Phenology refers to the study of the timing of seasonally recurring biological events. In temperate forests, seasonal variation of leaf area is one obvious and important aspect of phenology [Richardson et al., 2013]. However, there are other aspects of phenology that warrant attention. In particular, seasonal variation of leaf photosynthetic capacity has been shown to affect the functioning of temperate deciduous forests [Wilson et al., 2001; Xu and Baldocchi, 2003; Grassi et al., 2005; Wang et al., 2008; Ow et al., 2010] and temperate grasslands [Wolf et al., 2006]. In a synthesis of observations, Bauerle et al.  argued that seasonal variation of photosynthetic capacity for broadleaf deciduous trees was related to photoperiod, with photosynthetic capacity attaining a maximum around the summer solstice and then declining in concert with photoperiod. Although seasonal variation in photosynthetic capacity has been incorporated into a few terrestrial biosphere models [Krinner et al., 2005; Medvigy et al., 2009; Oleson et al., 2010], the specifics of the implementations have varied, and many models have not included it at all. Consequently, the broad implications of the seasonal variation of photosynthetic capacity are not well understood.
 The impacts of seasonal variation in photosynthetic capacity are likely to be sensitive to particular time scales. Consider a pair of forest stands, one of which has a seasonal variation of photosynthetic capacity that tracks photoperiod, and the other without seasonal variation of photosynthetic capacity (Figure 1). We propose here that it is possible for these two types of stands to yield, on average, similar annual gross primary productivity (GPP) but different monthly GPP. The essential prerequisite for this is that the stand with seasonal variation of photosynthetic capacity must have a larger maximum photosynthetic capacity than the stand without seasonal variation in photosynthetic capacity. Then, the stand that experiences seasonal variation in photosynthetic capacity would have less GPP during the spring and fall than a stand with constant photosynthetic capacity, more GPP in early summer, and similar annual average GPP.
 Here, we present three hypotheses based on these ideas. (1) If unfavorable climate anomalies were more likely to occur during times of long photoperiod, the GPP of seasonal photosynthetic capacity trees will be affected more strongly than that of constant photosynthetic capacity trees. Conversely, unfavorable climate anomalies occurring during times of short photoperiod will have less of an effect on the GPP of seasonal photosynthetic capacity trees than that of constant photosynthetic capacity trees. (2) Certain disturbances, such as defoliation by insects, tend to occur at particular times of the year. In the forests of the eastern US Atlantic coastal plain, defoliation by gypsy moth (Lymantria dispar L.) larvae tends to occur during times of relatively long photoperiod [Liebhold et al., 1992; Johnson et al., 2005, 2006]. Consequently, seasonal photosynthetic capacity trees will be more strongly affected by defoliation than constant photosynthetic capacity trees. (3) There will be interactions between seasonal variations of leaf area and seasonal variations of photosynthetic capacity. If global warming causes earlier budburst or delayed senescence [Menzel et al., 2008; Lebourgeois et al., 2010; Vitasse et al., 2011; Migliavacca et al., 2012; Jeong et al., 2013], the ability of vegetation to capitalize on the prolonged growing season and increase carbon uptake will hinge on photosynthetic capacity being sufficiently high at the start and end of the growing season. Thus, the carbon gains achieved by trees with seasonal variations of photosynthetic capacity will be less than the gains achieved by trees with constant photosynthetic capacity if increasing temperatures cause growing seasons to be lengthened.
 In this paper, we test these ideas using model simulations of a highly instrumented forested stand in the New Jersey Pinelands. In section 2, we describe the model, the forest stand, and the simulation design. In section 3, we evaluate model performance and assess how seasonal variations in photosynthetic capacity modulate the impacts of transient climate anomalies, defoliation, and increased temperatures. Section 4 contains a discussion of our results, and our conclusions are presented in section 5.