Nitrogen Dynamics in Forest Trees

Many wild tree species have economic values due to their use in the production of timber, biomass, and a variety of non-timber products. It is essential to learn more about the nutrient dynamics that can affect the health and productivity of forests, since we currently have little information on this topic. Nitrogen is an important driver of growth in woods, as it is in agriculture. Hence, research on the dynamics of this vital element is of utmost importance. Since trees are long-lived, investigative methods must be non-destructive to avoid affecting their continued growth. Fortunately, this is now possible with the help of a new line of instruments, which have miniaturized complicated analysis techniques.

Importance of Forests’ Trees

With the current emphasis on technological and economic development, it is easy to lose sight of how dependent we are on nature for our survival. Forests are essential for the welfare of humanity since they provide us with many ecosystem services that affect our quality of life. Forest and other natural vegetation keep the air clean, give us enough water, protect us from storms, and are a treasure-trove of genetic diversity of present and future medicines.

Sadly, at present, we continue to lose natural habitats every year for more farms and urban development. To be able to protect and preserve the forests that we have, we need to know more about trees and their functioning.

Forestry management of plantations grown to produce wood products is also under pressure to manage trees efficiently to avoid further deforestation and increase profits.

Nitrogen is a limiting factor for plants in the wild, too. We need to know more about nitrogen dynamics in trees, its use and storage within the tissues, and recycling in the ecosystem. The long life cycle of trees includes variations, due to seasons and many instances of stress.

So, the strategies and physiology of trees are likely to be different from short annual crops, which have been the focus of most of the plant research, especially in physiological processes.

The attempt to know more about tree physiology has begun, and the use of nitrogen by plants is being studied using various approaches. Below, we will discuss some of the basic research on the influence of nitrogen on the vegetative growth of trees.

Nitrogen use for Chlorophyll Production

In general, chlorophyll development and production depends on the levels of nitrogen.

Chlorophyll is a green-colored pigment found concentrated in chloroplasts. Pigments are known for the strong spectroscopic response to light. The spectroscopy of chlorophyll was used in one study to see how nitrogen content influenced chlorophyll levels in leaves of eucalyptus.

We know that chlorophyll absorbs visible light in two peaks: one in the blue range, with wavelengths between 400 – 500 nanometers (nm), and one in the red region between 620 – 700 nm. Previous research has found that the red and infrared region, with wavelengths between 700-750 nm and green light between 500 – 570 nm, were the most useful in chlorophyll spectroscopy in crop plants.

Since chlorophyll estimation was not made earlier in eucalyptus, scientists in Brazil had to first find out the light spectrum best suited for this particular species. The spectral data were used to derive vegetation indices, such as Normalized Difference Red-Edge (reNDVI) and Modified Red-Edge Normalized Difference Vegetation Index (mNDI). Then, the researchers checked for correlations of nitrogen levels on chlorophyll level and the vegetation indices.

Figure 1: “Average spectral reflectance by foliar color pattern. Each curve represents the average of 270 samples of leaf reflectance. Numbers near the curves indicate the average of N concentration (g kg−1). NIR = Near-infrared,” Ramalho de Oliveira et al., 2017. (Image credits: https://doi.org/10.1590/1678-992x-2015-0477)

The amount of chlorophyll was detected for leaves of three Eucalyptus clones by the CI-710 Miniature Leaf Spectrometer, produced by CID Bio-Science Inc. The device can measure absorbance, transmission, and reflectance of NIR and visible light. It is made for pigment analysis and to calculate vegetation indices. The CI-710 is lightweight and designed for use in the field, even for non-destructive measurements.

The nitrogen levels in the same leaves were analyzed by the Kjeldahl method.

In the case of eucalyptus, the reflectance of the light spectra by chlorophyll peaked at the edge of red and NIR (700 – 750 nm), as shown in Figure 1. Thus, tree leaf spectroscopy is different from crop leaf spectroscopy, which also responds to visible green light.

Among the vegetation indices, NDVI (calculated based on these data) was able to predict the levels of nitrogen in the leaves the best. The experiment showed that nitrogen levels in leaves positively correlated with chlorophyll levels, and the leaves with more nitrogen levels reflected less visible light but more NIR light. Lighter colored leaves reflected less NIR light, as shown in Figure 1.

This study confirms that the more nitrogen a tree has, the more chlorophyll it can produce.

Less nitrogen in trees means there is less chlorophyll and, therefore, less photosynthesis. This has a cascade of results, as it will affect the growth and development of trees not only in the vegetative phase but also influence the production of flowers and fruits.

Therefore, the importance of nitrogen and chlorophyll to trees cannot be overstated.

Gender Difference in the use of Nitrogen in Trees

How trees use the nitrogen can depend on their species, age, and even gender.

Plants, including trees, usually have both male stamens and female stigma within the same flower. A small percent of species have male and female flowers on different individuals. In these dioecious species, the entire tree/plant is, therefore, either male or female.

