Agroforestry systems may play a critical role in reducing the vulnerability of farmers' livelihood to droughts as tree-based systems provide several mechanisms that can mitigate the impacts from extreme weather events. Here, we use a replicated throughfall reduction experiment to study the drought response of a cacao/Gliricidia stand over a 13-month period. Soil water content was successfully reduced down to a soil depth of at least 2.5 m. Contrary to our expectations we measured only relatively small nonsignificant changes in cacao (À11%) and Gliricidia (À12%) sap flux densities, cacao leaf litterfall ( 1 8%), Gliricidia leaf litterfall (À2%), soil carbon dioxide efflux (À14%), and cacao yield (À10%) during roof closure. However, cacao bean yield in roof plots was substantially lower (À45%) compared with control plots during the main harvest following the period when soil water content was lowest. This indicates that cacao bean yield was more sensitive to drought than other ecosystem functions. We found evidence in this agroforest that there is complementary use of soil water resources through vertical partitioning of water uptake between cacao and Gliricidia. This, in combination with acclimation may have helped cacao trees to cope with the induced drought. Cacao agroforests may thus play an important role as a drought-tolerant land use in those (sub-) tropical regions where the frequency and severity of droughts is projected to increase.

We measured fluxes of three greenhouse gases (N 2 O, CO 2 and CH 4 ) from soils of six different land-use types at 27 temporary field sites in Jambi Province, Sumatra, Indonesia. Study sites included natural and logged-over forests; rubber plantation; oil palm plantation; cinnamon plantation; and grassland field. The ranges of N 2 O, CO 2 and CH 4 fluxes were 0.13-55.8 µg N m −2 h −1 ; 1.38-5.16 g C m −2 d −1 ; −1.27-1.18 mg C m −2 d −1 , respectively. The averages of N 2 O, CO 2 and CH 4 fluxes at 27 sites were 9.4 µg N mrespectively. The values of CO 2 and CH 4 fluxes were comparable with those in the reports regarding other humid tropical forests, while the N 2 O flux was relatively lower than those of previous reports. The N 2 O fluxes in each soil type were correlated with the nitrification rates of soils of 0-5 cm depth. In Andisols, the ratio of the N 2 O emission rate to the nitrification rate was possibly smaller than that of the other soil types. There was no clear relationship between N 2 O flux and the soil water condition, such as water-filled pore space. Seventeen percent of CH 4 fluxes were positive; according to these positive fluxes, we did not find a good correlation between CH 4 uptake rate and soil properties. Although we performed a chronosequence analysis to produce some hypotheses about the effect of land-use change by a limited amount of sampling at one point in time, further tests are required for the future.

It is generally assumed that declining soil fertility during cultivation forces farmers to clear forest. We wanted to test this for a rainforest margin area in Central Sulawesi, Indonesia. We compared soil characteristics in different landuse systems and after different length of cultivation. 66 sites with four major land-use systems (maize, agroforestry, forest fallow and natural forest) were sampled. Soils were generally fertile, with high base cation saturation, high cation exchange capacity, moderate pH-values and moderate to high stocks of total nitrogen. Organic matter stocks were highest in natural forest, intermediate in forest fallow and lowest in maize and agroforestry sites. In maize fields soil organic matter decreased during continuous cultivation, whereas in agroforestry it was stable or had the tendency to increase in time. The effective cation exchange capacity (ECEC) was highest in natural forest and lowest in maize fields. Base cations saturation of ECEC did not change significantly during cultivation both maize and agroforestry, whereas the contribution of K cations decreased in maize and showed no changes in agroforestry sites. Our results indicate that maize cultivation tends to reduce soil fertility but agroforestry systems are able to stop this decline of soil fertility or even improve it. As most areas in this rain forest margin are converted into agroforestry systems it is unlikely that soil degradation causes deforestation in this case. On the contrary, the relatively high soil fertility may actually attract new immigrants who contribute to deforestation and start agriculture as smallholders.

