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Papers by David Coomes

Research paper thumbnail of Responses of Juvenile Trees to above- and Belowground Competition in Nutrient-Starved Amazonian Rain Forest

Ecology, 1998

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Research paper thumbnail of Seed mass and nutrient content in nutrient-starved tropical rainforest in Venezuela

Seed Science Research, 1997

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Research paper thumbnail of Amazonian caatinga and related communities at La Esmeralda, Venezuela: forest structure, physiognomy and floristics, and control by soil factors

Plant Ecology, 1996

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Research paper thumbnail of Colonization, tolerance, competition and seed-size variation within functional groups

Trends in Ecology and Evolution, 2003

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Research paper thumbnail of Impacts of Root Competition in Forests and Woodlands: A Theoretical Framework and Review of Experiments

Ecological Monographs, 2000

... 1965). Although some shade-tolerant trees may establish under pine, the process of succession... more ... 1965). Although some shade-tolerant trees may establish under pine, the process of succession is greatly slowed by the soil infertility, persistence of the early-successional species, and by fire and drought (Faliński 1994). We ...

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Research paper thumbnail of Impacts of forest fragmentation on species composition and forest structure in the temperate landscape of southern Chile

Global Ecology and Biogeography, 2007

Aim Few studies have explicitly examined the influence of spatial attributes of forest fragments... more Aim Few studies have explicitly examined the influence of spatial attributes of forest fragments when examining the impacts of fragmentation on woody species. The aim of this study was to assess the diverse impacts of fragmentation on forest habitats by integrating landscape-level and species-level approaches.Location The investigation was undertaken in temperate rain forests located in southern Chile. This ecosystem is characterized by high endemism and by intensive recent changes in land use.Method Measures of diversity, richness, species composition, forest structure and anthropogenic disturbances were related to spatial attributes of the landscape (size, shape, connectivity, isolation and interior forest area) of forest fragments using generalized linear models. A total of 63 sampling plots distributed in 51 forest fragments with different spatial attributes were sampled.Results Patch size was the most important attribute influencing different measures of species composition, stand structure and anthropogenic disturbances. The abundance of tree and shrub species associated with interior and edge habitats was significantly related to variation in patch size. Basal area, a measure of forest structure, significantly declined with decreasing patch size, suggesting that fragmentation is affecting successional processes in the remaining forests. Small patches also displayed a greater number of stumps, animal trails and cow pats, and lower values of canopy cover as a result of selective logging and livestock grazing in relatively accessible fragments. However, tree richness and β-diversity of tree species were not significantly related to fragmentation.Main conclusions This study demonstrates that progressive fragmentation by logging and clearance is associated with dramatic changes in the structure and composition of the temperate forests in southern Chile. If this fragmentation process continues, the ability of the remnant forests to maintain their original biodiversity and ecological processes will be significantly reduced.

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Research paper thumbnail of Spatially explicit models to analyze forest loss and fragmentation between 1976 and 2020 in southern Chile

Ecological Modelling, 2008

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Research paper thumbnail of Rapid deforestation and fragmentation of Chilean Temperate Forests

Biological Conservation, 2006

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Research paper thumbnail of Factors Preventing the Recovery of New Zealand Forests Following Control of Invasive Deer

Conservation Biology, 2003

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Research paper thumbnail of The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering

Journal of Ecology, 2005

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Research paper thumbnail of Designing systems to monitor carbon stocks in forests and shrublands

Forest Ecology and Management, 2002

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Research paper thumbnail of Disturbances prevent stem size-density distributions in natural forests from following scaling relationships

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Research paper thumbnail of Effects of size, competition and altitude on tree growth

Journal of Ecology, 2007

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Research paper thumbnail of Disturbances prevent stem size-density distributions in natural forests from following scaling relationships: Scaling rules and size-density distributions

