Influence of tree canopies on grassland productivity and nitrogen dynamics in deciduous oak savanna

Jackson, Louise E.; Strauss, Rochelle; Firestone, Mary K.; Bartolome, James W.

doi:10.1016/0167-8809(90)90126-x

Cited by 120 publications

(102 citation statements)

References 12 publications

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“…Differences in the physical water holding characteristics of the soils at the two sites account for this difference in evaporation, and provide a partial explanation why the vegetation differs at the two sites. We also report that these findings support the hypothesis that the presence of trees improve the soil water holding capacity of savanna soils (Joffre and Rambal, 1988;Jackson et al, 1990).…”

Section: Resultssupporting

confidence: 77%

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How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

Baldocchi

Kiang

2004

Agricultural and Forest Meteorology

528

415

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Savannas and open grasslands often co-exist in semi-arid regions. Questions that remain unanswered and are of interest to biometeorologists include: how do these contrasting landscapes affect the exchanges of energy on seasonal and annual time scales; and, do biophysical constraints imposed by water supply and water demand affect whether the land is occupied by open grasslands or savanna? To address these questions, and others, we examine how a number of abiotic, biotic and edaphic factors modulate water and energy flux densities over an oak-grass savanna and an annual grassland that coexist in the same climate but on soils with different hydraulic properties.The net radiation balance was greater over the oak woodland than the grassland, despite the fact that both canopies received similar sums of incoming short and long wave radiation. The lower albedo and lower radiative surface temperature of the transpiring woodland caused it to intercept and retain more long and shortwave energy over the course of the year, and particularly during the summer dry period.The partitioning of available energy into sensible and latent heat exchanged over the two canopies differed markedly. The annual sum of sensible heat exchange over the woodland was 40% greater than that over the grassland (2.05 GJ m −2 per year versus 1.46 GJ m −2 per year). With regards to evaporation, the oak woodland evaporated about 380 mm of water per year and the grassland evaporated about 300 mm per year. Differences in available energy, canopy roughness, the timing of physiological functioning, water holding capacity of the soil and rooting depth of the vegetation explained the observed differences in sensible and latent heat exchange of the contrasting vegetation surfaces.The response of canopy evaporation to diminishing soil moisture was quantified by comparing normalized evaporation rates (in terms of equilibrium evaporation) with soil water potential and volumetric water content measurements. When soil moisture was ample normalized values of latent heat flux density were greater for the grassland (1.1-1.2) than for the oak savanna (0.7-0.8) and independent of moisture content. Normalized rates of evaporation over the grassland declined as volumetric water content dropped below 0.15 m 3 m −3 , which corresponded with a soil water potential of −1.5 MPa. The grassland senesced and quit transpiring when the volumetric water content of the soil dropped below −2.0 MPa. The oak trees, on the other hand, were able to transpire, albeit at low rates, under very dry soil conditions (soil water potentials below −4.0 MPa). The trees were able to endure such low water potentials and maintain basal levels of metabolism because * Corresponding author. Baldocchi et al. / Agricultural and Forest Meteorology 123 (2004) ecological forcings kept the tree density and leaf area index of the woodland low, physiological factors forced the stomata to close progressively and the trees were able to tap deeper water sources (below 0.6 m) than the grasses.

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Section: Resultssupporting

confidence: 77%

“…Joffre and Rambal (1988), for example, show that bulk density and moisture retention differed between tree-covered area and open area and soil water content was higher in tree covered areas. And Jackson et al (1990) report better soil hydraulic properties under trees in California oak woodlands.…”

Section: Weather and Climatementioning

confidence: 99%

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

Baldocchi

Kiang

2004

Agricultural and Forest Meteorology

528

415

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“…The mechanisms by which trees affect grass quality beneath their crowns are manifold. Trees can improve the conditions for grass layer growth directly, for example, by providing shade and reducing evapo-transpiration, by accumulating soil nutrients close to their root systems (Belsky 1994;Dean et al 1999), and by changing the micro-climate (Jackson et al 1990;Belsky 1994;Ludwig et al 2001;Power et al 2003). Recent studies have shown that both soil and grass nitrogen (N) and phosphorus (P) contents were elevated beneath tree canopies compared to inter-canopy sites (Ludwig et al 2004;Treydte et al 2007Treydte et al , 2008 whereas some studies did not find any or only slight differences in soil properties between canopy and inter-canopy sites (Witkowski and Garner 2000).…”

Section: Introductionmentioning

confidence: 99%

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

2009

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Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3.5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (\2.3 m). Moderate herbivory enhanced these effects slightly. Grass species composition differed beneath and beyond the tree canopy but not between tree size classes. As large trees significantly improve the grass nutrient quality for grazers in contrast to smaller trees, the decline of the former should be halted. The presence of trees further increases grass species diversity and patchiness by favouring shade-tolerant species. Both grazing wildlife and livestock will benefit from the presence of large trees because of their structural and functional importance for savannahs.

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“…This fine-scale heterogeneity is often the relevant scale at which many ecological processes occur. For example, species sorting at the local scale between oak understory and open grassland environments has been documented (Jackson et al 1990). Increased deposition to the oak canopy may partly explain these patterns.…”

Section: Model Comparisonmentioning

confidence: 99%

Nitrogen deposition across scales: hotspots and gradients in a California savanna landscape

Tulloss

Cadenasso

2015

Ecosphere

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Abstract. Nitrogen (N) deposition is a key component of global change. In order to predict the consequences of those changes, the spatial and temporal patterns of N deposition need to be quantified. We investigated patterns of inorganic-N deposition across two spatial scales-regional and local-in six oak savanna sites across north-central California during the 2008-2009 wet season. We found a regional-scale deposition gradient in which sites closer to urban and agricultural sources received higher rates of deposition than more remote sites. Additionally, sites closer to urban sources received a greater proportion of N as NO 3 À -N. At the local, within-site scale, canopy understories received greater N inputs compared to the adjacent open grassland. Temporal heterogeneity in N deposition was also found. Deposition exhibited a strong seasonal pattern, corresponding to the distinct wet-dry cycles characteristic of this Mediterranean ecosystem. While these spatial and temporal results were expected, it was surprising that the magnitude of the hotspot effect under oaks changed across the regional gradient. At the high deposition end of the regional gradient proportionally more N was received beneath canopies than at the low end of the gradient. This nonlinearity between local and regional scales demonstrates the importance of landscape context for understanding within-site dynamics and the need for multi-scalar research approaches. Incorporating regional-scale landscape context and local-scale vegetation structure, as well as temporal variability, into our understanding of deposition patterns is crucial for monitoring efforts as well as addressing potential ecological effects of increased N deposition.

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Influence of tree canopies on grassland productivity and nitrogen dynamics in deciduous oak savanna

Cited by 120 publications

References 12 publications

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

Nitrogen deposition across scales: hotspots and gradients in a California savanna landscape

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