Short-term manipulation of precipitation in Mongolian steppe shows vegetation influenced more by timing than amount of rainfall

QuestionsHow does plant productivity and community composition in the mountain steppe of northern Mongolia respond to increased precipitation? Do these changes differ with the timing of precipitation? LocationMountain steppe at ecotone between steppe and taiga, Dalbay Valley, Lake Hovsgol International Long Term Ecological Research Site (ILTER), northern Mongolia (51 degrees 01.405N 100 degrees 45.600E). MethodsWe experimentally applied water, representing a 25% increase in growing season precipitation, to plots at the drier end of a topographic moisture gradient in mountain steppe vegetation. In one treatment, supplemental water was applied weekly for four growing seasons. A second treatment started in year 2, in which three times the weekly supplement of water was applied once every 3wk for three growing seasons. Vascular plant species presence and percentage cover were used to assess water treatment effects on forb and graminoid composition. Plant species biomass and percentage cover in year 4 was used to determine treatment effects on total productivity, and on the abundances of legumes, non-leguminous forbs and graminoids, separately. Lichen, litter and root biomass, plant available N, P and K, and seedling demography were also examined in year 4. ResultsWeekly watering changed the relative abundances of forbs and increased the total abundance of legumes and non-leguminous forbs measured either as percentage cover or biomass. The 3-wk watering treatment did not induce the same changes in forb composition or abundance of functional groups but produced less compositional change in graminoids compared to controls. Neither treatment increased total productivity. Three-week watering increased plant available P and decreased litter. Seedling emergence, survival and size increased with one or both watering treatments. ConclusionIn mountain steppe, precipitation increases will produce immediate changes in composition and abundance of forbs, including legumes, but not in graminoids or total productivity. The steppe is more responsive to regular incremental increases in precipitation than to less frequent, larger storms whose effect would be timing-dependent within the short, temperature-limited growing season. Longer term investigations need to be conducted to explore if further lagged responses to water addition occur.