Background and Aims

How well plants reproduce near their geographic range edge can determine whether distributions will shift in response to changing climate. Reproduction at the range edge can be limiting if pollinator scarcity leads to pollen limitation, or if abiotic stressors affect allocation to reproduction. For many animal-pollinated plants with expanding ranges, the mechanisms by which they have overcome these barriers are poorly understood.

Methods

In this study, we examined plant-pollinator interactions hypothesized to impact reproduction of the black mangrove, Avicennia germinans, which is expanding northward in coastal Florida, USA. We monitored insects visiting A. germinans populations varying in proximity to the geographic range edge, measured the pollen loads of the most common insect taxa and pollen receipt by A. germinans stigmas, and quantified flower and propagule production.

Key Results

We found that despite an 84% decline in median floral visits by insects at northernmost vs. southernmost sites, range-edge pollen receipt remained high. Notably, local floral visitor assemblages exhibited substantial turnover along the study’s latitudinal gradient, with large-bodied bees and hover flies increasingly common at northern sites. We also observed elevated flower production in northern populations and higher per capita reproductive output at the range edge. Furthermore, mean propagule mass in northern populations was 18% larger than propagules from the southernmost populations.

Conclusions

These findings reveal no erosion of fecundity in A. germinans populations at range limits, allowing rapid expansion of mangrove cover in the region. These results also illustrate that substantial turnover in the assemblage of flower-visiting insects can occur at an expanding range edge without altering pollen receipt.

Data were collected over the course of three years at eleven sites containing A. germinans populations in eastern Florida. The southernmost sites (latitudes 27.1–27.9°N) contain mixed stands of all three Floridian mangrove species (with Rhizophora mangle L. [Rhizophoraceae] and Laguncularia racemosa [L.] C.F. Gaertn. [Combretaceae]), in varying stages of regeneration following impoundment for mosquito control in the first half of the 20th century. Farther north, in the southern reaches of the mangrove-marsh ecotone (latitudes 28.5–29.1°N), many sites are also recovering from impoundment or other human alteration, but here are characterized by mixed-mangrove stands edging waterways, with salt marsh vegetation (primarily Distichlis spicata [L.] Greene [Poaceae], Spartina alterniflora Loisel. [Poaceae], Batis maritima L. [Bataceae], and Salicornia sp. L. [Amaranthaceae]) landward. At the northern end of the study region (latitudes 29.6–29.9°N), sites contain some of Florida’s northernmost mangroves (Cavanaugh et al. 2019). Here, clusters of short (generally <3m tall) mangroves – almost exclusively A. germinans – are embedded in a salt marsh matrix consisting primarily of S. alterniflora, B. maritima, and Salicornia sp. Most of these northern sites fall within the Guana-Tolomato-Matanzas National Estuarine Research Reserve (GTM).

The identity and frequency of A. germinans’ floral visitors were assessed during the 2013, 2014, and 2015 flowering seasons. All 11 sites were monitored at least once over these three years, though only seven were monitored in any given year, with each site visited repeatedly within a flowering season. Across all sites in a given year, visitation data were collected over 105.5hrs, 67hrs, and 33.5hrs in 2013, 2014, and 2015, respectively.

A. germinans flowers were monitored for floral visitors during 15-minute observation periods, which we conducted in fair weather from mid-morning to mid-day. Focal A. germinans were selected haphazardly; trees were at least 10m apart and contained at least 10 open flowers. We conducted a single observation period per day at each focal tree (i.e. no tree was monitored twice on the same date). During each observation period, the observer selected 4–10 open flowers in close proximity to one another and recorded the identity of each floral visitor and the number of focal flowers visited by each individual insect. Insects were typically identified to family, with the exception of the Apidae (Hymenoptera) which were identified to genus or species. During each observation period, the observer recorded the focal tree’s height (to the nearest 0.5m) and estimated the total number of open flowers on the tree.

