Effects of planting density on tree growth and induced soil suction

Abstract

Plant evapotranspiration is recognised to affect soil suction of slopes and landfill covers. Previous work has focused on evapotranspiration-induced suction by a single plant, with little attention paid to the effects of planting density. The aim of this study is to quantify any changes in tree growth and tree-induced suction during evapotranspiration and rainfall under different planting densities for non-mixed-species plantations. A tree species, Schefflera heptaphylla, which is commonly found in Asia, was planted in silty sand at spacings of 60, 120 and 180 mm, representing three different planting densities. For each case, three replicates were tested to consider tree variability. In total, the responses of suction for 297 seedlings subjected to drying and a rainfall event with a 10-year return period were measured. The test results show that reducing the tree spacing from 180 to 60 mm induced greater tree-tree competition for water, as indicated by a 364% increase in peak suction upon evapotranspiration. Such tree-tree interaction led to: (a) a 19-35% reduction in the leaf area index; (b) a 17-36% decrease in root length; and (c) an obvious decay of roots. Upon the rainfall event, the infiltration rate for vegetated soil with trees planted at a spacing of 60 mm was up to 247% higher than those for soil with a wider tree spacing, where mainly fresh roots were found. Although most suction within the root zone (i.e. top 100 mm) was lost due to increased infiltration at 60 mm spacing, suctions in deeper depths below root zone were largely preserved.

Plant age effects on soil infiltration rate during early plant establishment

“…Studies that focus on relatively young plants found slowing of infiltration (Gish & Jury, 1983;Leung et al, 2015;Jotisankasa & Sirirattanachat, 2017) presumably due to root occupancy of soil pore space, which blocks water flow paths (Scholl et al, 2014;Ng et al, 2016). In contrast, increased infiltration rate is more often reported in mature plants (van Noordwijk et al, 1991;Mitchell et al, 1995;Ng et al, 2017), and is attributable to the formation of (a) root channel related macropores associated with root decay (Ghestem et al, 2011) or (b) desiccation cracks upon drying of medium-to high-plasticity clay (Zhan et al, 2007;Jotisankasa & Sirirattanachat, 2017;Song et al, 2017). The existing research often considered plant effects only at one particular plant age.…”

Changes in Leaf Characteristics with Tree Age in <i>Dryobalanops beccarii</i> Dyer in a Restored Forest of Sarawak, Malaysia

“…Meanwhile, the competition to absorb water in the soil through overlapping root system (Manoli et al, 2014) directly reduce water availability for transpiration (Prieto et al, 2012). The rate of photosynthesis declined due to sunlight obstructed by the canopy layer through selfshading or being shaded by nearby trees (Gundel et al, 2014;Manoli et al, 2014;Ng et al, 2016).…”

Unsaturated hydraulic properties of vegetated soil under single and mixed planting conditions

“…Plant roots cause changes in soil matric suction (Simon & Collison, 2002;Veylon et al, 2015;Ng et al, 2016a;Ni et al, 2017) through evapotranspiration and soil hydraulic properties, including soil water retention curve (SWRC) and soil hydraulic conductivity function (SHCF). Some studies (Table 1) showed an increase in water retention capability when plant roots are present in the soil (Scanlan & Hinz, 2010;Rahardjo et al, 2014;Leung et al, 2015;Ng et al, 2016a, b;Jotisankasa & Siririrattanachat, 2017), probably because of the blockage of soil pore space by roots (Buczko et al, 2007).…”

“…Some studies (Table 1) showed an increase in water retention capability when plant roots are present in the soil (Scanlan & Hinz, 2010;Rahardjo et al, 2014;Leung et al, 2015;Ng et al, 2016a, b;Jotisankasa & Siririrattanachat, 2017), probably because of the blockage of soil pore space by roots (Buczko et al, 2007). However, some studies reported an opposite result Jotisankasa and Sirirattanachat, 2017), arguably because of the formation of soil cracks due to, for instances, repeated soil shrinkage, swelling and root decay and growth (Vergani & Graf, 2015;Ng et al, 2016a;Ni et al, 2017;Leung et al, 2017).…”