The effect of management operations on the demography of Eucalyptus globulus seedlings

Abstract

There has been an increased concern about the invasiveness of Eucalyptus globulus Labill. Much research has been devoted to understanding the factors influencing E. globulus recruitment. Standard management operations, performed inside and outside plantations, cause profound disturbances on soil and vegetation, potentially influence E. globulus recruitment, but have been poorly studied. This study, therefore, aims to understand the impacts of standard management operations on seeds and capsules shed prior or after such operations, and, together with weather, their influence on the germination and early survival of E. globulus seedlings. We performed a sowing experiment in Portugal using E. globulus seeds, following a full factorial design with 16 treatments, resulting from three factors: management operations (burning, soil stripping, soil harrowing and control); sowing time (before or after management operations); sowing type (sowing seeds or capsules). Each treatment was replicated 10 times, giving a total of 160 plots (0.5 × 05 m), each sown with 100 seeds or 10 capsules. Seeds and capsules were monitored for approximately one year. We also tested for the presence of seed predation close to the experimental field. Seed sowing resulted in 4.8% germination (379 seedlings), but capsule sowing resulted in only two seedlings. The seed predation experiment, showed that nearly half of the E. globulus seeds went missing, suggesting that predation may contribute to the low germination results. <1.5 months after sowing, 96.3% of the total seedlings had emerged. A Multivariate Regression Tree and a Generalized Linear Mixed Model showed that the number of emerged seedlings was enhanced in plots sown with seeds, and after the management operations, particularly harrowing. Sowing before burning resulted in limited germination. The Cox Model showed that the survival probability was greatest for seeds shed after the soil treatments and in harrowing plots. A Linear Mixed Model showed the negative influence of soil harrowing and the positive effect of crop evapotranspiration on seedling mortality across time. This study shows that E. globulus seeds are able to germinate in distinct soil/vegetation management scenarios, even though some types of management operations, such as soil harrowing, may provide better conditions for germination and survival. Keeping dense competing vegetation is probably the most cost-effective option to minimize unwanted E. globulus recruitment and maximize seedling mortality.