Seasonal patterns of tropical forest leaf area index and CO₂ exchange

Abstract

[1] We used in situ and satellite measurements to investigate the seasonal patterns of leaf area index (LAI) and gross ecosystem CO 2 exchange (GEE) by an evergreen tropical forest. The forest experienced a dry season from June through November. The rates of light-saturated CO 2 uptake (GEE) were comparatively high from December through March and low from May through July. In situ measurements showed that LAI varied seasonally, with a minimum from May through September. Leaf production and leaf abscission were reduced from December through April. Leaf abscission increased in May, which reduced LAI. High rates of leaf abscission and production occurred from July through September associated with leaf turnover. Leaf abscission decreased abruptly in October, while production continued, which rapidly increased LAI. Leaf phenology was not directly correlated with changes in soil water. The seasonal cycle of in situ LAI differed markedly from the seasonal cycles of in situ normalized difference vegetation index (NDVI) and the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD15 LAI product. We hypothesize that the NDVI and MOD15 seasonality at the site is driven partly by seasonal changes in leaf age and leaf reflectance. We developed three simple models to investigate the causes of GEE seasonality. The first two models showed that the seasonal changes in LAI alone, and the effects of leaf age on leaf-level photosynthesis alone, could not account for the observed GEE seasonality. The third model showed that the combined effect of seasonal changes in LAI and seasonal changes in leaf age and leaf photosynthesis was sufficient to account for the observed GEE seasonality.

Spectroscopic Remote Sensing of Non-Structural Carbohydrates in Forest Canopies

“…We varied LAI from 1.0 to 8.0 units (Table 2), which not only incorporates a global range of LAI values [23], but is also far more variable than what is typically encountered in closed-canopy tropical forests [38]. Our leaf angle distributions and other model parameters were also structurally very wide ranging [18,20].…”

Drought impact on forest carbon dynamics and fluxes in Amazonia

“…In our seasonal estimates of NPP we 350 exclude several smaller components such as branchfall (although these data are shown in ED figure 6 351 and described in ED Tables 1-3), herbivory, coarse root, and small tree NPP (<10cm) that we have 352 included in previous estimates of these sites. We calculate leaf flush by calculating the change in leaf 353 area index, LAI (m 2 m -2 ), multiplied by the mean specific leaf area, SLA (m 2 g -1 ), and adding this to 354 leaf litterfall following a procedure from Doughty and Goulden (2008) 31 . Total estimated autotrophic 355 respiration consisted of rhizosphere respiration (i.e.…”

Carbon exchange in an Amazon forest: from hours to years

“…NEE is the difference between uptake from gross ecosystem productivity (GEP) and emission from ecosystem respiration (RE). The magnitudes of these gross fluxes are influenced both by exogenous environmental conditions such as light, moisture, and temperature (Collatz et al, 1991;Bolker et al, 1998;Fatichi et al, 2014;Kiew et al, 2018) and by endogenous biophysical properties such as canopy structure, phenology, and community composition (Barford et al, 2001;Melillo et al, 2002;Dunn et al, 2007;Doughty and Goulden, 2008;Stark et al, 2012;Frey et al, 2013;Morton et al, 2016;Wu et al, 2016).…”