Traits associated with leaf energy balance are especially important for
desert plants because decoupling leaf and
ambient temperatures can be critical for maximizing productivity and survival in
these hot, dry regions. Deserts
are also one of the most climatically unpredictable and heterogeneous biomes;
thus, variation of energy balance
characters due either to plasticity or genetic differentiation might also be
crucial to the success of desert plant
species. In a common garden environment we examined variation in the traits
associated with energy balance and
productivity in Encelia farinosa A. Gray (brittlebush) plants. Comparing
two populations from contrasting rainfall
and drought climates, we found that plants from the wetter site (Superior, Arizona)
always maintained higher leaf
absorptances than plants from the more xeric region (Oatman, Arizona) when at
similar water potentials. Superior
plants also increased stomatal conductance and photosynthesis in response to
mid-spring rainfall, whereas Oatman
plants did not. Oatman plants, however, tended to have greater leaf areas and
continuous spring growth, which
made them larger than Superior plants, yet both populations produced the same
number of flower heads. The
differences for these traits, and the associations among them, agree with
predictions based on the contrasting
drought and rainfall environments of these two populations. Our results suggest
that the differences might
represent alternative suites of characters of selective importance for maximizing
carbon gain over the climatically
variable range of this widespread species. Furthermore, these differences,
detected in common environment
conditions, might have a genetic basis, which might contribute to a greater
potential for local adaptation in this species.