The use of chronosequences in studies of ecological succession ...

732 L. R. Walker et al.Table 2. Relative appropriateness of the chronosequence approach varies depending on (a) predictability and trajectory type (divergent orconvergent) and (b) plant biodiversity and disturbance impact (frequency plus severity). ++Very useful, +useful, )not useful, ))potentiallymisleadingDivergentConvergentPlant succession Soil development Plant succession Soil development(a)Predictable + + ++ ++Unpredictable ) ) + +Low disturbanceHigh disturbancePlant succession Soil development Plant succession Soil development(b)High biodiversity ) + )) )Low biodiversity + + +* or )† +*Progressive succession.†Retrogressive succession.Table 3. Guidelines for developing appropriate chronosequence studies in terms of the elements needed and potential limitations of studies whenthese elements are missingElements neededTwo or more stages (duration of time series depends onparameter of interest)Multiple stand characteristics that vary across stagesAt least one independent verification of time seriesReplication within stages (number and spacing depends onspatial heterogeneity)Sampling intervals within life span of every dominant speciesof interest or duration of process of interestMultiple visits to study plotsSere-appropriate measurementsStandardized measurementsPotential limitations if element is missingChronosequence study of ecosystem parameters onlyReduced ability to interpret temporal dynamicsFaulty assumptions about temporal linkagesMisrepresentation of stage characteristicsMissed stages, inaccurate trajectoriesMissing verification of short-term dynamicsFailure to record relevant changesLack of ability to extrapolate to other studiesexperiments performed along both the progressive and retrogressivestages of the Hawaiian chronosequence (Vitousek2004) have greatly enhanced our understanding of how the relativeimportance of nitrogen and phosphorus limitation influencesecosystem development both above and below ground.Similarly, plant removal experiments along a 6000-year,fire-driven chronosequence in northern Sweden (Wardle &Zackrisson 2005; Gundale, Wardle & Nilsson in press) haveclarified the shifting linkages between plant community compositionand soil biogeochemical processes during succession.Although few manipulative experiments have been performedacross successional gradients, such studies offer tremendouspotential for better understanding the role of both biotic andabiotic factors in driving community and ecosystem changeduring succession.The appropriate use of chronosequences relies on at leastfive site-specific issues that serve as limitations, if not addressed(Table 3). First, chronosequences are most useful when thereis a clear pattern of temporal change between multiple stages.Secondly, there should be several lines of evidence about thehistory of the site. For short-term chronosequences, such evidencemight include oral histories, tree rings or historical maps,whereas for long-term chronosequences, these data mightinclude good geographical or stratigraphic dating or biologicalindicators such as micro- and macro-fossils. If such independentverification of a time series is present, the chronosequenceapproach is more likely to be justified. Thirdly, locating replicateplots randomly within each stage of the chronosequence(not just the progressive phase), when possible, can helpaddress the structure of the (non-age-related) variation amongÓ 2010 The Authors. Journal compilation Ó 2010 British Ecological Society, Journal of Ecology, 98, 725–736