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.