The interaction between substrate texture and rainfall generally mediates water availability to plants and thus plays an important role in ecosystem productivity. Over a rainfall gradient on the island of Hawaii, we examined how two lava types, 'a'a (rough) and pahoehoe (smooth), potentially alter water availability to early successional plants. Concurrently, we examined the relationship between plant water potential and photosynthesis within and among species to evaluate how water availability may affect vegetation differences found between these two substrates and to determine if physiological variation among species might also be a mechanism mediating productivity differences. Contrary to expectations, we found few differences in water potential and photosynthetic rates of plants on 'a'a vs. pahoehoe. Of the comparisons that were different, neither 'a'a nor pahoehoe was consistently more favorable. There was, however, a slight trend towards 'a'a communities having greater water stress than pahoehoe communities when average water availability was lower. And, in sites where rainfall is high, water stress was usually greater on the substrate with higher biomass. Within a community there were always distinct differences among species for water potential and photosynthesis, but the species-by-lava effect was rarely significant, indicating that each species was responding similarly to what differences there may be between 'a'a and pahoehoe.

Key words: ecosystem succession, growth substrate, Hawaii, photosynthesis, rainfall gradient, water potential