Effects of irradiance and nitrogen on Clematis vitalba establishment in a New Zealand lowland podocarp forest remnant

Abstract

The influences of light and nitrogen (N) on growth and physiological characteristics of C. vitalba seedlings were investigated in a New Zealand lowland podocarp forest remnant. Seedlings planted within undisturbed forest did not persist, even when supplied with additional nutrients, whereas seedlings planted outside the forest not only persisted but, with additional nutrients, achieved substantial growth. Under controlled conditions with irradiance over a range from full sunlight (100% I r) to 1% full sunlight (1% I r), seedlings achieved maximum growth at 100% I r, substantial growth as low as 10% I r , and little growth at 3% I r. Seedlings at 1% I r did not survive. The influence of irradiance on seedling growth is compared with light acclimation characteristics; seedlings at low compared with high irradiances had a lower chlorophyll a:b ratio, lower concentration of total carotenoids and soluble protein per unit leaf area and chlorophyll, and a greater shoot:root (S:R) ratio, specific leaf area (SLA), chlorophyll concentration per unit leaf area and dry mass, and xanthophyll cycle pigments (V+A+Z) and B‐carotene as a proportion of the total carotenoid pool. Limited growth or survival at irradiances <3% I r under controlled conditions and when seedlings were planted within undisturbed forest suggests that low irradiance is the primary factor limiting establishment within undisturbed parts of the forest remnant. At higher irradiances near the forest margin, a similar increase in growth when seedlings were supplied with N or N plus base (P, K, S, Ca) fertiliser suggest that N is the major nutrient limiting growth in this forest remnant. Seedlings also showed a substantial growth response to applied N (as NO3 ‐) under controlled conditions; increased growth coincided with increased nitrate reductase activity (NRA) and concentration of NO3 ‐ in plant tissue. The results are related to other work that reports on the distribution of C. vitalba in the same forest remnant and the influence of light and N on seed germination. We suggest that germination and growth in response to light and N can account for the pattern of establishment and success of C. vitalba in this native forest remnant.