Spatial patterns of CO 2 , CH 4 , and N 2 O flux were analyzed in the soil of a primary forest in Sumatra, Indonesia. The fluxes were measured at 3-m intervals on a sampling grid of 8 rows by 10 columns, with fluxes found to be below the minimum detection level at 12 points for CH 4 and 29 points for N 2 O. All three gas fluxes distributed log-normally. The means and standard deviations of CO 2 and CH 4 fluxes calculated by the maximum likelihood method were 3.68 Ϯ 1.32 g C m -2 d -1 and 0.79 Ϯ 0.60 mg C m -2 d -1 , respectively. The mean and standard deviation of N 2 O fluxes using a maximum likelihood estimator for the censored data set was 2.99 Ϯ 3.26 g N m -2 h -1 . The spatial dependency of CH 4 fluxes was not detected in 3-m intervals, while weak spatial dependency was observed in CO 2 and N 2 O fluxes. The coefficients of variation of CH 4 and N 2 O were higher than that of CO 2 . Some hot spots where high levels of CH 4 and N 2 O were generated in the studied field may increase the variability of these gases. The resulting patterns of variability suggest that sampling distances of Ͼ 10 m and Ͼ 20 m are required to obtain statistically independent samples for CO 2 and N 2 O flux in the studied field, respectively. But because of weak or no spatial dependency of each flux, a sampling distance of more than 10 m intervals is enough to prevent a significant problem of autocorrelation for each flux measurement.

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Abstract. Estimation of belowground carbon stocks in tropical wetland forests requires funding for laboratory analyses and suitable facilities, which are often lacking in developing nations where most tropical wetlands are found. It is therefore beneficial to develop simple analytical tools to assist belowground carbon estimation where financial and technical limitations are common. Here we use published and original data to describe soil carbon density (kgC m −3 ; C d ) as a function of bulk density (gC cm −3 ; B d ), which can be used to rapidly estimate belowground carbon storage using B d measurements only. Predicted carbon densities and stocks are compared with those obtained from direct carbon analysis for ten peat swamp forest stands in three national parks of Indonesia. Analysis of soil carbon density and bulk density from the literature indicated a strong linear relationship (C d = B d ×495.14+5.41, R 2 = 0.93, n = 151) for soils with organic C content > 40 %. As organic C content decreases, the relationship between C d and B d becomes less predictable as soil texture becomes an important determinant of C d . The equation predicted belowground C stocks to within 0.92 % to 9.57 % of observed values. Average bulk density of collected peat samples was 0.127 g cm −3 , which is in the upper range of previous reports for Southeast Asian peatlands. When original data were included, the revised equation C d = B d × 468.76 + 5.82, with R 2 = 0.95 and n = 712, was slightly below the lower 95 % confidence interval of the original equation, and tended to decrease C d estimates. We recommend this last equation for a rapid estimation of soil C stocks for well-developed peat soils where C content > 40 %.

Abstract. Climate change induced droughts pose a serious threat to ecosystems across the tropics and sub-tropics, particularly to those areas not adapted to natural dry periods. In order to study the vulnerability of cacao (Theobroma cacao) -Gliricidia sepium agroforestry plantations to droughts a large scale throughfall displacement roof was built in Central Sulawesi, Indonesia. In this 19-month experiment, we compared soil surface CO 2 efflux (soil respiration) from three roof plots with three adjacent control plots. Soil respiration rates peaked at intermediate soil moisture conditions and decreased under increasingly dry conditions (drought induced), or increasingly wet conditions (as evidenced in control plots). The roof plots exhibited a slight decrease in soil respiration compared to the control plots (average 13% decrease). The strength of the drought effect was spatially variable -while some measurement chamber sites reacted strongly (responsive) to the decrease in soil water content (up to R 2 = 0.70) (n = 11), others did not react at all (non-responsive) (n = 7). A significant correlation was measured between responsive soil respiration chamber sites and sap flux density ratios of cacao (R = 0.61) and Gliricidia (R = 0.65). Leaf litter CO 2 respiration decreased as conditions became drier. The litter layer contributed approximately 3-4% of the total CO 2 efflux during dry periods and up to 40% during wet periods. Within days of roof opening soil CO 2 efflux rose to control plot levels. Thereafter, CO 2Correspondence to: O. van Straaten (ostraat@gwdg.de) efflux remained comparable between roof and control plots. The cumulative effect on soil CO 2 emissions over the duration of the experiment was not significantly different: the control plots respired 11.1±0.5 Mg C ha −1 yr −1 , while roof plots respired 10.5±0.5 Mg C ha −1 yr −1 . The relatively mild decrease measured in soil CO 2 efflux indicates that this agroforestry ecosystem is capable of mitigating droughts with only minor stress symptoms.