2.1 STUDY AREA
The study took place in the Etosha Heights Private Reserve (EHPR)
(19.2451 °S, 15.1921 °E), located in the Kunene region in northern
Namibia, approximately 400 km north of the city of Windhoek. EHPR
spreads over 480 km2 and shares an approximately 65 km
long border with the Etosha National Park. The research site is part of
a large-scale wildlife area.
The climate in the region is semi-arid (Peel et al., 2007). It is
characterized by high air and soil temperatures with mean annual
temperatures above 22°C and high solar radiation. Annual rainfall
amounts to 300 - 350 mm and precipitation is highly variable in time and
space. Annual evaporation rate ranges 2,500 - 2,600 mm, and thus an
order of magnitude higher than the average levels of rainfall received
(Atlas of Namibia Team, 2022). Temperatures and precipitation are
changing seasonally, with the highest average precipitation during the
wet season (November - March) and lowest average precipitation during
the dry season (April - October). The soil is rich in carbonate and
consists of limestone and dolomite rocks as well as sand and calcrete
(Nortjé, 2019). The vegetation type in the area is a tree-and-shrub
savanna with varying density, from large open grasslands with scattered
trees up to dense, shrub-dominated landscapes (Atlas of Namibia Team,
2022). The most dominant shrub species in the region isColophospermum mopane , which occurs in high abundance throughout
the reserve (Bester, 1996). Other prominent species in the area includeSenegalia mellifera, Dichrostachys cinerea, Vachellia nebrownii,
Catophractes alexandri and Vachellia reficiens . The fauna within
EHPR is rich and includes large populations of species from various
taxonomic classes, including large mammalian grazers (Antelopes,
Giraffes), browsers (Elephants, Rhinos), and predators (Lions,
Leopards).
2.2 SHRUB SAMPLING AND
MEASUREMENTS
We sampled all prominent shrub species, in total six species, in an area
of approximately 164 km2 in central-east EHPR during
March and April 2021. Sampling was assigned to 60 blocks, with a minimal
distance between blocks of 500 m to control for local effects and to
avoid spatial autocorrelation. Measured individuals and stems reflect
the full range of sizes for each species occurring within EHPR. To avoid
biases in shrub dimensions due to browsing, we sampled only individuals
with minimal apparent damage, such as recently broken branches or a
large portion of missing foliage caused by e.g., rhinos or elephants.
To establish allometric
relationships between canopy area and sapwood area via stem diameter, we
measured canopy area (CA) and stem diameter (DStem) for
one individual of each of the six species (360 individuals in total, 60
per species) in each block. CA estimates relied on the average crown
spread method (ACS) (Blozan, 2004), using measuring poles with a 1 cm
scale. As stems are not necessarily round, DStem was
measured by averaging two perpendicular diameters using a vernier scale
with an accuracy level of 1 mm. In the case of a multi-stemmed
individual, we measured all stems. Although DStem is
commonly measured at breast height (i.e., 130 cm above ground level),
the morphology of most shrub species included in this study did not
allow for this measure, as shrubs are often either smaller or far too
branched at this height. Since species differ in size and morphology, we
selected appropriate heights for each species as follows: 20 cm
(Catophractes alexandri, Vachellia nebrownii ), 50 cm
(Dichrostachys cinerea, Senegalia mellifera, Vachellia
reficiens ), 130 cm (Colophospermum mopane ). The validity and
comparability of stem diameter measurements at different heights rely on
the foundations of Leonardo da Vinci’s area-preserving rule, which
states that the total cross-section area across all stems in an
individual plant remains constant along the height axis (Richter 1970).
This area-preserving rule has been repeatedly supported by different
studies and methods (Minamino & Tateno. 2014; Oppelt, 2001).
To study the relationship between stem diameter and sapwood area, we
randomly selected between 20 and 23 individuals per species (visiting 25
blocks: 0-1 individual per block per species, 130 individuals in total).
We removed one branch per individual using branch shears or a handsaw.
To ensure that our data set for developing the allometries contained
samples from the complete spectrum of Dstem values, we
sampled a large range of branch sizes (3.9 mm – 111.4 mm). Measurements
of stem diameter and sapwood area were achieved via staining of stem
cross sections. Immediately after cutting, the ends were shaved with
fresh razor blades, thus reducing the possibility that damage to the
xylem during harvest would affect our results. The sample was
immediately inserted into a tube containing a staining solution of
methylene blue 0.5% (w/v) and distilled water with a concentration
ratio of 1:20. The amount of diluted solution varied according to the
diameter of the sample and the tubes used for staining, between 10 ml
for very small samples (twigs) and 200 ml (large branches). Tubes were
sealed to prevent evaporation and samples remained in the solution for
at least 24 hours to allow uptake of the dye into the stem via active
transpiration by the remaining foliage. After removing and drying the
samples in the air, cross-sections were made approximately 1 cm above
the bottom of each sample using branch shears or an electric hand saw.
