Experimental simulation: using generative modeling and palaeoecological data to understand human-environment interactions

The amount of palaeoecological information available continues to grow rapidly, supporting improved descriptions of the dynamics of past ecosystems and enabling them to be seen from new perspectives. At the same time, there has been concern over whether palaeoecological enquiry needs to move beyond descriptive inference to a more hypothesis-focussed, or experimental approach. However, the extent to which conventional hypothesis-driven scientific frameworks can be applied to historical contexts (i.e., the past) is the subject of ongoing debate. In other disciplines concerned with human-environment interactions, including physical geography and archaeology, there has been growing use of generative simulation models, typified by agent-based approaches. Generative modeling encourages counter-factual questioning (“what if…?,”) a mode of argument that is particularly important in systems and time-periods, such as the Holocene, and now the Anthropocene, where the effects of humans and other biophysical processes are deeply intertwined. However, palaeoecologically focused simulation of the dynamics of the ecosystems of the past either seems to be conducted to assess the applicability of some model to the future or treats humans simplistically as external forcing factors. In this review we consider how generative simulation-modeling approaches could contribute to our understanding of past human-environment interactions. We consider two key issues: the need for null models for understanding past dynamics and the need to be able learn more from pattern-based analysis. In this light, we argue that there is considerable scope for palaeoecology to benefit from developments in generative models and their evaluation. We discuss the view that simulation is a form of experiment and by using case studies, consider how the many patterns available to palaeoecologists can support model evaluation in a way that moves beyond simplistic pattern-matching and how such models might also inform us about the data themselves and the processes generating them. Our emphasis is on how generative simulation might complement traditional palaeoecological methods and proxies rather than on a detailed overview of the modeling methods themselves

Demography of the long-lived conifer Agathis ovata in maquis and rainforest, New Caledonia

The endemic New Caledonian conifer Agathis ovata occurs as an emergent tree in fire-prone shrublands (maquis), and fire-sensitive rainforest. Growth, survivorship and recruitment over 5 yr were compared for populations from forest and maquis on ultramafic substrates in New Caledonia to investigate whether demographic behaviour varied in response to the strongly contrasting forest and shrubland environments. Growth of seedlings and of small (30–100 cm height) and large (100 cm height; 5 cm DBH) saplings was slow, but varied significantly among stages, site types and years. The greatest difference in growth rates was among stages, seedlings growing 0.34 cm.yr−1, small saplings 1.06 cm.yr−1 and large saplings 2.13 cm.yr−1. Tree DBH increased by only 0.05 cm.yr−1 and, based on these rates, individuals with DBH of 30 cm are estimated to be more than 700 yr old. Few trees (3.5%) produced cones in any year and seedling recruitment was low, but some recruitment was recorded each year in both maquis and forest. Rates of recruitment per parent were highest in forest (1.28.yr−1, cf 0.78.yr−1), but the higher density of trees in maquis meant that overall recruitment was greater there (92 ha−1.yr−1, cf 56 ha−1.yr−1). Seedling mortality ranged from 0.9 to 2.9% among years with no significant difference between maquis and forest. No sapling mortality was recorded, but annual tree mortality ranged from 0 to 1.4%. Evidence from a recently burned site indicated that while trees may survive fire, seedlings and saplings do not. Post-fire seedling recruitment per ha from surviving trees was four times lower than in unburned sites, but growth rates were four times higher. Similar demographic attributes, including high survivorship, low growth rate and low rates of recruitment over a long reproductive life, characterize Agathis ovata populations in both maquis and rainforest in New Caledonia and are indicative of a broad tolerance of light environments that is unusual among tree species. These demographic attributes help to explain the long-term persistence of the species in these strongly contrasting habitats

Spatial modelling of vegetation change in dynamic landscapes: a review of methods and applications

