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Guest post: The physics, remote detection and control of invasive species

What can physics, plants and remote sensors have in common? This is a question I frequently hear and a question I asked myself at the beginning of my PhD. But right now I am finishing that PhD about the potential use of spectroscopy by remote sensor to monitor invading plants and their interaction with organisms on the ground.

 

Spectroscopy is a way of measuring light that involves the relationship of light in relation to the structure and composition of matter. The basis of this method is in the physics of the materials and their interaction with the different wavelengths of light. The sun’s radiation penetrates the Earth’s atmosphere and, when reaching the Earth’s surface, the wavelengths are altered owing to the composition of the molecules of air.

Figura 1

Figure 1: The top chart represents the solar radiation after reaching the earth’s atmosphere. The lower chart shows the characteristics of the electromagnetic spectrum measured in the green leaves of ragwort (Jacobaea vulgaris). The molecules that influence some of the wavelengths are identified.

 

 

This effect results in a radiation that is more energetic in the visible spectrum than in the infra-red spectrum (Fig. 1). On this basis we can explain why plants are (generally) green. Plants have adapted to convert solar radiation into useful energy for their growth.

For greater efficiency in the conversion of solar energy the plants focus on the radiation that is more energetic when it reaches the ground, that is, the visible zone that we see as red and blue. This results in plants being green because these are wavelengths that they discard and reflect and that our eyes can “see”.

The spectrometers work partly like our eyes; they process the light that is reflected by the materials, but while the eyes translate that information into colours and textures, the spectrometer translates it into numbers that may afterwards be used to study the properties of the plants. This allows us to study the changes in the structure and chemical composition of the plants in detail.

A great benefit in using these techniques is that it allows us to study these phenomena on a large scale, once several satellites have similar sensors (Jackson, 1986; Asner et al, 2008).

The influence of soil organisms on plants is often difficult to assess due to the cryptic nature of their interactions. Soil organisms may influence the plants by means of symbiotic interactions (friendly) or pathogenic (conflicting). And it is increasingly recognized that the reduction of the interactions between plants and microorganisms can facilitate the invasion of plants (Van Der Putten, 2003).

Similarly, it is recognized that introduced exotic species can become increasingly controlled by soil organisms where they were introduced (Diez et al., 2010). A recurring challenge for ecologists in the study of these raids is to understand and predict when and where these plant species can be affected by organisms of the new habitat.

Because these interactions cause often changes in the physical and chemical structures of plants, the use of spectrometers to measure these changes is increasingly an option.

Figura 2

Figure 2: Flowering ragwort, Jacobaea vulgaris Gaertn. (Syn. Senecio jacobaea L.)

 

In the study I made, several ragwort species were selected as a study model, with particular emphasis on Jacobaea vulgaris Gaertn. (Syn. Senecio jacobaea L.), which is a native species in expansion in the Netherlands (Fig. 2). At the beginning of my study little was known about the impact of soil organisms in the patterns of spectral reflectance of plants.

 

It has been shown that plant reflectance patterns can provide information about their interactions with soil organisms as if we were examining different organs in the plant (leaves or flowers, for example) (Carvalho et al., 2013). The resulting plant-soil interaction discriminations (Carvalho et al., 2012), illustrate that the spectral patterns can provide interesting clues for remote sensing of these interactions on a broader scale.

If this technique becomes widely used, various areas of research may benefit, such as for example, biological control or monitoring of soil quality for agriculture. The future of remote sensing shows promise, and physics is a great ally in the study of plants.

Asner GP, Knapp DE, Kennedy-Bowdoin T, Jones MO, Martin RE, Boardman J, Hughes RF. 2008. Invasive species detection in Hawaiian rainforests using airborne imaging spectroscopy and LiDAR. Remote Sensing of Environment 112(5): 1942-1955.

Carvalho S, Macel M, Schlerf M, Skidmore AK, van der Putten WH. 2012. Soil biotic impact on plant species shoot chemistry and hyperspectral reflectance patterns. New Phytologist 196(4): 1133-1144.

Carvalho S, Schlerf M, van der Putten WH, Skidmore AK. 2013. Hyperspectral reflectance of leaves and flowers of an outbreak species discriminates season and successional stage of vegetation. International Journal of Applied Earth Observation and Geoinformation24(0): 32-41.

Diez JM, Dickie I, Edwards G, Hulme PE, Sullivan JJ, Duncan RP. 2010. Negative soil feedbacks accumulate over time for non-native plant species. Ecology Letters 13(7): 803-809.

Jackson RD. 1986. Remote sensing of biotic and abiotic plant stress. Annual Review of Phytopathology 24(1): 265-287.

Van Der Putten WH. 2003. Plant defense belowground and spatiotemporal processes in natural vegetation. Ecology 84(9): 2269-2280.

 

Sabrina Carvalho

Sabrina Carvalho completed her thesis titled “Jacobaea through the eyes of spectroscopy: Identifying plant interactions with the (a)biotic environment by chemical variation effects on spectral reflectance patterns” as part of her PhD in Ecology and Hyperspectral remote sensing.

If she wasn’t a scientist, she would be baker. She enjoys wandering through second-hand markets trying to find dresses of the 50’s.

Visit her webpage in at the Nederlands Instituut voor Ecologie.

 

 

This post is also available in: Portuguese (Portugal)

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