Nighttime transpiration in a seasonally dry tropical montane cloud forest environment

Abstract

Key message

Highly variable nighttime transpiration, with higher rates generally observed after preceding fog, is prevalent in dominant tree species of the nutrient-poor tropical montane cloud forest environment of central Veracruz, Mexico.

Abstract

Although nighttime transpiration (E _n) is prevalent in a wide range of species from cloud-affected forests, its magnitude relative to total daily transpiration (E _d) as reported in the literature is generally small (E _n/E _d is 0.12 on average). In the present study, we observed high dry-season E _n/E _d ratios with large night-to-night variation in dominant species from the tropical montane cloud forest (TMCF) zone of central Veracruz, Mexico: 0.22 ± 0.18 for Quercus lancifolia (old-growth TMCF); 0.26 ± 0.14 and 0.16 ± 0.16 for Alchornea latifolia and Alnus jorullensis, respectively (regenerating post-fire TMCF); and 0.30 ± 0.20 to 0.12 ± 0.21 for Pinus patula (young and mature pine plantations). E _n was determined as the difference between observed nocturnal sap flow and estimated refilling of stem water storage, the latter of which was on average: 21–25 % of nocturnal sap flow for Q. lancifolia; 6 and 5 % for A. latifolia and A. jorullensis, respectively; and 21–23 % for P. patula. Night-to-night variation in E _n was mostly due to large variation in vapor pressure deficit (VPD) related in turn to the alternation of cold fronts (producing fog events) and high pressure weather (producing nights with VPD up to 2 kPa). Moreover, in the hours following fog events without concurring rainfall, E _n was often higher as compared to fog-free nights with similar VPD across all species examined. Low-nutrient availability and high water content of the soils in the study area suggest a nutrient uptake benefit associated with the relatively high E _n rates observed.

Appendix 1: Physical and chemical soil properties

At each site, soil samples for laboratory determination of soil physical and chemical properties were collected from different horizons along ~1.5 m soil profiles. All laboratory analyses were performed at the Soil Laboratory of the Instituto de Ecología A.C., Xalapa, Veracruz. Soil bulk density (BD) was determined from undisturbed samples (three replicates) collected at each site and depth with stainless steel rings of 100 cm³. The samples were weighed and oven-dried at 105 °C for 24 h. Soil porosity (POR) was derived from BD and particle density. Particle-size distribution was determined with a combined sieve and pipette method, after removal of organic matter with hydrogen peroxide and dispersion with sodium hexametaphosphate (Van Reeuwijk 2002).

For chemical analysis, air-dried soil samples (three replicates) were sieved using 2-mm screens. Total organic carbon (C) and total nitrogen (N) were measured using a TruSpec dry combustion CN analyzer (LECO, USA). Extractable phosphorus (P) was determined by the Bray I method (Bray and Kurtz 1945) and phosphate retention (Pret) was measured following the procedure of Blakemore et al. (1987). Soil cation exchange capacity (CEC) was determined by the ammonium acetate pH7 method (Van Reeuwijk 2002). Base saturation (BS, %) was calculated as the portion of CEC that is occupied by exchangeable bases: (Ca + Mg + K + Na)/CEC. As recommended for soils with variable charge (Shoji et al. 1993), the effective cation exchange capacity (ECEC) was determined as the sum of exchangeable bases and exchange acidity (Al + H); the latter was determined by 1.0 M KCl extraction (Van Reeuwijk 2002).

The resulting chemical properties characterize the soil across sites as nutrient-poor. Topsoil was very acidic (MAT and SEC) or acidic (YREF and MREF), while deeper soil layers were moderately acid (Appendix Table 4). Across sites, organic carbon (C) and total nitrogen (N) content in the topsoil was high typically decreasing sharply below a depth of ~50 cm (Appendix Table 4), but the mineralization rate of N was moderate (as suggested by the relatively high C/N ratios; Appendix Table 4), indicating low N availability. Phosphate availability was also low as its retention was generally very high along the soil profiles (Appendix Table 4). Base saturation was generally very low in the topsoil across sites, rapidly decreasing even more with depth reaching <0.2 % (Appendix Table 4), indicating low availability of macronutrients (i.e. potassium, calcium and magnesium). Last, effective cation exchange capacity was typically low in the topsoil (<7 cmol_c kg⁻¹ across sites; data not shown) revealing the low capacity of the soil’s solution to provide nutrients to plants.

Table 4 Selected soil properties of the study sites and depths at which soil volumetric water content (VWC) was measured