Seasonal variation in photosynthetic capacity of montane conifers

The capacity to perform photosynthesis, given appropriate environmental conditions, is reflected by measurements of the maximum quantum yield of photosystem II (Fv/Fm). The seasonal course of Fv/Fm may help define the length of the annual photosynthetic period in temperate evergreen forests. Fv/Fm was measured 31 times from September to May on six conifer species located along an altitudinal gradient between 400 and 1400 m a.s.l. in northern Idaho, USA. The species were Western Redcedar (Thuja plicata Donn ex D. Donn), Douglas Fir (Pseudotsuga menziesii (Beissn.) Franco), Engelmann Spruce (Picea engelmannii Parry), Grand Fir (Abies grandis (Dougl.) Lindl.), Ponderosa Pine (Pinus ponderosa Laws.) and Lodgepole Pine (Pinus contorta Dougl.). Species differed in mean Fv/Fm, in magnitude of response to maximum and minimum temperature, and in altitude effects other than temperature. For two species Fv/Fm in spring differed from that in autumn, even when measured at the same air temperature. Regardless of temperature, most species maintained some photosynthetic capacity all winter. A lag of several days to 2 months was observed in the response of Fv/Fm to ambient temperature. The lag differed among species. It was not observed in Grand Fir, and was as long as 2 months in Western Redcedar. Over all species combined the best overall correlation was with a moving average of maximum temperature over the previous 30 days. The correlation between Fv/Fm and maximum temperature was generally stronger than that with minimum temperature (average root mean-squared error was reduced by 10%), presumably because maximum temperatures better reflect daytime photoinhibitory conditions. A reduced model predicted Fv/Fm based on species and maximum temperature; this model can be used to parameterize models describing the annual cycle of photosynthetic capacity for the six conifer species included in this study.