CARBON ALLOCATION AND CARBON TRANSFER BETWEEN BETULA-PAPYRIFERA AND PSEUDOTSUGA-MENZIESII SEEDLINGS USING A C-13 PULSE-LABELING METHOD

Here we describe a simple method for pulse-labeling tree seedlings wit h (CO2(gas)-C-13), and then apply the method in two related experiment s: (i) comparison of carbon allocation patterns between Betula papyrif era Marsh. and Pseudotsuga menziesii (Mirb.) France, and (ii) measurem ent of one-way belowground carbon transfer from B. papyrifera to Fl me nziesii. Intraspecific carbon allocation patterns and interspecific ca rbon transfer both influence resource allocation, and consequently dev elopment, in mixed communities of B. papyrifera and P. menziesii. In p reparation for the two experiments, we first identified the appropriat e (CO2(gas)-C-13) pulse-chase regime for labeling seedlings: a range o f pulse (100-mL and 200-mL 99 atom% (CO2(gas)-C-13)) and chase (0, 3 a nd 6 d) treatments were applied to one year-old B. papyrifera and P. m enziesii seedlings. The amount of (CO2)-C-13 fixed immediately after 1 .5 h exposure was greatest for both B. papyrifera (40.8 mg excess C-13 ) and P. menziesii (22.9 mg excess C-13) with the 200-mL pulse, but hi gher C-13 loss and high sample variability resulted in little differen ce in excess C-13 content between pulse treatments after 3 d for eithe r species. The average excess C-13 root/shoot ratio of B. papyrifera a nd P. menziesii changed from 0.00 immediately following the pulse to 0 .61 and 0.87 three and six days later, which reflected translocation o f 75% of fixed isotope out of foliage within 3 d following the pulse a nd continued enrichment in fine roots over 6 d. Based on these results , the 100-mL CO2(gas) and 6-d chase were considered appropriate for th e carbon allocation and belowground transfer experiments. In the carbo n allocation experiment, we found after 6 d that B. papyrifera allocat ed 49% (average 9.5 mg) and P. menziesii 41% (average 5.8 mg) of fixed isotope to roots, of which over 55% occurred in fine roots in both sp ecies. Species differences in isotope allocation patterns paralleled d ifferences in tissue biomass distribution. The greater pulse labeling efficiency of B. papyrifera compared to P. menziesii was associated wi th its two-fold and 13-fold greater leaf and whole seedling net photos ynthetic rates, respectively, 53% greater biomass, and 35% greater roo t/shoot ratio. For the carbon transfer experiment, B. papyrifera and P . menziesii were grown together in laboratory rootboxes, with their ro ots intimately mingled. A pulse of 100 mL (CO2(gas)-C-13) was applied to paper birch and one-way transfer to neighboring P. menziesii was me asured after 6 d. Of the excess C-13 fixed by B. papyrifera, 4.7% was transferred to neighboring P. menziesii, which distributed the isotope evenly between roots and shoots. Of the isotope received by P. menzie sii, we estimated that 93% was taken up through belowground pathways, and the remaining 7% taken up by foliage as (CO2(gas)-C-13) respired b y B. papyrifera shoots. These two experiments indicate that B. papyrif era fixes more total carbon and allocates a greater proportion to its root system than does Il menziesii, giving it a competitive edge in re source gathering; however, below-ground carbon sharing is of sufficien t magnitude that it may help ensure co-existence of the two species in mixed communities.