Xiaobao Deng

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.

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)).

Monodominance of Parashorea chinensis on fertile soils in a Chinese tropical rain forest

Monodominance in the tropics is often seen as an unusual phenomenon due to the normally high diversity in tropical rain forests. Here we studied Parashorea chinensis H. Wang (Dipterocarpaceae) in a seasonal tropical forest in south-west China, to elucidate the mechanisms behind its monodominance. Twenty-eight 20 × 20-m plots were established in monodominant and mixed forest in Xishuangbanna, Yunnan province. All individuals 1c m stem diameter and 16 soil variables were measured. Parashorea chinensis forest had a significantly higher mean tree dbh compared with mixed forest. Diversity did not differ significantly between the two forest types. However, within monodominantpatches,alldiversityindicesdecreasedwithanincreaseinP.chinensisdominance.Floristiccomposition of P. chinensis forest did differ significantly from the mixed forest. These differences were associated with more fertile soils (significantly higher pH, Mn, K and lower carbon pools and C:N ratio) in the P. chinensis forest than the mixed forest. In contrast to current paradigms, this monodominant species is not associated with infertile, but with fertile soils.Parashoreachinensisseemstobeespeciallyassociatedwithhighmanganeseconcentrationswhichitcantolerate, andwithedaphicconditions(water,K)thatallowthistallandexposedemergentspeciestomaintainitswaterbalance. This is in contrast with most previous studies on monodominance in the tropics that found either no effect of soil properties, or predict associations with nutrient-poor soils.

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

Impact of Striped-Squirrel Nectar-Robbing Behaviour on Gender Fitness in Alpinia roxburghii Sweet (Zingiberaceae)

Nectar-robbing has the potential to strongly affect male and female reproductive fitness of plants. One example of nectar theft is that shown by striped-squirrels (Tamiops swinhoei) on a number of ginger species, including Alpinia roxburghii and A. kwangsiensis (Zingiberaceae). In this study, we used a fluorescent dye as a pollen analogue, and measured fruit and seed output, to test the effect of squirrel nectar-robbing on A. roxburghii reproductive fitness. Pollen transfer between robbed and unrobbed flowers was assessed by comparing 60 randomly established plots containing robbed and unrobbed flowers. The frequency of squirrel robbing visits and broken styles were recorded from a number of flowers for five consecutive days. Two bee species (Bombus eximius and Apis cerana), were the primary pollinators, and their visitation frequency was recorded for six consecutive days. The results showed that fluorescent powder from unrobbed flowers was dispersed further, and to a greater number of flowers than that placed on robbed flowers. Additionally, robbing flowers caused significant damage to reproductive organs, resulting in lower fruit and seed sets in robbed than in unrobbed flowers and influencing both male and female fitness. The frequency of the primary pollinator visits (B. eximius) was significantly higher for unrobbed plants than for robbed plants. The present study clearly shows the negative impact of squirrel robbing on A. roxburghii male reproductive fitness and neutral impact on female reproductive fitness.