Article Title

Tree Recruitment during Dry Spells at an Oak Savanna in Minnesota

Journal Title/Source

Tree-Ring Research

Publication Date

6-2008

Volume

64

Issue (if applicable)

1

Page Numbers

47-54

DOI (if applicable)

dx.doi.org/10.3959/2007-17.1

Document Type

Journal Article

Department

Earth, Environmental and Geographical Sciences

Abstract

Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index.21), versus only 5 of 42 (12%) that established in wet years (PDSI.1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x2510.738, df51, p-value50.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation offine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion