NZ-2: Initial Burning Period (IBP) gradient/dynamics
Title: Documenting the geographic pattern and consequences of the IBP and Post-IBP vegetation succession dynamics
Investigators: McWethy, Whitlock, Wilmshurst, McGlone, Perry
Objectives: Reconstructions of past fire and vegetation in South Island New Zealand indicate a clear signal of increased fire activity following the first known Māori presence there (McWethy et al. 2009, 2010). Paleoecological data from the South Island point to anthropogenic burning as the cause of the vegetation change yet the structure and character of the IBP and the accompanying ecological response are still poorly understood. Records from 16 sites show an abrupt increase in fire activity soon after Māori arrival, yet the timing varies among watersheds (McWethy et al. 2010).
The IBP is followed by an absence of fires at some sites and periodic fires at others. Pollen records show little recovery of pre-IBP vegetation in the absence of fire, suggesting that succession of open vegetation to small tree and pre-IBP forests was delayed yet the factors responsible for this delay are still unknown. Work from northern NZ in modern landscapes shows positive feedbacks between vegetation, soil and fire may facilitate deforestation and slow succession. Ongoing modeling work by Wilmshurst, McGlone and Perry indicates that delayed successional development could be achieved by as few as one sizeable fire every 50-100 years in South Island landscapes following deforestation during the IBP. If this is true, then even infrequent small fires could arrest succession and facilitate conversion to pyrophyllic vegetation and lead to what McGlone terms “ecosystems in waiting”. Because open vegetation of grasses and bracken produces little charcoal, we hypothesize that even small peaks in the post-IBP charcoal time series could represent ecologically significant fires that suppressed reforestation. Alternatively, mass-wasting and consequent loss in soil stability and fertility could have worked alone, or in concert with episodic fires, to explain delayed succession. Some sites show a second wave of burning coincident with European arrival in New Zealand, but at other sites, this signal is weak or absent. This signal may relate to what was being converted by Europeans – some sites may have reverted to forest and been reburned while others may have still been seral when Europeans burned them. No doubt Europeans also burned previously intact forest.
The present number of sites with high-resolution charcoal and pollen records is confined to the south-central South Island. In the absence of a broader network of records, important questions related to the IBP remain unresolved: What spatial and temporal patterns of fire are evident during the IBP? Were the environmental consequences of the IBP largely shaped by the nature of the burning, the introduction of a novel disturbance, or by site-specific physical characteristics? Why was there little recovery of native New Zealand forests after the IBP? To address these questions, we will (1) retrieve lake-sediment cores from new sites that complete the gradient of samples from interior to coast on both South and North Island; and (2) examine the charcoal, pollen, and other paleoenvironmental proxy to better understand the ecosystem response during and following the IBP.
Work Plan: To increase the geographic coverage of high-resolution fire history records, we will obtain high-resolution pollen and charcoal records from 6-8 new sites in the coastal South Island and the North Island where Māori settlements and agriculture were well established. Site selection will be guided by empirical spatial fire models developed by Perry, Wilmshurst and McGlone that identify fire-vulnerable/fire-sensitive vegetation on the landscape. Comparison of model predictions and empirical data will test of the ability of the model to detect fire at decreasing spatial scales and help interest the charcoal data based on large landscape attributes.
With the additional records, we will (1) develop a GIS database, building on the LENZ and CINZAS datasets, that allows us to examine the pattern of burning with respect to physical, biological, and cultural attributes so that we can better understand the geographic pattern of the IBP and the motivation for burning; (2) analyze chironomids, diatoms, and geochemistry of lake sediments to better characterize the fire regime and watershed response to fire; and (3) employ spatial simulation models to explore potential landscape responses to fire under different fuel and climate conditions and topographic constraints.
To better understand the fire regimes of the post IBP period, we will select 2-4 sites where previous pollen and charcoal work suggests limited forest recovery and little fire activity following the IBP. We will analyze intermediate charcoal size fractions (50-125 microns) to determine whether smaller charcoal sizes can detect small surface fires not evident macroscopic charcoal (>125 mµ). We will also collaborate with C. Barbante at the University of Venice to conduct levoglucosan analyses to identify the source material of charcoal 1600-1800 AD.
This project will complement efforts by Perry, McGlone and Wilmshurst to better understand the vulnerability of New Zealand ecosystems to human-set fires and post-fire vegetation dynamics. In addition to PIRE support, the research is funded by an associated NSF grant (BCS-1024413; McWethy and Whitlock).
High-resolution charcoal, pollen, geochemical and limnobiotic analyses for new sites from South and North Island, New Zealand. These results will be coordinated and published with McWethy as lead author and Perry, McGlone, or Wilmshurst as lead author on related publications; and the data will be used in other synthetic activities. Post-IBP analysis of new and existing sediment cores will be led by McWethy; linkages to modeling efforts will be led by Perry and McGlone. McWethy will also work with Barbante on the analysis of fire biomarkers in the sediment cores.
Year 2 Update
This project seeks to understand the spatial and temporal variability of the fires set by Maori when they arrived in New Zealand 800 years ago. It builds on a complementary project led by McWethy and Whitlock through a grant from the NSF GSS program. The goal is to obtain high-resolution, well-dated charcoal records from lakes in areas where Maori settlements were known to exist, as a contrast to previously studied sites in more remote settings. The character of the initial burning period in areas of high population might help reveal whether early human-set fires were accidental or deliberate. Sediment cores were collected from twelve new lakes, including critical coastal sites in the North and South Island. Initial sampling and core descriptions were undertaken in the field and also at Landcare Research in Lincoln NZ. Charcoal and pollen analyses of these sites are underway to assess the Maori use of fire in areas of dense occupation. In addition, preliminary lithologic and geochemical (scanning XRF) analyses have been completed on these cores at the University of Minnesota to examine the possibility of watershed and limnologic changes associated with the initial burning period. Cores are being split and subsampled at the MSU Paleoecology Laboratory for paleoecological analyses. A significant collaborative paper on the dynamics of the IBP was submitted and published (Perry et al. 2012).
Introductions and discussions were also made with contemporary Maori communities and archeologists to gain information about traditional uses of fire and perceptions of nature. This information will be critical as the project continues to examine the role of humans in shaping New Zealand’s fire regime.