Ecology Letters, 2003

Enquist and Niklas propose that trees in natural forests have invariant size-density distribution... more Enquist and Niklas propose that trees in natural forests have invariant size-density distributions (SDDs) that scale as a −2 power of stem diameter, although early studies described such distributions using negative exponential functions. Using New Zealand and ‘global’ data sets, we demonstrate that neither type of function accurately describes the SDD over the entire diameter range. Instead, scaling functions provide the best fit to smaller stems, while negative exponential functions provide the best fit to larger stems. We argue that these patterns are consistent with competition shaping the small-stem phase and exogenous disturbance shaping the large-stem phase. Mortality rates, estimated from repeat measurements on 1546 New Zealand plots, fell precipitously with stem size until 18 cm but remained constant after that, consistent with our arguments. Even in the small-stem phase, where SDDs were best described by scaling functions, the scaling exponents were not invariantly −2, but differed significantly from this value in both the ‘global’ and New Zealand data sets, and varied through time in the New Zealand data set.

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Research paper thumbnail of Mortality and tree-size distributions in natural mixed-age forests

Journal of Ecology, 2007

1Tree-size distributions are changing in many natural forests around the world, and it is importa... more 1Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests.2We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands.3Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation.4Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands.5Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak.6The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure.7The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests.We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands.Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation.Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands.Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak.The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure.The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.

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Research paper thumbnail of On the mechanisms of coexistence among annual-plant species, using neighbourhood techniques and simulation models

Plant Ecology, 2002

Many studies have investigated the density-dependent regulation ofannual-plant populations on coa... more Many studies have investigated the density-dependent regulation ofannual-plant populations on coastal sand dunes, but few have explored theconsequences of competition for the coexistence of plants in these simplecommunities. We used neighbourhood techniques to parameterize competition anddispersal functions from field data collected for two species of dune annual(Aira praecox and Erodium cicutarium)over three successive years, and then combined these functions into spatiallyexplicit simulation models. The population size of Airavaried enormously between years, while Erodium remainedsteady. Competition with neighbours reduced the spike length ofAira plants only in one of the three years (when itspopulation density was highest), while competition with neighbouringErodium plants appeared to result in the local death ofAira plants. However, these density-dependent effects werefar too weak to generate the observed changes in the population size ofAira among years, or to maintain populations below theupper limits observed. The large-seeded Erodium wasaffected by intraspecific competition but was unaffected by small-seededAira plants. Therefore, the larger-seeded species wascompetitively superior to the smaller-seeded species, an affect that couldpromote coexistence (albeit weakly) by a competition-colonisation trade-off.Modal dispersion distances of Aira andErodium were 45 and 60 mm respectively,greater than the radius within which competitive interactions occurred (40mm). Theoretical studies suggest that under these conditions thespatial arrangement of plants should be nearly random. In factAira was spatially aggregated, especially when rare,suggesting that patchy mortality across the dunes was important in generatingspatial structure. The study suggests that density dependence only weaklyregulates dune annual communities, while year-to-year environmental variationexert major influences on population sizes and spatial structures.

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Research paper thumbnail of Are differences in seed mass among species important in structuring plant communities? Evidence from analyses of spatial and temporal variation in dune-annual populations

Oikos, 2002

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Research paper thumbnail of How spatial structure alters population and community dynamics in a natural plant community

