Sharp distribution boundaries of pines in sierra

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- Sharp distribution boundaries of pines in Sierra Nevadas in California US. (3 needle and 5 needle species) - The species with the same number of needles are morphologically and ecological similar and they overlap little in elevation on sites with similar slopes, exposures and soil types - Interspecific competition leading to competitive exclusion. - Highest density sometimes at ecotone - Not much evidence for the super-organismic view (graph) along resource gradients during succession. - But, some evidence for distribution patterns of communities based on (resource) gradient studies - Associations – groups of plant species commonly found in similar habitat (eg. Sugar maple – yellow birch) – not necessarily from succession. Community Structure – Paleoecological Perspective
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- Differences in ages of colonization suggest all tree species did NOT ‘arrive’ at the same time ( no super organism) - Recolonization following Pleistocene - Glaciation (1000y BP intervals) - N boundary similar but pattern of recolonization (migration) very different (note rate and current distribution) - Migration depended on seed dispersal and life histories that differed between species Community structure – succession – Two competing (three?) theories: Pattern, Process, Both? Points of agreement: within area of similar soil and climate, succession sis a directional process leading to the formation of a community containing a predictable set of dominant species. - While some groups of species shifted in concert, others became associated with different communities, while still others were unable to adapt and went extinct. - The reality is that while there is much individualism along gradients or the landscape, there are clear associations of species that exist on different sites. - Even if spatial or environmental boundaries may be fuzzy, identifiable communities, particularly dominance-type communities can often be delineated at many different spatial scales and provide a useful way to describe and study the geographic distribution of life Measuring community structure - Because of often indistinct nature of communities, much work done on identifying communities and associations and mapping their spatial boundaries - Other approaches may quantitatively survey plant species present and compare them to other stand or areas. o Species richness – total # of species o Dominant species o Species evenness – distribution of individuals between different species o Species diversity o Growth form – “structure” of plant o Primary Production (net and gross) – More about function - For forest trees, widely used set of quantitative measures of community structure. For a given species (x): Relative density = individual species x/ total individuals of all species x 100 Relative Frequency = frequency of species x/sum of frequency of all species x 100 Relative Dominance = basal area of species x/total basal area of all species x 100 Importance Value = Relative Density = Relative Frequency + Relative Dominance of Species x Beta diversity – the degree of difference in the species composition for two different habitat or
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