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Not all trees can make a forest: tree species composition and competition control forest encroachment in a tropical savanna

Citation

Flake, Samuel et al. (2022), Not all trees can make a forest: tree species composition and competition control forest encroachment in a tropical savanna, Dryad, Dataset, https://doi.org/10.5061/dryad.vq83bk3tm

Abstract

Forest encroachment into savannas is a widespread phenomenon, the rate of which may depend on soil conditions, species composition, or changes in stand structure. As savanna specialist trees are replaced by generalist species, rates of stand development may increase. Because generalists can persist in forests, they are likely to grow more quickly and survive longer in dense stands, compared to savanna specialists. Furthermore, the faster growth rates of generalists may allow them to overtop and outcompete savanna specialists, causing rapid species turnover.

We measured growth and survival of 6147 individuals of 112 species of savanna and generalist tree species over a period of 10 years in an ecological reserve in Assis, São Paulo State, Brazil. We modeled growth and mortality as a function of soil texture and nutrients, tree size, competitive neighborhood, and membership in savanna or generalist (species which can persist in forests and savannas) functional groups.

Tree growth and survival was strongly influenced by competition, as estimated by the basal area of trees taller than a focal tree. At the stand level, savanna species are unable to contribute basal area growth in closed stands, while generalist species continue to increase in basal area even at high stand basal area. This phenomenon is driven by differences in growth and mortality. Generalists grew faster than savanna species, both in height and diameter. This difference in growth rates led to savanna species becoming suppressed more rapidly than generalists. When suppressed, savanna species were more than twice as likely to die than were generalists. Soils had inconsistent and mostly weak effects which were difficult to separate from gradients of stand structure.

Synthesis: We demonstrate that the presence of generalist trees accelerates rates of basal area accumulation due to their greater growth rates and tolerance of shading. Generalists outcompete savanna trees by growing faster in the open and overtopping savanna specialists. Due to the slow growth and high mortality of savanna species in the shade, they are unable to form closed-canopy stands. Accounting for differences among functional types and development of vegetation structure is critical for modeling forest encroachment.

Methods

This dataset consists of three files: individual tree data, plot-level data, and soils data (also collected at the plot level).

Tree_data_EEcAssis.xlsx: Thirty plots were established in 2006 along a gradient from open savanna to closed forest. Each plot comprised 20 x 50 m, within which all trees > 5 cm DBH were sampled. For each tree, DBH, height, and species identity were recorded. Individual trees were tagged and resurveyed in 2011 and 2016, along with any new recruits into the 5 cm size class. 

Soil_EEcAssis_All data.xlsx: Within each plot, soil structure and chemistry were sampled at a depth of 0-20 cm and 60-80 cm. Samples were commingled to produce one sample per plot. 

Plot_data_EEcAssis.xlsx: Some variables are summarized to the plot level in this spreadsheet, to facilitate analysis. 

For details on data collection, see Assis, et al. (2011) and Honda & Durigan (2016). 

Assis, A. C. C., R. M. Coelho, E. da Silva Pinheiro, and G. Durigan. 2011. Water availability determines physiognomic gradient in an area of low-fertility soils under Cerrado vegetation. Plant Ecology 212:1135–1147.

Honda, E. A., and G. Durigan. 2016. Woody encroachment and its consequences on hydrological processes in the savannah. Phil. Trans. R. Soc. B 371:20150313.

Usage Notes

Plot_data_EEcAssis.xlsx
Plot-level summary data, including soils data and variables derived from tree inventories.
Rows are color-coded by vegetation structure: straw: savanna; orange: cerrado sensu stricto; green: cerradão.
Soil size classes follow Camargo, et al. (1986)

Columns:

Plot: plot identifier
TBA 2011 m2 ha-1 : toal tree basal area in 2011
Total sand %: soil % sand
Clay %: soil % clay
Silt %: soil % silt
Coarse sand % : soil % coarse sand
Fine sand %: soil % fine sand
Base saturation V% 0-20cm: base saturation in the 0-20 cm soil layer
Base saturation V% 60-80cm: base saturation in the 60-80 cm soil layer
Canopy cover %: canopy cover estimated from satellite images
Canopy oppeness %: 1 - canopy cover
Transprecipitation 2006 %: rain throughfall; estimated following Honda and Durigan (2016).
soil humidity 0-20cm %: soil moisture in 0-20cm soil layer
Diameter increment cm/yr: mean growth increment of trees, calculated using only positive growth

------------------------------------------
Soil_EecAssis_All_data.xlsx
Plot-level soil variables, estimated for the 0-20 cm layer and 60-80 cm layer (different tabs)
For details, see Assis et al. (2011). 

Columns:

Plot number: plot identifier
Soil Depth: soil layer (0-20 cm or 60-80 cm)
Coarse Sand: soil % coarse sand
Fine sand: soil % fine sand
Clay: soil % clay
Silt: soil % coarse silt
Clay + Silt: % total fines
Moisture: soil moisture
P(resin): Phosphorus concentration (measured using resin)
Organic Matter: Organic matter 
pH: pH
K: Potassium concentration
Ca: Calcium concentration
Mg: Magnesium concentration
H+Al: Sum of Hydrogen and Aluminum concentrations
Al: Aluminum concentration
Sum of bases: Sum of nutrient cation conetrations
Cation exchange capacity: Cation exchange capacity
Base Sat: Proportion of cation exchance sites bound with bases
Al Sat: Proportion of cation exchance sites bound with Aluminum
Cu: Copper concentration
Zn: Zinc concentration
Mn: Manganese concentration
Fe: Iron concentration
B: Boron concentration

----------------------------------------------------
Tree_data_EEcAssis.xlsx
Individual tree data from three inventories (2006, 2011, and 2016).

Columns:

Old number: Tree identifier
Current number: Tree identifier
Plot: Plot identifier
Subplot: Subplot identifier (subplots 1-10 within each plot)
a=2006; n=2011; nv=2016: indicates which year the tree was added to the inventory
Alive2011: indicates whether the tree was alive in 2011 (v) or dead (m)
Alive2016: indicates whether the tree was alive in 2016 (v) or dead (m)
Family: Family of the species
Species (names not updated): Scientific name and authority of species
C=Cerrado; G=generalist;: Indicates whether the species is a cerrado specialist or generalist
Shade Tolerance: Is the species shade tolerant (T) or intolerant (I)
H 2016 m: Height of tree in 2016
DAP1/16 : DAP7/16: Diameters at beast height of stems of individual tree
G2016 (m2): Cross-sectional area of tree, calculated from individual tree diameters
dg2016 (cm): Effective diameter, calculated from cross-sectional area columns repeat for each sample interval (2011 and 2006)
dbh annual increment(cm)_10 years: Change in tree diameter from 2006 to 2016. Only includes trees which were alive in 2006 and 2011.

References:

Assis, A. C. C., Coelho, R. M., da Silva Pinheiro, E., & Durigan, G. (2011). Water availability determines 
    physiognomic gradient in an area of low-fertility soils under Cerrado vegetation. Plant Ecology, 212(7), 
    1135–1147. https://doi.org/10.1007/s11258-010-9893-8

Honda, E. A., & Durigan, G. (2016). Woody encroachment and its consequences on hydrological processes in 
    the savannah. Phil. Trans. R. Soc. B, 371(1703), 20150313. https://doi.org/10.1098/rstb.2015.0313

Camargo AO, Moniz AC, Jorge JA, Valadares JMAS (1986) Métodos de análise química, mineralôgica e física de
    solos do Instituto Agronômico de Campinas. Instituto Agronômico, Campinas, SP (Boletim Técnico, 106)

Funding

National Science Foundation, Award: DEB1354943

Conselho Nacional de Desenvolvimento Científico e Tecnológico, Award: 475286/2007-3

Fundação de Amparo à Pesquisa do Estado de SP, Award: 2016/17888-2

Conselho Nacional de Desenvolvimento Científico e Tecnológico, Award: 303179/2016-3