ABSTRACT
Southern African savanna rangelands are facing a widespread degradation
pattern called bush encroachment. This is associated with implications
for various aspects of the water cycle and in particular canopy
transpiration. At the individual-tree scale, it is estimated by scaling
sap-flux density by sapwood area. However, the direct measurement of
sapwood area is impracticable at landscape scale and general allometric
equations of the West-Brown-Enquist (WBE) model relating sapwood area to
primary size measures seem to fail for some species and climates.
Therefore, we conducted intensive field measurements to establish
species-specific allometric relationships between sapwood area and sizes
(stem diameter, crown area) in six dominant shrub species involved in
bush encroachment in Namibia (Colophospermum mopane, Senegalia
mellifera, Vachellia reficiens, Dichrostachys cinerea, Vachellia
nebrownii, Catophractes alexandri ). We found strong allometric
relationships between sapwood area and stem diameter as well as between
sapwood area and crown area for all six species. These relations are
largely in line with the WBE theory but still provide estimates that are
more accurate. Only in D. cinerea , the sapwood area was
significantly smaller than predicted by the WBE theory, which might be
caused by a larger need for stabilizing heartwood. Our results are
useful to estimate water loss via transpiration at a large scale using
remote sensing techniques and can promote our understanding of the
ecohydrological conditions that drive species specific bush encroachment
in savannas. This is particularly important in the light of climate
change, which is considered to have major implications on
ecohydrological processes in savannas.
INTRODUCTION
Bush encroachment is a prominent form of rangeland degradation in arid
and semiarid regions worldwide (Stevens et al. 2016). In sub-Sahara
Africa, where rangelands cover over two thirds of the land area, it
leads to various problems, which are often linked through
ecohydrological feedback loops (Trimble & van Aarde, 2014). Problems
include faunal and floral biodiversity loss (Blaum et al., 2007, 2009;
Chown, 2010; Dreber et al., 2018; Hering et al., 2019), decreased
carrying capacities of rangelands (Angassa 2005; reviewed in Ayalew &
Mulualem, 2018; De Klerk, 2004), lowered infiltration rates and related
soil moisture depletion, and presumably changed evapotranspiration
patterns (Chartier et al., 2011; Huxman et al., 2005; Geissler et al.,
2019; Groengroeft et al., 2018; Wilcox et al., 2022). High temperatures
and solar radiation make evapotranspiration the major form of water
loss, which accounts for water loss amounting to over 90% of the annual
precipitation (Haan et al., 1994). It includes water evaporating from
the soil and from the plant foliage after being intercepted as well as
water transpired by plants following soil water uptake. Thus, even
relatively small changes of vegetation composition and structure could
have important consequences, not only for soil moisture budgets but also
for groundwater recharge, land-atmosphere energy exchange, local
climate, and primary production including carbon accumulation (Huxman et
al., 2005; Lubczynski, 2009). Hence, estimating evapotranspirative water
loss is critical for managing the environmental effects of bush
encroachment. A common constraint is, however, the difficulty in
obtaining evapotranspiration estimates and its partitioning across
widespread and sometimes inaccessible savannas. This difficulty is often
closely associated with species-specific traits of water use of the
woody species, which are involved in a particular bush encroachment
process (Lubczynski et al., 2017).
It has become increasingly apparent that the estimation of
evapotranspiration depends on sufficient understanding and accurate
modelling, including validation, of the two main processes underlying
evapotranspiration, namely transpiration and evaporation. While
estimating evaporation is relatively straightforward, determining the
rate of vegetation water use is far more complex (Sun et al., 2019).
Transpiration is influenced by many factors including species-specific
rooting depth, conductive sapwood area and canopy structure (Sohel,
2022).
A common method to estimate transpiration of woody vegetation relies on
measurements of sap-flux density (SFD) across the active
fluid-transporting tissue (the xylem), also known as sapwood.
Multiplication of SFD with sapwood area provides the total water-use of
an individual tree, while the multiplication with total sapwood area of
all woody plants per ground area gives provides an estimate measure of
woody stand transpiration at landscape scale. In general, the
per‐species SFD variability among trees of different size and age is
relatively low (Jaskierniak, et al., 2016; Kumagai et al., 2007;
Reyes‐Acosta & Lubczynski, 2014). Therefore, tree water use depends
mainly on the conductive sapwood area. A reliable estimate of sapwood
area, therefore, is a key component in quantifying transpiration of the
woody vegetation components in bush encroached savannas.
However, measuring sapwood area is time consuming, often destructive and
impractical, both at the small scale of a plot or stand and at landscape
level. This is particularly true for shrub stands occurring in semiarid
savannas, which often comprise of multi-stemmed species. While some
variability of sapwood patterns among species exists (Carrodus, 1972),
fortunately, sapwood area scales reliably with various plant dimensions,
such as leaf area, stem diameter and crown dimensions, which are easier
to assess (Lubczynski et al., 2017; Mitra 2020; West et al., 1999).
Particularly crown properties can later be detected and quantified at
landscape scale using aerial imagery, light detection and ranging
(LiDAR). Although previous attempts to establish allometric
relationships between sapwood area and crown dimensions, such as crown
area or crown diameter, have found that they can in fact be strong
(Fregoso, 2002; Mitra et al., 2020), stem diameter achieves still the
most accurate values of sapwood area. A convenient model for explaining
characteristic allometric relations of different species between
specific size measures, such as stem diameter, and traits, such as
sapwood area, is the West-Brown-Enquist-model of West et al. (1999,
hereafter WBE). It predicts that sapwood area scales with stem diameter
with a third power law of 7/3 (2.33). Reasons for the constant scaling
are geometric and hydrodynamic constraints that limit the size in living
organisms (West et al., 1997). Despite many findings supporting the WBE
model’s predictions, empirical results showed that variations from the
predicted scaling value do occur (Daba & Soromessa, 2019; Schoppach et
al., 2021). Therefore, establishing species-specific relationships
between sapwood area and stem diameter may be essential to accurately
estimating transpiration rates (Ter-Mikaelian & Korzukhin, 1997;
Yaemphum et al., 2022). This may be particularly important in savanna
shrubs, which may increase sapwood area relative to the total wood area,
avoiding cavitation-embolism under drought (Brodersen & McElrone,
2013).
Allometric equations for estimating sapwood area and eventually
whole-tree and stand transpiration using tree size parameters have been
developed for various woody ecosystems, such as tropical and temperate
forests (Yaemphum et al., 2022; Wang et al., 2010), but only a few exist
for arid and semiarid bush encroached savannas. Studies for Southern
Africa are particularly rare (Lubczynski et al., 2017).
The primary goal of this study was therefore to test whether allometric
relations between sapwood area and stem diameter or crown area exist for
six dominant bush encroacher species of Namibia. Relying on extensive
field measurements, we derived species-specific equations, which can
later be used in combination with sap flux monitoring to estimate canopy
transpiration for different bush encroachment scenarios at plot and
landscape scale, with the latter based on remote sensing of crown area.
Second, we tested whether the allometric relations between sapwood area
and stem diameter agree with scaling predictions from the WBE model. We
predicted that deviations from the model are attributed to drought
induced species-specific changes of relative investments in the area of
sapwood.
METHODS