Abstract
Climate change is increasing the risk of drought to forested ecosystems. Although drought impacts are often anecdotally noted to occur in discrete patches of high canopy mortality, the landscape effects of drought disturbances have received virtually no study. This study characterized the landscape configuration of drought impact patches and investigated the relationships between patch characteristics, as indicators of drought impact intensity, and environmental gradients related to water availability to determine factors influencing drought vulnerability. Drought impact patches were delineated from aerial surveys following an extreme drought in 2011 in southwestern Australia, which led to patchy canopy dieback of the Northern Jarrah Forest, a Mediterranean forest ecosystem. On average, forest gaps produced by drought-induced dieback were moderate in size (6.6 ± 9.7 ha, max = 85.7 ha), compact in shape, and relatively isolated from each other at the scale of several kilometers. However, there was considerable spatial variation in the size, shape, and clustering of forest gaps. Drought impact patches were larger and more densely clustered in xeric areas, with significant relationships observed with topographic wetness index, meteorological variables, and stand height. Drought impact patch clustering was more strongly associated with the environmental factors assessed (R2 = 0.32) than was patch size (R2 = 0.21); variation in patch shape remained largely unexplained (R2 = 0.02). There is evidence that the xeric areas with more intense drought impacts are ‘chronic disturbance patches’ susceptible to recurrent drought disturbance. The spatial configuration of drought disturbances is likely to influence ecological processes including forest recovery and interacting disturbances such as fire. Regime shifts to an alternate, non-forested ecosystem may occur preferentially in areas with large or clustered drought impact patches. Improved understanding of drought impacts and their patterning in space and time will expand our knowledge of forest ecosystems and landscape processes, informing management of these dynamic systems in an uncertain future.
Highlights
Ecological disturbances are known to be important drivers of the patch-mosaic structure of vegetation communities across a landscape
We investigated a drought-induced forest dieback event characterized by extremely patchy canopy mortality in the Northern Jarrah Forest of southwestern Australia [25]
The above discussion focused on the spatial occurrence of drought impacts, we suggest that the spatial properties of drought impact patches, especially patch size and patch clustering, provide measures of drought impact intensity that are more meaningful than the presence/absence of drought impacts alone
Summary
Ecological disturbances are known to be important drivers of the patch-mosaic structure of vegetation communities across a landscape. Because disturbances occur heterogeneously in space and time, they produce variability in landscape composition, influencing patterns of the types of vegetation present, as well as their successional stages and vertical structure, among other factors [1]. The spatial configuration of landscapes is strongly dependent on disturbances, and varies between disturbance agents and disturbance regimes. For a given disturbance agent, variation in disturbance regime due to differences in environment [4] or management (such as fire suppression [5,6,7]) produces differences in the spatial characteristics of disturbed patches. One type of disturbance that has received surprisingly little consideration as a driver of landscape structure is drought. Many drought-induced dieback and tree mortality events have already been observed worldwide, in diverse forest systems [16,17]
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