Nitrogen and phosphorus additions negatively affect tree species diversity in tropical forest regrowth trajectories

Abstract

Nutrient enrichment is increasingly affecting many tropical ecosystems, but there is no information on how this affects tree biodiversity. To examine dynamics in vegetation structure and tree species biomass and diversity, we annually remeasured tree species before and for six years after repeated additions of nitrogen (N) and phosphorus (P) in permanent plots of abandoned pasture in Amazonia. Nitrogen and, to a lesser extent, phosphorus addition shifted growth among woody species. Nitrogen stimulated growth of two common pioneer tree species and one common tree species adaptable to both high- and low-light environments, while P stimulated growth only of the dominant pioneer tree Rollinia exsucca (Annonaceae). Overall, N or P addition reduced tree assemblage evenness and delayed tree species accrual over time, likely due to competitive monopolization of other resources by the few tree species responding to nutrient enrichment with enhanced establishment and/or growth rates. Absolute tree growth rates were elevated for two years after nutrient addition. However, nutrient-induced shifts in relative tree species growth and reduced assemblage evenness persisted for more than three years after nutrient addition, favoring two nutrient-responsive pioneers and one early-secondary tree species. Surprisingly, N + P effects on tree biomass and species diversity were consistently weaker than N-only and P-only effects, because grass biomass increased dramatically in response to N + P addition. The resulting intensified competition probably prevented an expected positive N + P synergy in the tree assemblage. Thus, N or P enrichment may favor unknown tree functional response types, reduce the diversity of coexisting species, and delay species accrual during structurally and functionally complex tropical rainforest secondary succession.

Cluster planting facilitates survival but not growth in early development of restored tropical forest

“…The same patterns have been reported for grass interactions (Wedin & Tilman 1993) and for adult trees in regeneration plots in Amazon (Siddique et al 2010), where pioneer species were favored by nutrient addition. According to Burslem, Pinard and Hartely (2005), pioneers could outcompete non-pioneers over the short term, as they monopolize and pre-empt resources, yet non-pioneers would prevail in the long term due to competitive success related to low resource requirements.…”

“…Moreover, rapidly colonizing, fast-growing species (pioneers) are typically weaker competitors for nutrients (Wedin & Tilman 1993). Therefore, responses to biotic interaction and resource addition may differ according to the successional status, and it would be expected that pioneer species are most likely to benefit from nutrient enrichment (Lawrence 2003;Siddique et al 2010), because of subsequent reductions in competition.…”

Nutrient addition effects on tropical dry forests: a mini-review from microbial to ecosystem scales

“…pasture with N and P in a highly seasonal region in Amazonia favored the growth of a few nutrient-responsive earlysuccessional tree species, decreasing tree assemblage evenness (Siddique et al, 2010). We hypothesize that higher nutrient availability could impact species composition through similar mechanisms in regenerating TDFs (Figure 1), though there is a dearth of long-term experimental data that can clarify community-wide impacts ( Table 1).…”

Effects of nutrient addition and soil drainage on germination of N-fixing and non-N-fixing tropical dry forest tree species

“…Much of our current knowledge of the effects of increased nutrient availability on vegetation comes from studies conducted in temperate ecosystems (Matson et al 1999;Siddique et al 2010). …”