Journal of Ecology, 2007

1We quantify the effects of spatial structure on individual, population and community biomass wit... more 1We quantify the effects of spatial structure on individual, population and community biomass within a natural community of annuals for two years (1994 and 1996) in which competition operated differently.2Using field-parameterized neighbourhood models in which we had previously estimated the spatial scale and magnitude of competitive effects, we perform virtual experiments in which we manipulate spatial structure relative to that observed. We first remove segregation by randomly re-labelling all annual plants in a focal plant's neighbourhood and then give individuals of all species the same average number of neighbours by re-locating individuals within the overall pattern of annuals.3The natural spatial structure generally acted to promote species coexistence. In 1994, when the strength of individual-level competition within species was already greater than that between species, the natural spatial structure enhanced this effect for six out of seven species by increasing the relative frequency of intraspecific contacts. Average plant biomass increased by 0.24–38% when spatial locations were randomized.4In 1996, when a single competitive hierarchy operated, the natural spatial structure systematically benefited small-seeded, weakly competitive species while having weak negative effects on large-seeded, strong competitors. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 12% but increased the biomass of the species with the largest seeds by < 1%. This asymmetry increased dramatically when we used the model to increase the competitive advantage of large seeds artificially. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 37% but increased the biomass of the species with the largest seeds by only 2.7%.5Effects of spatial structure on total community biomass were always small (maximum 7%). This was because common species always have weak spatial structure, and they draw down the effect on the community as a whole.6Our analysis, for the first time, quantifies the magnitude and direction of the effects of spatial structure in a natural community, and supports the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics.We quantify the effects of spatial structure on individual, population and community biomass within a natural community of annuals for two years (1994 and 1996) in which competition operated differently.Using field-parameterized neighbourhood models in which we had previously estimated the spatial scale and magnitude of competitive effects, we perform virtual experiments in which we manipulate spatial structure relative to that observed. We first remove segregation by randomly re-labelling all annual plants in a focal plant's neighbourhood and then give individuals of all species the same average number of neighbours by re-locating individuals within the overall pattern of annuals.The natural spatial structure generally acted to promote species coexistence. In 1994, when the strength of individual-level competition within species was already greater than that between species, the natural spatial structure enhanced this effect for six out of seven species by increasing the relative frequency of intraspecific contacts. Average plant biomass increased by 0.24–38% when spatial locations were randomized.In 1996, when a single competitive hierarchy operated, the natural spatial structure systematically benefited small-seeded, weakly competitive species while having weak negative effects on large-seeded, strong competitors. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 12% but increased the biomass of the species with the largest seeds by < 1%. This asymmetry increased dramatically when we used the model to increase the competitive advantage of large seeds artificially. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 37% but increased the biomass of the species with the largest seeds by only 2.7%.Effects of spatial structure on total community biomass were always small (maximum 7%). This was because common species always have weak spatial structure, and they draw down the effect on the community as a whole.Our analysis, for the first time, quantifies the magnitude and direction of the effects of spatial structure in a natural community, and supports the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics.

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Research paper thumbnail of Identifying Aggregation and Association in Fully Mapped Spatial Data

Ecology, 1999

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Research paper thumbnail of Seed mass and the competition/colonization trade-off: competitive interactions and spatial patterns in a guild of annual plants

Journal of Ecology, 2004

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Research paper thumbnail of Responses of Juvenile Trees to above- and Belowground Competition in Nutrient-Starved Amazonian Rain Forest

Ecology, 1998

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Research paper thumbnail of Seed mass and nutrient content in nutrient-starved tropical rainforest in Venezuela

Seed Science Research, 1997

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Research paper thumbnail of Amazonian caatinga and related communities at La Esmeralda, Venezuela: forest structure, physiognomy and floristics, and control by soil factors

Plant Ecology, 1996

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Research paper thumbnail of Colonization, tolerance, competition and seed-size variation within functional groups

Trends in Ecology and Evolution, 2003

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Research paper thumbnail of Impacts of Root Competition in Forests and Woodlands: A Theoretical Framework and Review of Experiments

Ecological Monographs, 2000

... 1965). Although some shade-tolerant trees may establish under pine, the process of succession... more ... 1965). Although some shade-tolerant trees may establish under pine, the process of succession is greatly slowed by the soil infertility, persistence of the early-successional species, and by fire and drought (Faliński 1994). We ...

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Research paper thumbnail of Impacts of forest fragmentation on species composition and forest structure in the temperate landscape of southern Chile

Global Ecology and Biogeography, 2007

Aim Few studies have explicitly examined the influence of spatial attributes of forest fragments... more Aim Few studies have explicitly examined the influence of spatial attributes of forest fragments when examining the impacts of fragmentation on woody species. The aim of this study was to assess the diverse impacts of fragmentation on forest habitats by integrating landscape-level and species-level approaches.Location The investigation was undertaken in temperate rain forests located in southern Chile. This ecosystem is characterized by high endemism and by intensive recent changes in land use.Method Measures of diversity, richness, species composition, forest structure and anthropogenic disturbances were related to spatial attributes of the landscape (size, shape, connectivity, isolation and interior forest area) of forest fragments using generalized linear models. A total of 63 sampling plots distributed in 51 forest fragments with different spatial attributes were sampled.Results Patch size was the most important attribute influencing different measures of species composition, stand structure and anthropogenic disturbances. The abundance of tree and shrub species associated with interior and edge habitats was significantly related to variation in patch size. Basal area, a measure of forest structure, significantly declined with decreasing patch size, suggesting that fragmentation is affecting successional processes in the remaining forests. Small patches also displayed a greater number of stumps, animal trails and cow pats, and lower values of canopy cover as a result of selective logging and livestock grazing in relatively accessible fragments. However, tree richness and β-diversity of tree species were not significantly related to fragmentation.Main conclusions This study demonstrates that progressive fragmentation by logging and clearance is associated with dramatic changes in the structure and composition of the temperate forests in southern Chile. If this fragmentation process continues, the ability of the remnant forests to maintain their original biodiversity and ecological processes will be significantly reduced.