The cross-sections were placed on a clear background adjacent to a ruler
with 1 mm marks for consistent scaling. Images were taken using a
digital Canon SX540 HS camera placed on a tripod. Visual analysis was
done using ImageJ (Fiji is Just) version 2.1.0/1.53c (Abramoff, 2004;
Schindelin et al., 2012). Representative examples for staining samples
for each species are illustrated in Fig 1. In every sample, the borders
of the following three areas were marked manually and the resulting
areas measured digitally with an accuracy of 1 mm2: A)
total area (the entire area of the cross-section including bark), B)
wood area, and C) heartwood area (Fig. 2). The outer limits of the
heartwood were defined as the first innermost tree ring of the
cross-section not containing traces of blue staining (Fig. 2). While our
staining approach dyed the active part of the sapwood only, this
procedure enabled us to identify the entire sapwood area that has been
formed. Sapwood area was mathematically obtained by subtracting the
heartwood area from the wood area. Stem diameter (Dstem)
was derived based on total area (Atot), assuming
circularity of the cross-section.
2.3
DATA ANALYSIS
Species-specific allometric relationships were modeled using the power
law equation (Niklas, 1994)
\(Y\ =\ {b\bullet X}^{a}\), (1)
where \(X\) is the size measure (Dstem or CA), \(b\) a
normalization constant, \(a\) the scaling exponent, and \(Y\) the
estimated sapwood area (SA or SAtot). We considered the
total sapwood area (SAtot) of each study shrub for the
allometry between canopy area (CA) and SA. To obtain
SAtot we measured the Dstem of all
stems, applied the corresponding species-specific allometric
relationship between Dstem to SA to each stem, and
summed up the estimates. To identify the scaling exponent and to enable
a comparison with the WBE prediction, we applied log-transformation and
fitted a linear model, as
\(\ln\left(Y\right)=\ a\bullet ln\ \left(X\right)+ln(b)\). (2)
Models were assessed using both, R2 and the regression
coefficient \(a\) (the scaling exponent) and its significance (P
< 0.05 of F-value). Additionally, 95% confidence intervals of
the scaling exponent were calculated and plotted for each
species-specific model.
The equations were back-transformed into their ‘natural’ exponential
form to simplify the application of the allometric relationships. To
prevent underestimation bias of the logarithmic regression estimate
(Niklas, 1994), a back-correction factor CF was introduced based on
Sprugel (1983), as
\(CF\ =exp(\frac{\text{SEE}^{2}}{2})\) (3)
where SEE is the standard error of the scaling exponent estimate \(a\).
The final equation for the prediction of sapwood area from the
respective size measure will then have the following form:
\(\ln\left(Y\right)=CF\bullet\left(\text{\ ln}\left(b\right)+\ a\bullet ln\ \left(X\right)\right)\overset{\Rightarrow}{}Y=b^{\text{CF}}\bullet X^{a\bullet
CF}\),
(4)
To identify species-specific structural characteristics of sapwood
formation due to drought and their relation to plant age, we also
calculated the relative sapwood area
(\(\frac{\text{SA}}{\text{wood\ area}}\)) and fitted an additional
linear model for the relationship between relative sapwood area and
Dstem for each species.
Data processing, data visualization and statistical analysis were
performed with R version 4.0.5 (2021-03-21) and R-Studio version 1.1.423
(R Core Team, 2019). Used packages included ‘car’ (Fox & Weisberg,
2019), ‘dplyr’ (Wickham et al., 2023a), ’FSA’ (Ogle et al., 2023)
‘ggplot2’ (Wickham, 2016), ‘ggpubr’ (Kassambara, 2020), and ‘tidyr’
(Wickham, 2023b).
RESULTS
In total, we stained and measured 130 individual plants for sapwood area
and stem diameter, and measured canopy area and total stem diameter of
360 individuals of six savanna bush encroacher species. A detailed
summary of the measurements is found in Appendix Table A1.
In all six species, sapwood area was positively correlated with stem
diameter. Dstem accounted for almost the entire
variation in C. alexandri (99%), C. mopane (98%),S. mellifera (97%), V. reficiens (97%), V.
nebrownii (97%) and D. cinerea (78%) (Fig. 3, Table 1). The
very high values of R2 for all species established
Dstem as a strong independent variable to estimate
sapwood area. Apart from D. cinerea , all species included the
regression line predicted by the WBE model within the 95% confidence
interval of the slope.
The relative sapwood area to Dstem relationship varied
substantially between species (Fig. 4). While this ratio was constant inC. mopane, S. mellifera , and V. reficiens , a
significant positive relationship was found in C. alexandri ,V. nebrownii and a significant negative one in D. cinerea .
