A scaled, contextual perspective of woody structure and dynamics across a savanna riperian landscape
Date
2009-02-05T08:44:20Z
Authors
Levick, Shaun Robert
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Abstract
Sound understanding of the influence of scale and context on ecological patternprocess
relationships is lacking in many systems. The hierarchical patch dynamics
paradigm (HPDP) provides a framework for addressing spatio-temporal heterogeneity,
but the range of systems in which, and scales at which, its principles apply are
largely unknown. Furthermore, it does not explicitly account for the influence of
spatial context. Recent developments in remote sensing science show potential for
bridging this gap by enabling the exploration of landscape heterogeneity at multiple
scales and across a wide range of systems and contexts, but the ecological application
of these new techniques is lagging. The savanna riparian landscapes of the
northern Kruger Park, South Africa, provided a unique platform in which to explore
the influence of spatial context, and to test the pattern-process-scale and metastability
principles of the HPDP, to further its potential as a unifying framework in
landscape ecology.
LiDAR and high-resolution aerial imagery were integrated through object-based
image analysis to create spatial representations of woody structure (canopy height,
canopy cover, canopy height diversity and canopy cover diversity) across a portion
of the savanna landscape (60 000ha). Temporal change in woody cover and heterogeneity
(number and size of woody patches) was assessed from a historical aerial
photography record, that spanned 59 years from 1942 to 2001. Spatial relationships
between environmental variables and patterns of woody structure and dynamics
were tested at broad (100ha), medium (10ha) and fine-scales (1ha) through canonical
correspondence analysis (CCA). The relative contribution of different categories
of environmental variables, to the total explained variation in woody structure, was
assessed at each scale through partial canonical correspondence analysis (PCCA).
Spatial variation in environmental variables, and the influence of spatial context on woody structure-environment relationships, was explicitly tested through geographically
weighted regression (GWR).
LiDAR results provided an unprecedented basis from which to explore spatial
patterns of woody structure in an African savanna. Standard approaches to generating
normalized canopy models (nCM) from LiDAR suffered interpolation artifacts
in the heterogeneous landscape, but an object-based image analysis technique was
developed to overcome this shortfall. The fusion of LiDAR with aerial imagery
greatly enhanced the structural description of the landscape, and the accuracy of
canopy height estimates varied between different vegetation patch types.
Woody structure and dynamics displayed distinct spatial trends across the landscape
with high diversity and variability occurring in the alluvial riparian zones.
Woody canopy height, canopy cover and cover dynamics exhibited scale variance
in their relationship with environmental variables, but woody structural diversityenvironment
relationships were scale invariant across the analysis patch hierarchy.
These findings from different woody attributes both support and contradict
the pattern-process-scale principle of the HPDP, which hypothesizes that ecological
processes shift with scale, but that spatial variance measures exhibit stepwise
patterns of change with scale, along a patch hierarchy.
Percentage woody cover was stable over time across the landscape, despite high
variability at smaller scales. However the metastability principle cannot be considered
generally applicable in this system, as a broader view of the woody component
revealed a marked decline in woody heterogeneity over time. Although losses
of woody cover on the diverse alluvial substrates were countered by increases of
cover in the uplands, analysis of current woody structure in the context of historical
change revealed that the increases took place in the form of shrub encroachment
and not the replacement of tall trees. The vertical structure of woody vegetation,
and therefore both the biodiversity and ecological functioning of the system, has
changed over time across the landscape. The metastability principle of theHPDP may not be applicable in spatially heterogeneous systems, where ecological processes
act differentially across the landscape, but may apply within specific patch
types at certain temporal scales.
Spatially localized analysis models revealed significant spatial non-stationarity
in the majority of processes correlated with woody structure, and showed that both
the magnitude and direction of woody structure-environment relationships varied
in different spatial contexts across the landscape. These results have fundamental
implications for the manner in which both science and conservation measures
are conducted in heterogeneous systems. Global analysis models, that assume stationarity,
are widely accepted and employed in ecological research but may greatly
misrepresent ecological relationships that are context-dependent. These findings
question the level of system understanding that field studies can provide, by revealing
the dangers of inferring patterns and relationships from measurements of limited
spatial representation. Leveraging the latest remote sensing technologies, that provide
large-extent but fine-grain coverage, in a scaled and context conscious manner,
will enhance ecological understanding by spatially quantifying the full spectrum of
system heterogeneity.
The heterogeneous patterns, scaled relationships and context-dependent patterns
identified in this study are challenging from both ecological research and biodiversity
conservation points of view. Traditional approaches to science and conservation
are ill equipped to address these issues. The HPDP provides an excellent conceptual
construct for meeting such challenges, but the influence of spatial context needs to
be more explicitly incorporated within the framework.
A catchment-based hierarchy is suggested for guiding future research and conservation
efforts in heterogeneous landscapes, where context-dependency of ecological
processes may be the norm.