Wide variation of winter-induced sustained thermal energy dissipation in conifers: A common-garden study

Andrew Walter-McNeill, Barry A. Logan, David Bombard, Jaret S. Reblin, Sophia Lopez, Charles Southwick and Elena Sparrow, Biology Department, Bowdoin College, Brunswick, ME, Maria Garcia and David Bowling, School of Biological Sciences, University of Utah, Salt Lake City, UT

Background/Question/Methods
Evergreen-dominated boreal forests account for around 10% of all forested area globally, and evergreen conifers can be found in abundance at temperate latitudes. Measuring photosynthetic seasonality in these forests is difficult because the presence of green foliage, a commonly used indicator for photosynthetic activity, becomes decoupled from photosynthesis in evergreens during winter. To address this, an expanding range of remote sensing techniques report seasonal signals driven by phenological changes in needle carotenoid content underlying the induction of sustained photoprotective thermal energy dissipation in winter and its subsequent relaxation at the onset of the growing season. One of the remaining challenges associated with these methods is understanding how the implementation of sustained thermal energy dissipation and its relationship to needle pigment content varies among different evergreen taxa.
We measured chlorophyll fluorescence and pigment content (carotenoids and chlorophylls) in summer and winter in 70 evergreen individuals growing outdoors at Red Butte Garden in Salt Lake City, Utah. Our common garden experimental design allows us to explore the influence of species traits, such as family affiliation and temperature hardiness, on sustained thermal energy dissipation and associated acclimation of needle pigment content.
Results/Conclusions
Under common-garden conditions, we observed a wide range in engagement of sustained thermal energy dissipation across the 70 conifers. Quantified as the percentage decrease in dark-acclimated Fv/Fm from summer to winter (‘winter photoinhibition’; WPI), values ranged from 5 to 95 percent. Among examined pigment parameters, the increase in zeaxanthin content from summer to winter (expressed on a chlorophyll basis) most strongly correlated with WPI; however, this relationship accounted for only slightly more than 1/3 of the variation in WPI (r2 = 0.37). We observed no difference in the range of WPI values between members of the Cupressaceae and the Pinaceae, nor did we find a relationship between WPI and a species’ USDA temperature hardiness rating. We did not find evidence of a consistent wintertime decrease in evergreen chlorophyll content on a needle fresh-mass basis, indicating that evergreen conifers, in general, do not decrease the amount of light absorbed by photosynthetic tissue during winter, when cold temperature reduces photochemical energy use. This finding has important implications in the interpretation of remotely sensed solar-induced fluorescence (SIF) and measures of “greenness” derived from repeat digital photography, both which have been shown to decrease in winter in evergreen needleleaf forests.