Differential Effects of Climate Warming on the Nectar Secretion of Early- and Late-Flowering Mediterranean Plants

Abstract

Floral nectar is a vital resource for pollinators, thus having a very important role in ecosystem functioning. Ongoing climate warming could have a negative effect on nectar secretion, particularly in the Mediterranean, where a strong temperature rise is expected. In turn, decreased nectar secretion, together with shifts in flowering phenology can disrupt plant–pollinator interactions and consequently affect the entire ecosystem. Under fully controlled conditions, we tested how temperature influenced nectar secretion (through nectar volume, sugar concentration, sugar content, and number of flowers produced) in six Mediterranean plant species flowering from winter to summer (viz. Asphodelus ramosus, Ballota acetabulosa, Echium plantagineum, Lavandula stoechas, Rosmarinus officinalis, and Teucrium divaricatum). We compared the changes in nectar secretion under temperatures expected by the end of the century and estimated the effect of climate warming on nectar secretion of plants flowering in different seasons. We found a significant effect of temperature on nectar secretion, with a negative effect of very high temperatures in all species. Optimal temperatures for nectar secretion were similar to the mean temperatures in the recent past (1958–2001) during the respective flowering time of each species. Increasing temperatures, however, will affect differently the early-flowering (blooming in winter and early spring) and late-flowering species (blooming in late spring and early summer). Temperature rise expected by the end of the century will shift the average temperature beyond the optimal range for flower production and the sugar produced per plant in late-flowering species. Therefore, we expect a future decrease in nectar secretion of late-flowering species, which could reduce the amount of nectar resources available for their pollinators. Early-flowering plants will be less affected (optimal temperatures were not significantly different from the future projected temperatures), and may in some cases even benefit from rising temperatures. However, as many earlier studies have found that early-flowering species are more prone to shifts in phenology, the plant–pollinator interactions could instead become affected in a different manner. Consequently, climate warming will likely have a distinctive effect on both plant and pollinator populations and their interactions across different seasons.

Global warming and plant–pollinator mismatches

“…For example, stresses such as more frequent and intense heatwaves could predominantly impact summerflowering communities through decreasing nectar production ( particularly in Mediterranean regions where the temperatures are already very high) because the optimal temperature of nectar production is more likely to be exceeded. On the other hand, spring-flowering communities could be more severely impacted by phenological mismatches with their host plants that to date have been more frequently observed during the spring [100,101].…”

Geography, climate, ecology: What is more important in determining bee diversity in the Aegean Archipelago?

“…Our findings, although at smaller extent, fit well in the above context, demonstrating that bee richness is controlled by water–energy interaction, either directly or via flowering plant abundance. Thus, bee richness is probably related to more diverse floral resources (mainly nectar) available within the better rainfed habitats of the generally hot and dry Archipelago (Petanidou, Goethals, & Smets, ; Petanidou & Smets, ; Takkis, Tscheulin, & Petanidou, ; Takkis, Tscheulin, Tsalkatis, & Petanidou, ).…”

Species‐specific responses to combined water stress and increasing temperatures in two bee‐pollinated congeners (Echium, Boraginaceae)

“…Water and temperature stresses are particularly deleterious when they occur early during the reproductive phase and during the blooming period (Barnabás, Jäger, & Fehér, 2008;Scheepens, Deng, & Bossdorf, 2018). The number and size of flowers decrease under water and temperature stresses (Carroll, Pallardy, & Galen, 2001;Descamps, Quinet, & Baijot, 2018;Glenny, Runyon, & Burkle, 2018;Phillips et al, 2018;Takkis, Tscheulin, & Petanidou, 2018). Nectar resources are generally reduced under these conditions, mainly because nectar volume decreases; even when the nectar sugar concentration rises, the total nectar sugar production declines (Carroll et al, 2001;Descamps et al, 2018;Phillips et al, 2018;Takkis et al, 2018).…”

“…The number and size of flowers decrease under water and temperature stresses (Carroll, Pallardy, & Galen, 2001;Descamps, Quinet, & Baijot, 2018;Glenny, Runyon, & Burkle, 2018;Phillips et al, 2018;Takkis, Tscheulin, & Petanidou, 2018). Nectar resources are generally reduced under these conditions, mainly because nectar volume decreases; even when the nectar sugar concentration rises, the total nectar sugar production declines (Carroll et al, 2001;Descamps et al, 2018;Phillips et al, 2018;Takkis et al, 2018). Despite the consequences for food production and wild species survival, studies on floral biology under combined abiotic stresses for bee-pollinated species are rare.…”