Impacts of Nutrient Availability on Calystegia Spithamaea at Harvard Forest 2013

Low bindweed (Calystegia spithamaea (L.) Pursh ssp. spithamaea, Convolvulaceae), is a low-growing perennial plant of the morning glory family that ranges from Georgia north to Nova Scotia. It is recorded from 3 extant and 8 historic stations in Massachusetts, and 18 total extant populations across New England, where it inhabits dry, open sites with sandy to rocky soils, including sandy roadsides and path edges, inland sandplains, power line rights-of-way, loose talus slopes, and gravel pits. Massachusetts lists the species as S1, Endangered. Factors promoting reproduction in this rare species are largely unknown. Although Calystegia spithamaea has been noted to produce short rhizomes, its ability to spread vegetatively had not been determined as of 2013. Sexual reproduction is very rare in extant New England populations; although herbarium specimens show flowers, fruits are rare and seeds have not been collected at any population. Field studies have been conducted since 2007 of a population of several thousand ramets of Calystegia spithamaea in a minimally managed field on the Army Corps of Engineers Birch Hill Dam property, Royalston, Massachusetts. The population occurs on excessively drained, sandy loam, which supports otherwise low plant diversity and appears to be nutrient-poor. We tested the hypothesis that nutrient limitation may hinder sexual reproduction and ramet growth in this species. From May to August 2013, a greenhouse study was conducted at Harvard Forest to determine the effects of nutrient availability on Calystegia spithamaea growth and reproduction: Ramets were excavated from the field and were found to be propagating on long rhizomes, confirming for the first time that the species is capable of at least limited asexual reproduction. Forty-eight ramets of Calystegia spithamaea were planted in pots in the greenhouse and randomly allocated to one of two treatments: control and nutrient-amendment with 20:20:20 N:P:K fertilizer. Five ramets from a newly-discovered population of Calystegia were also planted in the greenhouse in June and maintained under control conditions. Ramets grown in the greenhouse and receiving N:P:K nutrient amendments showed significantly higher relative growth rates than control plants from Birch Hill (but not Montague), but did not differ in leaf production, average leaf area, or specific leaf area from control plants of either the Birch Hill or the Montague populations. However, four nutrient-amended plants flowered, whereas control plants did not, and herbivore damage was significantly lower on nutrient-amended plants relative to controls. Conclusions: Nutrient amendments had few significant effects on growth, but may have triggered flowering and promoted herbivore defense by Calystegia spithamaea.

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In 2012, Hickler and Farnsworth had recommended that a greenhouse study of Calystegia spithamaea ramets would be beneficial for understanding limits to growth in this species. Per our findings from the field, we hypothesized that growth, flowering, and fitness would be maximized in high-light conditions with nutrient-poor soils, in the absence of plant competitors. To test this hypothesis, we initiated a study at the Torrey Greenhouse of Harvard Forest (Petersham, MA). On 31 May 2013, we collected 48 ramets (with rhizomes and roots) of Calystegia plants at Birch Hill. Plants with at least 3 fully expanded leaves and minimal herbivore damage were collected, with half coming from full sun sites in the field and half collected from nearby copses. Plants were transported immediately to the Harvard Forest greenhouse, and planted into 4-inch square pots with their native soil. All plants were moved to a greenhouse table with ambient light filtered by 20% shade-cloth to moderate greenhouse temperatures. Half of the ramets (N = 24) were given biweekly nutrient amendments 1.7 g L–1 of 20:20:20 fertilizer (J. R. Peters, Inc., Allentown, PA) in 100 ml per pot; 24 “control” plants were watered with 100 ml distilled water. Initial size measures were taken at planting to ensure that plants in the nutrient and control cohort did not differ significantly in stem length (nutrient cohort = 12.1 cm ± 3.5 [S.D.]; control = 13.1 ± 4.3), number of leaves (nutrient = 5.5 ± 1.2; control = 5.8 ± 1.4), or length of longest leaf (nutrient = 3.8 cm ± 0.7; control = 4.1 ± 0.8). Plants were watered automatically from overhead sprinklers twice per day. Predominant soil texture (sand versus loam) was noted at planting time in case it was a significant covariate in plant performance (it was subsequently found not to be). Three extra plants were potted at the same time, to replace any plants dying in either treatment. Noting in early June that plants were suffering water stress, we placed water-holding pans under the pots. These, however, caused leaves to become chlorotic; thus, the pans were removed in mid-July but watering frequency was increased. Three plants in the Nutrient cohort died within 2 weeks of transplant and were replaced with the additional ramets. On June 15, five additional plants from a newly-discovered (in 2012) population in Montague, MA (see below) were transplanted into 4-inch pots at the Harvard Greenhouse and maintained in control conditions, to characterize differences in growth between populations of Calystegia. Plants were measured non-destructively for total stem length, number of leaves produced, number of new branches and root-sprouting ramets, and percentage of leaves damaged by herbivores on June 15 and July 28, 2013. Relative growth rates and leaf production from the June to July sampling dates (when mortality stabilized and watering treatments were increased to prevent desiccation of plants) were calculated as the change in total stem length divided by initial stem length and number of days between measurements. Three leaves from each plant were harvested on 28 July 2013 and areas measured with a Li-Cor Leaf Area Meter at Harvard Forest. Leaf tissue was then dried at 70oC for three days and weighed to obtain specific leaf area. Average leaf area was 11.7 cm2 ± 0.1 (S.E); average specific leaf area was 36.9 m2/kg ± 0.2 (somewhat higher than SLA recorded in field-grown plants); neither area nor SLA differed significantly among cohorts.