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Research paper thumbnail of Spatially explicit models to analyze forest loss and fragmentation between 1976 and 2020 in southern Chile

Ecological Modelling, 2008

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Research paper thumbnail of Rapid deforestation and fragmentation of Chilean Temperate Forests

Biological Conservation, 2006

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Research paper thumbnail of Factors Preventing the Recovery of New Zealand Forests Following Control of Invasive Deer

Conservation Biology, 2003

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Research paper thumbnail of The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering

Journal of Ecology, 2005

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Research paper thumbnail of Designing systems to monitor carbon stocks in forests and shrublands

Forest Ecology and Management, 2002

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Research paper thumbnail of Disturbances prevent stem size-density distributions in natural forests from following scaling relationships

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Effects of size, competition and altitude on tree growth

Journal of Ecology, 2007

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Research paper thumbnail of Disturbances prevent stem size-density distributions in natural forests from following scaling relationships: Scaling rules and size-density distributions

Ecology Letters, 2003

Enquist and Niklas propose that trees in natural forests have invariant size-density distribution... more Enquist and Niklas propose that trees in natural forests have invariant size-density distributions (SDDs) that scale as a −2 power of stem diameter, although early studies described such distributions using negative exponential functions. Using New Zealand and ‘global’ data sets, we demonstrate that neither type of function accurately describes the SDD over the entire diameter range. Instead, scaling functions provide the best fit to smaller stems, while negative exponential functions provide the best fit to larger stems. We argue that these patterns are consistent with competition shaping the small-stem phase and exogenous disturbance shaping the large-stem phase. Mortality rates, estimated from repeat measurements on 1546 New Zealand plots, fell precipitously with stem size until 18 cm but remained constant after that, consistent with our arguments. Even in the small-stem phase, where SDDs were best described by scaling functions, the scaling exponents were not invariantly −2, but differed significantly from this value in both the ‘global’ and New Zealand data sets, and varied through time in the New Zealand data set.

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Research paper thumbnail of Mortality and tree-size distributions in natural mixed-age forests

Journal of Ecology, 2007

1Tree-size distributions are changing in many natural forests around the world, and it is importa... more 1Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests.2We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands.3Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation.4Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands.5Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak.6The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure.7The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests.We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands.Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation.Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands.Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak.The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure.The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.

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Research paper thumbnail of On the mechanisms of coexistence among annual-plant species, using neighbourhood techniques and simulation models

Plant Ecology, 2002

Many studies have investigated the density-dependent regulation ofannual-plant populations on coa... more Many studies have investigated the density-dependent regulation ofannual-plant populations on coastal sand dunes, but few have explored theconsequences of competition for the coexistence of plants in these simplecommunities. We used neighbourhood techniques to parameterize competition anddispersal functions from field data collected for two species of dune annual(Aira praecox and Erodium cicutarium)over three successive years, and then combined these functions into spatiallyexplicit simulation models. The population size of Airavaried enormously between years, while Erodium remainedsteady. Competition with neighbours reduced the spike length ofAira plants only in one of the three years (when itspopulation density was highest), while competition with neighbouringErodium plants appeared to result in the local death ofAira plants. However, these density-dependent effects werefar too weak to generate the observed changes in the population size ofAira among years, or to maintain populations below theupper limits observed. The large-seeded Erodium wasaffected by intraspecific competition but was unaffected by small-seededAira plants. Therefore, the larger-seeded species wascompetitively superior to the smaller-seeded species, an affect that couldpromote coexistence (albeit weakly) by a competition-colonisation trade-off.Modal dispersion distances of Aira andErodium were 45 and 60 mm respectively,greater than the radius within which competitive interactions occurred (40mm). Theoretical studies suggest that under these conditions thespatial arrangement of plants should be nearly random. In factAira was spatially aggregated, especially when rare,suggesting that patchy mortality across the dunes was important in generatingspatial structure. The study suggests that density dependence only weaklyregulates dune annual communities, while year-to-year environmental variationexert major influences on population sizes and spatial structures.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Are differences in seed mass among species important in structuring plant communities? Evidence from analyses of spatial and temporal variation in dune-annual populations

