Photosynthetic nitrogen utilization of Robinia pseudoacacia, an invasive species, grown in contrasting light conditions: A leaf scale approach

Robinia pseudoacacia (black locust), an alien species in Japan, has been commonly planted to rehabilitate degraded land and as a resource for the production of honey. Although very few R. pseudoacacia adult trees are found in established mature forests, the management of R. pseudoacacia is a vital and urgent matter because of astounding growth potential, which negatively affects biodiversity and native vegetation. For this reason, we evaluated the seasonal shade-acclimation capacity of black locusts from the viewpoint of photosynthetic nitrogen use (PNU) and their partitioning. We assessed in situ photosynthetic N use traits of R. pseudoacacia leaves at open sites (relative photosynthetic photon flux density: (rPPFD) > 90%) and shaded sites (rPPFD < 22%) on the forest floor in larch (Larix kaempferi Carr.) plantations. Leaf unfolding started in late May and leaves turned yellow by mid-October just before leaf shedding. Shoot length at open sites was three times longer than at shaded sites. R. psudoacacia at open sites has more the leaflet number of each pinnate compound leaf than shaded sites from mid-August to October, and the size of leaflets at shaded sites was about 20% smaller than it was at the open sites. Leaf mass per area showed (mean +/- standard deviation) 40.00 +/- 2.14 g m(-2) at the open sites and 22 +/- 1.56 g m(-2) at the shaded sites from June to October. The light-saturated photosynthetic rate peaked at 22 mu mol m(-2) s(-1) at the open sites in July. At the shaded sites, it gradually increased to 7-10 mu mol m(-2) s(-1) around August, and then decreased at both sites toward zero in mid-October. The total N content during the growth period was 1.60 +/- 0.17 g m(-2) at open sites and 1.16 +/- 0.15 g m(-2) at shaded sites. Unlike other tree species, N partitioning to photosynthetic organs was not affected by light conditions. These findings suggested R. psudoacacia cannot acclimate to shady conditions.