We measured needle pigment content and photosynthetic rates of 1-year-old western larch (Larix occidentalis Nutt.) during autumn foliar senescence. Chlorophyll (Chl) and carotenoid (xanthophyll + b-carotene) contents of needles declined 11 and 17%, respectively, before CO(2) assimilation rate began to decline. Chlorophyll a/b ratio, Chl/carotenoid ratio, photochemical efficiency (F(v)/F(m)), and photochemical quenching did not begin to decline until late in senescence. Internal CO(2)/ambient CO(2) did not change during needle yellowing. In seedlings in warmed soil (average 3 degrees C above natural conditions), the decline in needle chlorophyll content was delayed by 10 days and the decline in CO(2) assimilation rate was delayed by 5 days, compared with seedlings in soil at ambient temperature. In seedlings exposed to an extended 16-h photoperiod, the decline in needle chlorophyll content was delayed by 32 days, and the decline in CO(2) assimilation rate was delayed by 21 days, compared with seedlings exposed to natural day lengths. In addition to delaying the onset of needle senescence, the treatments affected the sequence of events during senescence. Differences among treatment groups provide evidence that the onset of pigment loss and photosynthetic decline and the sequence of events during needle senescence are affected by soil temperature and day length.

We induced foliar senescence in western larch (Larix occidentalis Nutt.) needles (100 to 400 d old) in chambers set to combinations of photoperiod (8 or 16 h) and air temperature (8 or 15°C). We used regression analysis to estimate the effects of air temperature, photoperiod, provenance and leaf age on the date of decline in chlorophyll, photosynthesis, and chlorophyll a/b ratio. Increased air temperature delayed the decline in chlorophyll and photosynthesis without affecting chlorophyll a/b: seedlings with 200‐d‐old needles placed under 8h days and 8°C lost chlorophyll, photosynthesis and chlorophyll a/b in 22, 37 and 63d, respectively, while those senescing under 15°C lost them in 54, 61 and 63d. Extended photoperiod delayed the decline in chlorophyll but not photosynthesis or chlorophyll a/b ratio: the 200‐d‐old needles placed in 8°C and 16 h photoperiod lost chlorophyll, photosynthesis, and chlorophyll a/b in 34, 37 and 63 d, compared with 22, 37 and 63 d for seedlings in 8 h days. Our analysis shows that it is possible to quantify the extent to which the timing and process of senescence vary with environmental conditions.

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