Because of the spatiotemporal scales involved and the logistical constraints in collecting landscape-level data, spatially explicit simulation models have become important tools in ecological and biogeographical studies conducted over broad extents. Here we review the methods used and some of the applications of landscape-level models of succession and disturbance dynamics. Mechanistic and stochastic models are compared and contrasted and the development, over the last 15 years, of spatial landscape models of ecological change is discussed. Coarse-grained spatial landscape models are compared with finer-grained individual-based approaches (eg. forest gap models). Management and monitoring applications of landscape models are considered alongside a discussion of the appropriate use of models in this context. A key area where spatial landscape models of the type described here need to develop is improved integration with the social sciences - both in terms of the parameters and the processes that the models incorporate. Finally issues related to scale and scaling are outlined and, in particular, the utility of methods for linking ecological models operating at disparate scales (eg. forest gap models versus landscape models) is examined

Humans, fire and landscape pattern: understanding a maquis-forest complex, Mont Do, New Caledonia, using a spatial 'state-and-transition' model

Aim: The research explores how changes in disturbance regime resulting from human settlement may affect landscape structure. A spatially explicit grid-based simulation model is used to explore the interplay between humans, fire regime and landscape composition. Location: The study site for this research is the botanical reserve at Mont Do, New Caledonia. The endemic conifer Araucaria laubenfelsii (Araucariaceae) forms a key component of the landscape at Mont Do. This species is unusual in that it is found scattered as an emergent in maquis and as a canopy species in adjacent rain forest patches. Although now dominated by a low maquis, prior to human settlement of New Caledonia, montane landscapes such as Mont Do are likely to have been heavily forested. Methods: A spatially explicit simulation model, based on field data and palaeoecological information, was used to explore interactions between disturbance regime and the landscape. The model is described briefly here and more fully in Perry & Enright (2002) Ecological Modelling, 152, 279. Results: The model suggests that human-influenced changes to the fire regime at Mont Do have been important in generating the current landscape structure. The origin and maintenance of forest landscapes and maquis-forest mosaic landscapes are considered in the context of alternative stable states. Strong feedback loops between fire size and landscape composition, mediated at the smaller scale by other similar mechanisms, are capable of driving landscape change. The utility of a spatial state and transition modelling approach is demonstrated. Main conclusions: The current landscape pattern on Mont Do is likely the result of changes to the fire regime occurring since human settlement. The specific mechanisms for this change outlined here may occur in a number of other similar systems. Understanding the origin and persistence of these 'fire landscapes' in New Caledonia and in the southwest Pacific in general is crucial for effective management

Spatial modelling of landscape-scale vegetation dynamics, Mont Do, New Caledonia

The coniferous tree Araucaria laubenfelsii forms a key component of vegetation structural assemblages on ultramafic substrate at Mont Do, New Caledonia. It is the sole species to be found both as an emergent in maquis and as a common canopy species in adjacent rainforest patches. This paper describes a spatially explicit, landscape-level model developed to investigate the vegetation pattern on Mont Do and provides preliminary results of model analyses. Results indicate that the interaction between fire, terrain and the long residence times of fire-prone successional stages may be largely responsible for the landscape patterns visible. Maquis is found predominantly on hillsides where fire spread is likely to be most rapid, while maquis with emergent A. laubenfelsii, and A. laubenfelsii woodland, are restricted to rocky areas and rainforest margins of likely intermediate fire frequency. The model indicates that if fires were to become either more frequent or increase in size, maquis would become increasingly prevalent in the landscape. Pattern metrics, such as fractal dimension, indicate that as a result of such a change, the spatial complexity and fragmentation of the landscape may be reduced. Conversely, if fire frequency decreases or fire sizes become smaller, then rainforest becomes more dominant. Although the presence of A. laubenfelsii on Mont Do is not threatened under such a scenario, the persistence of certain 'transient' structural assemblages at their current abundances seems less certain. While the role that ultramafic soil conditions might play at the landscape level is not clear, several possible interactions between the plant-soil relationship and broader scale dynamics are identified. In particular, soil chemical constraints on plant growth may be responsible for the very long time-scales associated with succession from maquis to forest, and this slowed succession increases the potential role of fire as a major determinant of landscape-scale vegetation pattern relative to that on nearby non-ultrabasic substrates

Spatial modelling of "alternative" future landscapes under climate change and fire suppression, Mont Do, New Caledonia

