Effects of experimental nitrogen additions on plant diversity in an old‐growth tropical forest

Abstract

Response of plant biodiversity to increased availability of nitrogen (N) has been investigated in temperate and boreal forests, which are typically N-limited, but little is known in tropical forests. We examined the effects of artificial N additions on plant diversity (species richness, density and cover) of the understory layer in an N saturated old-growth tropical forest in southern China to test the following hypothesis: N additions decrease plant diversity in N saturated tropical forests primarily from N-mediated changes in soil properties. Experimental additions of N were administered at the following levels from July 2003 to July 2008: no addition (Control); 50 kg N ha À1 yr À1 (Low-N); 100 kg N ha À1 yr À1 (Medium-N), and 150 kg N ha À1 yr À1 (High-N). Results showed that no understory species exhibited positive growth response to any level of N addition during the study period. Although low-to-medium levels of N addition ( 100 kg N ha À1 yr À1 ) generally did not alter plant diversity through time, high levels of N addition significantly reduced species diversity. This decrease was most closely related to declines within tree seedling and fern functional groups, as well as to significant increases in soil acidity and Al mobility, and decreases in Ca availability and fine-root biomass. This mechanism for loss of biodiversity provides sharp contrast to competitionbased mechanisms suggested in studies of understory communities in other forests. Our results suggest that high-N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition.

Effects of experimental nitrogen additions on plant diversity in tropical forests of contrasting disturbance regimes in southern China

“…Although soil acidification process can contribute to decline of diversity (Gilliam, 2006;Bobbink et al, 2010;Lu et al, 2010), there was no evidence that it was important in the present study. In both disturbed and rehabilitated forests, we found no negative effects of N additions on leaching of base cations (K, Ca, and Mg) and release of soil Al (Table 2).…”

“…The understory layer, defined here as all vascular plants 1 m in height (similar to the herbaceous layer sensu Gilliam and Roberts, 2003;Lu et al, 2010), was monitored within the two permanent 1-m 2 sub-plots in each plot. We chose this layer and definition because (1) it is widely used in the literature, (2) this stratum is sensitive to changes in nutrient availability, and (3) most of the plant biodiversity of forests is typically found there (see Gilliam, 2007 for a review).…”

“…Extensive work in tropical forests of the Hawai'i archipelago (see Vitousek, 2004) has shown that much of this arises from loss of available P through the long-term pedogenic processes of tropical soil formation. A limited number of studies (e.g., Lu et al, 2008Lu et al, , 2010 on the effects of N deposition on tropical forest plant diversity in natural ecosystems have shown that there are regional differences on this response. Lu et al (2010) found that high levels of N additions (e.g., 150 kg N ha À1 yr À1 ) significantly reduced understory plant diversity in an N-saturated mature forest of tropical China.…”

“…A limited number of studies (e.g., Lu et al, 2008Lu et al, , 2010 on the effects of N deposition on tropical forest plant diversity in natural ecosystems have shown that there are regional differences on this response. Lu et al (2010) found that high levels of N additions (e.g., 150 kg N ha À1 yr À1 ) significantly reduced understory plant diversity in an N-saturated mature forest of tropical China. They concluded that the mechanism for this decline was N deposition-mediated soil acidification, rather than competition-based mechanisms commonly found in studies of temperate ecosystems (Gilliam, 2006).…”

“…We hypothesize that N additions decrease plant diversity in both rehabilitated and disturbed forests mainly from competition exclusion among species. Because the understory layer makes an important contribution to plant diversity (Gilliam, 2007;Lu et al, 2010), we focus on the diversity of understory layer in this study. Furthermore, because reforested tropical ecosystems play an important role as a reservoir of biodiversity, identifying the effects of N deposition on plant diversity will improve our understanding on forest ecosystem management and biodiversity protection in the future.…”