Seasonal variation in photosynthetic capacity of montane conifers

Abstract

SummaryThe 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 ofFv/Fmmay help define the length of the annual photosynthetic period in temperate evergreen forests.Fv/Fmwas 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 plicataDonn ex D. Donn), Douglas Fir (Pseudotsuga menziesii(Beissn.) Franco), Engelmann Spruce (Picea engelmanniiParry), Grand Fir (Abies grandis(Dougl.) Lindl.), Ponderosa Pine (Pinus ponderosaLaws.) and Lodgepole Pine (Pinus contortaDougl.).Species differed in meanFv/Fm, in magnitude of response to maximum and minimum temperature, and in altitude effects other than temperature. For two speciesFv/Fmin 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 ofFv/Fmto 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 betweenFv/Fmand 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 predictedFv/Fmbased 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.