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Relationships between gross primary productivity (GPP) and the remotely sensed photochemical reflectance index (PRI) suggest that time series of foliar PRI may provide insight into climate change effects on carbon cycling. However, because a large fraction of carbon assimilated via GPP is quickly returned to the atmosphere via respiration, we ask a critical question—can PRI time series provide information about longer term gains in aboveground carbon stocks? Here we study the suitability of PRI time series to understand intra‐annual stem‐growth dynamics at one of the world's largest terrestrial carbon pools—the boreal forest. We hypothesized that PRI time series can be used to determine the onset (hypothesis 1) and cessation (hypothesis 2) of radial growth and enable tracking of intra‐annual tree growth dynamics (hypothesis 3). Tree‐level measurements were collected in 2018 and 2019 to link highly temporally resolved PRI observations unambiguously with information on daily radial tree growth collected via point dendrometers. We show that the seasonal onset of photosynthetic activity as determined by PRI time series was significantly earlier (p < .05) than the onset of radial tree growth determined from the point dendrometer time series which does not support our first hypothesis. In contrast, seasonal decline of photosynthetic activity and cessation of radial tree growth was not significantly different (p > .05) when derived from PRI and dendrometer time series, respectively, supporting our second hypothesis. Mixed‐effects modeling results supported our third hypothesis by showing that the PRI was a statistically significant (p < .0001) predictor of intra‐annual radial tree growth dynamics, and tracked these daily radial tree‐growth dynamics in remarkable detail with conditional and marginal coefficients of determination of 0.48 and 0.96 (for 2018) and 0.43 and 0.98 (for 2019), respectively. Our findings suggest that PRI could provide novel insights into nuances of carbon cycling dynamics by alleviating important uncertainties associated with intra‐annual vegetation response to climate change.

Light availability drives vertical canopy gradients in photosynthetic functioning and carbon (C) balance, yet patterns of variability in these gradients remain unclear. We measured light availability, photosynthetic CO2 and light response curves, foliar C, nitrogen (N) and pigment concentrations, and the photochemical reflectance index (PRI) on upper and lower canopy needles of white spruce trees (Picea glauca) at the northern and southern extremes of the species' range. We combined our photosynthetic data with previously published respiratory data to compare and contrast canopy C balance between latitudinal extremes. We found steep canopy gradients in irradiance, photosynthesis, and leaf traits at the southern range limit, but clear convergence across canopy positions at the northern range limit. Thus, unlike many tree species from tropical to mid-latitude forests, high latitude trees do not require vertical gradients of metabolic activity to optimize photosynthetic C gain. Consequently, accounting for self-shading is less critical for predicting gross primary productivity at northern relative to southern latitudes.Northern trees also had a significantly smaller net positive C balance than southern trees suggesting that, regardless of canopy position, low photosynthetic rates coupled with high respiratory costs may ultimately constrain the northern range limit of this widely distributed boreal species.SHORT TITLE: Variation in photosynthetic canopy gradients of white spruce at the species' range limits.

&lt;p&gt;The &amp;#8216;growing season&amp;#8217; of trees is often assumed to be coupled with climatology (e.g., summer vs winter) and visual canopy phenology cues (e.g., leaf emergence in spring and senescence in autumn). However, green leaves are not always photosynthetically active and actual tree radial growth via cambial cell division is &amp;#8216;invisible&amp;#8217; since it is hard to see and occurs at micrometer resolution. Therefore, despite the presence of apparently green vegetation, trees may not be assimilating carbon or growing. Here, we study photosynthesis and tree-growth at near-instantaneous timescales using &lt;em&gt;in-situ&lt;/em&gt; and satellite remote sensing, point dendrometers, quantitative wood anatomy, and Pulse Amplitude Modulated chlorophyll fluorescence. Tree and leaf-level measurements are being made on eight oak (&lt;em&gt;Quercus spp.&lt;/em&gt;) trees in a temperate forest in southern New York, USA. We find that oak trees commence radial growth in the first week of April approximately one-month prior to canopy development that is not completed until the first week of May. Additionally, the development of foliar photosynthetic capacity lags leaf expansion by nearly two weeks. Further, we find that oak growth for the season is completed by late July while photosynthetic activity is maintained for three additional months until early November. Finally, we examine the growth climate sensitivity across a network of 16 oak tree-ring width chronologies distributed across the northeastern US. These relationships suggest that oak earlywood growth relies on carbon assimilated in prior year autumn while oak latewood relies on current year assimilated carbon. Therefore, photosynthesis and tree-growth in Northeastern US oaks occurs asynchronously, since trees don&amp;#8217;t reach peak photosynthetic performance the moment leaves emerge or grow through the &amp;#8216;growing season&amp;#8217;.&lt;/p&gt;