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.

矮平卧打碗花（Calystegia spithamaea (L.) Pursh ssp. spithamaea，旋花科（Convolvulaceae））是旋花科的低矮多年生草本植物，分布范围从佐治亚州向北延伸至新斯科舍省。该物种在马萨诸塞州有3个现存分布点与8个历史分布点记录，新英格兰地区总计18个现存种群，其生境为砂质至岩质土壤的干燥开阔地带，包括沙质路边、步道边缘、内陆砂质平原、电力线路行道带、松散岩屑坡与砾石坑。马萨诸塞州将该物种列为S1级濒危物种。

目前对于这一稀有物种的繁殖促进机制仍知之甚少。尽管有记录显示平卧打碗花可产生短根状茎，但截至2013年，其无性繁殖扩散能力尚未得到证实。新英格兰地区现存种群的有性繁殖极为罕见：尽管蜡叶标本中可见该物种的花，但果实极为稀少，且从未在任何种群中采集到种子。

自2007年起，研究人员针对马萨诸塞州皇家斯顿市美国陆军工程兵团（Army Corps of Engineers）比奇山大坝地块内一处低管理强度田野中的数千株分株种群开展了野外研究。该种群生长在过度排水的砂质壤土中，该生境的植物多样性整体偏低且养分匮乏。我们提出假说：养分限制可能阻碍了该物种的有性繁殖与分株生长。

2013年5月至8月，研究团队在哈佛森林（Harvard Forest）开展温室试验，以探究养分有效性对平卧打碗花生长与繁殖的影响：从野外挖掘的分株被发现依托长根状茎进行繁殖，这首次证实该物种至少具备有限的无性繁殖能力。研究人员将48株平卧打碗花分株种植于温室盆钵中，并随机分为两组：对照组与施加20:20:20氮磷钾（N:P:K）复合肥的处理组。此外，于6月从新发现的打碗花属（Calystegia）种群中采集5株分株种植于温室，维持常规养护条件作为对照。

结果显示：温室中施加氮磷钾养分的分株相对生长率显著高于比奇山种群的对照组分株（但与蒙塔古种群对照组无显著差异），但在叶片产量、平均叶面积与比叶面积方面，与比奇山及蒙塔古种群的对照组均无显著差异。然而，养分处理组中有4株植株开花，而对照组均未开花；且养分处理组植株的草食动物取食损伤程度显著低于对照组。

结论：养分添加对平卧打碗花的生长仅产生少量显著影响，但可能触发其开花进程，并促进其草食动物防御能力。