Seasonal changes in leaf area of Amazon forests from leaf flushing and abscission

Abstract

.[1] A large increase in near-infrared (NIR) reflectance of Amazon forests during the light-rich dry season and a corresponding decrease during the light-poor wet season has been observed in satellite measurements. This increase has been variously interpreted as seasonal change in leaf area resulting from net leaf flushing in the dry season or net leaf abscission in the wet season, enhanced photosynthetic activity during the dry season from flushing new leaves and as change in leaf scattering and absorption properties between younger and older leaves covered with epiphylls. Reconciling these divergent views using theory and observations is the goal of this article. The observed changes in NIR reflectance of Amazon forests could be due to similar, but small, changes in NIR leaf albedo (reflectance plus transmittance) resulting from the exchange of older leaves for newer ones, but with the total leaf area unchanged. However, this argument ignores accumulating evidence from ground-based reports of higher leaf area in the dry season than the wet season, seasonal changes in litterfall and does not satisfactorily explain why NIR reflectance of these forests decreases in the wet season. More plausibly, the increase in NIR reflectance during the dry season and the decrease during the wet season would result from changes in both leaf area and leaf optical properties. Such change would be consistent with known phenological behavior of tropical forests, ground-based reports of seasonal changes in leaf area, litterfall, leaf optical properties and fluxes of evapotranspiration, and thus, would reconcile the various seemingly divergent views.

Impacts of Leaf Age on Canopy Spectral Signature Variation in Evergreen Chinese Fir Forests

“…New leaf expansion also contributes to negligible increase in canopy Red REF from March to July ( Figure 11B1). EVI2 trajectories follow the same patterns ( Figure 11C1) as NIR reflectance trajectories, arising from the linear dependence of EVI2 on NIR reflectance, as proved by a previous study [23]. We can conclude that both new and mature leaves contribute to the seasonality of forest albedo, which is independent of changes in other canopy attributes.…”

“…However, the situation is very different for mature evergreen forests, which show a relatively stable total leaf area every year. Thus, changes in canopy reflectance do not necessarily imply changes in LAI [23]. In evergreen forests, leaves have more than a one-year lifespan, and current-year new leaves remain throughout the winter.…”

“…Leaves affects the radiation field through its LOPs, including leaf reflectance and transmittance characteristics, which are wavelength-dependent [25,26]. A few supporting studies have confirmed the combined effects of LAI and LOPs on the seasonality of gross ecosystem CO 2 exchange (GEE), photosynthesis and NIR reflectance for Amazonian forests [23,27,28]. Many efforts aimed at accounting for effect of stand age on canopy reflectance with forest succession have improved the interpretation of canopy signals [29][30][31][32][33].…”

“…In addition to leaf area, LOPs are another significant factor that can change with time, which may strongly affect canopy reflectance [19,23,24], and leaf-age effects on canopy signals require more attention. Leaves affects the radiation field through its LOPs, including leaf reflectance and transmittance characteristics, which are wavelength-dependent [25,26].…”

Carbon exchange in an Amazon forest: from hours to years

“…The seasonally asynchronous nature of phenologymediated LUE establishes a middle ground in debates over whether the eastern Amazon canopy is enhanced or "greens up" during the dry season (Huete et al, 2006;Myneni et al, 2007;Samanta et al, 2012;Morton et al, 2014;Bi et al, 2015;Guan et al, 2015;Saleska et al, 2016). Changes to the canopy's LUE do indeed occur, but not synchronously with the dry season at our site (Fig.…”