Phloem flow modeling

Seasonal changes in climate are accompanied by shifts in carbon allocation
and phenological changes in woody angiosperms, the timing of which can
have broad implications for species distributions, interactions, and
ecosystem processes. During critical transitions from autumn to winter and
winter to spring, physiological and anatomical changes within the phloem
could impose a physical limit on the ability of woody angiosperms to
transport carbon and signals. There is a paucity of literature that
addresses tree (floral or foliar) phenology, seasonal phloem anatomy and
seasonal phloem physiology together, so our knowledge of how carbon
transport could fluctuate seasonally, especially in temperate climates is
limited. We review phloem phenology focusing on how sieve element anatomy
and phloem sap flow could affect carbon availability throughout the year
with a focus on winter. To investigate whether flow is possible in the
winter, we constructed a simple model of phloem sap flow and investigated
how changes to the sap concentration, pressure gradient and sieve plate
pores could influence flow during the winter.  Our model
suggests that phloem transport in some species could occur year-round,
even in winter, but current methods for measuring all the parameters
surrounding phloem sap flow make it difficult to test this hypothesis. We
highlight outstanding questions that remain about phloem functionality in
the winter and emphasize the need for new methods to address gaps in our
knowledge about phloem function.