Managing forb diversity in temperate grassy ecosystems

Abstract

Native forbs contribute significantly to grassland species diversity, with flow-on benefits for habitat value (eg. for birds, reptiles, invertebrates), ecosystem services (eg. pollinator diversity, pest control) and resistance to invasion by exotic plants. Humans have modified grasslands around the world with agricultural land use and/or poor management, often resulting in significant loss of native forb diversity. Research over several decades identifies elevated soil nutrients, exotic species and changed disturbance regimes as threats to grassland forb diversity, but despite this knowledge, forb restoration remains difficult and forb diversity in temperate grasslands remains low. My research aimed to further our understanding of forb ecology and the challenges of maintaining and restoring native forb diversity, focusing on grassy ecosystems in south-eastern Australia. The research was predominantly conducted within an area of White Box – Yellow Box – Blakely’s Red Gum woodland and derived grassland in a nature reserve in Canberra, in south-eastern Australia. This ecosystem is listed as critically endangered, largely on the basis of lost ground-layer diversity.

This thesis includes an introduction (or context statement) outlining my research program and how it contributes to the literature, three chapters describing empirical research projects I conducted for the PhD, and a final chapter representing a synthesis of my results in the form of a conceptual model with management recommendations. Chapters two to five are written as manuscripts submitted or accepted for publication in scientific journals.

Chapter 2: A comparison of native and exotic forb responses to eight variables that influence forb habitat quality in temperate grasslands The second chapter (submitted) is based on a survey of the study area, in which I collected data on ground-layer vegetation, soil chemistry and soil physical properties, and topography. I analysed forb responses to eight environmental variables representing landscape, soil, and vegetation biomass. I identified thresholds of grass cover, litter cover and soil phosphorus above which native forbs are much less likely to occur than exotic forbs.

Chapter 3: Fine-scale variables associated with the presence of native forbs in natural temperate grassland The third chapter (submitted) is based on a case-control survey designed to determine the fine-scale variables associated with the presence/absence of individual native forbs. Consistent with the results from our other research, I identified that dead biomass cover, grass cover, and exotic forb cover were negatively associated with the odds that a native forb would be present in temperate grassland.

Chapter 4: Seed addition and biomass removal key to restoring native forbs in degraded temperate grassland The fourth chapter (manuscript accepted by Applied Vegetation Science) is based on a controlled field experiment designed to test hypotheses arising from our previous results. I applied treatments that reduce above ground biomass (grass tussocks, leaf litter, and exotic species) and added seed of 14 native forb species. Native and exotic forb seedlings were both negatively affected by living grass cover, however, native forb seedlings were more restricted by litter than exotic forb seedlings. Our results suggest that naturally occurring seed in disturbed grassland is likely to be inadequate for restoration.

Chapter 5: Maintaining or restoring forb diversity in temperate grasslands The fifth chapter (submitted) provides a synthesis of our PhD results as a conceptual model of habitat suitability for native forbs in relation to available phosphorus and biomass cover. I propose some thresholds for these variables and discuss appropriate restoration actions. This chapter targets practitioners aiming to conserve native diversity in temperate grasslands. Show more