The resilience of a landscape is defined here to mean the capacity for the landscape to influence recovery of biodiversity after perturbation back to its pre-disturbance state (Holling 1973). Hence the biodiversity will recover more quickly after disturbance in a patch within a more resilient landscape compared with a similar patch in a less resilient landscape. Clearly there are characteristics of the patch which will enable biodiversity to recover more or less quickly, but for this discussion it is the
Chapter 2 – Landscape context variation characteristics of the landscape that enable them to influence biodiversity that is of greater interest here.
Numerous ecological studies have demonstrated associations between landscape composition and the capacity of forest patches to maintain diversity particularly of species dependent on late stage forest habitats (Forman and Moore 1992; Laurance et al. 1998b; Mesquita et al. 1999; Laurance et al. 2000; Lindenmayer and Fischer 2006; Baker et al. 2013b; Farmilo et al. 2014). In the area studied here there was evidence of changing landscape composition in the results which determined that there had been a reduction through time in the mean, range and heterogeneity of LCI scores. Both the empirical and simulation literature describing landscape ecology provides a
foundation for the hypothesis that the observed changes in landscape composition within the study area may be associated with changes in recruitment and mortality rates for at least some species. Evidence from previously published studies within the forests of this study area have already demonstrated that distance to the nearest mature forest edge has an influence on the recruitment rates of several rainforest tree species (Tabor et al. 2007) and the recovery rate of beetle communities (Fountain- Jones et al. 2015). The mechanisms by which proximity to mature forest can facilitate recovery of mature forest species have been described in detail by Baker et al.
(2013b).
The LCI metric provides a measure of the relative abundance of mature forest species habitats in the surrounding landscape of a site. Although not a direct measure of proximity, the more abundant mature forest habitat is then the closer it is likely to be. Biota (especially mature forest biota) in sites with higher LCI scores are likely to recover faster following disturbance compared with sites with lower LCI scores, all else being equal. The reduction in LCI scores and reduced abundance of oldgrowth forests observed in the study area since 1947 is therefore likely to have increased distances to the nearest mature forest and particularly an increased distance to oldgrowth forests. This is likely to have resulted in some loss of landscape resilience in many parts of the study area, but particularly eastern areas where agricultural land
and plantations are concentrated. Although the response of biota to landscape is
Chapter 2 – Landscape context variation the LCI metric at any scale will give some measure of landscape resilience. Should wildfire occur now the recovery of at least some mature forest species may be slower than if LCI scores were higher and old growth forests formed a greater portion of the region.
Previous studies have also demonstrated that the direct effects of settlement,
agriculture and logging also interact with and affect disturbance, including pollution, pest, weed and disease invasion on forest frontiers (Hobbs and Saunders 1994; McIntyre and Hobbs 1999; Saunders et al. 1999). In particular, fire regime change is common to many frontiers and has serious ramifications for biodiversity (McIntyre and Hobbs 1999; Shearman et al. 2012; Spies et al. 2012; Taylor et al. 2014). Small and landscape-scale fires were numerous in this region following settlement until the last recorded landscape-scale fire in 1967, observations which accord with those of Alcorn et al. (2001), Hickey et al. (1999) and Podger et al. (1988) for adjacent and overlapping regions. The limited available evidence suggests that fire was less
frequent in wet forest and rainforest, at least for this area, prior to European settlement
(Podger et al. 1988, Alcorn et al. 2001). More compelling evidence for vegetation
shifts caused by both reduced and increased fire frequencies are provided for other parts of Tasmania (Podger et al. 1988; Ellis and Thomas 1988; Marsden-Smedley 1998; di Folco and Kirkpatrick 2013).
Fire extent and frequency after 1967 were much less than in the earlier post- settlement period. This is explained at least partially by increased awareness and regulation of ignition sources during high fire danger weather and improved fire suppression capacity. The reduction of slash by high intensity burning after clear- felling may also have reduced available fuel loads within youngest silvicultural forests compared with selectively logged areas. Since 1947 there has been a
significant shift in demographic structure towards younger and more even-aged forest due to logging. Younger forests are more flammable than mature forests (Jackson 1968) and it is possible that higher densities of eucalypts in silvicultural regeneration create a more flammable forest-type than equivalent aged multi-cohort forest
regenerated by wildfire. Shifts in the demographics of forest in the northwest pacific and southeast USA have resulted in increased continuity of fuel loads, increasing the probability of landscape scale and stand-replacing fires (Covington and Moore 1994a;
Chapter 2 – Landscape context variation
Hessburg et al. 2000). Taylor et al. (2014) detected a difference in the severity of fires
burning in Eucalypt regnans forest depending on the age of the silvicultural
regeneration, providing evidence that the most severe fires were associated with forests aged between seven and 36 years.
Although the results of the present study suggest the study area may be less resilient now compared with 1947, this is not surprising given that most of it is dedicated for timber production. The results also suggested that despite ongoing exploitation, government regulation in recent decades has contributed to the mitigation of LCI score reductions in coupes following logging in western areas of the study area. By dispersing coupes more widely and reducing their size, the LCI scores following harvest are now higher following logging compared with previous practices and it is likely that the mature forest biodiversity which is likely to be sensitive to disturbance, may have the potential to recover more quickly. Field survey work is required to test this hypothesis and to determine the nature of any relationship that may exist between native species and LC influence within the wet forest of this study area.
"... the first law of geography: everything is related to
everything else, but near things are more related than distant things."
Waldo Tobler (1970) A computer movie simulating urban growth in the Detroit region. Economic Geography, 46(2): 234–240.
Chapter 3 – Distance from the mature forest edge