Physiological and environmental differences between the top and bottom of a Pinus strobus canopy during cold hardening

Background/Question/Methods

As canopy dominant trees approach their maximum height, greater extremes in temperature and radiation environments, as well as lower water availability, may make it more difficult for the top of the canopy to accumulate biomass for further growth.  Most previous studies of physiological height limitation have focused on stresses during the growing season.  Maximum heights for eastern North American forest trees tend to decrease with increasing latitude, implying that winter-related stresses may also play a role.  We compared the thermal environment, needle relative water content (RWC), and photosynthetic efficiency (QY) near the top (16 m) and bottom (8 m) of the canopy of a Pinus strobus tree in northern Minnesota during the cold hardening period.  Thermal environment was measured with temperature loggers in black painted copper globes, integrating the effect of the radiant energy environment with air temperature.  QY measurements were collected twice weekly, and RWC was measured once before cold hardening and once after trees were fully hardened.

Results/Conclusions

Before cold hardening, the RWC was 87% at the top of the canopy, and 89% at the bottom of the canopy (difference significant at p < 0.001).  After hardening, RWC was at about 87% for both the top and the bottom of the canopy. Water content has generally been found to decrease during cold hardening, but the top of the canopy was already at its post-hardening water content. As temperature decreased, overall QY decreased in both the top and the bottom of the canopy, but the top of the canopy hardened more rapidly.  After 45-50% hardening, the top and the bottom followed the same trend, increasing during warm spells and decreasing during cold spells.  After cold hardening was complete, QY still responded to spikes in temperature, but with much less sensitivity.  In spite of the greater sky exposure at the top of the canopy, the distribution of black globe temperatures did not differ significantly between the top and the bottom of the canopy.  This implies that temperature is not the driving factor for the differences in QY between the top and the bottom of the canopy, but light and water content differences cannot be ruled out.