Hua-Zheng Lu

Heterotrophic respiration does not acclimate to continuous warming in a subtropical forest.

As heterotrophic respiration (RH) has great potential to increase atmospheric CO2 concentrations, it is important to understand warming effects on RH for a better prediction of carbon–climate feedbacks. However, it remains unclear how RH responds to warming in subtropical forests. Here, we carried out trenching alone and trenching with warming treatments to test the climate warming effect on RH in a subtropical forest in southwestern China. During the measurement period, warming increased annual soil temperature by 2.1 °C, and increased annual mean RH by 22.9%. Warming effect on soil temperature (WET) showed very similar pattern with warming effect on RH (WERH), decreasing yearly. Regression analyses suggest that WERH was controlled by WET and also regulated by the soil water content. These results showed that the decrease of WERH was not caused by acclimation to the warmer temperature, but was instead due to decrease of WET. We therefore suggest that global warming will accelerate soil carbon efflux to the atmosphere, regulated by the change in soil water content in subtropical forests.

Organic nitrogen uptake is a significant contributor to nitrogen economy of subtropical epiphytic bryophytes.

Without any root contact with the soil, epiphytic bryophytes must experience and explore poor, patchy, and heterogeneous habitats; while, the nitrogen (N) uptake and use strategies of these organisms remain uncharacterized, which obscures their roles in the N cycle. To investigate the N sources, N preferences, and responses to enhanced N deposition in epiphytic bryophytes, we carried out an in situ manipulation experiment via the 15N labelling technique in an Asian cloud forest. Epiphytic bryophytes obtained more N from air deposition than from the bark, but the contribution of N from the bark was non-negligible. Glycine accounted for 28.4% to 44.5% of the total N in bryophyte tissue, which implies that organic N might serve as an important N source. Increased N deposition increased the total N uptake, but did not alter the N preference of the epiphytic bryophytes. This study provides sound evidence that epiphytic bryophytes could take up N from the bark and wet deposition in both organic and inorganic N forms. It is thus important to consider organic N and bark N sources, which were usually neglected, when estimating the role of epiphytic bryophytes in N cycling and the impacts of N deposition on epiphytic bryophytes in cloud forests.

Adaptation of epiphytic bryophytes in the understorey attributing to the correlations and trade-offs between functional traits

This study explores adaptive strategies of epiphytic bryophytes in the understorey by investigating the photosynthetic characteristics, pigment concentrations and nutrient stoichiometry, as well as other functional traits of three trunk-dwelling bryophytes in a subtropical montane cloud forest in SW China. The results showed that their light-saturated net photosynthetic rate (Anmax−L), light saturation point (Isat), light compensation point (Ic) and dark respiration rate (Rd) were ca 0.55, 106.72, 4.17 and 0.25 μmol m−2 s−1, respectively. Furthermore, the samples demonstrated photosynthetic down-regulation under high irradiance. These photosynthetic characteristics can be explained by higher total chlorophyll concentrations, specific leaf area, chlorophyll per unit leaf N (Chl/N), lower ratio of chlorophyll a to chlorophyll b (Chl a/b) and photosynthetic nitrogen-use efficiency. We suggest that the bryophytes adapted to the shaded understorey microhabitats through a series of correlations and trade-offs between functional traits.

Survival and Growth of Epiphytic Ferns Depend on Resource Sharing

Locally available resources can be shared within clonal plant systems through physiological integration, thus enhancing their survival and growth. Most epiphytes exhibit clonal growth habit, but few studies have tested effects of physiological integration (resource sharing) on survival and growth of epiphytes and whether such effects vary with species. We conducted two experiments, one on individuals (single ramets) and another on groups (several ramets within a plot), with severed and intact rhizome treatments (without and with physiological integration) on two dominant epiphytic ferns (Polypodiodes subamoena and Lepisorus scolopendrium) in a subtropical montane moist forest in Southwest China. Rhizome severing (preventing integration) significantly reduced ramet survival in the individual experiment and number of surviving ramets in the group experiment, and it also decreased biomass of both species in both experiments. However, the magnitude of such integration effects did not vary significantly between the two species. We conclude that resource sharing may be a general strategy for clonal epiphytes to adapt to forest canopies where resources are limited and heterogeneously distributed in space and time.

Higher clonal integration in the facultative epiphytic fern Selliguea griffithiana growing in the forest canopy compared with the forest understorey

BACKGROUND AND AIMS The advantage of clonal integration (resource sharing between connected ramets of clonal plants) varies and a higher degree of integration is expected in more stressful and/or more heterogeneous habitats. Clonal facultative epiphytes occur in both forest canopies (epiphytic habitats) and forest understories (terrestrial habitats). Because environmental conditions, especially water and nutrients, are more stressful and heterogeneous in the canopy than in the understorey, this study hypothesizes that clonal integration is more important for facultative epiphytes in epiphytic habitats than in terrestrial habitats. METHODS In a field experiment, an examination was made of the effects of rhizome connection (connected vs. disconnected, i.e. with vs. without clonal integration) on survival and growth of single ramets, both young and old, of the facultative epiphytic rhizomatous fern Selliguea griffithiana (Polypodiaceae) in both epiphytic and terrestrial habitats. In another field experiment, the effects of rhizome connection on performance of ramets were tested in small (10 × 10 cm(2)) and large (20 × 20 cm(2)) plots in both epiphytic and terrestrial habitats. KEY RESULTS Rhizome disconnection significantly decreased survival and growth of S. griffithiana in both experiments. The effects of rhizome disconnection on survival of single ramets and on ramet number and growth in plots were greater in epiphytic habitats than in terrestrial habitats. CONCLUSIONS Clonal integration contributes greatly to performance of facultative epiphytic ferns, and the effects were more important in forest canopies than in forest understories. The results therefore support the hypothesis that natural selection favours genotypes with a higher degree of integration in more stressful and heterogeneous environments.