LBA-ECO ND-02 SOIL GAS FLUX, RAINFALL EXCLUSION, KM 67, TAPAJOS NATIONAL FOREST

Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Nino-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 (Tapajos 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 similar to 890 mm of throughfall during the exclusion period of 2000 (late January to early August) and similar to680 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 similar to100 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 similar to140 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 the stemwood increment that determines the amount of carbon stored in the forest. These results suggest that the net accumulation of carbon in mature Amazon forests indicated by recent permanent plot and eddy covariance studies may be very sensitive to small reductions in rainfall. The soil water reductions were also sufficient to increase soil emissions of N2O and to increase soil consumption of CH4-both radiatively important gases in the atmosphere.The possible reduction of tree reproductive activity points to potentially important effects of drought on the long-term species composition of Amazon forests.亚马孙及全球其他区域的湿润热带森林，正经由厄尔尼诺-南方涛动（El Nino-Southern Oscillation, ENSO）循环，以及可能由森林砍伐驱动的降雨减少过程，面临愈发严峻的干旱事件。此类趋势对热带森林林冠动态、温室气体排放及其他生态功能的潜在影响规模可观，但当前相关认知仍较为匮乏。 为阐明这些影响的作用机制，我们在亚马孙中东部的巴西塔帕若斯国家森林公园内的森林样地中，开展了穿透雨排除实验（throughfall exclusion experiment）。在对两块1公顷的森林样地完成为期1年的校准周期后，我们在其中一块样地的林下安装了塑料面板与木质水槽，以此在2000年的排除周期（1月下旬至8月初）内截留约890毫米的穿透雨；在2001年的排除周期（1月上旬至5月下旬）中，截至当前已累计截留约680毫米的穿透雨。 2000年排除周期内，到达土壤表层的日均穿透雨量在处理组与对照组样地中分别为4.9毫米与8.3毫米；2001年该数值则分别为4.8毫米与8.1毫米。在首个排除周期中，0-2米深度的表层土壤含水量下降约100毫米，而2-11米深度的深层土壤含水量未受显著影响。在第二个排除周期——该周期于旱季后期、土壤含水量处于低位时启动——中，表层与深层土壤含水量分别下降约140毫米与160毫米。 尽管此次土壤水分耗竭未引发可检测到的叶片干旱胁迫（即黎明前叶水势（predawn leaf water potential）未出现可观测下降），但部分物种的光合能力出现了显著下降；在第二个排除周期内，林冠出现疏伐现象，表现为林冠开放度提升、叶面积指数降低；高度低于15米的树木茎径向生长量出现下降，处理组样地中的细枯落物产量与树木结实量均有所减少。 处理组样地的地上净初级生产力（NPP，涵盖茎木增量与细枯落物产量）下降了四分之一，从15.1 Mg ha⁻¹ yr⁻¹降至11.4 Mg ha⁻¹ yr⁻¹；对照组样地的NPP则仅出现小幅下降，从11.9 Mg ha⁻¹ yr⁻¹降至11.5 Mg ha⁻¹ yr⁻¹。茎呼吸作用呈现季节性变化规律，且与茎径向生长呈显著正相关，但未表现出显著的处理组效应差异。 对穿透雨排除及其引发的表层土壤水分亏缺响应最快的介质为土壤本身。相较于对照组样地，处理组样地的N₂O排放有所降低，而CH₄氧化速率有所提升，这一现象大概率由土壤干燥带来的土壤通气性改善所导致。我们提出的"排除穿透雨后NO排放会增加"的研究假说未得到实验数据支持。 穿透枯落物层与深度达200厘米的矿质土壤的渗漏水的电导率与碱度，在处理组样地中更高，这或许是因为该样地中流经这些土层的径流水量更少。枯落物的分解速率在两组样地间未观测到显著差异。 综上，在部分穿透雨排除实验的前两年中，土壤水分的小幅减少已足以降低地上净初级生产力，包括决定森林碳储存量的茎木增量。本研究结果表明，近期永久样地与涡度协方差（eddy covariance）观测研究显示的成熟亚马孙森林碳净累积量，可能对降雨的小幅减少极为敏感。土壤水分减少还足以提升土壤N₂O排放与CH₄氧化速率——这两种气体均为大气中具有重要辐射强迫效应的温室气体。树木繁殖活动的潜在减少，也暗示干旱可能对亚马孙森林的长期物种组成产生重要且深远的影响。