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

The influences of light and nitrogen (N) on growth and physiological charac teristics of C. vitalba seedlings were investigated in a New Zealand lowlan d podocarp forest remnant. Seedlings planted within undisturbed forest did not persist, even when supplied with additional nutrients, whereas seedling s planted outside the forest not only persisted but, with additional nutrie nts, achieved substantial growth. Under controlled conditions with irradian ce 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 a s 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 acc limation characteristics; seedlings at low compared with high irradiances h ad 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 bet a-carotene as a proportion of the total carotenoid pool. Limited growth or survival at irradiances <3% I-r under controlled conditions and when seedli ngs were planted within undisturbed forest suggests that low irradiance is the primary factor limiting establishment within undisturbed parts of the f orest remnant. At higher irradiances near the forest margin, a similar incr ease 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 thi s forest remnant. Seedlings also showed a substantial growth response to ap plied N (as NO3-) under controlled conditions; increased growth coincided w ith increased nitrate reductase activity (NRA) and concentration of NO3- in plant tissue. The results are related to other work that reports on the di stribution of C. vitalba in the same forest remnant and the influence of li ght and N on seed germination. We suggest that germination and growth in re sponse to light and N can account for the pattern of establishment and succ ess of C. vitalba in this native forest remnant.