Oikos, 2002

Bookmarks Related papers MentionsView impact

Research paper thumbnail of How spatial structure alters population and community dynamics in a natural plant community

Journal of Ecology, 2007

1We quantify the effects of spatial structure on individual, population and community biomass wit... more 1We quantify the effects of spatial structure on individual, population and community biomass within a natural community of annuals for two years (1994 and 1996) in which competition operated differently.2Using field-parameterized neighbourhood models in which we had previously estimated the spatial scale and magnitude of competitive effects, we perform virtual experiments in which we manipulate spatial structure relative to that observed. We first remove segregation by randomly re-labelling all annual plants in a focal plant's neighbourhood and then give individuals of all species the same average number of neighbours by re-locating individuals within the overall pattern of annuals.3The natural spatial structure generally acted to promote species coexistence. In 1994, when the strength of individual-level competition within species was already greater than that between species, the natural spatial structure enhanced this effect for six out of seven species by increasing the relative frequency of intraspecific contacts. Average plant biomass increased by 0.24–38% when spatial locations were randomized.4In 1996, when a single competitive hierarchy operated, the natural spatial structure systematically benefited small-seeded, weakly competitive species while having weak negative effects on large-seeded, strong competitors. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 12% but increased the biomass of the species with the largest seeds by < 1%. This asymmetry increased dramatically when we used the model to increase the competitive advantage of large seeds artificially. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 37% but increased the biomass of the species with the largest seeds by only 2.7%.5Effects of spatial structure on total community biomass were always small (maximum 7%). This was because common species always have weak spatial structure, and they draw down the effect on the community as a whole.6Our analysis, for the first time, quantifies the magnitude and direction of the effects of spatial structure in a natural community, and supports the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics.We quantify the effects of spatial structure on individual, population and community biomass within a natural community of annuals for two years (1994 and 1996) in which competition operated differently.Using field-parameterized neighbourhood models in which we had previously estimated the spatial scale and magnitude of competitive effects, we perform virtual experiments in which we manipulate spatial structure relative to that observed. We first remove segregation by randomly re-labelling all annual plants in a focal plant's neighbourhood and then give individuals of all species the same average number of neighbours by re-locating individuals within the overall pattern of annuals.The natural spatial structure generally acted to promote species coexistence. In 1994, when the strength of individual-level competition within species was already greater than that between species, the natural spatial structure enhanced this effect for six out of seven species by increasing the relative frequency of intraspecific contacts. Average plant biomass increased by 0.24–38% when spatial locations were randomized.In 1996, when a single competitive hierarchy operated, the natural spatial structure systematically benefited small-seeded, weakly competitive species while having weak negative effects on large-seeded, strong competitors. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 12% but increased the biomass of the species with the largest seeds by < 1%. This asymmetry increased dramatically when we used the model to increase the competitive advantage of large seeds artificially. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 37% but increased the biomass of the species with the largest seeds by only 2.7%.Effects of spatial structure on total community biomass were always small (maximum 7%). This was because common species always have weak spatial structure, and they draw down the effect on the community as a whole.Our analysis, for the first time, quantifies the magnitude and direction of the effects of spatial structure in a natural community, and supports the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Identifying Aggregation and Association in Fully Mapped Spatial Data

Ecology, 1999

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Research paper thumbnail of Seed mass and the competition/colonization trade-off: competitive interactions and spatial patterns in a guild of annual plants

Journal of Ecology, 2004

Bookmarks Related papers MentionsView impact