US EPA

1764627 

Journal Article 

Tree fine root Ca/Al molar ratio - Indicator of Al and acidity stress 

Vanguelova, EI; Hirano, Y; Eldhuset, TD; Sas-Paszt, L; Bakker, MR; Puttsepp, U; Brunner, I; Lohmus, K; Godbold, D 

2007 

1 

Plant Biosystems
ISSN: 1126-3504
EISSN: 1724-5575 

141 

3 

460-480 

10.1080/11263500701626192 

WOS:000252341900017 

High soil acidity and elevated soil Al concentrations limit
plant growth in many terrestrial ecosystems. Aluminium toxicity can be ameliorated by Ca. Thus,
Ca/Al molar ratios in soil solution and in plant tissues have been proposed as superior
indicators than Al concentration itself for evaluating the Al toxicity stress to trees (Cronan &
Grigal, J Environ Qual 1995;24:209-226). This article presents an overview of publications since
1995 where the reduced Ca/Al ratio in fine tree roots has been used as an indicator of stress for
Al and/or soil acidity. The main aim of this review was to evaluate the use and the critical
threshold of the fine root Ca/Al ratio as a potential indicator for Al toxicity stress to trees
in acid soils. Based on the reviewed literature, the fine root Ca/Al molar ratio was strongly
negatively related to Al stress in small tree seedlings in controlled environments, whereas the
response was not clear under field conditions where other environmental factors interact. Fine
root Ca/Al ranged from 0.03 to 17 in tree seedlings and from 0.1 to 18 in mature trees depending
on experimental and site conditions, as well as the tolerance and uptake mechanisms of the
different tree species. Fine root Ca/Al was positively related to the soil solution Ca/Al molar
ratio. Fine root Ca/Al ratios were related positively to fine root length, growth, specific root
length, and biomass, and negatively to root diameter, callose formation, respiration chain
activity, starch concentration, and root necromass. A number of relationships have been also
found between the fine root Ca/Al and above-ground seedling and/or mature tree growth and
nutrient uptake. The critical thresholds for the Ca/Al fine root ratio of 0.2 suggested by Cronan
and Grigal (1995) is estimated to represent 90% risk of inverse impact on root and above-ground
tree growth. Values of Ca/Al molar ratio in the fine roots of mature trees were only rarely
determined below the critical 0.2. The caveats for the use and the interpretation of Ca/Al ratio
in fine roots have been addressed in detail. A protocol for root processing and elemental
analysis to obtain reliable and comparable results of Ca and Al concentrations in roots is also
provided. The article concludes with recommendations for a wider use of the Ca/Al ratio in roots
as a bioindicator of Al toxicity to trees in acid soils. 

aluminium; Ca/Al molar ratio; environmental indicator; fine roots; soil acidity; tree health