EFFECTS OF MOBILE TREE ISLANDS ON ALPINE TUNDRA SOILS

Abstract

Tree islands or krummholz found above tree line in the Front Range of Colorado are of interest because of their ability to migrate across the tundra landscape. This unique characteristic is caused by dieback of exposed, windward plant tissues, growth of the plant to the leeward, and the ability of these plants to produce new roots from branches buried in organic debris. This mobility also allows for the study of tree-induced changes on properties of soils initially formed by tundra vegetation. Soil characteristics, root productivity, decomposition of litter, and nitrogen flux in soils beneath mobile tree islands and adjacent alpine meadow tundra were studied to test whether tree islands modify the physical and chemical properties of soils formed in alpine tundra. Rates of surface decomposition of a standardized litter substrate were significantly increased leeward of tree islands and significantly decreased within these islands. Soil pH was not significantly reduced by tree island passage. We found significant reductions in amounts of soil organic matter (SOM) and KCl-extractable NH4+ in soils influenced by tree islands. No recovery in soil organic matter or NH4+ was observed at locations that correspond to 250-500 yr since the passage of a tree island. Cation exchange capacity was correlated with percentage SOM but was not reduced by tree island passage. Soil texture differences, when present, did not support the hypothesis that surface soils lose clay content with tree island passage but were consistent with a reduction in the depth of the A horizon of these locations. The soil carbon isotopic signature (6'3C) is only temporarily influenced by the passage of tree islands, suggesting that tree carbon is not stabilized in soils. Moreover while trees have numerous roots within the 0 horizon of soil, root ingrowth bags demonstrated large reductions in fine root production in the A horizons beneath trees (149 g.m-2-yr-' in tundra vs. 25 g.m-2 yr-1 beneath trees). Resin bag nitrogen collections indicated that soils beneath trees had higher NH4+ losses than adjacent tundra soils. Tree island passage therefore reduced SOM and the nitrogen storage potential of former tundra soils, and these effects likely persist for centuries following the passage of a tree island.