Seasonal biochemical changes in coniferous forest canopies and their response to fertilization

Abstract

Seasonal changes in concentrations of total nitrogen, free amino acids, chlorophyll, starch and sugar were measured in foliage from fertilized and unfertilized conifer forests in New Mexico and Oregon. In the New Mexico Douglas-fir (Pseudotsuga menziesii var glauca (Beissn.) Franco) forest, fertilization resulted in elevated foliar nitrogen concentrations on all dates, from an average of 9 mg g(-1) in unfertilized trees to 14 mg g(-1) in fertilized trees. In the Oregon western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest, fertilization increased total N by only 15%, from 13 mg g(-1) in unfertilized trees to 15 mg g(-1) in fertilized trees. Foliar nitrogen concentrations on a weight basis were lowest in winter and spring, but did not vary seasonally when expressed on a leaf area basis. Chlorophyll concentrations increased with fertilization and had greater seasonal variation than did total nitrogen concentrations. Chlorophyll concentrations were significantly higher during the growing season than in the winter and spring months. Fertilization did not result in major changes in the proportion of total nitrogen in chlorophyll at either the Oregon or the New Mexico site. Concentrations of free amino acids varied with date and fertilization treatment; in New Mexico, amino acids were highest in the winter sample, whereas in Oregon, they were lowest in winter and spring. At both sites, amino acid concentrations were significantly higher in fertilized trees than in control trees on most dates and the ratios of amino acid-N to total N were also significantly higher in fertilized trees. For both sites, starch concentrations were nearly zero for most of the year, but increased sharply just before bud break and initiation of new growth in the spring. Although fertilization resulted in increased nitrogen concentrations in foliage at both sites, the response in New Mexico was much greater than in Oregon. These results are in agreement with forest productivity data that suggest that growth in the New Mexico site is limited by nitrogen, whereas in the Oregon site it is not.

Does turgor limit growth in tall trees?

“…When buds swell in Douglas‐fir, foliar sugars are metabolized more rapidly than they are produced and therefore tend to be limited (Krueger & Trappe 1967; Billow, Matson & Yoder 1994). The rapid decline in osmotic solute concentration of previous years’ foliage of Douglas‐fir and other evergreen conifers observed during early spring may be a manifestation of this phenomenon and suggests that sugars and other soluble carbohydrates may comprise a significant fraction of cellular osmolytes.…”

“…By July, when needles are normally fully expanded, symplast solute content was greater at all heights and increased with height, resulting in a doubling of solute content at 55.6 m between May and July. Billow et al . (1994) found that sugar concentrations of unfertilized Douglas‐fir foliage nearly doubled between May and October…”

Greenness indices from digital cameras predict the timing and seasonal dynamics of canopy‐scale photosynthesis

“…Even though ENF sites exhibited well-defined seasonality in greenness, GCC was only 533 weakly correlated to GPP at these sites. Conifers undergo seasonal changes in chlorophyll 534 content, with winter minima approximately 40% lower than summer maxima (Billow et al 1994, 535…”

Responses of Early Successional Northern Hardwood Forests to Changes in Nutrient Availability

“…Although the higher foliar N concentrations might be associated with higher activity of RuBP carboxylase, which should stimulate light-saturated (darkreaction-limited) photosynthesis (Evans 1989), it is possible that much of the additional N in leaves was in the form of amino acids (Billow et al 1994). Although the higher foliar N concentrations might be associated with higher activity of RuBP carboxylase, which should stimulate light-saturated (darkreaction-limited) photosynthesis (Evans 1989), it is possible that much of the additional N in leaves was in the form of amino acids (Billow et al 1994).…”