On average, D. cinerea invested clearly the least of all species
in water conducting sapwood compared to heartwood.
Moreover, sapwood area was positively correlated with canopy area. CA
accounted for most of the variation of SAtot in C.
alexandri (82%), C. mopane (84%), S. mellifera (80%),V. reficiens (86%), V. nebrownii (81%) and a bit less
again in D. cinerea ( 71%) (Fig. 5, Table 2).Although R2 values were high for all species, they
were lower than for the correlation between Dstem and SA
(Fig. 3, Table 1).
DISCUSSION
Sapwood area is a key component in quantifying canopy transpiration and
allometric equations for estimating sapwood area using tree size
parameters have been developed for various woody ecosystems. And yet, in
the case of Southern African savannas, only few exist (Lubczynski et
al., 2017), although various allometric relationships have been
extensively studied in recent years (Fregoso, 2002; Issoufou et al.,
2015; Moncrieff et al., 2011; Tredennick et al., 2013). Scaling
exponents of these size-correlated trends often rely on the universal
value predicted by the West-Brown-Enquist model (West et al., 1999;
Niklas, 1994). Nevertheless, the model itself is a point of dispute
(Brown et. at., 2005; Kozlowski & Konarzewski, 2004) because it does
not incorporate features specific for different plant taxa or for
different environments. As sapwood formation is likely governed by low
water availability, establishing species-specific relationships between
sapwood area and tree dimension for Southern African savanna woody
species may be essential to accurately estimating transpiration rates
(Ter-Mikaelian & Korzukhin, 1997; Yaemphum et al., 2022). In this
study, we established allometric relationships between tree dimension
and sapwood area in six shrub species involved in savanna bush
encroachment in Namibia.
Our results illustrate that sapwood area can be reliably predicted based
on stem diameter in all six species tested. Comparing our results with
the prediction of the WBE model (West et al., 1999) showed that the
model holds true in most cases.
Various findings illustrate that despite the unique conditions in
savannas and other drylands, the scaling relationships between stem
diameter and sapwood area in drought resistant plants are similar to
those of plants found in other climates (Fregoso, 2002; Gebauer et al.,
2009; Patino et al., 1995; Wang et al., 2010). However, the relationship
between stem diameter and sapwood area in D. cinerea revealed an
exponent substantially lower than the other five bush species tested.
This is not exclusive for the Etosha region. In the eastern Kalahari in
Botswana, D.cinerea revealed the smallest slope of nine species
(Lubczynski et al., 2017), although authors fitted a linear function
rather than a power function (in accordance to the WBE model) to the
relation between sapwood area and stem diameter. Petit and Anfodillo
(2009) illustrate an anatomical reason why actual sapwood area fails to
scale with the stem diameter raised to 2.33 sometimes. What we usually
consider as sapwood does not actually correspond to the conductive
tissues in the WBE model. Conduits (vessels and tracheids) are always
embedded in a matrix of wood fibers (stabilization function) and
parenchyma (storage and embolism repair function). If the general
assumptions of the WBE theory regarding geometric and hydrodynamic
constrains (West et al., 1997) are valid, smaller allometric exponents
as in D. cinerea arise if there is more genuine conductive tissue
and less parenchyma and wood fibers on the same surface. More genuine
conductive tissue can arise either with wider conduits or with more
conduits in total. Such a sapwood composition at the expense of
parenchyma is, however, more vulnerable to cavitation and -embolism
(Brodersen & McElrone, 2013), even if the same conductivity across the
stem is reached compared to species exhibiting the allometric exponent
of 2.33 of the WBE theory. Parenchyma cells in sapwood serve as large
water storage reservoirs and are linked to osmotically driven embolism
repair mechanism (Broderson & McElrone, 2013). The vulnerability to
cavitation and embolism might be partly compensated by a decrease in
conduit size. The higher wood density of D. cinerea compared to
other savanna species (Fernandez-Ortuño et al., 2015) supports indeed a
smaller conduit size. That these vessels are small, although probably
high in number, finds also support since D. cinerea has a low sap
flow velocity relative to other savanna shrub species (Zziwa 2003), and
estimations of transpiration rates of vegetation plots in areas
predominated by D. cinerea were lower than areas with similar
shrub density predominated by other common shrubs (Chavarro-Rincon,
2009). Although the size and number of conducting vessels per stem area
of D. cinerea is unknown, the allometric exponent and sap flow
velocity taken together indicate that the species might be relying on
this to compensate for a reduced sapwood area. Thus, D. cinereaseems to be less efficient in water uptake and might be more water
dependent than the other bush encroacher species in our study. IndeedD. cinerea does almost not increase in biomass and is less
abundant in areas with less soil moisture and higher temperature (de
Klerk, 2004; Shikangalah et al., 2021). Its main distribution in Namibia
as bush encroacher extends to savannas with MAP > 550mm (de
Klerk, 2004). Questions remain regarding the underlying causes of its
dominance in these moister areas as well as the functional driver of the
unusual small allometric exponent. In any case, D. cinerea ’s
striking anatomy is supported by our results on low relative sapwood
area and by previous results showing that D. cinerea has an
unusual small sapwood to heartwood ratio than other woody savanna
species (Shikangalah et al., 2021; Zziwa, 2003). A large heartwood area
might relate to an adaptation mechanism, which increases the stem
stability, because heartwood is generally stiffer than sapwood. A strong
argument for this view is our observation that relative sapwood area
decreased with age (stem diameter as a proxy of age or growth). However,
if central heartwood was to increase flexural stiffness, the heartwood
would need to have a inordinately lower elasticity than sapwood (Niklas,
1997), a trait which we do not know. Moreover, high wind velocities as a
driver of stability traits are not typical for the main distribution
areas of D. cinerea (Atlas of Namibia Team, 2022).
