Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 2: Spartina Distribution and Spreadwater supply is not limiting, photosyn<strong>the</strong>sis in C4 plants iscarbon dioxide saturated. However higher atmosphericcarbon dioxide is likely to increase photosyn<strong>the</strong>sis, andhence growth, in C3 plants such as Puccinellia due to higherconversion efficiency.The different effects <strong>of</strong> elevated temperatures andcarbon dioxide on <strong>the</strong> two species were modelled by Long(1990). The model predicts <strong>the</strong> primary production <strong>of</strong>Spartina and Puccinellia to be expected in <strong>the</strong> year 2050,assuming a 3 degree rise in temperature and a doubling <strong>of</strong>atmospheric carbon dioxide. It was validated by comparing<strong>the</strong> predictions <strong>of</strong> production under 1978 conditions wi<strong>the</strong>mpirical field data. The increase in annual net production <strong>of</strong>Spartina from 1.3 kilograms per square meter (kg/m 2 ) in1978 to 2.1 kg/m 2 in 2050 was largely attributable totemperature-driven increases in leaf area, enabling <strong>the</strong> pointwhere <strong>the</strong> leaf area index is sufficient to intercept 30% <strong>of</strong> <strong>the</strong>incoming solar radiation to be reached 50 days earlier. Theincrease in Puccinellia, from 1.4 kg/m 2 in 1978 to 2.5 kg/m 2in 2050, was largely attributed to higher conversionefficiency in a high carbon dioxide environment but <strong>the</strong>model indicated that Puccinellia would gain from highertemperatures in spring and autumn.Long’s model, as he acknowledges, excludes severalfactors which may change with climate and will impact onplant growth. These include salinity, nutrient availability andwater level. Also excluded is a consideration <strong>of</strong> <strong>the</strong>competitive interactions between <strong>the</strong> two species underchanging conditions.A COMPETITION EXPERIMENT BETWEEN THE TWO SPECIESIn order to gain insight into how changing climate mayaffect <strong>the</strong> distribution <strong>of</strong> Spartina and Puccinellia <strong>the</strong> twospecies were grown in a competition experiment undercontrolled conditions <strong>of</strong> carbon dioxide and temperature.The experiment and full results are described in Gray andMogg (2001) and only a summary is given below.Plants <strong>of</strong> <strong>the</strong> two species were originally sampled froma salt marsh in Morecambe Bay (at 54° 10’ N) and grown incommon conditions for 18 weeks before 160 tillers <strong>of</strong> eachspecies were removed, size-matched and transplanted intopots containing standard soils. The experimental design wasa replacement series (de Wit 1960), each series comprisingfive pots <strong>of</strong> four tillers (4 Spartina, 3 Spartina + 1Puccinellia, 2 Spartina + 2 Puccinellia, 1 Spartina + 3Puccinellia, and 4 Puccinellia). Eight series were set up inlarge pots and eight in smaller pots to give two density levelsand, following a three-week establishment period, <strong>the</strong> potswere transferred to <strong>the</strong> experimental conditions. Thesecomprised eight hemispherical glasshouses (<strong>the</strong>‘solardomes’) in which atmosphere and temperature arecontrolled to a high degree <strong>of</strong> precision. The eight domesallowed two replicates <strong>of</strong> each treatment and <strong>the</strong> fourtreatments were: ambient temperature + ambient CO 2 ;ambient temperature + a CO 2 concentration <strong>of</strong> 340 parts permillion; temperature elevated by 3° C and trackedcontinuously above ambient + ambient CO 2 ; and elevatedtemperature + elevated CO 2 . The pots were maintained withnon-limiting water supplies, fertilized, and harvested after 11months growth when each plant was separated into aboveand below ground material before weighing. The results aresummarized in Table 1.The main conclusions to be drawn from <strong>the</strong> experimentare (1) most yield variables were significantly affected bytreatment, ei<strong>the</strong>r by temperature or carbon dioxide level or<strong>the</strong> interaction between <strong>the</strong>se treatments and density(Spartina height and inflorescence number beingexceptions), (2) <strong>the</strong> main competitive interaction effectswere due to <strong>the</strong> competitive superiority <strong>of</strong> Puccinellia,which had a significant negative effect on Spartina’s heightand above ground weight and displayed strong intraspecificcompetitive effects on tiller number and biomass, (3)Spartina responded to higher temperature as predicted, butalso, at low plant density, to carbon dioxide concentration.In both cases <strong>the</strong> response was mainly an increase in belowgroundgrowth including rhizomes, (4) Puccinelliaresponded to higher atmospheric carbon dioxide and highertemperature mainly by increasing above-ground growth, (5)Spartina had an unexplained poor performance in <strong>the</strong> ++treatment, an effect seen at low density and <strong>the</strong>reforeunlikely to be explained by competition with Puccinellia,Table 1. Individual plant means for six yield variables at final harvest.Trait Species Amb +T +CO 2 ++ Sig mn effectsSpa 4.10 6.17 6.00 3.48 D*, CT*Till no.Pucc 31.46 17.69 25.76 25.98 P**, CTD*Fl. TillHt(cm)S wt(g)BG wt(g)Biom(g)Spa 0.13 0.11 0.28 0.01 nsPucc 1.79 0.42 1.34 1.53 D***, DP*Spa 15.37 16.69 19.98 17.88 P*Pucc 44.47 44.08 48.24 54.98 D***, CD*Spa 0.45 0.70 0.89 0.37 D*, P*Pucc 1.77 1.28 1.61 2.70 D***, CDT**Spa 0.61 0.97 1.38 0.60 CT*, CTD**Pucc 0.42 0.24 0.36 0.37 CTP**Spa 1.04 1.67 1.98 0.97 CT**, CTD**Pucc 2.19 1.53 1.97 3.08 D***, CTP*The treatments (see text) were: Amb = ambient temperature and CO 2 , +T =elevated temperature and ambient CO 2 , +CO 2 = ambient temperature andelevated CO 2 , ++ = elevated temperature and elevated CO 2 . The variableswere: Till no. = number <strong>of</strong> tillers, Fl. Till = number <strong>of</strong> flowering tillers,Ht(cm) = plant height, S wt(g) = shoot weight, BG wt(g) = weight <strong>of</strong>below-ground material, Biom(g) = total biomass. The significance levelsare from a generalised ANOVA <strong>of</strong> log transformed values in which D =effect <strong>of</strong> density (pot size) (1df), C = effect <strong>of</strong> elevated CO 2 (1df), T =effect <strong>of</strong> elevated temperature (1df), and P = effect <strong>of</strong> <strong>the</strong> proportion <strong>of</strong>Puccinellia (which is a measure <strong>of</strong> competitive interaction effects) (3df).Only main interaction effects are given. Significance * p