Biomass Allocation of Montane and Desert Ponderosa Pine: An Analog for Response to Climate Change

We measured aboveground biomass allocation of Pinus ponderosa on hydrothermally altered andesite in montane and desert climates. Thus substrates was held constant while climate varied. Trees from montane climates had higher leaf mass per unit cross—sectional area of sapwood (functional conducting xylem) than trees from desert climates, suggesting that a functional response to differences in climate had occurred. Our results also indicate that sapwood mass; leaf mass ratios of P. ponderosa may increase 2°50% with a 5°C change in mean growing—season temperature, approximately the difference between our montane and desert sites. High proportional allocation of biomass to sapwood may improve water relations of P. ponderosa, but because sapwood contains living parenchyma, respiratory costs may be high. Site—specific regression equations were used to calculate aboveground biomass allocation for simulated montane and desert trees with the same diameter at breast height, dbh. Simulated montane trees were 46—52% taller than desert trees, and montane trees 10 cm in dbh had twice the total aboveground mass of desert counterparts. Simulated 50—cm montane and desert trees were almost identical in total mass, even though the montane tree was 46% taller. The predicted proportion of biomass allocated to bole sapwood increased with size for both montane and desert models; however, the 50—cm desert model contained 8% more total sapwood mass than the taller montane model. Total biomass of branches was similar for paired models of all size classes; however, biomass of primary and secondary branches differed considerably. The 50—cm desert model had twice as much biomass in primary branches in comparison to the montane model, whereas the montane model had 3 times more biomass in secondary branches than the desert model. For 10—cm trees of the desert and montane models 29 and 33% of the biomass were leaves, respectively. In larger trees, leaf allocation decreased to 5 and 7% for desert and montane models, respectively. The effects of climate on biomass allocation, such as reported here, and corresponding changes in whole—plant assimilation rates must be incorporated into growth—response models used to predict future fluctuations in forest productivity due to global climate change.