Divaricating shrubs in Patagonia and New Zealand

There are at least three hypotheses to account for the abundance of divaricating shrubs in New Zealand: 1) Ratites in the form of 11 species of moa, led to divarication for browse protection (Greenwood and Atkinson, 1977); 2) Divarication evolved as a microclimatic shield (McGlone and Webb, 1981); 3) Divarication evolved to aid leaves in light harvesting (Kelly, 1994). In Patagonia before human arrival, there were browsing mammals in addition to the ratite rhea. To examine the possible influence of the different grazing animals on the degree of divarication in Patagonian shrubs, a transect was established across Argentine Patagonia at c.40 degrees S lat., from Andean forests to the shrub desert of the east, providing a rainfall gradient from 3000mm to 134mm annual precipitation. Divarication Indices of Atkinson (1992), I-ATK and Kelly (1994), I-KEL were calculated for all shrubs encountered at 20 sites along this gradient. As I-KEL gave zero values for four leafless shrubs, including the important Mulinum spinosum, this index was not further used for distributional analyses. I-ATK gave 18 species as fully divaricate (I-ATK > 14) and 8 as semi-divaricate (I-ATK <14) The highest values of I-ATK were lower than in New Zealand (Atkinson 1992), a function of a lower number of wide-angle branches (>90 degrees) in Patagonia. All except two species were spiny, as were most other shrubs on the traverse. Unlike the majority of divaricates in New Zealand which retain divarication in dense forest, none of the Patagonian shrubs are divaricate in forest and only two species divaricate in more open forest and scrub. Divaricate cover increased steeply along the traverse through drier, open forest and seral scrub. Once out of the seral scrub zone into arid country, a different suite of divaricate taxa was encountered. The diversity and cover of divaricates rose to reach a maxima at 134 mm annual precipitation. In Patagonia, divarication and spininess could be responses to the indigenous browsing mammals that are common in the semiarid and arid zones, or to climate.