Seasonal variation in the canopy color of temperate evergreen conifer forests

Abstract

 Evergreen conifer forests are the most prevalent land cover type in North America. Seasonal changes in the color of evergreen forest canopies have been documented with near-surface remote sensing, but the physiological mechanisms underlying these changes, and the implications for photosynthetic uptake, have not been fully elucidated.  Here, we integrate on-the-ground phenological observations, leaf-level physiological measurements, near surface hyperspectral remote sensing and digital camera imagery, towerbased CO 2 flux measurements, and a predictive model to simulate seasonal canopy color dynamics.  We show that seasonal changes in canopy color occur independently of new leaf production, but track changes in chlorophyll fluorescence, the photochemical reflectance index, and leaf pigmentation. We demonstrate that at winter-dormant sites, seasonal changes in canopy color can be used to predict the onset of canopy-level photosynthesis in spring, and its cessation in autumn. Finally, we parameterize a simple temperature-based model to predict the seasonal cycle of canopy greenness, and we show that the model successfully simulates interannual variation in the timing of changes in canopy color.  These results provide mechanistic insight into the factors driving seasonal changes in evergreen canopy color and provide opportunities to monitor and model seasonal variation in photosynthetic activity using color-based vegetation indices.

Multi‐Decadal Carbon Cycle Measurements Indicate Resistance to External Drivers of Change at the Howland Forest AmeriFlux Site

“…PhenoCam‐derived (2010‐onwards) metrics for start‐ and end‐of‐season, which have been shown to be correlated with the onset and cessation of evergreen photosynthesis (Seyednasrollah et al., 2019, 2021), are also highly variable from year‐to‐year. The mean start‐of‐season date for the evergreens is April 11, although in 2012 start‐of‐season occurred almost 3 weeks earlier (March 23).…”

“…Imagery, recorded every 30 min from sunrise to sunset, is processed to provide daily measures of canopy greenness, from which start‐ and end‐of‐season transition dates are extracted using established methods based on dates when the canopy greenness reaches (rising in spring, falling in autumn) 25% of the seasonal amplitude, where amplitude is calculated as the difference between the summer maximum and winter minimum (Richardson et al., 2018). For conifers, seasonal changes in canopy greenness are driven by leaf‐level changes in pigmentation, specifically the carotenoids:chlorophyll ratio (Bowling et al., 2018; Seyednasrollah et al., 2021); in late winter and early spring, increases in canopy greenness occur independently of (and well in advance of) the production of new foliage. By comparison, for deciduous species, seasonal change in canopy greenness are driven by budburst and leaf expansion in spring, and leaf coloration and leaf fall in autumn.…”

“…of evergreen photosynthesis (Seyednasrollah et al, 2019(Seyednasrollah et al, , 2021, are also highly variable from year-to-year. The mean start-of-season date for the evergreens is April 11, although in 2012 start-of-season occurred almost 3 weeks earlier (March 23).…”

Local scale evaluation of the simulated interactions between energy, water and vegetation in land surface models

“…This was illustrated by the differences in the simulations and observations 30% of the LAI was attributed to the understory during the summer (Rivalland, 2003). The seasonal cycle in the remote sensed LAI seems exaggerated (ranging between 1 m 2 m −2 in winter and 4 m 2 m −2 in summer).However, considering the understory and seasonal variation in needleleaf greenness (Seyednasrollah et al, 2021), assuming a flat LAI does not seem accurate nether, in the context of simulating GPP.…”

Disentangling the Relative Drivers of Seasonal Evapotranspiration Across a Continental‐Scale Aridity Gradient

“…These linkages are a product of increased water loss via stomata during photosynthesis, and while seasonal changes and phenology of LAI are recognized as a key driver of ET in deciduous broadleaf forests, there has been less work evaluating these linkages across a broader range of plant functional types (PFTs) and among different hydrologic regimes (i.e., energy‐ vs. water‐limited systems). As an example, in evergreen needleleaf forests, there are clear seasonal signals in photosynthetic activity and ET (Bowling et al., 2018; Seyednasrollah et al., 2021; Stoy et al., 2006), even though annual leaf turnover is considerably less compared to deciduous forests. However, it remains unclear how influential phenology is relative to other factors, particularly atmospheric and surface dryness represented by vapor pressure deficit (VPD) and precipitation, in evergreen needleleaf forests (e.g., Samuels‐Crow et al., 2020; Stoy et al., 2006).…”