Austral Ecology (2005) 30, 827–833 *Corresponding author. Present address: School of Biological Sciences, Tamaki Campus, University of Auckland, Private Bag 92019, Auckland, New Zealand. (Email: ibas005@ec.auckland.ac.nz). Accepted for publication June 2005. Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration I. E. BASSETT, 1 * R. C. SIMCOCK 2 AND N. D. MITCHELL 1 1 School of Geography and Environmental Science, University of Auckland, and 2 Landcare Research, Auckland, New Zealand Abstract Soil compaction can affect seedling root development by decreasing oxygen availability and increasing soil strength. However, little quantitative information is available on the compaction tolerances of non-crop native species. We investigated the effects of soil compaction on establishment and development of two New Zealand native species commonly used in restoration programmes; Cordyline australis (Agavaceae) (cabbage tree) a fleshy rooted species, and Leptospermum scoparium (Myrtaceae) (manuka) a very finely rooted species. Seedlings were grown in a range of soil compaction levels in growth cabinet experiments. Low levels of soil compaction (0.6 MPa) reduced both the number and speed of C. australis seedlings penetrating the soil surface. In contrast, L. scoparium seedlings showed improved establishment at an intermediate compaction level. Root and shoot growth of both species decreased with increasing soil strength, with L. scoparium seedlings tolerating higher soil strengths than did C. australis. Despite these results, soil strength accounted for only a small amount of variation in root length (R 2 < 0.25), due to greater variability in growth at low soil strengths. Soil strengths of 0.6 MPa are likely to pose a barrier to C. australis regeneration. This is consistent with adaptation to organic and/or soft, waterlogged soils. Active intervention may be necessary to establish C. australis from seed on many sites previously in farmland. Key words: Cordyline australis, Leptospermum scoparium, regeneration, seedling establishment, soil compaction. INTRODUCTION Soil compaction occurs in a variety of rural and urban situations, as a result of a range of human activities, including construction work, the use of farm machin- ery, vehicular and pedestrian traffic, and trampling by livestock. Soil compaction mainly affects plant devel- opment through increased soil strength, decreased oxygen availability, and altered (either increased or decreased) water storage and availability. Greater mechanical resistance increases the force required for the plant root to push its way through the soil. This is compounded by the reduction in size and continuity of soil macropores through which roots preferentially grow (Kozlowski 1999), leading to slower root elongation, reduced root length and reduc- tion in soil volume exploited (Materechera et al. 1991; Panayiotopoulos et al. 1994). A soil strength of 2 MPa is commonly cited as limiting for the growth of crop plants (Materechera et al. 1991; Day & Bassuk 1994). However, the literature also shows plant species vary considerably in their tolerance of soil compaction. The seedling stage may be particularly vulnerable to com- paction, with increased soil strength delaying both ger- mination and penetration of the soil surface by the radicle (Court 1985; Smith et al. 2001; Soyelu et al. 2001). A reduction in the volume and continuity of soil pores may decrease oxygen availability, and allow build up of toxic gases (Hillel 1971). Ten per cent air- filled porosity at -10 kPa tension is commonly cited as limiting for plant growth (Day & Bassuk 1994; Penfold 1998; Drewry & Paton 2000), although this is influenced by factors such as soil temperature, water table height and plant adaptations. Impeded drainage may exacerbate oxygen deficits within the soil, as water takes up more of the available pore space. Compaction may also reduce infiltration of water into the soil, thus increasing run-off and mechanical resistance. Restricted rooting may disadvantage the plant in a number of ways; plants may be more susceptible to drought and environmental fluctuations (Liang et al. 1999), or nutrient deficiencies of particularly immo- bile nutrients such as phosphorus (Greacen & Sands 1980). Reduced seedling growth may further contrib- ute to restricted natural regeneration at compacted sites by decreasing competitiveness and increasing sus- ceptibility to weed invasion. Restricted rooting may impair plant anchorage, increasing susceptibility to wind throw (Kodrik & Kodrik 2002). Soil compaction may present a barrier to seedling establishment and growth where land managers seek