Nectar and pollen in Acer trees can contribute to improvement of food resources for pollinators

Abstract

In the present study, we quantified floral resources (nectar and pollen production) and their quality (nectar sugar composition, pollen protein content, pollen amino acid composition) in five Acer species (f. Sapindaceae) growing in forests and commonly planted in urban areas in the temperate zone. Acer trees provide high amounts of sugars and/or pollen. No nectar was produced by A. negundo flowers. The other species produced nectar in functionally female flowers. The floral nectar was composed of sucrose, glucose, and fructose and was classified as hexose-rich or sucrose-rich. The pollen of all the Acer species contained essential amino acids. Acer trees should be planted for improvement of cost-effective food resources in various landscape types (agroforestry, urban areas), with the exception of A. negundo (an invasive species with no nectar available). However, maple trees alone are not sufficient to support pollinators, and other plant species flowering before and after Acer spp. should be planted to ensure a continued supply of food for pollinators.

Fig. 1

Calendar of flowering of five Acer species in 2018–2019.

Fig. 2

Acer negundo (A–C) and Acer pseudoplatanus (D–G). Male inflorescence; bar = 1 cm. (B) Male flower; bar = 1 mm. (C) Female flower; bar = 2 mm. (D) Hermaphroditic inflorescence with functionally male flowers and functionally female flowers; bar = 4 cm. (E) Functionally female flower (a-anthers, st-style, s-stigma); bar = 1 mm. (F,G) Functionally male flower with a reduced pistil (a-anthers, s-stigma); bars = 2 mm (F), 5 mm (G).

Fig. 3

Interspecific variations in nectar characteristics in functionally female Acer flowers; (I) nectar amount secreted during flower life-span (mg); (II) nectar concentration (%); (III) amount of nectar sugar produced during flower life-span (mg). Means from 2018–2019. Interspecific significant differences (P < 0.05) are marked with different letters. Tukey’s HSD test was applied for log transformed data; however, the presented data are not transformed. The dots within the boxes indicate mean values, the boxes show ± SD, and the whiskers show the standard error (± 0.95).

Fig. 4

Comparison of pollen production in functionally female and functionally male flowers (mean from the Acer species studied). Notice: the anthers in ca. 80% of the functionally female flowers did not dehisce and pollen was retained therein. Statistically significant differences (P < 0.05) are marked with different letters; Tukey’s test was applied for log transformed data; however, the presented data are not transformed. The dots within the boxes indicate mean values, the boxes show ± SD, and the whiskers show the standard error (± 0.95).

Fig. 5

Pollen mass established in (I) functionally male flowers and (II) functionally female flowers (means from 2018–2019). Tukey’s HSD test was applied for log transformed data; however, the presented data are not transformed. The dots within the boxes indicate mean values, the boxes show ± SD, and the whiskers show the standard error (± 0.95).

Fig. 6

Comparison of total nectar sugar (I) and pollen (II) resources in Acer species available per m² of tree crown and calculated for the all-blooming season in 2018–2019. Statistically significant differences (P < 0.05) are marked with different letters; capital letters (A–D) indicate interspecific differences; small letters (a,b)—inter-year differences within a species. Bars ± SD (standard deviation). One-way ANOVA and Tukey’s HSD test was applied for log transformed data; the presented data are not transformed. Note: the data on A. negundo sugar resources are not available as the species does not produce nectar.

Fig. 7

Protein content in the pollen of five Acer species. Small letters represent non-significant interspecific differences at (P < 0.05) according to the Kruskal–Wallis test: H (4, N = 10) = 4,307, P = 0.3659. Data represent means ± SD.