Easter Island Deforestation: At The Hands of Mother Nature?

As discussed in the previous post, within a matter of a few hundred years, the forested landscape that existed on Easter Island since the Pleistocene disappeared to the grassy stoney landscape that exists today. The collapse of Easter Island is widely percieved as murder or suicide by the settlers themselves…but what if it was murdered by mother nature?

This post aims to assess the theory proposed by Mann et al (2008) in the paper: ‘Drought, vegetation change and human history on Rapa Nui (Isla de Pascua, Easter Island)’, which suggests that climate variability and change caused deforestation on Easter Island.

Mann et al (2008) reports on sediment, charcoal and pollen stratigraphy in the Rano Raraku lake core on Easter Island, and use their results to help identify the timing of widespread forest clearance and help test the hypotheses about involvement of drought. Stratigraphic records from lake sediment cores and slope deposits on Rapa Nui indicate that the Rano Raraku lake basin dried out after 4090-4410 cal yr BP (BC 2140-2460). A depositional hiatus (in this case an angular unconformity whereby younger sediment lies on the eroded surface of older rocks) in the sediment record suggests that drought occurred between 1180-3990 cal yr BP (BC 2040 to AD 770) due to the fact that the soil structure originated from dessication-cracking of the dry lake bed. So what caused these droughts?

There are three meteorological phenomena that are important for controlling rainfall patterns in southeast pacific; Subtropical southeast Pacific high, tracks taken by cyclonic storms carried in the westerlies, and ENSO

1.    Subtropical southeast Pacific High: can block approach of storms carried by westerlies, therefore when it weakens, cyclonic storms track further north (equatorwards) it reduced rainfall and lake levels at Easter.

2.    Cyconlic Storms: Most of the rains come from these cylconic storms moving eastward across the Pacific. Mann et al (2008) argure that shifts in the latitude of storm tracks are probably an important trigger for drought on Rapa Nui, as they shift latitudinally depnding on the overall pole-equator temp gradient.

3.    ENSO: During phases of La Nino, Easter Island experience warmer sea surface temperatures and extended droughts.

Mann et al (2008) conclude that:

“latitudinal shifts in the subtropical storm track and resultant changes in the intensity and frequency of cyclonic storms provide feasible, though untested, climatological trigger for radical changes in moisture balance on Rapa Nui”

They also provide another line evidence based on the knowledge that the geography of the subtropical high pressure system, due to the fact we would expect synchronous droughts on Easter Island and mainland Chile. Based on the Rano Raraku sediment record, the refilling of the lake basin at AD 1180-1290 coincides with the end of a warm and dry climate episode over central Chile, which thus strengthens evidence for drough found in the Rano Raraku sediment record.

There is some literature to support the conclusions made in this paper, for example Stenseth and Voje (2009) present evidence that the ENSO causes reduced marine biomass and resources on Easter, and thus during extended La Nina phases, and other climate variability, reduced marine resources leads to more intensive agriculture on land, or the search for new fishing grounds, both of which require wood of some sort, consequently increasing deforestation.

However, even at a first glance, it is clear to see that many issues surround this theory. Firstly, the trees and shrubs on Easter Island survived climate variability due to ENSO, cyclonic storms and changes in the Subtropical southeast Pacific pressue belt for tens of thousands of years before (Flenley and Bahn 2007), therefore it seems very unlikely that the droughts proposed by Mann et al (2008) could have caused such extensive and widespread deforestation. A paper by Saez et al  (2009) that investigates the sedimentary record of the Rana Raraku lake basin on Easter Island also supports this point, whereby the sedimentary record shows intense drought periods occurred from the mid to late Holocene, yet trees and shrubs survived throughout this period. They also go on to suggest that the Medieval Warm Period aided population expansion, howver because population expansion and deforestation are supposed to go hand in hand, climate is not likely to have had any impact on deforestation.

Secondly, the depositional hiatus in the sediment record is assumed to be caused by drought, however because there is no sediment record, it is impossible to say what the cause of this hiatus is, as we simply have no evidence. As a result, detailed records and evidence of past climate variability on Easter Island remains (and accordingly conclusions as to the effect of climate on Easter’s deforestation) unfeasible to date.

So, did drought and climate variability cause deforestation? From looking at the evidence it is clear that before we can make any solid conclusions on the relationship between climate variability and deforestions, a more accurate sedimentary record is needed. For now though, it is safe to say that it seems likely that climate did not cause deforestation, BUT it could have exacerbated it.