ALBERT

Description: Root elongation of greenhouse-grown Alaskan taiga tree seedlings increased with increasing root temperature in all six species examined and was most temperature sensitive in warm-adapted aspen (Populustremuloides Michx.). Root elongation was slower in fine than large roots and in black spruce (Piceamariana (Mill.) B.S.P.) was less temperature sensitive in fine than in large roots. Root elongation in the laboratory was slowest in black spruce, which has an inherently slow growth rate, and most rapid in poplar (Populusbalsamifera L.) and aspen, which grow more rapidly. In contrast, field root elongation rates tended to be highest in black spruce from cold wet sites, suggesting that site factors other than soil temperature (e.g., moisture) predominated over genetic differences among species in determining field root elongation rates. The seasonal pattern of root elongation was closely correlated with soil temperature and reached maximum rates in July for all tree species (except aspen medium-sized roots). Most roots of each species were in the top 20 cm of soil. However, root growth penetrated to greater depth in warm compared with cold sites. Root biomass in a 130-year black spruce forest (1230 g/m2) comprised only 15% of total tree biomass. Root biomass of 25-year aspen and 60-year poplar sites (517 and 5385 g/m2, respectively) comprised a greater proportion (57% in poplar) of total tree biomass than in spruce.

Description: Seedlings of Alaskan floodplain species (Populusbalsamifera L. (balsam poplar), Alnustenuifolia Nutt. (thinleaf alder), and Piceaglauca (Moench) Voss (white spruce)) and an upland species (Populustremuloides Michx. (trembling aspen)) were grown in early-successional floodplain soils treated with a floodplain salt (calcium sulfate, CaSO4), an osmoticant (polyethylene glycol), and nitrogen. CaSO4 reduced the growth of aspen relative to controls but also reduced the growth of some typical floodplain colonizers (alder at low nitrogen and poplar at high nitrogen). Aspen and poplar were the most rapidly growing species, even when grown with salt or polyethylene glycol. Effects of CaSO4 on growth, therefore, do not explain why aspen is less abundant on the floodplain than are typical floodplain colonizers. CaSO4 reduced growth directly in salt-sensitive species, judging from the insensitivity of water potential, transpiration, and photosynthesis to CaSO4 addition. Tissue concentrations of nitrogen and phosphorus were unaffected by CaSO4, suggesting that the declines in nutrient accumulation by salt-sensitive species in response to CaSO4 addition reflected a decline in nutrient demands for growth rather than being the cause of the reduction in growth. Growth and nutrient accumulation were stimulated by nitrogen addition in all species. We suggest that floodplain salts may be important in succession by slowing the establishment and growth of alder, which is responsible for most of the nitrogen acquired by plants during succession.

Description: Seedlings of six Alaskan taiga tree species and one tall shrub were grown in sand at three phosphate levels. There was a positive correlation between the growth rate of a species at the high-phosphate level in sand culture and its productivity in the natural environment. Poplar (Populusbalsamifera L.), which had highest growth rate under high phosphate, was most sensitive to reduction in phosphate supply, followed by birch (Betulapapyrifera (Reg.) Fern, and Raup) and aspen (Populustremuloides Michx.), whereas growth of conifers (larch (Larixlaricina (Du Roi) K. Koch), white spruce (Piceaglauca (Moench) Voss), and black spruce (P. mariana (Mill.) B.S.P.)) from late successional sites was slow and unaffected by phosphate supply. Similarly, when birch and white spruce seedlings were transplanted into natural forest stands, the maximum growth rate of birch was greater than that of white spruce, but birch growth was curtailed more by unfavorable conditions than was that of white spruce. We conclude that a slow growth rate reduces nutrient requirement and therefore minimizes nutrient stress on infertile sites, whereas a rapid growth enables nutrient-demanding species to dominate fertile sites.

