The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m(-2) year(-1)) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.

Abstract:Lianas are important components of tropical forests and have significant impacts on the diversity, structure and dynamics of tropical forests. The present study documented the liana flora in a Chinese tropical region. Species richness, abundance, size-class distribution and spatial patterns of lianas were investigated in three 1-ha plots in tropical seasonal rain forests in Xishuangbanna, SW China. All lianas with ≥ 2 cm diameter at breast height (dbh) were measured, tagged and identified. A total of 458 liana stems belonging to 95 species (ranging from 38 to 50 species/ha), 59 genera and 32 families were recorded in the three plots. The most well-represented families were Loganiaceae, Annonceae, Papilionaceae, Apocynaceae and Rhamnaceae. Papilionaceae (14 species recorded) was the most important family in the study forests. The population density, basal area and importance value index (IvI) varied greatly across the three plots. Strychnos cathayensis, Byttneria grandifolia and Bousigonia mekongensis were the dominant species in terms of IvI across the three plots. The mean aboveground biomass of lianas (3 396 kg/ha) accounted for 1.4% of the total community aboveground biomass. The abundance, diversity and biomass of lianas in Xishuangbanna tropical seasonal rain forests are lower than those in tropical moist and wet forests, but higher than those in tropical dry forests. This study provides new data on lianas from a geographical region that has been little-studied. Our findings emphasize that other factors beyond the amount and seasonality of precipitation should be included when considering the liana abundance patterns across scales. Rev. Biol. Trop. 57 (1-2): 211-222. Epub 2009 June 30.Key words: biodiversity, liana abundance, liana biomass, species richness, tropical forests.The woody climbing plants, lianas, are important components of tropical forests. They typically rely on other plants for mechanical support and are abundant, diverse, and make a significant contribution (10-25%) to the overall plant diversity in the tropical forests (Gentry and Dodson 1987, Nabe-Nielsen 2001). Before the 1980s, limited attention is paid to lianas in the tropical forest. Recent years, however, more and more studies about lianas have been conducted due to the awareness of the important role of lianas in the tropical forest ecosystems (Schnitzer 2005, Gerwing et al. 2006. Even though, the ecology of lianas in most forests is still poorly understood (Mascaro et al. 2004).Lianas have been found to play a vital role in many aspects of forest ecosystem dynamics and functioning, including reducing treegrowth rates (Putz 1984), suppressing tree regeneration , providing valuable food sources for insects and animals (Nabe-Nielsen 2001), physically linking trees together and thereby increasing community stability and providing canopy-to-canopy access for arboreal animals (Schnitzer and Bongers 2002). However, we still have only a vague idea of the liana composition of most tropical forests and know little about the role th...

Decomposition of woody detritus is an important but often ignored process in forest ecosystems. Moisture and temperature regimes are dominant controls over woody decay, contributing to significant variability at local, regional, and global scales. Our focus was on local variability in woody decay rates depending on their physical position. Woody detritus may decay on the forest floor, aboveground, or combination of both, depending on the mortality agent. In this study, we measured decay rates of logs, large branches on the forest floor, and snags over a three-year period. We also collected monthly respiration estimates, and analyzed woody detritus N and P content throughout the study. Logs exhibited the greatest mass loss with a decay-rate constant of k = 0.606 ± 0.020), followed by large branches (k = 0.316 ± 0.012) and snags (k = 0.268 ± 0.008). Heterotrophic respiration was greatest prior to the peak of rainy season, and was greatest for snag material during the first two years of sampling, probably a result of water saturation in ground material. Both N and P were released in all materials, but the rate of P release was much slower in snags. There were large differences of P concentration and C:P among the materials, but their value became similar after three years, indicating P limitation on microbial activities. Our results presented robust evidence for the physical-position-dependence of coarse woody detritus decomposition in the forests.