Natural Variation in Volatile Emissions of the Invasive Weed Calluna vulgaris in New Zealand

Abstract

Invasive plants pose a threat to natural ecosystems, changing the community composition and ecological dynamics. One aspect that has received little attention is the production and emission of volatile organic compounds (VOCs) by invasive plants. Investigating VOCs is important because they are involved in vital ecological interactions such as pollination, herbivory and plant competition. Heather, Calluna vulgaris, is a major invasive weed in New Zealand, especially on the Central Plateau, where it has spread rapidly since its introduction in 1912, outcompeting native species. However, the chemical behaviour of heather in its invaded ranges is poorly understood. We aimed to explore the natural variation in volatile emissions of heather and the biotic and abiotic factors influencing them on the Central Plateau of New Zealand. To this end, foliar volatiles produced by heather at four different sites were collected and analysed using gas chromatography coupled to mass spectrometry. Soil properties, herbivory and other environmental data were also collected at each site to investigate their effects on VOC emissions using generalised linear models (GLMs). Our results reveal significant differences in VOC emissions between sites and suggest that soil nutrients are the main factor accounting for these differences. Herbivory and temperature had only a minor effect, while soil water content had no impact. Further studies are needed to investigate how these variations in the invasive plant's foliar volatiles influence native species.

Keywords: heather; invasive species; plant scents; plant secondary metabolites; soil nutrients; volatile organic compounds.

Figure 1

The distribution of heather in the Central North Island. (a) North and South Islands of New Zealand. (b) Invasion of heather in the Central Plateau. Heather was first planted in the white region (FI) but has now spread through all the regions in purple. The boundary of Tongariro National Park (TNP) is shown in red.

Figure 2

Volatile organic compound (VOC) classes identified from the headspace of heather at four different sites. Bars show mean ± SE of total (a) monoterpenoids, (b) homoterpenes, (c) sesquiterpenes, (d) fatty acid derivatives, (e) aldehydes and (f) total volatile emissions measured from target plants from each site (n = 5). Letters indicate pairwise comparisons between sites. Abbreviations: fatty acid derivatives (FADs).

Figure 3

Principal components analysis (PCA) biplot showing PC scores of individuals and loadings of variables. PCA was based on 33 VOCs emitted by heather from all sites. The numbers in the graph indicate the following compounds; (1) hexyl acetate, (2) 1-hexanol, (3) (Z)-2-hexenol, (4) (Z)-3-hexenol, (5) (Z)-3-hexenyl 2-methylbutyrate, (6) (Z)-3-hexenyl acetate, (7) (Z)-3-hexenyl benzoate, (8) (Z)-3-hexenyl butyrate, (9) (Z)-3-hexenyl hexanoate, (10) (Z)-3-hexenyl isobutyrate, (11) (Z)-3-hexenyl valerate, (12) α-pinene, (13) α-terpineol, (14) β-myrcene, (15) β-pinene, (16) limonene, (17) linalool, (18) (Z)-β-ocimene, (19) (E,E)-α-farnesene, (20) α-gurjunene, (21) (E)-β-caryophellene, (22) δ-cadinene, (23) γ-elemene, (24) copaene, (25) germacrene B, (26) germacrene D, (27) humulene, (28) (E)-β-famesene, (29) (Z,E)-α-farnesene, (30) (E)-4,8-Dimethyl-1,3,7-nonatriene, (31) 1-octen-3-ol, (32) decanal, (33) nonanal. Compounds were assigned to the following classes: fatty acid derivatives (1–11), monoterpenes (12–18), sesquiterpenes (19–29), homoterpene (30), alcohol (31), aldehydes (32–33).

Figure 4

Comparison of (a) ambient daytime temperature, (b) soil water content and (c) soil temperature between sites. The y-axis showing mean ± SE values and x-axis representing the four study sites. Different letters indicate significant differences.

Figure 5

Arthropod community composition from study sites determined by beating a branch of target heather plants on a tray (n = 5). Bars show mean number ± SE of individuals in the respective arthropod orders. Arthropod groups with asterisks (*) were significantly different between sites.

Figure 6

(a) Visible herbivore damage on heather. Sections in white circles were counted as two separate damage events. (b) Mean number of damage marks counted on target plants (n = 5) for each site expressed as damage per gram dry weight.