Tree height–diameter allometry of Populus euphratica across riparian Tugay forests along a gradient of groundwater table depths

Deeper groundwater tables restrict taller, slenderer tree forms, pinpointing the importance of hydraulic path length presumably dictated by groundwater depth in regulating tree growth and architecture.

How tree dimensions are interrelated (i.e., allometry) is a key question in biomechanics and forest ecology. Yet, the functional significance of tree allometry in coping with groundwater availability in drylands has been rarely explored. This gap of knowledge is particularly acute for riparian forests in Central Asia (known as Tugay forests), where groundwater table depth (GWD) is one of the most limiting factors. Based on the rationale of the hydraulic constraint hypothesis in explaining tree height growth, we posited that GWD should dictate hydraulic path length and eventually limit height (H) more than diameter (D) growth. Therefore, we predicted that there would be a lower increment in H with a unit increment in D (i.e., a small scaling exponent), or lower H at a given D, above deeper groundwater tables. Here we gathered a dataset of paired H–D measures for > 6500 trees of Populus euphratica, a keystone phreatophyte species of riparian Tugay forests, along a gradient of GWD from 1.5 m to 10 m across 16 sites in the Tarim River corridor (NW China). We quantified the H–D allometries at individual sites using standardized major axis regression, and tested for any significant shift in scaling exponents or expected H at a given D in relation to GWD. Results showed that the H–D scaling exponents in P. euphratica varied widely across sites, with an average of 0.65 (i.e., H ∝ D^0.65) close to the “canonical” value of 2/3 as posited by theory. With increasing GWD, the scaling exponents did not vary predictably, but the expected H at the grand mean of D decreased as anticipated. This study highlights the functional importance of shorter, stouter tree architectures in persisting above deeper groundwater tables within and perhaps beyond riparian forests in Central Asia’s drylands.