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