The vegetation dynamics and disturbance regimes of the south-west Pacific have been significantly altered following human settlement. Previously forested landscapes are now dominated by a matrix of flammable early successional vegetation within which patches of mesic, fire-sensitive forest are embedded. Future environmental change, and in particular climate change, will further affect disturbance regimes in these ecosystems. If ignition frequency and fire extent increase, then the persistence of these landscapes in their current composition and structure is uncertain. Using a spatially explicit landscape ecological model, we explored the implications of climatically altered fire regimes for landscape composition and structure in a mountain-top reserve in New Caledonia. The outcomes of the modeling suggest that increased ignition probability and vegetation flammability would lead to a maquis (heathland)-dominated landscape structurally simpler than that seen today. The feasibility of fire suppression as a means of managing altered fire regimes was explored using a series of model experiments. Fire suppression has been problematic in some systems, especially those where fire hazard increases over time. However, in this ecosystem, and others in the south-west Pacific, it may be a viable alternative for managing fire because fire hazard, in terms of flammability, peaks early in the succession and then decreases over successional time

Contrasting outcomes of spatially implicit and spatially explicit models of vegetation dynamics in a forest-shrubland mosaic

Much contemporary ecology emphasises the importance of taking a spatial perspective in linking ecological patterns and processes. However, collecting and analysing spatial data is expensive. Here we compare spatially implicit and spatially explicit versions of a model of successional dynamics in a forest-shrubland mosaic in a mountain-top reserve in New Caledonia. The models are used to (i) understand the circumstances driving change in abundance of forest and shrubland, and (ii) compare the outcomes of spatially explicit and spatially implicit models of the same system. The spatially explicit model is grid-based and uses a spatially implemented 'state-and-transition' approach, with fire spread and seed dispersal the main spatial processes considered. The spatially implicit model is based on a transition matrix approach. Two alternative transition matrices were constructed, one based on field measurements and the other parameterised using output from the spatially explicit model. Although the averaged dynamics of the two models appear similar, the models make very different qualitative predictions about the landscape. Under the same initial parameter conditions two alternative landscape states emerge from the spatially explicit model; this is not the case for the spatially implicit model. Further, the spatial model produces outcomes much closer to those documented historically and inferred from the palæoecological record. The differences between the non-spatial and spatial models arise because, in this system, fine-scale interactions between landscape pattern and process are drivers of coarser scale dynamics, and such interactions are not included in the spatially implicit model. More generally, in order to understand coarse-scale spatial dynamics it may be important to consider local spatial patterns; spatially explicit models are those most likely to incorporate these

A comparison of methods for the statistical analysis of spatial point patterns in plant ecology

We describe a range of methods for the description and analysis of spatial point patterns in plant ecology. The conceptual basis of the methods is presented, and specific tests are compared, with the goal of providing guidelines concerning their appropriate selection and use. Simulated and real data sets are used to explore the ability of these methods to identify different components of spatial pattern (e.g. departure from randomness, regularity vs. aggregation, scale and strength of pattern). First-order tests suffer from their inability to characterise pattern at distances beyond those at which local interactions (i.e. nearest neighbours) occur. Nevertheless, the tests explored (first-order nearest neighbour, Diggle's G and F) are useful first steps in analysing spatial point patterns, and all seem capable of accurately describing patterns at these (shorter) distances. Among second-order tests, a density-corrected form of the neighbourhood density function (NDF), a non-cumulative analogue of the commonly used Ripley's K-function, most informatively characterised spatial patterns at a range of distances for both univariate and bivariate analyses. Although Ripley's K is more commonly used, it can give very different results to the NDF because of its cumulative nature. A modified form of the K-function suitable for inhomogeneous point patterns is discussed. We also explore the use of local and spatially-explicit methods for point pattern analysis. Local methods are powerful in that they allow variations from global averages to be detected and potentially provide a link to recent spatial ecological theory by taking the 'plant's-eye view'. We conclude by discussing the problems of linking spatial pattern with ecological process using three case studies, and consider some ways that this issue might be addressed