter, and tended to decrease or remain stable just before budbreak. Starch and total nonstructural carbohydrate concentrations of needles and stems of fertilized seedlings were lower just before budbreak and sugar concentrations of fine roots of fertilized seedlings were lower when data from all harvests were combined. The reduction in carbohydrate reserves following fertilization probably reflects increased respiration associated with the synthesis and maintenance of higher levels of enzymes.

Description: Increasing stomatal closure was exhibited by two conifer and six broadleaf species as vapor pressure deficit increased. Conifers were more sensitive to high vapor pressure deficit than were the broadleaved species. One shrub, snowbrush (Ceanothusvelutinus Dougl. ex Hook.), exhibited no stomatal closure as vapor pressure deficit increased. These traits, when interpreted in terms of known soil moisture depletion patterns, help explain why broadleaved species initially colonize disturbed areas in western Oregon, but are later replaced by long-lived conifers.

Description: The 3-PGS (physiological principles for predicting growth using satellite data) model generates monthly estimates of transpiration, photosynthesis, and net primary production (NPP), the latter derived as a fixed proportion (0.47) of gross photosynthesis. To assess the reliability of a simplified process model (3-PGS) to predict the productive capacity of coniferous forest across diverse landscapes in southwestern Oregon, we first used a geographic information system to display and manipulate basic data. This involved the following steps: (i) extrapolate monthly mean weather data to reflect topographic variation; (ii) transform monthly temperature extremes to spatial resolution of 4 ha and estimate incoming solar radiation, subfreezing days per month, daytime vapor pressure deficits, and mean temperatures; (iii) convert statewide soil survey maps into topographically adjusted estimates of soil fertility and water storage capacity (θ); and (iv) acquire satellite-derived estimates of the faction of light intercepted by vegetation during midsummer. Model predictions of soil water availability during summer months compared well with those reported from published measurements of predawn water potentials at three contrasting sites and with measurements acquired at the end of seasonal drought at 18 sites (r2 = 0.78 with mean monthly modeled drought index; r2 = 0.57 with seasonal modeled drought index). Similarly, seasonal shifts in the relative importance of various climatic and edaphic variables closely matched those defined in previously published studies. Finally, model predictions of maximum annual aboveground growth were compared with those derived from forestry yield tables based on height-age relationships with a resulting r2 of 0.76, and a standard error of 1.2 m3·ha-1·year-1 (P 〈 0.01).