THE high incidence of failure when late-successional conifer species are replanted on disturbed forest sites is a considerable problem1–3. Here we advance a hypothesis that might explain many of these reforestation problems on a physiological basis, within the framework of forest succession. It is known that the chemical speciation of inorganic nitrogen in forest soils changes from predominantly ammonium (NH+4) in late-successional (mature forest) soils to mostly nitrate (NO–3) after disturbances such as clearcut harvesting2–6. The capacity of plant roots to take up and use these two sources of nitrogen is therefore very important for species establishment on successionally different sites. We have used kinetic and compartmental-analysis techniques with the radiotracer 13N to compare the efficiency of nitrogen acquisition from NH+4 and NO–3 sources in seedlings of white spruce, an important late-successional conifer. We found that uptake of NH+4 was up to 20 times greater than that of NO–3 from equimolar solution, cytoplasmic concentration of NH+4 was up to 10 times greater than that of NO–3, and physiological processing of NO–3 was much less than that of NH+4. This reduced capacity to use NO–3 is thought to present a critical impediment to seedling establishment on disturbed sites, where species better adapted to NO-3 would have a significant competitive advantage.

Conifer root discrimination against soil nitrate and the ecology of forest succession

SUMMARY
Across Europe, total nitrogen deposition is increasing and, of this total, atmospheric ammonia can contribute up to 50–80%. Average deposition of ammonia in the UK is likely to be around 15–20 kg ha−1 yr−1, while in The Netherlands, which has some of the highest rates of deposition, this value is likely to be between 40 and 50 kg ha−1 yr−1. It is argued that because of the processes of assimilation and nitrification this ammonia is an acidifying pollutant.

Ammonia taken up by plants is most likely to be directly assimilated and this uptake can have a strong effect on the nutrient imbalances of the plant. With root uptake in particular, anions are taken up in preference to cations. However, simple soil/plant nutrient measurements are unlikely to be a definitive means of monitoring ammonia pollution. This is because the processes of ammonia metabolism and acidification affect soil ion activity, mycorrhizas, plant uptake, and foliar leaching. These effects interact with acidity per se, and are compounded by the strong correlative co-deposition of ammonia with sulphur. Evidence for uptake of gaseous and wet deposited ammonia by leaves is presented.

The exact mechanism of ammonia toxicity is still not really clear, but could be due to physiological perturbation, rather than to the direct toxicity of the ion. Assimilation of ammonia by leaves releases protons which can cause cellular acidosis, and has important implications for acid-base regulation in cells. This regulation depends on intrinsic features of the plant's metabolism, that is in turn dependent on the ecology of root versus leaf nitrogen nutrition under normal conditions. Certain species are more acidic in a leaf physiological sense and tend to be prone to damage by pollutants.

Likewise, acidic habitats are particularly prone to damage through both eutrophication and the different capacities of plants both to utilize and to buffer against this nitrogen enrichment. The current evidence from The Netherlands suggests that the part this plays in perturbing the ecosystem should not be underestimated.

The deposition of atmospheric ammonia and its effects on plants

Publisher Summary This chapter discusses the present limits to interpretation for uncultivated systems with original data from Scottish old fields in reference to terrestrial plant ecology and I5N natural abundance. The use of I5N natural abundance (δ15N) is a powerful tool for obtaining insights through δI5N pattern analysis, and for deriving new questions to be tested, but it is not a reliable tracer of nitrogen fluxes in soils or in plants growing in soils. To interpret δ15N in terms of sources and sinks, one must know the source δ15N values, be able to quantify the fractionations occurring between source and sink, and frequently be able to estimate nitrogen fluxes. Given the widespread absence of such knowledge, question arises whether much of the published literature on δ15N for terrestrial systems can have been interpreted correctly. Moreover, the techniques of δ15N analyses are ideally suited to research in remote locations but not the interpretations. Nitrogen isotope interpretations are complicated and depend on a thorough knowledge of the site under study and on controlled experimentation. The isotopic values do not, themselves, supply the answers. The answers are supplied through examining patterns of isotopic signatures in the context of what is already known about the system's ecology and through testing hypotheses which arise from analysis of the δ15N patterns.

Terrestrial plant ecology and 15N natural abundance : The present limits to interpretation for uncultivated systems with original data from a Scottish old field

. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus e, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (A max ), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.

/pdf/linking-hydraulic-traits-to-tropical-forest-function-in-a-52cyvdood2.pdf

Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro)

Using abiotic data for conservation assessments over extensive regions : quantitative methods applied across New South Wales, Australia

In a study of the plant communities of two Australian rainforests, it was found that pioner species had high levels of nitrate reductase (EC 1.6.6.1) and were predominantly leaf nitrate assimilators. Under- and over-storey species had low levels of shoot and root nitrate reductase activity, and many of them showed little capacity for nitrate reduction even when nitrate ions were freely available. Although closed-forest species have lower levels of nitrate reductase than those of gaps and forest margins, their total nitrogen contents were similar, suggesting the former utilize nitrogen sources other than nitrate ions. Glutamine synthetase (EC 6.3.1.2) was present in the leaves of all species examined. In the leaves of pioneer species the chloroplastic isoform of glutamine synthetase predominted, while in most of the species typical of closed-forest the cytosolic isoform accounted for at least 40% of total leaf activity. Low levels of chloroplastic glutamine synthetase were correlated with a low capacity for leaf nitrate reduction, and both are characteristic of many species that regenerate and grow for some time in shade. Low levels of chloroplastic glutamine synthetase imply that, in some of these woody plants, photorespiratory ammonia is re-assimilated via cytosolic glutamine synthetase.

Inorganic nitrogen assimilation in plants of Austrlian rainforest communities

Conservation atlas of plant communities in Australia

A study of the sun leaves of two closed-forest (rainforest) and eight open-forest communities in subtropical southeast Queensland, Australia, showed that a large number of pioneer woody species in closed-forests had high levels of nitrate reductase (EC 1.6.6.1), whereas only a few herbaceous species in the open-forests showed high levels. There was a continuously declining gradient in nitrate reductase activity from pioneer to mature-forest species in all communities, associated with a decrease in Leaf Specific Area. The level of nitrate reductase activity was lower in certain plant families (including sclerophyllous monocotyledons, small-leaved composites and legumes), but still showed the same general relationship with Leaf Specific Area. The decrease in Leaf Specific Area is associated with an increase in both the dry weight: fresh weight ratio and the chlorophyll a: chlorophyll b ratio of the leaves. Three groups of plants can be recognised by nitrate reductase activity plotted against water content (% fresh weight) of their leaves-(1) pioneer, (2) mature-forest and (3) semi-sclerophyllous species. As the proportion of cytoplasm to structural tissue (indicated by water content) in leaves increases, there is a continuous increase in (a) nitrate reductase activity (b) total chlorophyll (per unit dry weight) (c) the proportion of chlorophyll b to chlorophyll a and (b) chloroplastic isoform of glutamine synthetase. These attributes are associated with high nitrogen content in the leaves and high photosynthetic potentials, resulting in rapid growth rates of pioneer species.

E. E. Hegarty

Papers

Inorganic nitrogen assimilation in plants of Austrlian rainforest communities

Conservation atlas of plant communities in Australia

Nitrate reductase activity and chlorophyll content in sun leaves of subtropical Australian closed-forest (rainforest) and open-forest communities.