Description: Seasonal patterns of biomass, nitrogen (N), and phosphorus (P) were determined for major plant parts of the deciduous shrub Vacciniumuliginosum L. and the evergreen shrub Ledumgroenlandicum Oeder. in a black spruce (Piceamariana (Mill.) B.S.P.) forest in interior Alaska. New growth comprised 52 ± 7% of aboveground biomass in Vaccinium compared with the evergreen Ledum for which a maximum of 38 ± 3% of aboveground biomass was new growth. In Vaccinium the spring decline in leaf N and P concentration was due to dilution by increasing leaf biomass, whereas the autumn decline in N and P concentration was due to retranslocation, at which time 68–72% of leaf N and P was retranslocated from leaves. In contrast, the entire decline in N and P concentration of new growth in Ledum was due to dilution by increasing leaf biomass. Uptake contributed 60–68% of the maximum N and P requirement for aboveground growth of Vaccinium, with the remainder coming from stored reserves. Ledum supported 71–79% of its aboveground nutrient requirement by direct uptake from soil and may have been less dependent upon stored nutrient reserves. Vaccinium and Ledum together comprised only 0.8–2.8% of the standing crop of aboveground vascular biomass and N and P pools at Washington Creek but contributed 16% of vascular aboveground production and 19–24% of the N and P cycled annually by vascular plants. The importance of understory shrubs is due to their small support structure and rapid turnover of biomass and nutrients (34–43% of aboveground pools annually) relative to that of the trees (2–5% annually). Understory shrubs at Washington Creek and in other evergreen forests are much more important in nutrient cycling than their small biomass would suggest.

Description: Four evergreen and four deciduous trees and shrubs were sampled from habitats with differing soil temperature regimes in interior Alaskan forests to examine the relative importance of habitat and leaf habit in determining seasonal patterns of shoot growth, tissue nutrient concentration, respiration rate, and phosphate absorption rate. Leaf habit was the primary determinant of shoot growth, with deciduous species producing leaf area and leaf biomass earlier in the season than evergreens. Deciduous trees produced more biomass per shoot and per unit ground area than did evergreens. The seasonal pattern of leaf nitrogen and phosphorus concentration was correlated closely with patterns of leaf growth, declining through the growing season in deciduous species first as nutrient concentrations were diluted by increasing leaf biomass and later as nutrients were retranslocated from senescing leaves. In evergreens the seasonal decline in nutrient concentration was entirely due to dilution by increasing leaf biomass, and there was no evidence of autumn retranslocation from 1st-year leaves. In contrast to seasonal pattern, the magnitude of leaf phosphorus and root nitrogen and phosphorus concentrations was correlated more closely with habitat than with leaf habit, generally being lower in cold sites. Leaf respiration was highly correlated with leaf nitrogen concentration, so that the seasonal pattern of leaf respiration was determined primarily by leaf habit, whereas the magnitude of respiration was more closely correlated with habitat. Root respiration showed no consistent correlation with either habitat or leaf habit but was lower than leaf respiration, as would be expected from low root nitrogen concentration. Phosphate absorption rate was determined more strongly by habitat than by leaf habit, being lower in cold sites characterized by slow plant growth and consequently low annual nutrient requirement. Evergreen species were more effective at absorbing phosphate at low solution concentrations than were deciduous species. Phosphate absorption was less temperature sensitive than root respiration, so that roots of all species absorbed more phosphorus per unit of carbon respired at low root temperature.

Description: Xylem (water) pressure potential was measured through one winter in evergreen black spruce (Piceamariana (Mill.) B.S.P.) and deciduous larch (Larixlaricina (Du Roi) K. Koch) in Fairbanks, Alaska. Larch values averaged about −1.0 MPa, with occasional dips to −1.5 MPa. Black spruce showed similar values until May, when values dropped to −2.5 MPa. Regression models indicate that desiccation of black spruce responds primarily to cumulative vapor pressure deficit (drought), which becomes severe as spring daylight rapidly increases (R2 = 80%). In larch, the effect of cumulative drought was offset by increased spring air temperatures.