Is root maintenance limited by carbon availability?

Background/Question/Methods

Tree roots are heterotrophic organs—their growth and maintenance ultimately depend on the transport of carbohydrates produced by leaves. The tight coupling of root metabolism with recently assimilated carbon (i.e., photosynthate) is an emerging paradigm in the ecophysiological literature. However, it has long been realized that stored nonstructural carbohydrates (NSC) can buffer the asynchrony between carbohydrate supply and demand in a variety of tree organs. We tested the hypothesis that root maintenance is not carbon limited in longleaf pine (Pinus palustris L.)—a species that characteristically maintains large belowground nonstructural carbohydrate pools, presumably as an evolutionary adaptation to chronic disturbance. We experimentally manipulated the transfer of photosynthate belowground using stem girdling (applied in May and August) and root trenching. Trenching was applied in May and differed from stem girdling in that it not only terminated the supply of photosynthate belowground, but also severed connections between the root branching network and the tap root—a potential reservoir for stored carbon. We monitored the impact of belowground photosynthate exclusion treatments on root metabolism and root NSC concentrations of different root functional categories for one year following treatment application.

Results/Conclusions

Root metabolism, as measured via soil CO₂ efflux, was not influenced by photosynthate exclusion treatments and seasonal dynamics were independent of treatment. Root NSC concentrations were influenced by photosynthate exclusion treatments, but responses depended on the specific treatment, the root functional category, and the time since treatment application. The trench treatment resulted in the most rapid depletion of root NSC, suggesting that tap roots may be an important NSC storage reservoir. Regardless of photosynthate exclusion treatment, starch concentrations of the larger, higher order root categories were influenced sooner than smaller, lower order roots. In fact, NSC concentrations of the smallest, most metabolically active roots were not influenced until nearly one year after treatment application. Some root mortality occurred approximately one year after treatment application and recently dead roots contained relatively high starch concentrations (75-86% of that measured in live roots of the same root functional category). We cannot be certain if the remaining NSC's were inaccessible or if something other than carbon was limiting their metabolic functioning and causing mortality. The large starch concentrations in longleaf pine along with the capacity for starch transport from larger to smaller order roots maintained root metabolic functions. We therefore conclude that root maintenance is not limited by carbon availability when NSC reserves are sufficiently large.