de Vere 2007 Biol Flora C. dissectum.pdf - The Whitley Wildlife ...

886N. de Veregreater productivity had higher seed production andpercentage germination was greater under greenhouseconditions. These factors should allow greater colonizationcapacity of nearby sites but this will only bepossible if there are safe sites for seedling establishment;other research suggests that this is very rarely thecase (see sections VI(C) and VIII(D)). Seed dispersalability decreased with greater productivity, reducingthe possibility of longer distance dispersal.(C) EFFECT OF FROST, DROUGHT, ETC.Cirsium dissectum is found in moist habitats, althoughsome of its sites such as the heaths of the New Forest,Culm grasslands of south-west England, well-drainedSchoenus fens on limestone and sand-dune slacks willdry out to an extent during the summer. Ross (1999)showed that plants grown in growth-room conditionswere able to survive more than 32 h but less than 64 h ina post-wilting state but that this had a negative impacton relative growth rate (see section VI(E)(ii)). It thereforeseems likely that C. dissectum can survive limitedperiods of summer drought even though it will affectsubsequent growth.(C) PERENNATION: REPRODUCTIONCirsium dissectum is a long-lived perennial; in productivesites it can flower in its second year at the earliest.Rosettes die after flowering, but generally plantsreproduce vegetatively before this.Survival and growth of seedlings in the field is veryrare and clonal propagation is the dominant form ofreproduction (Jongejans 2004; Jongejans et al. 2006b).In a 5-year study of three grasslands in the Netherlands,Jongejans (2004) found only three seedlings. SimilarlyKay & John (1994) and de Vere (2007) found noseedlings in their surveys of populations in the BritishIsles. The low number of seedlings is caused primarilyby very low establishment rates in vegetation stands(Jongejans et al. 2006b; de Vere 2007), as seedlings aremore abundant in restoration areas where the topsoil has been removed near C. dissectum populations(Jongejans 2004).(D) CHROMOSOMESMaterial from Port Ellen, Islay was found to be 2n = 34(Morton 1977).© 2007 The AuthorJournal compilation© 2007 BritishEcological Society,Journal of Ecology,95, 876–894VI. Structure and physiology(A) MORPHOLOGYCirsium dissectum has a hemi-cryptophyte basal rosettegrowth form. It typically produces new rosettes at theend of long rhizomes but new rosettes can also beformed at the base of existing ones. Rhizomes are a palestraw colour and smooth, with small brown scales atthe nodes. Rhizome lengths vary, from close to the parentplant to up to 40 cm, and they grow in a downwardcurve through the soil, seemingly unaffected by rootsor tussocks of other species or soil type (Jongejans2004). After 2 years, plants have produced a large caudex,approximately 10 × 2 cm (de Vere 2007). Jongejans(2004) investigated the relationships between rosettesize, flowering probability, rhizome formation andsite characteristics within five grasslands in theNetherlands. The number of rhizomes produced by aplant varied from 0 to 5 and was generally positivelycorrelated with its caudex weight; caudex weight inturn was positively correlated with site productivity.The most productive grassland had the greatestpercentage of rosettes that flowered; flowering rosetteshad heavier caudices than vegetative rosettes andproduced more rhizomes. Rhizome length and depthdid not differ systematically between sites.(B) MYCORRHIZAArbuscular mycorrhizas are found, with the youngestfine roots being the most heavily colonized and olderroots having very little detectable arbuscule development(Ross 1999).(E) PHYSIOLOGICAL DATA(i) Response to shadeRoss (1999) compared relative growth rate in C. dissectumand Helianthus annuus at two light levels, 350 and150 µmol m –2 s –1 photon flux density, in a growth roomwith a 16 h day at 22 °C and 15 °C night. Plants weregrown in conical flasks containing Rorison nutrientsolution. The mean relative growth rate was 0.063 g g –1day –1 for C. dissectum and 0.125 g g –1 day –1 for H. annuusat the higher light, and 0.055 g g –1 day –1 for C. dissectumand 0.105 g g –1 day –1 for H. annuus at the lower light;Ross (1999) concluded that C. dissectum was relativelytolerant of shade. Ellenberg (1988) gave C. dissectum alight indicator value of 7, representing a species ofwell-lit places that also occurs in partial shade, but Hillet al. (1999) gave C. dissectum within Britain a lightindicator value of 8, representing a light-loving plantrarely found where relative illumination in summer isless than 40%.(ii) Water relationsRoss (1999) investigated water uptake and water-useefficiency using a gravimetric method; plants weregrown in conical flasks containing Rorison nutrientsolution and the use of water determined by weighingthe plants and the conical flasks containing thesolutions at regular intervals. Helianthus annuus wasalso grown so that the water use of this mesophyticspecies could be compared to C. dissectum. Theexperiment was carried out in a growth room with a16 h day at 22 °C and 15 °C night with day-time light