Needle longevity, shoot growth and branching frequency in relation to site fertility and within-canopy light conditions in Pinus sylvestris

Abstract

Changes in needle morphology, average needle age, shoot length growth, and branching frequency in response to seasonal average integrated quantum flux density (Qint) were investigated in Pinus sylvestris L. in a fertile site (old-field) and an infertile site (raised bog). In the fertile site, the trees were 30 years old with a dominant height of 17-21 m, and with average ± SD nitrogen content (% of dry mass) of 1.53 ± 0.11 in the current-year needles. In the infertile site, 50 to 100-yr-old trees were 1-2 m tall, and needle N content was 0.86 ± 0.12%. Relationships between the variables were studied using linear correlation and regression analyses. With increasing irradiance, shoot length (Ls) and shoot bifurcation ratio (Rb, the number of current-year shoots per number of shoots formed in the previous year) increased in the fertile site, but not in the infertile site. Despite greater branching frequency, apical control was enhanced at higher irradiance in the fertile site. The shoot length distributions became more peaked (positive kurtosis) and biased towards lower values of Ls (positive skewness) with increasing Qint in this stand. The shoot distributions were essentially normal in the infertile site. Large values of Rb combined with the skewed distributions of shoot length resulted in conical crowns in the fertile site. In contrast, lower bifurcation ratio, normal shoot length distributions and low rates of shoot length growth led to flat-topped crowns in the bog. Average needle age was independent of Qint, but was larger in the infertile site. Thus, reduced rates of foliage production in the infertile site were somewhat compensated for by increased foliage longevity, and we suggest that shoot growth rates may have directly controlled the needle life span via reduced requirements for nutrients for the growth and via reduced self-shading within the canopy. Needle age and Qint independently affected needle structure. Needle age only moderately altered needle nutrient contents, but the primary age-related modification was the scaling of needle density with age. The density was similarly modified by age in both sites, but the needles were denser in the infertile site. Given that denser needles are more resistant to mechanical injury, larger density may provide an additional explanation for enhanced longevity in the infertile site. Our study demonstrates that site fertility is an important determinant of the plastic modifications in crown geometry, and needle life span in P. sylvestris.