Global patterns of root turnover for terrestrial ecosystems

Richard A. Gill and Robert B. Jackson

Duke University, Department of Botany, Durham, North Carolina, USA

Summary

We tested global controls on root turnover across climatic gradients and for plant functional groups using a database of 150 published studies. Root turnover rates increased exponentially with mean annual temperature for fine roots (<5 mm diameter) of grasslands and forests, and total root shrubland root systems (P<0.001 for grasslands and shrublands; P=0.03 for forests). Surprisingly there was no such global relationship for precipitation and root turnover (P>0.25 in all cases). The slowest average turnover rates were observed for entire tree root systems (10% annually), followed by 34% for shrublands, 53% for grasslands, 55% for wetland species, and 56% for forest fine roots. Root turnover decreased from tropical to high-latitude systems for all plant functional groups. To test the ability of the global relationships to explain inter-annual variability in root turnover, we evaluated 14 years of published root turnover data from the shortgrass steppe long-term ecological research site in north central Colorado, USA. At that site there was no correlation between interannual variability in mean annual temperature and root turnover. Rather, turnover was positively correlated with the ratio of growing season precipitation and maximum monthly temperature (P<0.001). We conclude that there are global patterns in rates of root turnover among plant groups and across climatic gradients but that these patterns may not always be used predict the consequences of climate change at a particular site.

Objectives

(1) Synthesize root turnover data for major biomes and plant life forms globally

(2) Assess broad-scale patterns of root turnover along climatic gradients

(3) Contrast regional and local controls over root turnover

Defining Root Turnover

Ratio of belowground production to maximum biomass (from Dahlam and Kucera 1967)
In cases where only mean biomass provided, used a convention to convert between mean and maximum biomass:
Maximum Biomass = 0.45*BNPP + Mean Biomass

Controls Over Root Turnover

Results

Objective 1: Synthesize root turnover data for major biomes and plant life forms globally

Figure 1: Root turnover for vegetation types separated by latitudinal zone.

Objective 2: Assess broad-scale patterns of root turnover along climatic gradients

Figure 2: (a) Relationship between root turnover and mean annual temperature by vegetation type (p<0.001, r² = 0.63 for grasslands; p<0.001, r² = 0.75 for shrublands; p<0.03, r² = 0.10 for forest fine roots) (b) Relationship between root turnover and mean annual precipitation by vegetation type (p<0.02, r² = 0.32 for grasslands)

Objective 3: Contrast regional and local controls over root turnover

Figure 3: (a)Relationship between mean annual temperature and root turnover for grasslands globally (r²=0.63, p<0.001) and for the Central Plains Experimental Range in northeastern Colorado, USA (p>0.15); (b) Relationship between root turnover and Growing season PPT/ Maximum Monthly Temperature at the CPER (r²=0.72, p<0.001)

The Database

~150 papers from journals, book chapters, technical reports, and unpublished manuscripts
Coverage included all continents except Antarctica, although most sites were in the Northern Hemisphere
Used root standing crop and belowground net primary productivity to determine root turnover
Supplementary data included location, climate, method, vegetation, and soils information
Averaged turnover across years and within plant functional types for each study to avoid biasing the dataset
Analyzed data based on vegetation and latitudinal location
Data from shortgrass steppe long-term ecological research site come from 14 years of belowground biomass studies

Conclusions

Root turnover increases exponentially with mean annual temperature for Grasslands, Shrublands, and Forest Fine Roots

Relative turnover rate by plant functional type: Whole trees < Shrublands < Grasslands = Wetlands = Forest Fine Roots

Turnover increases from the poles to the equator for all vegetation types

Global patterns may not be useful in predicting consequences of climate change at an individual site

Acknowledgements

This work was supported by the National Science Foundation, Department of Energy, and the Andrew W. Mellon Foundation. We thank Jason Tullis for his help in making the site location map.

Contact Information:

R.A. Gill
Duke University, Department of Botany
Durham, NC 27708
rgill@duke.edu
Ph: (919) 660-7290; Fax: (919) 660-7425

This poster was presented at a New Phytologist sponsored workshop on Root Dynamics and Global Change in October 1999.

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Last modified 25 October 2000