Hard mineral clast sediments applied to dry archaeological fields in the distant apical islands of the Polynesian triangle are frequently associated with sweet potato / kumara (Ipomoea batatas (L) Lain.) cultivation. In a novel variation on this practice, north-western South Island Maori deposited tuangi (Austrovenus stutchburyi (Wood, 1828)) mollusc beach valves to cap 10-20 cm deep planting holes or pits by or around the sixteenth century AD at Triangle Flat, Golden Bay. Discrete tuangi beach valve sediment had been extended over much of the larger field surface in association with shallow (<10 cm deep) planting depressions. Surface shell deposits would have suppressed weed growth, redirected radiant energy onto young kumara plants, and conserved planting pit soil and moisture against disruptive and desiccating winds, respectively. The temporal extension of Triangle Flat shell sediment could be related to socio-economic and political pressures to improve kumara production from a fixed land unit. However, since climate change beginning in the sixteenth century brought cooler temperatures and eventually, stronger westerly winds to this region, it seems more likely that shell mulch was extended to maintain shallow planting soils and production in a collier, windier period. This local, kumara-focused development may have been encouraged by the opportunity to apply ritually safe and perhaps spiritually potent, uncooked tuangi beach shell to tapu cultivation surfaces.
Keywords: South Island Maori, molluscan mulch, kumara, climate change, tapu.
An ancient cultivation technique involving the addition of hard, predominantly mineral clasts to field surfaces is reported globally, if infrequently. This agronomy is glossed as "lithic mulching" by Lightfoot (1994, 1995, 1997; see also Lightfoot & Eddy 1994). Archaeologists have identified the practice in dry cultivation settings on islands at the margins of the Polynesian triangle that were settled by cognate (originally western Pacific) populations. In archaeological studies, Polynesian hard mulch practices are most often associated with sweet potato / kumara (Ipomoea batatas (L) Lain.) cultivation (Allen 2004: 211-12; Barber 2010; Ladefoged et al. 2010; Louwagie et al. 2006; Stevenson et al. 1999, 2006; Wozniak 1999, 2001; Figure 1).
For subtropical Rapa Nui (Easter Island) in south-eastern Polynesia, the term "lithic mulch" describes volcanic stone applications (grain size >2 mm) to cultivation field surfaces (Figure 1). Researchers have argued that these rocky sediments acted to regulate temperature and conserve cultivation soils and moisture against desiccating winds and erosion, especially following island-wide deforestation (Baer et al. 2008; Bork et al. 2004; Ladefoged et al. 2010; Louwagie et al. 2006; Mieth & Bork 2003; Mieth & Bork 2004: 83-4; Mieth & Bork 2005; Stevenson et al. 1999, 2006; Wozniak 1999, 2001, 2005; Wozniak & Stevenson 2008). Recently, it has been suggested that the addition of fresh basalt clasts may have added important mineral nutrients to old, leached volcanic soils as well (Hunt & Lipo 2011: 46-8, 191-6; Ladefoged et al. 2010: 83). Wallin et al. (2005) associate lithic mulching with the intensive production of kumara surpluses to support the construction of Rapa Nui's monumental landscape (see also Stevenson & Haoa 1998; Stevenson et al. 1999). Mieth and Bork (2005: 62) even argue that "the labor intensity of the stone mulching phase" on Rapa Nui "probably exceeded the labor efforts of the ahu/moai phase by far". It is possible that the practice was intended variously to ameliorate environmental effects, especially increased wind impacts over time, and to improve production results from a fixed land unit in other cases ("intensifcation", after Brookfield 1984; see also Kirch 1994, and discussion in H.M. Leach 1999).
Applications of stone (generally gravel/pebble to small cobble class, 2-100 mm grain size) or coarse sand in Maori crop production are documented from temperate climate, central to south-eastern New Zealand locations as far south as Banks Peninsula (Figure 1; Barber 2004, 2010; McFadgen 1980; Walton 1983; Walton 2000: 20). …