The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest

Abstract

Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Niño–Southern Oscillation (ENSO) and possibly through deforestation‐driven reductions in rainfall. The effects of this trend on tropical forest canopy dynamics, emissions of greenhouse gases, and other ecological functions are potentially large but poorly understood. We established a throughfall exclusion experiment in an east‐central Amazon forest (Tapajós National Forest, Brazil) to help understand these effects. After 1‐year intercalibration period of two 1‐ha forest plots, we installed plastic panels and wooden gutters in the understory of one of the plots, thereby excluding ∼890 mm of throughfall during the exclusion period of 2000 (late January to early August) and ∼680 mm thus far in the exclusion period of 2001 (early January to late May). Average daily throughfall reaching the soil during the exclusion period in 2000 was 4.9 and 8.3 mm in the treatment and control plots and was 4.8 and 8.1 mm in 2001, respectively. During the first exclusion period, surface soil water content (0–2 m) declined by ∼100 mm, while deep soil water (2–11 m) was unaffected. During the second exclusion period, which began shortly after the dry season when soil water content was low, surface and deep soil water content declined by ∼140 and 160 mm, respectively. Although this depletion of soil water provoked no detectable increase in leaf drought stress (i.e., no reduction in predawn leaf water potential), photosynthetic capacity declined for some species, the canopy thinned (greater canopy openness and lower leaf area index) during the second exclusion period, stem radial growth of trees <15 m tall declined, and fine litterfall declined in the treatment plot, as did tree fruiting. Aboveground net primary productivity (NPP) (stemwood increment and fine litter production) declined by one fourth, from 15.1 to 11.4 Mg ha−1 yr−1, in the treatment plot and decreased slightly, from 11.9 to 11.5 Mg ha−1 yr−1, in the control plot. Stem respiration varied seasonally and was correlated with stem radial growth but showed no treatment response. The fastest response to the throughfall exclusion, and the surface soil moisture deficits that it provoked, was found in the soil itself. The treatment reduced N2O emissions and increased CH4 consumption relative to the control plot, presumably in response to the improved soil aeration that is associated with soil drying. Our hypothesis that NO emissions would increase following exclusion was not supported. The conductivity and alkalinity of water percolating through the litter layer and through the mineral soil to a depth of 200 cm was higher in the treatment plot, perhaps because of the lower volume of water that was moving through these soil layers in this plot. Decomposition of the litter showed no difference between plots. In sum, the small soil water reductions provoked during the first 2 years of partial throughfall exclusion were sufficient to lower aboveground NPP, including th...

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…4e) and other models (Supplement Fig D2) whose dry season E rates were sustained in part by capillary fluxes from below the simulated rooting zone. Absence of shallow groundwater in the Tapajós region is also corroborated by anecdotal evidence reported in the literature (reported at depths of ∼100 m in Nepstad et al, 2002;Belk et al, 2007), but it is important to note that water tables this deep are not characteristic of Amazonia in general (MiguezMacho and Fan, 2012). The observations further bound the degree of seasonal variation in soil moisture predicted by deep-root models (e.g., variation SiB3 is too large; Fig.…”

Widespread Amazon forest tree mortality from a single cross‐basin squall line event

“…In Manaus, the effects of rainfall anomalies were observed late in the dry season (September-October) [Marengo et al, 2008], whereas the elevated mortality reported here from both Landsat images and forest plot data occurred before the late dry season. Also, for drought to kill trees, multi-year severe rainfall reduction is needed to produce droughtinduced tree mortality [Nepstad et al, 2002;Meir et al, 2008]. Furthermore, individual snapped and windthrown trees comprised a significant fraction of the observed mortality at the BIONTE plots (data collected in July).…”

Comparison of four light use efficiency models for estimating terrestrial gross primary production

“…Specifically, the EC-LUE performed best (compared to the GLO-PEM and VPM) in the nonforest sites (e.g., savanna and grassland that usually have semi-arid climates), since the fw used by it (evaporative fraction, EF) related closely to the surface soil water content, which was documented as the major water stress on plant growth in those kinds of ecosystems (Sala et al, 1988;Kurc and Small, 2004). By contrast, the GLO-PEM and VPM performed better in the forest sites (characterized by the high annual precipitation and the deep roots system for accessing deep soil water in dry seasons), since the fw utilized by them (soil water condition (W s ) and atmosphere moisture condition (VPD s ), and the land surface water index (LSWI), respectively) can reflect both the leaf and canopy water content, which were believed to mainly limit the light use efficiency of plants in those ecosystems (Nepstad et al, 2002;Xiao et al, 2005b). Moreover, the VPM had better performance than GLO-PEM in DBF because the fw in it included the phenology scalar (P s ), which can account for the effects of leaf age on photosynthesis (Xiao et al, 2004b).…”