Transpiration and stomatal conductance in a young secondary tropical montane forest: contrasts between native trees and invasive understorey shrubs

Abstract

It has been suggested that vigorous secondary tropical forests can have very high transpiration rates, but sap flow and stomatal conductance dynamics of trees and shrubs in these forests are understudied. In an effort to address this knowledge gap, sap flow (thermal dissipation method, 12 trees) and stomatal conductance (porometry, six trees) were measured for young (5-7 years) Psiadia altissima (DC.) Drake trees, a widely occurring species dominating young regrowth following abandonment of swidden agriculture in upland eastern Madagascar. In addition, stomatal conductance (gs) was determined for three individuals of two locally common invasive shrubs (Lantana camara L. and Rubus moluccanus L.) during three periods with contrasting soil moisture conditions. Values of gs for the three investigated species were significantly higher and more sensitive to climatic conditions during the wet period compared with the dry period. Further, gs of the understorey shrubs was much more sensitive to soil moisture content than that of the trees. Tree transpiration rates (Ec) were relatively stable during the dry season and were only affected somewhat by soil water content at the end of the dry season, suggesting the trees had continued access to soil water despite drying out of the topsoil. The Ec exhibited a plateau-shaped relation with vapour pressure deficit (VPD), which was attributed to stomatal closure at high VPD. Vapour pressure deficit was the major driver of variation in Ec, during both the wet and the dry season. Overall water use of the trees was modest, possibly reflecting low site fertility after three swidden cultivation cycles. The observed contrast in gs response to soil water and climatic conditions for the trees and shrubs underscores the need to take root distributions into account when modelling transpiration from regenerating tropical forests.

Modelling non‐stationary water ages in a tropical rainforest: A preliminary spatially distributed assessment

“…Our findings resulted in an average transpiration of 65% of the total actual ET flux, which is close to that noted by Schlesinger and Jasechko (2014) for tropical forest transpiration (70%). The average annual transpiration flux accounted for 288 mm (p 25 = 258 mm and p 75 = 296 mm), which is similar to values reported for a tropical forest catchment in Madagascar (Ghimire et al, 2018) and is also in the range established by Bruijnzeel and Veneklaas (1998) for a tropical montane forest (250–645 mm/yr). However, our daily simulated values were slightly lower compared to measured transpiration in several trees in a nearby, but much wetter (mean annual rainfall >5,000 mm) low montane rainforest catchment (1.4 ± 0.7 mm/day) using heat dissipation sap flow sensors (Moore et al, 2018).…”

“…All maps are given at a 10 m × 10 m pixel size and different scales in months noted by Schlesinger and Jasechko (2014) for tropical forest transpiration (70%). The average annual transpiration flux accounted for 288 mm (p 25 = 258 mm and p 75 = 296 mm), which is similar to values reported for a tropical forest catchment in Madagascar (Ghimire et al, 2018) and is also in the range established by Bruijnzeel and Veneklaas (1998) for a tropical montane forest (250-645 mm/yr).…”

Contribution of understory evaporation in a tropical wet forest during the dry season

“…Additionally, the soil water reservoir used by understory shrubs and overstory trees differs. Shrub plants are more dependent on soil water, whereas the trees can access deeper water reservoirs (Ghimire et al, 2018). The number of plant species in TFE can exceed 50 species ha −1 (Eilu et al, 2004;Naidu and Kumar, 2016) with densities above 500 trees ha −1 (Crowther et al, 2015).…”

“…The structural complexity of TFE is defined by environmental variables such as altitude, climate and geomorphology (Holdridge and Tosi, 1967;Gomez, 1986;Hartshorn, 2002;Guariguata and Ostertag, 2002). The forest canopy can be segmented into four layers according to vegetation height and light requirements.…”

Contribution of understory evaporation in a tropical wet forest

“…Additionally, the soil water reservoir used by understory shrubs and overstory trees differ. Shrub plants are more dependant on soil water, whereas the trees can access deeper water reservoirs (Ghimire et al, 2018). The number of plant species in TFE can exceed 50 species ha −1 (Eilu et al, 2004;Naidu and Kumar, 2016) with densities above 500 trees ha −1 (Crowther et al, 2015).…”