Scientists have found that male and female plants can react differently, especially in times of stress due to nutrient deficiency.

Trees can suffer from nitrogen deficiency due to anthropogenic pressures that lead to topsoil erosion. Topsoil rich in organic matter is the place where maximum nutrient recycling takes place, and its loss reduces nutrients available for trees.

In general, trees with nitrogen deficiency will spend more of their resources to develop deeper roots in search of food and have less above-ground growth. So, there is less chlorophyll and less photosynthesis. The metabolites’ levels produced by plants during stress will also differ.

In China, the dioecious and fast-growing Populus cathayana was used as a model to study both nitrogen and phosphorus deficiency. To eliminate individual differences in response, cuttings were taken from mother trees and grown in a liquid medium. Then, forty male and forty female clones were transplanted into pots and grown in greenhouses with different levels of nitrogen supply.

The vegetative growth parameters that were studied included tree height and leaf area. Leaf area was measured by the portable CI-203 Handheld Laser Leaf Area Meter, produced by CID Bio-Science Inc. The instrument records the length, width, area, perimeter, shape factor, void count, and aspect ratio of leaves within seconds. A built-in GPS can tag leaves for subsequent measurements, too.

Chlorophyll content, fluorescence, and photosynthetic rates were also measured.

Assays estimated reduced glutathione (GS), hydrogen peroxide, and ascorbic acid levels. Similarly, the activities of enzymes involved in nitrogen metabolism—like peroxidase (POD), nitrate reductase (NR), and glutamate dehydrogenase (GDH)—were also analyzed by assays.

The results showed that young female trees were more sensitive to nutrient deficiency than male trees. The difference is due to selection pressure on female trees, which have to invest resources to produce seeds. Variations between the genders seem to begin during vegetative growth itself.

Figure 2: “Morphological traits of P. cathayana leaves (the fourth expanded leaf, counted from the top) as affected by 60 days of N and P deficiencies. MC, control males; MN, N-deficient males; MP, P-deficient males; MNP, NP-deficient males; FC, control females; FN, N-deficient females; FP, P-deficient females; FNP, NP-deficient females,” Zhang et al., 2014. (Image credits: https://academic.oup.com/treephys/article/34/4/343/2338218)

Trees that suffered nitrogen deficiency were pale yellow, and the leaves were smaller in both the sexes than in the control, as shown in Figure 2. However, the impact of nitrogen deficiency was more significant on the female trees. Male trees had more biomass, larger leaves, and a better photosynthetic rate than females.

Female trees store more starch during nitrogen deficiency than males. This disrupts chloroplast, which has less chlorophyll and, therefore, photosynthesis.

NR, GS, and GDH, which are needed to assimilate and move nitrogen in the plants, show less activity in female trees affected by nitrogen deficiency; this is not the case in male trees. Moreover, phosphorus deficiency in females further affects the assimilation of nitrogen in females and not males, so on the whole female trees suffer more when there is nutrient deficiency.

Nitrogen deficiency produces more hydrogen peroxide in females than males. The mechanisms used to deal with the stress of accumulated hydrogen peroxide is also different. Female trees use non-enzymatic methods, like ascorbic acid, while males use enzymatic antioxidants.

Thus, the study shows that the two genders have very different physiology as well as tolerance to stress. Male trees can deal better with nitrogen (and phosphorus) deficiency than female trees, as the latter need more nitrogen for the extra reproductive roles they have to perform.

Nitrogen’s Influence on Vegetation

By studying the plant parameters and physiological functions that nitrogen influences, researchers were able to trace the role that nitrogen plays in trees. This was made possible by small portable devices that can be carried into the field to collect and analyze data, giving results in real-time. Nitrogen also influences root growth and physiology, and we will need other instruments to study these aspects. There are still a vast range of topics that have to be covered in forest ecology and forestry research if we are to understand how trees and the whole forest ecosystem functions.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature Image Courtesy of David Fulmer

Sources

Cánovas, F. M., Cañas, R. A., Torre, F. N. D. L., Pascual, M. B., Castro-Rodríguez, V., & Avila, C. (2018). Nitrogen Metabolism and Biomass Production in Forest Trees. Frontiers in Plant Science, 9. doi: 10.3389/fpls.2018.01449

Oliveira, L. F. R. D., Oliveira, M. L. R. D., Gomes, F. S., & Santana, R. C. (2017). Estimating foliar nitrogen in Eucalyptus using vegetation indexes. Scientia Agricola, 74(2), 142–147. doi: 10.1590/1678-992x-2015-0477

Zhang, S., Jiang, H., Zhao, H., Korpelainen, H., & Li, C. (2014). Sexually different physiological responses of Populus cathayana to nitrogen and phosphorus deficiencies. Tree Physiology, 34(4), 343–354. doi: 10.1093/treephys/tpu025