Seasonal variation in photosynthetic capacity of montane conifers

1. The capacity to perform photosynthesis, given appropriate environmental conditions, is reflected by measurements of the maximum quantum yield of photosystem II ( F v / F m ). The seasonal course of F v / F m may help define the length of the annual photosynthetic period in temperate evergreen forests. 2. F v / F m 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.). 3. Species differed in mean F v / F m , in magnitude of response to maximum and minimum temperature, and in altitude effects other than temperature. For two species F v / F m 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. 4. A lag of several days to 2 months was observed in the response of F v / F m 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. 5. The correlation between F v / F m 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. 6. A reduced model predicted F v / F m 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.