Limits to recruitment of a rare conifer: Wollemia nobilis

Abstract

Wollemia nobilis (Wollemi pine, Araucariaceae) is an Australian conifer and one of the world’s rarest trees, with a known population of 83 mature trees and 200-300 seedlings in the wild. Wollemia nobilis is a high-profile threatened species because of its discovery near a major capital city (Sydney) in 1994. My research began with the observation that the W. nobilis population had few individuals between the seedling and canopy tree size classes (i.e., between 2-20 m in height), and questions about potential recruitment limitation in W. nobilis.

I found that inter-annual variation in seed production, estimated from photographs of W. nobilis mature tree canopies, was not likely to limit the recruitment of new seedlings (Chapter 2). Through monitoring W. nobilis seedlings in the wild (Chapter 3), I observed that new seedlings germinated each year, but most of these seedlings died soon after germination (65% of seedlings lived less than one year) and (44% of established juveniles survived the entire 16-year monitoring period). However, for the remaining seedlings, survival rates were much higher, although growth rates remained low. Shade-tolerant trees, such as W. nobilis, commonly need increased light to grow rapidly. Tree rings from established W. nobilis suggested rapid growth from early establishment and greenhouse studies showed W. nobilis increased growth with increased light. It is likely that canopy gap creation is required for increased growth of W. nobilis seedlings in the wild.

Fire and drought are key threats to the survival of established W. nobilis seedlings. Through ex situ burning experiments I found that W. nobilis could resprout after fire, providing further evidence to challenge to the historical idea that all rainforests are fire sensitive (Chapter 4). Alternatively, W. nobilis was intolerant of prolonged drought, compared with other species from the Araucariaceae family (Chapter 5).

To extend my knowledge of W. nobilis recruitment, I then set about establishing a new W. nobilis population (Chapter 6). I planted 191 W. nobilis into a new site, similar to the wild site, but with a greater range of light availabilities. Two years after planting I found that survival, but not growth, was improved at higher light sites. Monitoring is ongoing. The life history of W. nobilis (particularly slow growth) in conjunction with its environment (deeply shaded rainforest) results in infrequent recruitment – similar to many threatened conifers. Wollemia nobilis differs from most threatened conifers in that it is not affected by human and land-use change threats, such as timber harvesting. However, humans can play a role in the protection and recovery of W. nobilis, such as through the active establishment of new W. nobilis populations in the wild. Beyond its scientific value as a relict population, the high profile of W. nobilis and its potential to raise awareness of biodiversity issues provide further impetus for maintaining W. nobilis population persistence in the wild.