To approximate the relative importance of individual taxa as pollinators of A. germinans, we captured floral visitors and measured the size of their pollen loads. Using hand nets, we collected individual insects representing 11 of the 12 most frequently observed floral visitor taxa, directly from A. germinans flowers (one frequent taxon – Pompilidae – was not encountered during this time). We collected as close to 10 individuals per taxon as possible. We quickly immobilized the insects in coolers with ice packs and later transferred them to a freezer to kill them. They were subsequently pinned and inspected using a dissecting microscope. In the lab, each insect was swabbed with a ~2mm³ cube of fuchsin jelly for up to 10 minutes to sample its pollen load. In the case of Apis mellifera L. (Hymenoptera:Apidae) and Bombus (Hymenoptera:Apidae), hydrated corbicular pollen was avoided, as it is thought to be unlikely to contribute to pollination (Thorp 2000). The fuchsin jelly was then transferred to a microscope slide, where the number of Avicennia and non-Avicennia pollen grains could be counted using a compound microscope at 100X and 400X magnification.

To assess pollen deposition, we collected A. germinans stigmas from six sites in 2015. Each site was visited twice – once in the early flowering season and once in the mid-flowering season. During each site visit, up to 12 trees were haphazardly selected, and we collected up to eight flowers with mature stigmas from throughout the tree canopy. Mature stigmas could be identified by their spread lobes, which open on approximately day 3 of anthesis (personal observation); flowers are syncarpous, with a single stigma per flower. In total, we collected 768 stigmas from all six sites. In the lab, each stigma was removed from its flower and mounted in fuchsin jelly on a microscope slide, which we inspected at 100X and 400X magnification on a compound microscope. Given the sometimes-large number of pollen grains present, we counted the number of A. germinans pollen grains three times on each stigma and used the average of these three counts for analysis.

Flower production per tree was measured during each observation period for floral visitors, as described above. In late October-early November of 2014 and 2015, when propagules were maturing on parent trees, we established transects at multiple sites to measure the densities of reproductive A. germinans trees and fecundity (propagule production per tree). In each year, transects were established at six sites (though not the same set of sites in both years). At each site, we identified three areas of high A. germinans density and laid out one 20m transect in each area (oriented to maximize the number of trees intersected), resulting in three transects per site. We recorded the number of reproductive A. germinans over 0.5m tall that occurred within 1m of the transect tape, on either side.

To measure propagule production per tree, or fecundity, we randomly selected five reproductive trees along each transect (or in close proximity to the transect, if fewer than five were immediately adjacent) and estimated the number of propagules present by counting the number on a representative portion of the tree and extrapolating to the entire canopy. Because different sites were in different stages of propagule drop, we then corrected these estimates to account for propagules that had already fallen. We did this using infructescences at each site that had been covered in mesh bags prior to the onset of propagule drop (three bags per tree on up to 30 haphazardly selected A. germinans per site); these bags collected falling propagules and allowed us to calculate the fraction of total propagules that remained attached to the pedicel. By multiplying our original propagule counts by the inverse of this fraction, we estimated the total number of propagules originally produced by the trees.

Fruit set rates, defined as the number of mature propagules produced per floral bud, were also measured using these same bags. Once each bag was collected, we counted the number of propagules (both abscised and still attached to the pedicel) as well as the total number of floral bud scars. Fruit set rate was calculated as the total number of propagules produced by a given inflorescence, divided by the number of floral bud scars.

In 2015, propagules were collected from seven sites and weighed. To avoid bias resulting from earlier phenology at the northern sites, we collected only mature propagules that had dropped from their pedicels. To do this, we haphazardly selected 13–22 A. germinans at each site in late summer, once flowering had ended; we installed large-mesh (~1.0cm) bags over three inflorescences, each on a separate branch. In late October/early November, we returned to the sites, collected a total of 326 mature propagules that had dropped from the pedicels into the bags, and measured their fresh weight to the nearest 0.001g.

Data files are CSVs and can be opened with any software compatible with this file type. The programming script is in R, originally written in R version 3.5.0.