PhD Thesis (2008) Ilyas Siddique, School of Integrative Biology, The University of Queensland, Australia:

“Interactions between tree species composition and nutrient relations in tropical and subtropical forest recovery”

Abstract (English):

While rapid deforestation in the tropics and subtropics continues unabated, natural forest regrowth has been delayed or arrested on millions of hectares. Accelerated forest recovery for multiple functions is in growing demand, but requires ecological understanding of tradeoffs among inherent ecosystem processes and services to humans in forest rehabilitation. Low availability of soil nitrogen (N) and/or phosphorus (P) is considered to widely limit productivity in secondary forests, and tree species may affect and be affected by nutrient relations. I used three complementary approaches to investigate interactions between tree species composition, diversity and N and P relations on strongly weathered soils in Brazil and Australia. Thus, I analyzed (I) effects of fertilization on tree community dynamics in natural forest regrowth, (II) effects of contrasting functional tree species composition on N and P relations in unfertilized mixed-species tree plantings, and (III) belowground complementarity and inhibition among tree species with contrasting nutritional physiologies in microcosms, in relation to field plantations.

(I) Floristic trajectories of the tree assemblage in abandoned Amazonian pasture were analyzed in response to large, repeat-additions of N versus P in permanent plots. N, and to a lesser extent P, shifted species woody biomass, favoring three responsive tree species. This delayed increases in tree species richness and reduced assemblage evenness. Fertilization effects on tree growth rates lasted only approximately two years, presumably due to fertilizer immobilization in the soil. However, nutrient-induced shifts in relative tree species growth and possibly reduced assemblage evenness persisted for >3 years post-fertilization. Surprisingly, N+P effects on tree biomass and species diversity were consistently weaker than the sum of N-only and P-only, probably due to the dramatic grass biomass response observed only after N+P addition. In conclusion, nutrient enrichment may adverse effects on tree species diversity, even during complex, highly dynamic tropical moist forest secondary succession. However, positive N+P synergy might be prevented by competition among plant life forms.

(II) Effects of contrasting functional tree species composition on nutrient relations were assessed in unfertilized, adjacent mixed plantings in Southeast Brazil, where regrowth of Atlantic forest had been inhibited. N and P relations of four native tree species were compared between a legume-dominated, species-poor mixture of early-successional species (“legume mixture”), and a species-diverse, legume-poor mixture of all successional groups (“diverse mixture”). After 7 years, the legume mixture had six-fold higher abundance of N₂-fixing trees, 177% higher total tree basal area, 22% lower litter C:N, six-fold higher in situ soil resin-nitrate, and 40% lower in situ soil resin-P, compared to the diverse mixture. All non-N₂-fixing species studied showed plastic adjustment in leaf nutrient concentrations and proportional resorption to diverged N:P supply ratios between mixtures, whereas nutrient relations in the N₂-fixer did not respond to planting mixtures. Rapid N accumulation in the legume mixture caused excess soil nitrification over nitrate immobilization and tighter P recycling compared with the diverse mixture. The legume mixture attained more rapid tree growth and canopy closure, but might result in periods of N losses and/or P limitation. Incorporation of locally adapted species with efficient nitrate uptake and P mobilization from resistant soil pools offers potential to optimize these tradeoffs.

(III) Effects of interspecific belowground interactions on biomass partitioning along a soil fertility gradient were systematically assessed in binary mixtures of six tree species with contrasting nutritional physiologies. All pairwise combinations of tree species as saplings were restricted to small volumes of two soil types of contrasting fertility in glasshouse microcosms. Biomass accumulation after one year was primarily determined by soil type and neighbor biomass. Expected, linear reductions in target tree biomass with increasing neighbor biomass indicated species-specific and soil type-specific responses to competition across a diverse range of competing tree functional types. Positive or negative deviations from these specific response profiles to competition identified complementarity/facilitation or inhibition, respectively, in species combinations. This analysis reproduced several positive and negative species interactions reported from long-term field plantations of the same species in monocultures and mixed in Eastern Australia. In microcosms, rhizosphere phosphatase activities in monocultures differed dramatically among species, and some mixtures suggested non-additive effects. Thus, microcosms offer a cost-efficient screening tool for the identification of synergistic species mixtures which maximize productivity relative to the specific potential of each component species, while enabling identification of some of the mechanisms potentially responsible for synergisms. However, the utility of such a screening framework in microcosms relies on close integration with field experiments for cyclical hypothesis generation and testing.

In tropical secondary forest, dominance by N₂-fixing tree species may cause rapid N enrichment and increase N:P ratios, whereas ecosystem N or P enrichment may negatively affect tree species diversity. Future research should assess whether N₂-fixer dominance also has direct, adverse effects on subsequent accrual of species diversity. Furthermore, integrating hypothesis generation and testing in microcosms and field experiments may reveal mechanisms of nutritional complementarity and facilitation among tree species with potential to optimize tradeoffs between productivity, species diversity, nutrient accumulation and retention in tropical forest rehabilitation.

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