Soil and vegetation response to thinning White Cypress Pine (Callitris glaucophylla) on the North Western Slopes of New South Wales, Australia

Abstract

Dense White Cypress Pine (Callitris glaucophylla J. Thompson and L.A.S. Johnson) regrowth occurs frequently across previously cleared landscapes in New South Wales (NSW), and is thought to adversely affect agricultural production and to cause land degradation. The NSW Native Vegetation Act (2003) requires that management of native vegetation including pre-1990 regrowth must 'improve or maintain' site condition, yet there is currently limited information regarding techniques for the optimum management of C. glaucophylla in this regard. We conducted a preliminary study to examine floristic composition, soil condition (to 50 cm) and carbon storage under 'Dense' (dense regrowth), 'Thinned' (dense regrowth thinned 2000/2001) and 'Un-colonised' (pasture not yet recolonised by C. glaucophylla) plots on private lands in NSW. Reduced tree density from thinning resulted in increased biomass of the remaining individual trees. Un-colonised plots had significantly more groundcover than thinned plots, which had significantly more groundcover than dense plots. Differences in plant diversity however, were explained by site factors rather than land use. Soils in the dense plots were the most acid but soil pH was significantly higher in thinned plots and pH was highest in soil of the un-colonised plots. Mean values for carbon, nitrogen, sulphur and extractable phosphorus varied among sites, although each were significantly more abundant in the mineral soil of dense and thinned plots compared with un-colonised plots, suggesting that thinning had had a minimal effect on the soil parameters assessed. Accounting for all site components, site carbon storage was significantly higher in dense and thinned plots compared with un-colonised plots due to elevated levels of soil and litter carbon as well as the presence of trees. The results indicate that thinning dense C. glaucophylla can maintain and (by some measures) improve site condition. However, given the variability in some of the parameters assessed, further study across a wider range of soil types and rainfall gradients is proposed.

Population structures of the widespread Australian conifer Callitris columellaris are a bio-indicator of continental environmental change

“…32,000 saplings ha −1 in Fig. 5b; see also McHenry et al (2006) and Harris and Lamb (2004)). This contrasts dramatically with a chronic lack of regeneration at other sites, especially in the arid zone (Table 1), where there are sometimes few trees <20 cm dbh.…”

“…Such prolific regeneration suppresses stem growth rates (Horne and Robinson, 1987) and thus foresters have undertaken extensive thinning programs to increase timber production (Lacey, 1973;Horne and Robinson, 1987). Practical means of thinning are also required to create more structural complexity in heavily stocked stands now set aside for conservation (McHenry et al, 2006). Fires are less frequent than in the tropical zone, but when they do occur they are typically intense, stand-replacing fires (Lacey, 1973;Prober and Thiele, 2004;Fig.…”

“…Fire can also reduce seed production of surviving trees for 5 years or more (Hawkins, 1966). In the absence of fire, C. columellaris can reproduce rapidly, and it is classed as an invasive native species in some agricultural areas (Harris and Lamb, 2004;McHenry et al, 2006). By contrast, frequent fires have resulted in a decline in abundance or extirpation in some areas Graham, 2001;Russell-Smith, 2006;Prior et al, 2007Prior et al, , 2010.…”

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

“…However, the impact that dense stands have on understory vegetation remains largely unresolved (Dwyer et al, 2010b). The negative impacts of high stand density may include suppression of understory floristic richness and cover (Aguiar et al, 1996;Briggs et al, 2005;Harrington and Edwards, 1999;Hobbs and Mooney, 1986;Lett and Knapp, 2003;McHenry et al, 2006;Price and Morgan, 2008;Wienk et al, 2004), reduction in stand growth rate (Dwyer et al, 2010a;Kenkel, 1988;McHenry et al, 2006;Sala et al, 2005;Vesk et al, 2008), delayed provision of desirable habitat features such as large boughs and hollows (Vesk et al, 2008), reduced stand fecundity (Vesk et al, 2010), and increased risks of fire, pathogens and insect attack (Sala et al, 2005;Wallin et al, 2004) and soil degradation (McHenry et al, 2006). There is also concern that regrowth stands may stabilise as degraded novel ecosystems (Cramer et al, 2008;Fensham, 2008;Geddes et al, 2011).…”

“…Eventually, dense stands will self-thin (Kenkel, 1988;Olson et al, 2014;Westoby, 1984). However, intervention with mechanical or chemical thinning has been demonstrated to benefit tree growth, hasten the development of structural diversity, reduce pest attack risk and tree mortality, and increase carbon storage (Comfort et al, 2010;Dwyer et al, 2010b;Harrington and Edwards, 1999;Horner et al, 2010;McHenry et al, 2006;Pollock and Beechie, 2014;Wallin et al, 2004). It is often assumed that reducing stem densities will maintain or increase understory condition, and in combination with increasing the growth rate of remaining trees, maintain or increase ecosystem diversity, function and structural complexity (Czembor and Vesk, 2009;Fensham, 2008;Good et al, 2011;Good et al, 2012;Horner et al, 2010;Stanturf et al, 2014).…”