Recruitment limitation in dry sclerophyll forests: Regeneration requirements and potential density-dependent effects in Eucalyptus tricarpa (L.A.S. Johnson) L.A.S. Johnson & K.D. Hill (Myrtaceae)

Abstract

Recruitment limitation in canopy trees is receiving increasing attention as restoration of tree cover for connectivity, biodiversity offsets, carbon-trading and improved catchment health becomes more prominent. Recruitment limitation is often addressed by examining seed traits and germination requirements. Distance between trees is also often explored as a factor, particularly in agricultural landscapes where forest structure has been altered, with large distances between fragments generating Allee effects linked to pollen limitation. Fewer studies have examined how short distances between trees (high stem densities), which can characterize regenerated forests, might affect recruitment. This study examined recruitment limitation by exploring germination requirements of Eucalyptus tricarpa (red ironbark) and the effects of stem density on seed characters and germination. Eucalyptus tricarpa is a canopy species in the box-ironbark forests of central Victoria, Australia. During the early European settlement phase in the 1800s these forests were heavily impacted by human disturbances including mining, logging and livestock grazing. In many regenerated stands, now more than 100 years old, current stem density exceeds those documented in 19th century reports and there is little regeneration of key canopy species such as E. tricarpa. In laboratory trials seed germinated at 18°C in darkness, and while viability varied among populations, overall it remained high (91-96%) and did not differ among populations with different stem densities. An in situ germination trial addressing sowing season, ground-cover, soil-ripping, canopy-cover and stem density recorded no germination. High stem density in coppicing box-ironbark forest was hypothesized to limit resources for reproduction; however, no relationship was found between nearest neighbour distance, allocation to selected reproductive structures or seed viability. The relative insensitivity of reproductive allocation to intraspecific competition (high stem density) may be interpreted as a trait linked to unpredictable environments and consistent with long-lived species. In terms of management, recruitment limitation in E. tricarpa appears not primarily attributable to seed viability, tested seedbed treatments or stem density so further factors, including fire, now require exploration.

Environmental effects on germination phenology of co-occurring eucalypts: implications for regeneration under climate change

“…Our results highlight that these two species are vulnerable to climate change as they may be unable to regenerate in the areas investigated in this study. Our outcomes for E. tricarpa are supported by Orscheg et al (2011) who found that recruitment of E. tricarpa is restricted and climate change may have adverse effects on its germination. The rapid decline in establishment potential for E. tricarpa is also supported by Byrne et al (2013) who found that the species can only perform well under a narrow range of conditions and Newell et al (2009) who predicted that the distribution of E. tricarpa may decline under climate change and that the species may be lost from most of its current distribution.…”

“…Six Eucalyptus species were selected from dry and wet sclerophyll forest types located in the temperate region of Victoria, south-eastern Australia. The co-occurring Symphyomyrtus (subgenera) species E. microcarpa (grey box), E. polyanthemos (red box) and E. tricarpa (red ironbark) represent the dry open 'box-ironbark' of the warm temperate region of Victoria (Newman 1961;Orscheg et al 2011). Annual rainfall in this region ranges from 400 to 1000 mm and the average monthly maximum/minimum temperature range from 23 to 33 °C/ -1 to 7 °C, respectively (Boland et al 2006).…”

Herbarium records identify sensitivity of flowering phenology of eucalypts to climate: Implications for species response to climate change

“…The co‐occurring species E. microcarpa (grey box), E. polyanthemos (red box) and E. tricarpa (red ironbark) represent the dry open forests of the warm temperate region of Victoria (Newman ; Orscheg et al . ). Annual rainfall in this region ranges from 400 to 970 mm and the maximum/minimum temperature range from 23°C to 33°C/−1°C to 4°C (Boland et al .…”

Environmental effects on growth phenology of co-occurring Eucalyptus species

“…Six Eucalyptus species were selected from both dry and wet sclerophyll forest types located in the temperate region of Victoria, south-east Australia. The co-occurring species E. microcarpa (grey box), E. polyanthemos (red box) and E. tricarpa (red ironbark) represent the dry open "box-ironbark" sclerophyll forest of the warm temperate region of Victoria (Newman 1961;Orscheg et al 2011). Annual rainfall in this region ranges from 400 to 970 mm and the maximum/minimum temperature ranges from 23 to 33°C / -1 to 4°C respectively (Boland et al 2006).…”

“…Growth of E. tricarpa may benefit under a defined range of temperature increase but if drier conditions arise then the species may be outcompeted by E. polyanthemos. The recruitment of E. tricarpa in Victorian "box ironbark" forests has recently been found to be highly restricted (Orscheg et al 2011). Combining this with the species limited growth under increasing temperatures, decreased moisture availability and lower air humidity(all predicted to occur due to climate change), suggests that E. tricarpa is at risk of being out competed by its current co-occurring species due to their divergent growth phenologies.…”