Effects of invasive versus native grasses on soil moisture and woody plant seedling recruitment across contrasting soils and summer precipitation regimes in a temperate semi-arid savanna.
Weltzin, Jake*,1, 1 University of Tennessee, Knoxville, TN, USA
ABSTRACT- Invasions of semi-arid grasslands by woody plants are likely dependent upon biotic and abiotic factors that control spatial and temporal availability of soil moisture. To test the hypothesis that establishment of woody plant seedlings depends on identity of neighboring (native and non-native invasive) grasses, soil texture, and amount of rainfall during the summer growing season, we conducted a large-scale field experiment in southeastern Arizona between 2002 and 2004. Six large precipitation shelters, constructed on highly contrasting clay-rich and sandy soils, covered a total of 72 experimental plots that were planted with either Eragrostis lehmanniana (a non-native invasive grass) or Heteropogon contortus (a native grass), or that were left bare. Plots were irrigated by hand to receive the equivalent of 50% more or 50% less than mean historic summer rainfall. Soil water potential (SWP) clearly reflected differences between soil type, watering treatment, and grass presence and identity. SWP at 15 cm was lower on sandy than clay-rich soils, particularly in dry plots. SWP on clay-rich soils was lower under stands of Eragrostis than under Heteropogon, and both were lower than in bare plots. Emergence of mesquite (Prosopis velutina) seedlings from seed planted into each plot in 2002 averaged 40%, did not differ between soil types, was greater in wet plots than in dry plots, and within wet plots was facilitated by the presence of grasses. By 2004, seedling survival on the sandy soil was greatest in bare plots, and was unaffected by watering regime. On the clay-rich soil, seedling survival was greatest on the bare, wet plots. Thus, although native and non-native invasive grasses exhibited differential effects on spatial and temporal patterns of soil moisture, the importance of grass species identity for woody plant recruitment was attenuated within two growing seasons. Substantial short-term recruitment of Prosopis seedlings on clay-rich soils without grass competitors may reflect the relative availability of soil moisture stored within the fine-textured matrix of these soils.
Key words: seedling recruitment, precipitation, soil moisture, grass invasions
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