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Floristic response to landscape context in vascular plant communities in Eucalyptus obliqua and Eucalyptus regnans wet forest, southern Tasmania

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Balmer, J (2016) Floristic response to landscape context in vascular plant communities in Eucalyptus obliqua and Eucalyptus regnans wet forest, southern Tasmania. PhD thesis, University of Tasmania.

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Abstract

Theories developed within the paradigm of landscape ecology propose that
biodiversity within any given patch will be influenced by the surrounding landscape
context (LC). Here LC is defined as the vegetation or land covers surrounding the site.
This thesis used empirical vascular plant abundance data from the Huon forest district
in southern Tasmania to test the hypothesis that LC influences the floristic
composition and successional trajectory of patches within Eucalyptus obliqua and
E. regnans wet forest. Secondary objectives included measuring the spatial and
temporal variation in LC of the study area and its association with timber harvesting;
describing differences in the response to LC between species and plant groups;
comparing the effect size of plant responses to LC with other environmental
predictors; finding the spatial and temporal scale at which plants respond most
strongly to LC; and determining whether the effect of LC varied in response to
disturbance regime differences.
Most studies in landscape ecology have explored the effects of fragmentation on
native vegetation in an agricultural matrix. In contrast, forest patches sampled in this
study comprised native forest of various ages and successional stages within intact to
variegated landscapes (sensu McIntyre and Hobbs 1999) in a frontier region
dedicated substantially to timber production. Variation in LC was investigated for the
study region using a new metric: the Landscape Context Index (LCI). This metric
provided a relative scale of vegetation maturity in areas surrounding any given 50 x
50 m pixel, measured in 500 m, 1 km and 2 km radii. LCI was mapped for three years
(1947, 1985 and 2009). The average LCI score was lower in 1985 than 1947, and was
lower again in 2009. The greatest declines in LCI score were associated with timber
harvesting, although in settled areas conversion to plantations was also an important
contributor to LC score decline. The extreme reductions in LCI score observed in the period between 1947 and 1985 were not observed in the second period, possibly due
in part to the mitigating effect of the Forest Practices Code, which resulted in changes
to timber harvesting practices, including increased dispersal and reduced size of clearfelled
patches.
Distance to the nearest mature forest edge (DMFE), was used as a surrogate for
mature forest influence, a component of LC. Between 15 to 200 m DMFE, there was
an observed gradient in assemblage variation, species richness and diversity in all
three age classes of silvicultural regrowth forest studied (4–9, 22–28, and 41–45 years
since regeneration). These trends were mainly driven by declines in richness and
cover of mature forest affiliated species. Factors considered likely to contribute to
differences in plant species response to LC are their sensitivity to micro-climatic
variation (influenced by proximity to mature forest, and topography) and macroclimatic
variation, their capacity to persist through disturbance, and their dispersal
mode.
Individual species abundance models for both mature forest affiliated species and
pioneer species were stronger (e.g. 38 out of 56 common species using beta regression
modelling) when they included both LC metrics and site environmental variables.
Although LC was important in explaining variance in species abundance, it typically
contributed less to model strength than other environmental predictors, such as soil,
climate, topography and disturbance history. Abundance of most pioneer species was
associated negatively with mature forest metrics, while the abundance of mature
forest affiliated species was associated positively with these metrics. There was little
evidence that plant species with bird or wind dispersed seed or that had seed able to
persist through disturbance in the soil or in woody capsules were any less associated
with LC metrics than species sensitive to disturbance with shorter dispersal capacity.
It is therefore possible that such species were responding indirectly to LC through
inter species competition, or because they were dependent on animal species sensitive
to LC for their pollination or dispersal. LC effects on site micro-climate, soil and
browsing pressure, which all vary with distance from edges of mature forest could
also be influencing the distribution of species in ways that are associated with the
distance from and proportion of mature forest in the landscape. Auto-correlation between LC metrics and site history are other factors that may be contributing to the
observed responses of all plant species to LC metrics.
There was only equivocal support for the hypothesis that floristic responses within
regrowth forests are more strongly associated with the LC of the patch at the time that
they were last disturbed rather than current LC. Interpretation of the response of
vegetation to both the spatial and temporal scale of LC was hampered by the strong
autocorrelation in LC scores between scales, and by the sampling methods chosen.
The balance of evidence suggests that the LC in the years following disturbance and
prior to canopy closure strongly influence the trajectory rate of succession, however
colonization and extinction within sites may occur infrequently at any time so that
later successional stages are still likely to be influenced by LC at all times.
An interactive effect between proximity to mature forest and fire frequency was
observed within secondary regrowth forests, such that patches burnt twice or more
since 1898 exhibited greater assemblage differences with distance from the boundary
compared with forest burnt only once. This was evidence that the response to LC is
partly dependent on disturbance regimes at the site and that succession towards
mature forest occurs more slowly in frequently disturbed patches. There was also a
poor representation of the dominant rainforest trees and epiphytic ferns in regrowth
forest that has colonized abandoned pastures (old fields), irrespective of the LC.
Barriers to the colonization by rainforest tree species within old field regrowth may
include the maintenance of more open canopy structure for longer periods than typical
in uncleared regrowth forest disturbed by single fire events.
An examination of all results in conjunction with those of other empirical studies
suggest that successional trajectories in the species composition of wet eucalypt
forests may be altered by changes in landscape configurations in response to
silvicultural practices and changes in climate and associated fire regimes. Given the
association between LC and succession in forest patches, the observed reductions in
mean LCI scores across the study region may signal that recovery from the effects of
wildfire, extreme climate events, and harvesting may be slower at the site level. If so,
then current LC patterns reflect a reduction in resilience at the landscape scale.
Predicted changes in climate and associated fire regimes may make local extinctions more likely in areas of low LCI score. In the context of landscape planning, although
dispersal and pollination distances for most vascular plants is best measured in tens of
metres rather than kilometres, landscape planning at 500 m to one kilometre may be
sufficient to avoid local extinctions at finer scales and thereby prevent range
contractions. Silvicultural methods and land management practices that may avert
future losses in landscape resilience are discussed.

Item Type: Thesis (PhD)
Keywords: landscape context, wet eucalypt forest, mature forest influence, eucalyptus regnans wet forest, southern Tasmania
Copyright Information:

Copyright 2016 the author

Date Deposited: 13 Oct 2016 22:28
Last Modified: 13 Oct 2016 22:28
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