Background/Question/Methods Southern California black walnut (Juglans californica) is a winter deciduous tree endemic to Southern California, a region that recently suffered its driest season (July 2006 through June 2007) on record, with most of lower elevation sites receiving > 10 cm of rain, which is about 25% of the average seasonal rainfall.   Because the species occurs in a wide variety of habitats, this study considered the physiological responses of J. californica to this drought and the subsequent season with respect to microclimate.  In particular, the water relations of J. californica was compared between a coastal site and an inland site, and within the inland site water relations was compared between trees on north-facing and south-facing slopes in order to address the following hypothesis: 1) coastal plants should experience greater drought stress than inland plants  because they received less rainfall during the season and 2) plants on south-facing slopes should experience greater drought stress than those on north slopes, due to a greater evaporative loss from the leaves and the soil on the south-facing slopes. 

Results/Conclusions In general, coastal plants experienced less water stress than inland plants on similar slopes; predawn and midday water potentials were -0.375 ±0.18 MPa higher, dark-adapted fluorescence (F_v/F_m) was typically 5% higher, and stomatal conductance was typically 50 % higher for coastal trees compared to inland trees. It is now believed that although coastal plants received a less rainfall than inland plants, the greater humidity and moderate temperatures allowed the coastal plants to maintain higher water potentials, F_v/F_m, and stomatal conductance.  Trees on south-facing slopes experienced significantly less water stress than their north-facing counterparts, with pre-dawn water potentials -0.26 ±0.16 MPa higher, F_v/F_m 3% higher, but stomatal conductance 27.3% higher for trees on south-facing slopes versus north-facing slopes. Midday water potentials were also greater for south-facing slopes versus north-facing slopes in the summer and fall.  Although establishment on the south-facing slope may be difficult due to exposure to sun competition for water appears to be a greater for established plants on north-facing slopes because, in general, plant frequency and groundcover are greater on north-facing slopes than for south-facing slopes.