Does above-average rainfall stimulate a recruitment pulse in semi-arid woodlands of southeastern Australia?

Prepared by Sally A. Kenny and Claire Moxham

Seedlings, young plants and mature trees within a Slender Cypress Pine semi-arid woodland at Wyperfeld National Park, southeastern Australia. Photo credit: Sally Kenny

Without management intervention, semi-arid woodlands are at risk of decline in southeastern Australia. Semi-arid woodlands are found in the drier regions of the world. They are generally made up of several dominant tree species and a sparse understorey of various shrubs, herbs (e.g. daisies) and grasses. These plants are all adapted to dry climates and can often survive long periods of lower-than-normal rainfall or drought.

Today, the condition of many semi-arid woodlands has declined due to human land use (e.g. agricultural uses, tree removal, livestock grazing) and a drying climate. While all semi-arid woodland plants have been affected, trees are more vulnerable than shrubs, herbs or grasses as they often rely on above-average rainfall for successful seed germination and maturing into adult trees (known as recruitment). Due to poor germination and recruitment, many woodlands are now dominated by older trees. As trees age beyond their prime, they often produce less seed. This exacerbates problems by further reducing germination and recruitment in the woodland. If this cycle continues, many of these woodlands will continue to decline until humans intervene to improve survival through revegetation and herbivore management.

From late 1996 to mid-2010, southeastern Australia was in the grips of the Millennium drought. Our study examined the effect of drought-breaking rain in 2010- 2011 on the seed germination of four dominant tree species – Belah (Casuarina pauper), Buloke (Allocasuarina luehmannii), Slender Cypress Pine (Callitris gracilis) and Sugarwood (Myoporum platycarpum). We wanted to know whether rainfall stimulated a pulse of seed germination and growth of seedlings into young trees. Since we recorded low numbers of seedlings and young trees for each tree species individually, we grouped the four trees into one dataset. This allowed us to determine if rainfall, the number of adult trees, and damage from grazing animals (rabbits/hares, kangaroos, goats), influenced the number of seedlings and young trees present within these woodlands.

Our results showed that the number of seedlings and younger trees were influenced by the above-average rainfall and other factors such as the number of adult trees and grazer damage. The number of young trees was also influenced by the number of seedlings. Overall, we found limited seed germination and development into young trees following the above-average rainfall. This is because seeds may not be germinating at all or seedlings and young trees were not surviving.

Our research indicates that the long-term survival of these woodlands is in jeopardy without a concerted effort to maintain and improve their health. Land managers will need to consider planting native species (targeting trees and shrubs), removing weeds, and managing herbivores.

This is a plain language summary for the paper of Kenny and Moxham published in the Journal of Vegetation Science (