Leaf biochemical and optical properties of Metrosideros polymorpha across environmental gradients in Hawaii

Leaf biochemical and biophysical traits and related optical properties were measured for 32 populations of Metrosideros polymorpha (Guad. Myrtaceae) across gradients of soil fertility, climate, and in a common garden.  Climate caused larger changes in specific leaf area (SLA) and SLA-linked traits, including area-based nitrogen (N), and pigment concentrations than did substrate, except under conditions of highest fertility such as those found on 20 ky old soils.  Biochemical constituent ratios containing chlorophyll (Car/Chl, Chl/N) varied more across climate than substrate gradients, while the Chl a/b ratio was similarly influenced by climate (2.75-3.13) and substrate (2.46-2.90).  In contrast to area-based constituents, mass-based biochemical concentrations displayed similar degrees of variation stemming from both climatic and soil age differences.  Optical indices indicative of light capture and use (PRI and CRI) were closely related to the measured biochemical changes, but the quantity of photosynthetically active radiation absorbed by leaves was fairly constant, varying only 5%.  Variation in photosynthetic function as expressed in the PRI and CRI was nearly six times greater in response to climate stimuli than across substrate ages, again with the exception of the high fertility site which had the highest optical index values.  Biochemical and physiological variations were not completely retained when plants were grown in a common garden, however, morphological adaptations (measured as changes in SLA) and the intrinsic balance of the photosynthetic apparatus (measured via Chl a/b ratio) were preserved.  Together, these results suggest that there is strong genetic control over morphological and photosynthetic traits, coupled with a high degree of environmental plasticity in M. polymorpha biochemistry, which allows this species to grow across a wide range of environmental conditions.