Seedling establishment and growthSeedling establishment and growth

Seedling establishment and growth

Seedlings have been identified in the wild population by the presence of cotyledons and cotyledon scars. They occur in the wild on a variety of substrates including rocks, logs, tree ferns and in the soil litter layer (W. Jones, J. Allen, field obs.). Seedlings and juveniles occur in all four wild Wollemi Pine stands. It appears that the Wollemi Pine maintains a slow growing juvenile bank, as is common in rainforest trees, and other Araucariaceae (Zimmer et al. 2014). Seedlings germinate in most years, but most of these seedlings die during their first year. Potential explanations for this mortality include fungal attack and soil pathogens, adverse microclimates (too dry, too hot) and herbivory. However, once seedlings become established their survival rates are high, although growth rates are low.

Light mediates the growth of seedlings under glasshouse conditions, with best growth rates in cultivation between 25-75% ambient light (Offord et al. 2014). Glasshouse trials demonstrated increasing growth rates from 5%, 15% to 50% sunlight (Offord et al. 2014). Growth rates of seedlings in the wild are very slow, with less than < 1 cm growth in height per year (Zimmer et al. 2014), compared with up to 60 cm per year under glasshouse conditions (Offord et al. 1999) and in translocated seedlings (Mackenzie et al. unpubl. data). This can, to some extent, be attributed to light availability. However, Wollemi Pine can exhibit symptoms of photoinhibition (sunburn) and are susceptible to heat stress (Offord 2011). High growth rates of Wollemi Pine in cultivation suggest that Wollemi Pine seedlings in the wild are resource-limited by light, nutrients and rooting space. Increases in availability of these resources may coincide with canopy gap openings (such as via treefall) (Zimmer et al. 2014). The understorey of wild Stand 1 receives about 3% full sunlight and, in cultivation, increasing growth has been recorded in up to 50% full sunlight (Offord et al. 2014). Resource availability may play a role in limiting the transition from juvenile to mature tree, although fire return intervals are also likely to be key (Mackenzie et al., unpubl. data).