Zhenghong Tan

Carbon balance of a primary tropical seasonal rain forest

The role of primary tropical rain forests in the global carbon cycle is under active debate. By combining long-term forest inventory data with physiological measurement data in a 1 ha permanent ecological research plot beneath an eddy covariance flux tower in a primary tropical seasonal rain forest, the ecosystem carbon balance was investigated and a detailed site-specific carbon budget was established. The studied ecosystem was a carbon sink as determined by both eddy covariance (1.19 Mg C ha(-1) yr(-1)) and biometric methods (3.59 Mg C ha(-1) yr(-1)). Biometric-and eddy covariance-based net ecosystem production showed no convergence in our investigation period. The large biomass increment, caused by the rapid annual growth rate of large trees, primarily accounted for the large ecosystem carbon sink derived from the biometric method. High leaf respiration in relation to carbon allocation and low ecosystem carbon use efficiency (0.34) were observed at our site.

Using digital cameras for comparative phenological monitoring in an evergreen broad-leaved forest and a seasonal rain forest

article i nfo Article history: Digital cameras have been used in phenological observations for their high accuracy and low labor cost. Most studies successfully use greenness indices derived from digital images for timing the events related to leaf de- velopment. However, when timing the leaf senescence events, wide discrepancies between actual and esti- mated dates are common. In this study, images of three species (two from an evergreen broad-leaved forest and one from a seasonal rain forest) were used to estimate three phenological events of leaf develop- ment and senescence. Other than the greenness index, a redness index was also employed. Different annual patterns in color indices developed among the species. The redness index was more accurate when estimat- ing leaf senescence, while the greenness index was more accurate for estimating leaf development events in Acer heptalobum and Machilus bombycina. The absolute differences in estimations of phenological events ran- ged from �3 to 1 day, which is more accurate than estimates based on the greenness index only (� 2t o 27 days). With the introduction of the redness index, this technique has been much improved and is possible to be applied to more species. Furthermore, variations of color indices during periods of phenological events were highly related to the climatic factors with a time lag of around 10 days. Because of the ease of use and efficiency (i.e., automatic daily data output), digital cameras are expected to be used in ecosystem process modeling, networks of phenology assessment and validation of the remote sensing results from satellites.

An observational study of the carbon-sink strength of East Asian subtropical evergreen forests

Relatively little is known about the effects of regional warming on the carbon cycle of subtropical evergreen forest ecosystems, which are characterized by year-round growing season and cold winters. We investigated the carbon balance in three typical East Asia subtropical evergreen forests, using eddy flux, soil respiration and leaf-level measurements. Subtropical evergreen forests maintain continuous, high rates of photosynthetic activity, even during winter cold periods. Warm summers enhance photosynthetic rates in a limited way, because overall ecosystem productivity is primarily restrained by radiation levels during the warm period. Conversely, warm climates significantly enhance the respiratory carbon efflux. The finding of lower sensitivity of photosynthesis relative to that of respiration suggests that increased temperature will weaken the carbon-sink strength of East Asia subtropical evergreen forests.

Effects of continuous drought stress on soil respiration in a tropical rainforest in southwest China

Background and aimsDrought is predicted to have a profound impact on soil respiration. This study aimed to assess the effects of long-term precipitation decrease on soil respiration in a tropical rainforest.MethodsA precipitation reduction experiment was conducted in a tropical forest in southwest China at the beginning of 2011. Soil respiration and environmental parameters were measured monthly for three years.ResultsThe continuous precipitation reduction treatment did not affect the seasonal patterns of soil respiration, but it significantly increased soil respiration in the study plot during the rainy season, and the relationship between soil respiration and soil moisture differed in the control and reduction treatment in the rainy season. Compared with the net ecosystem exchange of carbon in this system, the increment of annual soil carbon emissions in the reduction treatment was considerable and should not be ignored.ConclusionsOur results indicate that the responses of soil respiration to precipitation decrease may vary seasonally and the variation of volumetric water content in different seasons may be an important factor leading to the seasonal variation. The variation of soil moisture among different ecosystems as well as in different seasons should be taken into consideration when predicting the future response of soil respiration to drought globally.

Interannual and seasonal variability of water use efficiency in a tropical rainforest: Results from a 9 year eddy flux time series

We used a continuous 9 year (2003-2011) eddy flux time series with 30min resolution to examine water use efficiency in a tropical rainforest and determine its environmental controls. The multiyear mean water use efficiency (W-ue) of this rainforest was 3.160.33 gC per kg H2O, which is close to that of boreal forests, but higher than subtropical forests, and lower than temperate forests. The water vapor deficit (V-PD) had a strong impact on instantaneous W-ue, in the manner predicted by stomatal optimization theory. At the seasonal scale, temperature was the dominant controller of W-ue. The negative correlation between temperature and W-ue was probably caused by high continuous photosynthesis during low-temperature periods. The V-PD did not correlate with W-ue at the interannual scale. No interannual trend was detected in W-ue or inherent water use efficiency (W-ei), either annually or seasonally. The fact that no increasing trend of W-ei was found in the studied tropical rainforest, along with other evidence of CO2 stimulation in tropical rainforests, requires special attention and data validation. There was no significant difference between W-ue during a drought and the 9 year mean values in the forest we studied, but we found that dry season transpiration (T-r) was consistently lower during the drought compared to the mean values. Finally, whether W-ue increases or decreases during a drought is determined by the drought sensitivity of gross primary production (G(PP)).

Partial net primary production of a mixed dipterocarp forest: Spatial patterns and temporal dynamics

We examined how and why partial net primary production (NPPpart) varies across time and space in a Chinese dipterocarp forest. We hypothesize that (1) soil geochemistry explains the spatial pattern of NPPpart within the plot and (2) NPPpart can be used to measure the degree of drought resilience of a natural forest. Spatially, NPPpart was autocorrelated in the range of 75.3m and homogenous. This spatial pattern could not be well explained by any of the soil properties individually or in combination. If drought sensitivity is defined by marked reduction in NPPpart, the studied forest is drought resilient even when a longer and drier than usual drought hit. Although annual NPPpart was unchanged (vary within 18.24 and 18.52tha(-1)yr(-1)) after the drought, the allocation of NPPpart to short-lived litterfall increased, which has further effects on the ecosystem net carbon balance. Key Points