Identifying thresholds and ceilings in plant community recovery for optimal management of widespread weeds

A substantial body of work underlies the theory and practice of early intervention in the management of invasive alien plants, but less attention has been paid to the strategic management of widespread weeds, especially in the context of natural asset recovery. The assumption lingers amongst some researchers and land managers that removing weeds will automatically lead to positive biodiversity outcomes, with the more weed removed, the better the outcome. However, this is often not the case, particularly for long-established weed species whose dominance has created impoverished communities with little capacity for passive recovery. A common result may be wasted investment in weed control and, in the extreme, net negative impacts upon asset values. We present a conceptual model for the management of weed-impacted assets, plus guidance for its application, with a view to improving asset recovery practice. Weed removal should be calibrated by asset recovery, which may mean not seeking to completely remove a weed at a given spatial scale. Our model focusses on weed removal that is enough to initiate asset recovery, but not more than is necessary to promote maximum expression of asset resilience, particularly in the context of secondary invasions. Optimal management efficiency will involve a proportional allocation of resources to control, monitoring and revegetation activities that is appropriate to the stage of asset recovery, as well as a willingness to revise a management goal if the original one cannot be achieved within existing constraints on resources.

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The y-axis represents a sliding scale of vegetation community condition between an average invaded state (red zone) and an average non-invaded, native reference state (green zone). The origin at point 1 represents a state 100% dissimilar to the non-invaded, native condition (e.g. a weed-dominated area that contains none or a different suite of native species to those found in non-invaded reference areas). It is assumed that a non-invaded, native state comprises higher asset condition than weed-dominated areas. Impact of invasion is measured as the magnitude of difference between the average non-invaded state (green zone) and the average invaded state (red zone), represented by interval iii.

The x-axis represents a sliding scale of weed removal. The curves A, B, C and D represent different patterns of vegetation community response to weed removal (i.e. regeneration trajectories). Vegetation community regeneration in response to weed removal will vary as a function of resilience to invasion, which is defined as the extent of recovery of the asset post invasion . Blue curves (A, B and D) represent communities with relatively high resilience to invasion, while the orange curve C represents a community with relatively low resilience (see description below). For simplicity, we have presented resistance to invasion (i.e. degree of community change in response to invasion, interval iii) as equal for all curves, hence why the level of impact is equivalent at point 1 for all curves at the maximum level of weed abundance before removal of the weed commences.

Regeneration trajectories

Curve A represents the most resilient community (the best-case scenario for managers), because the native vegetation begins to recover very soon after weed removal commences (i.e. the asset recovery threshold occurs close to point 1), any amount of weed removal facilitates native vegetation recovery, and full recovery to the reference native state is achieved. An asset recovery ceiling occurs at point 2, beyond which asset condition will not improve with any further investment in weed removal. This point can also be considered as a weed removal ceiling, the latter being causal and asset recovery its effect.

Curve B is similar initially to curve A in that removal of the weed initiates rapid, linear asset recovery near point 1. However, for curve B, the community is relatively less resilient to invasion because the response trajectory does not reach the maximum level of the desired native reference state by the time the asset recovery ceiling is reached. This results in a recovery deficit (interval i) and represents the model space in which active intervention would be required to facilitate full community recovery (e.g. planting nursery-grown seedlings or seed addition).

Curve C represents a community with a much-reduced level of resilience, since regeneration of the asset only commences after a much greater proportion of the weed is removed from the invaded site (i.e. asset recovery threshold at point 3).

Curve D represents a scenario where weed removal promotes asset recovery until a weed removal ceiling is reached, but thereafter inhibits asset recovery due to disturbance effects of the control technique being used to remove the weed. In this case point 4 represents a management impact threshold and interval ii represents the net effects of management that balances benefits of weed removal (maximum at point 4) with negative effects of control action used to remove the weed.

Panetta, F.D., O’Loughlin, L.S. & Gooden, B. (2019) Identifying thresholds and ceilings in plant community recovery for optimal management of widespread weeds. NeoBiota 42, 1-18 // PDF DOI

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