Rainfall, land use and woody vegetation cover change in semi-arid Australian savanna (original) (raw)
Remote Sensing of Environment
The phenology of a landscape is a key parameter in climate and biogeochemical cycle models and its correct representation is central to the accurate simulation of carbon, water and energy exchange between the land surface and the atmosphere.Whereas biogeographic phenological patterns and shifts have receivedmuch atten- tion in temperate ecosystems, much less is known about the phenology of savannas, despite their sensitivity to climate change and their coverage of approximately one eighth of the global land surface. Savannas are complex assemblages of multiple tree, shrub, and grass vegetation strata, each with variable phenological responses to seasonal climate and environmental variables. The objectives of this study were to investigate biogeographical and inter-annual patterns in savanna phenology along a 1100 km ecological rainfall gradient, known as North Australian Tropical Transect (NATT), encompassing humid coastal Eucalyptus forests and woodlands to xeric inland Acaciawoodlands and shrublands.Key phenology transition dates (start, peak, end, and length of seasonal greening periods) were extractedfrom13 years (2000–2012) ofModerateResolutionImaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) data using Singular SpectrumAnalysis (SSA). Two distinct biogeographical patterns in phenologywere observed, controlled by different climate systems. The northern (mesic) portion of the transect, from 12°S, to around 17.7°S, was influenced by the Inter-Tropical Convergence Zone (ITCZ) seasonal monsoon climate system, resulting in strong latitudinal shifts in phenology patterns, primarily associated with the functional response of the C4 grass layer. Both the start and end of the greening (enhanced vegetation activity) season occurred earlier in the northern tropical savannas andwere pro- gressively delayed towards the southern limit of the Eucalyptus-dominated savannas resulting inrelatively stable length of greening periods. In contrast, the southern xeric portion of the study areawas largely decoupled from monsoonal influences and exhibited highly variablephenology thatwas largely rainfallpulse driven. The season- al greening periodswere generally shorter but fluctuatedwidely fromno detectable greening during extended drought periods to length of greening seasons that exceeded those in the more mesic northern savannas in somewet years. Thiswas in part due tomore extremerainfall variability, aswell as a C3/C4 grass-forbunderstory that provided the potential for extended greening periods. Phenology of Acacia dominatedsavannasdisplayed a much greater overall responsiveness to hydroclimatic variability. The variance in annual precipitation alone could explain 80% of the variances in the length of greening season across the major vegetation groups. We also found that increased variation in the timing of phenology was coupled with a decreasing tree-grass ratio. We further compared the satellite-based phenology results with tower-derived measures of Gross Ecosystem Production (GEP) fluxes at three sites over two contrasting savanna classes. We found good convergence be- tweenMODIS EVI and tower GEP, thereby confirming the potential to link these two independent data sources to better understand savanna ecosystem functioning.