Empirically validating a dense woody regrowth ‘problem’ and thinning ‘solution’ for understory vegetation

Abstract

a b s t r a c tIn landscapes with a short history of intensive land use, woody plant regrowth on cleared land is often favorably received as a shift back to a more natural state. However, it is common for these regrowth stands to be much denser than undisturbed forest. High stem density can adversely affect stand structure, understory composition, and habitat for dependent fauna. Thinning to reduce stem density is one common silvicultural method used to manage dense stands for ecological or restoration objectives. The effect of thinning on the stand structure is well understood but those on the understory vegetation are not. We address this knowledge gap in anticipation of an increasing call for public investment in 'ecological' thinning across public and private land. Our case study is from the eucalypt woodlands and forests of central Victoria, Australia, an ecosystem in which dense woody regrowth is common. From a broad survey of 98 sites, spanning a range of stem densities, we explored the effect of density on understory vegetation. High densities of small trees (<20 cm DBH) caused the greatest suppression of native and exotic cover and species richness. We compared our observations with benchmarks and found that sites with stem densities exceeding their benchmark had median values approximately one-seventh of the benchmark native understory cover, which was also less than a quarter of the cover of those sites with benchmark or lower stem density. We conducted an additional targeted survey of 11 thinned sites paired with non-thinned sites to evaluate the effects of thinning. We built models combining broad and targeted survey data relating understory response to stem density, thinning, land tenure and environmental covariates. These models predicted that thinning is likely to elicit positive responses from the understory plant community in the short term. This is the desired response from native species, but we caution that thinning can equally favor exotic plant species.

High-severity wildfires in temperate Australian forests have increased in extent and aggregation in recent decades

“…Resprouters' seed viability decreases with fire intensity [113] and therefore regeneration in high-severity patches may depend on dispersal from adjacent moderate-severity or unburned patches (although see [20] indicating prolific regeneration from seed of resprouter eucalypts after a single high-severity wildfire). Increases in highseverity patch size though aggregation as observed in this study could hinder post-fire tree establishment by increasing distances from seed source and also altering the regeneration abiotic environment [114] contributing to feedbacks that result in an increased risk of forest-type conversion [115,116]. Spatial configuration of high-severity patches can also influence regeneration of obligate seeder forests burnt by recurrent fires in quick succession (~20 years; [103]).…”

Spatially designed revegetation—why the spatial arrangement of plants should be as important to revegetation as they are to natural systems

“…For example, modeling by Vesk et al () found that recruitment was nearly three times more likely for trees and five times more likely for shrubs in low density (<250 stems/ha) compared to higher density sites (>250 stems/ha) in revegetated woodlands in southeastern Australia (Vesk et al ). However, there is a trade‐off between maximizing regeneration and growth of understory species and managing weeds (Jones et al ).…”

The value of information for woodland management: updating a state–transition model

“…The aim of the trial was to investigate whether ET could be used to restore structural diversity of habitat types and the functioning and persistence of key communities and species populations (Pigott et al 2010). Short-term observations imply that a diverse shrubby understory is enhanced under a program of ET (Jones et al 2015).…”

“…Short‐term observations imply that a diverse shrubby understory is enhanced under a program of ET (Jones et al. ).…”