Why Are Mangroves Expanding Into Saltmarshes In Eastern .
Why are Mangroves expanding intoSaltmarshes in eastern Australia?Leila Eslami- Andargoli 1, Pat ER Dale11Environmental Futures Centre, GriffithSchool of Environment, Griffith University,Nathan, Queensland, Australia
Outline Establishing encroachmentFactors that may influence encroachmentAim of the researchMethodsResultsConclusion
Mangroves encroaching into saltmarshThis is leading to regional decline in salt marsh area throughouteastern AustraliaWhy?
How are they encroaching? There has been much research* along theeastern seaboard of Australia showingmangrove expansion at the expense of saltmarsh19722004Mangroves (Avicennia)Saltmarsh (Sporobolus/SarcocorniaE.g., McTainsh et al. 1986, Saintilan & Williams 1999, Saintilan & Wilton 2001, Jones et al. 2004
Salt marsh losses of up to 100% havebeen reported (Saintilan and Williams2000)LocationSaltmarsh LossPeriodQueenslandBRISBANE- GOLD COAST areaOyster Point75% (Saltpan)1944-198365 hectares11%1944-19881974-1987Tweed River72%1947-1986SYDNEY areaWeeney Bay,Botany BayWoolaware Bay,Botany BayTowra Point,Botany Bay100%63%30%1950-19941950-19941942-1997Moreton BayCoolangatta-CaloundraNSW
What affects encroachment?There are many and often interacting factorssuch as: Climate change (Co2, Rainfall, Sea Level) Human activities – modifying the environment– Direct impact on wetlands (e.g., filling)– Land use/land cover changes/effects ofpopulation change
Modifying environmentIndirect impact on theintertidal zoneRainfallWhat is the appropriatetime frame?MangroveSaltmarshChangingClimate
Research aimTo assess the relationship between mangroveencroachment into salt marsh and: Rainfall pattern; Land use/cover; Population changes; and MSI index (the proportion of mangrove forestadjoining salt marsh relative to boundary length)Between 1972 and 2004 (1972 - earliest Landsatdata)
MethodsStudy sites: weselected 10sites and theirsub-catchmentsin northernMoreton Baywith similarclimate.To the south isthe denselypopulated StateCapital,Brisbane;northwardspopulationdensity is lower.
Data Published data– Daily rainfall data (Australian Bureau of Meteorology)– Population (Australian Bureau of Statistics) Remote sensing – Aerial photos and Landsat from1972, 1990 and 2004 used to map:– Land use (Landsat classification - pixel 30m *)– Mangrove/salt marsh spatial patterns (air photoanalysis at 1:24000 and 1:12000, scanned with1000 and 500 dots per inch to produce digitalimages with a resolution of nearly 0.6 m per pixel)* 1972 imagery at 80m pixel was resampled to a 30 m approximation
Landsat Satellite Imagery: MoretonBay
Aerial photo mosaics for mangroveand salt marsh mapping
Analysis:1. identifying patterns Change analysis identified 1990 as a significant changepoint in Rainfall pattern: this was used to define thetime frame for mangrove spatial analysis Mangrove distribution was mapped from air photoanalysis for the wet (pre-1990) and dry period (post 1990) for the study sites using ArcGIS The Mangrove – Salt marsh Interface (MSI index) wascalculated (boundary length related to mangrove area) Change in mangrove distribution encroaching intosaltmarsh was calculated as the annual increase % foreach period *Seaward expansion was also calculated but was relatively small
Analysis: 2. identifying relationshipsPartial least square regression (PLSR) generalizesand combines features from principal componentanalysis and multiple regressionWe used PLSR to analyse and identify the relationbetween the rate of mangrove expansion andrainfall, land use and population density and itspotential expansion (represented by the MSIindex*),during the wet (pre-1990) and the dry (post-1990)periods.* The Mangrove – Saltmarsh – Interface index
Results Rainfall change point in 1990: Pettit-Mann-Whitneytest (Probability of the change point)The‘cumulative sum technique’ (CUSUM) which detectschanges in the mean value of a time series dataset,
Mangrove/saltmarsh rate of change: summary bysite
SiteMangrove area changebetween 1972 and 2004 (ha)Cabbage Tree Ck (S area)13.4Bald Hills Ck21.48Pine Rivers5.06Hays Inlet8.31Little Burpengary Ck6.68Burpengary Ck6.47Southern Caboolture20.37Lagoon Ck6.16Ningi Ck9.46Glass Mt Ck (N area)TOTAL 10 sites21.76119.11How muchincrease inmangrove area?Over the same period117 ha of salt marshhas been lostSo far we have not seenany loss at the seawardedge – will sea levelrise change this? Overwhat time frame?
Mangrove increase /salt marshdecrease
Mangrove spatial change example:Glass Mt CreekPre 1990 mangrove change 2.15%/yrPost 1990 mangrove change 1.02%/yr
Boundaries matter- the opportunity for expansion –Mangrove Saltmarsh Interface index (MSI)1972MSI is based on theratio between thelength of boundaryand mangrove area: alarge MSI indexindicates a longboundary and henceopportunity forexpansion. A smallindex is the reverse.(a) Scattered patches at Ningi Creek with large MSI 1972 (15.76);rate of expansion pre 1990 1.09%/yr(b) Aggregated cover at Pine River with small MSI 1972 (0.75);rate of expansion pre 1990 0.41%/yr
1972Land cover/use change19902004
Mangrove expansion PLSR analysis:wet and dry periods comparedThe PLSR results of Components 1and 2 (Comp)VariableRainfall medianMSI indexPopulation densityAgriculture (%)Built up (%)Plantation forest (%)Variance x-block2RP-valueMangroveexpansion 590.1540.0010.005Mangrove 80.1870.5610.1710.0120.053PLSR weights whose squares are larger than 0.2 are shown in bold type, asthey retain relatively high information content of each component.
Conclusion The research established a significant relationshipbetween rainfall pattern and the landward expansionof mangroves in Moreton Bay’s subtropical estuaries; A key finding of this research was that the contributionof landscape variables to spatial changes in themangroves changed following a reduction in rainfall During wet periods mangrove expansion was related tosub-catchment-wide land use/cover pattern andpopulation density During drier periods it was more affected by local effectsof nearby land use/cover (within 500m)
Thank you
Weeney Bay,Botany Bay 100% 1950-1994 Woolaware Bay,Botany Bay 63% 1950-1994 Towra Point,Botany Bay 30% 1942-1997 What affects encroachment? . mangroves changed following a reduction in rainfall During wet periods mangrove expansion was
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