Decadal dynamics of tree cover in an Australian tropical savanna

Caroline Lehmann, L Prior, David Bowman

    Research output: Contribution to journalArticleResearchpeer-review

    Abstract

    Spatio-temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi-model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 � 0.88%, but was spatio-temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20-year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic Savannas. � 2009 Ecological Society of Australia.
    Original languageEnglish
    Pages (from-to)601-612
    Number of pages12
    JournalAustral Ecology
    Volume34
    Issue number6
    Publication statusPublished - 2009

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    Lehmann, C., Prior, L., & Bowman, D. (2009). Decadal dynamics of tree cover in an Australian tropical savanna. Austral Ecology, 34(6), 601-612.
    Lehmann, Caroline ; Prior, L ; Bowman, David. / Decadal dynamics of tree cover in an Australian tropical savanna. In: Austral Ecology. 2009 ; Vol. 34, No. 6. pp. 601-612.
    @article{ad87116aec8041999cbe5930c849a22f,
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    year = "2009",
    language = "English",
    volume = "34",
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    Lehmann, C, Prior, L & Bowman, D 2009, 'Decadal dynamics of tree cover in an Australian tropical savanna', Austral Ecology, vol. 34, no. 6, pp. 601-612.

    Decadal dynamics of tree cover in an Australian tropical savanna. / Lehmann, Caroline; Prior, L; Bowman, David.

    In: Austral Ecology, Vol. 34, No. 6, 2009, p. 601-612.

    Research output: Contribution to journalArticleResearchpeer-review

    TY - JOUR

    T1 - Decadal dynamics of tree cover in an Australian tropical savanna

    AU - Lehmann, Caroline

    AU - Prior, L

    AU - Bowman, David

    PY - 2009

    Y1 - 2009

    N2 - Spatio-temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi-model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 � 0.88%, but was spatio-temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20-year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic Savannas. � 2009 Ecological Society of Australia.

    AB - Spatio-temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi-model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 � 0.88%, but was spatio-temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20-year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic Savannas. � 2009 Ecological Society of Australia.

    KW - aerial photography

    KW - decadal variation

    KW - field survey

    KW - fire behavior

    KW - historical ecology

    KW - national park

    KW - patch dynamics

    KW - rainfall

    KW - remote sensing

    KW - savanna

    KW - spatiotemporal analysis

    KW - vegetation cover

    KW - Australasia

    KW - Australia

    KW - Kakadu National Park

    KW - Northern Territory

    M3 - Article

    VL - 34

    SP - 601

    EP - 612

    JO - Australian Journal of Ecology

    JF - Australian Journal of Ecology

    SN - 1442-9985

    IS - 6

    ER -

    Lehmann C, Prior L, Bowman D. Decadal dynamics of tree cover in an Australian tropical savanna. Austral Ecology. 2009;34(6):601-612.