Effects of Quercus emoryi on herbaceous vegetation in a semi-arid savanna

Haworth, Kathryn; McPherson, Guy R.

doi:10.1007/bf00044689

Cited by 26 publications

(18 citation statements)

References 36 publications

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“…(plains lovegrass). Herbaceous biomass beneath mature Q. emoryi is reduced by 40% relative to interspaces between trees (Haworth and McPherson 1994). More detailed information about the site is provided by McPherson (1994, 1995).…”

Section: Methodsmentioning

confidence: 99%

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Spatial and temporal soil moisture resource partitioning by trees and grasses in a temperate savanna, Arizona, USA

1997

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Stable isotope analysis was used to determine sources of water used by coexisting trees and grasses in a temperate savanna dominated by Quercus emoryi Torr. We predicted that (1) tree seedlings and bunchgrasses utilize shallow sources of soil water, (2) mature savanna trees use deeper sources of water, and (3) trees switch from shallow to deep water sources within 1 year of germination. We found that Q. emoryi trees, saplings, and seedlings (about 2 months, 1 year, and 2 years old), and the dominant bunchgrass [Trachypogon montufari (H.B.K.) Nees.] utilized seasonally available moisture from different depths within the soil profile depending on size/age relationships. Sapling and mature Q. emoryi acquired water from >50 cm deep, 2-month-old seedlings utilized water from <15 cm, and 1- and 2-year-old seedlings and grasses used water from between 20 cm and 35 cm. This suggests that very young seedlings are decoupled from grasses in this system, which may facilitate germination and early establishment of Q. emoryi within extant stands of native grasses. The potential for subsequent interaction between Q. emoryi and native grasses was evidenced by similar patterns of soil water use by 1- and 2-year-old seedlings and grasses. Q. emoryi seedlings did not switch from shallow to deep sources of soil water within 2 years of germination: water use by these seedlings apparently becomes independent of water use by grasses after 2 years of age. Finally, older trees (saplings, mature trees) use water from deeper soil layers than grasses, which may facilitate the stable coexistence of mature trees and grasses. Potential shifts in the seasonality of precipitation may alter interactions between woody plants and grasses within temperate savannas characterized by bimodal precipitation regimes: reductions in summer precipitation or soil moisture may be particularly detrimental to warm-season grasses and seedlings of Q. emoryi.

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Section: Methodsmentioning

confidence: 99%

“…Overstory tree cover, as estimated from aerial photographs, is 11% (Haworth and McPherson 1994), and consists almost entirely of the evergreen Q. emoryi Torr. (emory oak).…”

Section: Methodsmentioning

confidence: 99%

Spatial and temporal soil moisture resource partitioning by trees and grasses in a temperate savanna, Arizona, USA

1997

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“…Competitive interactions between the woody and herbaceous components of savannas, involving mainly available soil water as the primary determinant of production, have been reported worldwide (Australia: Walker et al, 1986Walker et al, , 1989Harrington and Johns, 1990;Scanlan and Burrows, 1990;North America: Scifres et al, 1982;Scifres, 1987;Archer et al, 1988;Bozzo et al, 1992;Haworth and McPherson, 1994; southern and east Africa: Donaldson and Kelk, 1970;Dye and Spear, 1982;Scholes, 1987;Belsky et al, 1989;Smit, 1994;Smit and Swart, 1994;Smit and Rethman, 1999;2000;Richter et al, 2001).…”

Section: Negative Influences Of Trees On Herbaceous Plants and Their mentioning

confidence: 96%

An approach to tree thinning to structure southern African savannas for long-term restoration from bush encroachment

Smit

2004

Journal of Environmental Management

108

116

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“…Effects of mature Q. emoryi on subcanopy microenvironments are well documented. Q. emoryi tree canopies suppress herbaceous biomass production by as much as 40%, although herbaceous species composition is not affected (Haworth and McPherson 1994). Seasonal temperature extremes are lower beneath trees than in interstitial zones: subcanopy temperatures are relatively cool in summer and warm in winter (Nyandiga andMcPherson 1992, Haworth and.…”

Section: Q Emoryi Effects On Subcanopy Microenvironmentsmentioning

confidence: 99%

“…Microenvironments beneath woody plants within these systems often differ markedly from adjacent grassland zones (see reviews by Vetaas [1992], Belsky and Canham [1994], , Scholes and Archer [1997], Wilson [1998]). Woody plant canopies alter subcanopy precipitation distribution, solar radiation, evapotranspiration, and ambient and soil temperature regimes (e.g., Tiedemann and Klemmedson 1973, Joffre and Rambal 1988, Belsky et al 1989, Haworth and McPherson 1994, and concentrate soil nutrients relative to adjacent grassland zones (e.g., Garcia-Moya and McKell 1970, Tiedemann and Klemmedson 1973, Belsky et al 1989, Schlesinger et al 1990, McPherson et al 1991. However, the relative importance of these different and potentially opposing canopy effects, and their interaction with patterns of seed dispersal, are seldom determined with respect to subcanopy seedling recruitment.…”

Section: Introductionmentioning

confidence: 99%

Facilitation of Conspecific Seedling Recruitment and Shifts in Temperate Savanna Ecotones

Weltzin

McPherson

1999

Ecological Monographs

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114

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In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopies and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19‐fold and increased recruitment rates between 30‐ and 60‐fold. We conclude that rates of Q. emoryi recruitment within grasslands below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat‐specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self‐enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland–grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one‐sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century‐scale climatic perturbation. The observed shift in tree line in the last millennium was less li...

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Effects of Quercus emoryi on herbaceous vegetation in a semi-arid savanna

Cited by 26 publications

References 36 publications

Spatial and temporal soil moisture resource partitioning by trees and grasses in a temperate savanna, Arizona, USA

Spatial and temporal soil moisture resource partitioning by trees and grasses in a temperate savanna, Arizona, USA

An approach to tree thinning to structure southern African savannas for long-term restoration from bush encroachment

Facilitation of Conspecific Seedling Recruitment and Shifts in Temperate Savanna Ecotones

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