An adaptation in flexural stiffness or any other reason for relatively
more heartwood was not detected in C. mopane , S. melliferaand V. reficiens . Furthermore, the small shrubs V.
nebrownii and C. alexandri even decrease the relative amount of
central heartwood with stem age, promoting a higher flexibility instead
of stiffness, which might be affordable if the increase in overall woody
biomass contributes to stability already. If sapwood area is correlated
with storage capacity for water (Scholz et al., 2008), lower water
availability could be a driver distinguishing their local occurrence.
While definitive conclusions regarding C. mopane , S.
mellifera and V. reficiens based on these findings are currently
hard to make, our results could help understanding what conditions drive
which shrub species during bush encroachment.
Based on the overall strong relationship between stem diameter and
sapwood area, we were also able to establish strong relationships
between crown area and sapwood area in all six species. This result is
meaningful because it supplies further support for the potential of
estimating sapwood area based on aerial imagery (Mitra et al., 2020).
Whereas previous studies found a linear relationship between crown size
and properties of water conductance (Ahongshangbam et al., 2020;
Quiñonez-Piñón & Valeo, 2019; Tziaferidis, 2021), the relationships
detected in our study were slightly exponential, presumably due to the
inclusion of smaller samples. Another reason for a divergent allometric
relationship might be the time of sampling. We carried out data
collection and measurements at the end of the rainy season. Deciduous
phenology of savanna trees and shrubs has been shown to be strongly
influenced by seasonality (Dahlin, 2016), meaning that the relationships
between crown area and total sapwood area may vary seasonally.
Continuous measurements of crown area and stem diameter as a base for
total sapwood area are therefore necessary to adapt the relationship
described above to different seasons and years.
In addition, since our measurements of plant dimensions were taken
within a nature reserve, where browsing pressure by large African
mammals such as Giraffes, Elephants, and Rhinos is present, a comparison
of crown area across different land-use types with different browsing
pressures might be required to understand to what degree crown area is
affected by the presence of large browsers. Nevertheless, we believe
that our allometric relations are sufficiently reliable and universal
due to our selection of apparently unharmed individuals, as well as due
to the sampling of shrubs from multiple plots covering a large area with
many different local conditions.
CONCLUSIONS
The assessments of six main bush encroacher species of Namibia proposed
here indicate that sapwood area can be reliably predicted based on stem
diameter, crown dimensions and the universal WBE model. Our allometric
equations offer the prospect of vastly increasing our knowledge about
transpirational water losses in semiarid bush-encroached savannas.
Robust species-specific models predicting individual shrub sapwood area
from shrub size are, however, critical for accurate estimating of stand-
and landscape-scale transpiration in shrub encroached regions, in
particular when D. cinerea is involved. A subsequent extension of
our research may apply these species-specific total sapwood area-crown
area or sapwood area-stem diameter allometric equations to estimate
woody transpiration for different bush encroachment scenarios in the
studied region. Such analysis remains highly relevant for all Southern
African savannas given the widespread shift in vegetation composition
via bush encroachment by different species but also considering climate
change, in particular as droughts are projected to become longer and
more frequent (IPBES, 2018; IPCC, 2023).
ACKNOWLEDGMENTS
This work was supported by the German Federal Ministry of Education and
Research (BMBF) project ORYCS (FKZ 01LL1804A). We thank the Ministry of
Environment, Tourism and Forestry (Namibia) and the Namibian National
Commission for granting research permission [certificate number
RCIV00032018 with authorization numbers: 20190602, 20190808] and Andre
Nel (the owner) and the team of Etosha Heights Private Reserve, for
their manifold support.
We gratefully acknowledge the German Research Foundation (DFG) for their
generous support through open access funding, enabling us to freely
disseminate our research findings.