Hydraulic conductance and photosynthetic capacity of a chronosequence of Juniperus virginiana on a shale barren in West Virginia, US

Background/Question/Methods Understanding why net primary productivity declines with tree age is a fundamental problem in forest ecology.  Several hypotheses have been developed to possibly explain this phenomenon, but it is difficult to separate the effects of age from the effects of size because tree size usually increases with age.   We have been examining hydraulic conductance and photosynthetic capacity in a stand of red cedar (Juniperus virginiana) trees which are approximately the same height (6.77 ± 0.54 m) yet differ greatly in age (98 – 481 years).  This chronosequence is found on a limestone outcrop along the South branch of the North Fork of the Potomac River in the Central Appalachian Mountains of West Virginia.  During the summer of 2008 we made monthly measurements of photosynthesis and stem and leaf hydraulic parameters.

Results/Conclusions

Hydraulic conductance of lateral branches showed reduction in flow of up to 60% with large amounts of variation across the summer, but there was no relationship between hydraulic conductance and tree age (r²⁼ 0.017, p= 0.546).  Reductions in hydraulic conductance seemed to be related to low amounts of precipitation in June after which a steady monthly increase in conductance occurred until October where only one sample showed reduced stem conductance.  Hydraulic conductance was not significantly different across months (p = 0.09, F = 2.3387).  There was no relationship between net photosynthesis (r²= 0.006, p= 0.5648), foliar N (r²= 0.007, p= 0.493, gas exchange-derived estimates of rubisco activity (r²= 0.035, p= 0.1472), and electron transport (r²= 0.030, p= 0.1778) with tree age.   These data suggest that tree age is not a large factor in reduced productivity and supports the hypothesis that increased tree height or reduced soil nutrient availability are more than likely the primary factors causing a decline in forest productivity.  However, since this stand has no red cedar trees younger than 98 years old, we can not preclude the possibility that physiological preconditioning may be occurring in trees older than 98 and that there is an age related decline in these parameters that occurs earlier in the life of the trees.  This study also seems to lend evidence for repair of embolisms in J. virginiana since we observed the recovery of hydraulic conductivity of these trees over the growing season after an initial dry June.