Fire and grazing influences on rates of riparian woody plant expansion along grassland streams.

Allison M Veach,Adam Skibbe,Walter K Dodds,Paul Adam

doi:10.1371/journal.pone.0106922

Abstract

Grasslands are threatened globally due to the expansion of woody plants. The few remaining headwater streams within tallgrass prairies are becoming more like typical forested streams due to rapid conversion of riparian zones from grassy to wooded. Forestation can alter stream hydrology and biogeochemistry. We estimated the rate of riparian woody plant expansion within a 30 m buffer zone surrounding the stream bed across whole watersheds at Konza Prairie Biological Station over 25 years from aerial photographs. Watersheds varied with respect to experimentally-controlled fire and bison grazing. Fire frequency, presence or absence of grazing bison, and the historical presence of woody vegetation prior to the study time period (a proxy for proximity of propagule sources) were used as independent variables to predict the rate of riparian woody plant expansion between 1985 and 2010. Water yield was estimated across these years for a subset of watersheds. Riparian woody encroachment rates increased as burning became less frequent than every two years. However, a higher fire frequency (1–2 years) did not reverse riparian woody encroachment regardless of whether woody vegetation was present or not before burning regimes were initiated. Although riparian woody vegetation cover increased over time, annual total precipitation and average annual temperature were variable. So, water yield over 4 watersheds under differing burn frequencies was quite variable and with no statistically significant detected temporal trends. Overall, burning regimes with a frequency of every 1–2 years will slow the conversion of tallgrass prairie stream ecosystems to forested ones, yet over long time periods, riparian woody plant encroachment may not be prevented by fire alone, regardless of fire frequency.

Highlights

Grasslands and wooded grasslands historically covered,30% of the world’s total land area, are responsible for,20% of global runoff, [1] and are threatened worldwide
Woody plant encroachment is occurring across numerous grassland ecosystems converting them into shrublands and forests
Watersheds are named by fire frequency (1, 2, 4, 20 years between burns), the inclusion or exclusion of native or cattle grazers (N or C), as well as specific drainage basin (K = Kings Creek north branch, S = Shane Creek) with the final letter assigned based on replicate number (A–D)

Summary

Introduction

Grasslands and wooded grasslands historically covered ,30% of the world’s total land area, are responsible for ,20% of global runoff, [1] and are threatened worldwide. Woody plant encroachment is occurring across numerous grassland ecosystems converting them into shrublands and forests. The timing required for this conversion may largely be attributed to interactions between climate, fire regime, herbivory, nitrogen deposition, and increases in CO2 concentrations [3,9,10,11]. Conversion of grasslands to shrublands and forest may lead to shifts in terrestrial ecosystem functioning [6,12], such as heightened carbon sequestration [13] and reductions in carbon mineralization [14]. Woody plant encroachment is leading to widespread ecosystem changes which may not be reversible [15]

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