Examining the influence of environmental parameters on sap flow across stand ages and slope positions in a tropical montane cloud forest
Water flow through plants is controlled by water availabilty from different potential sources (e.g., soil, groundwater, and fog), as well as the strength of climatic drivers that influence the rate of this exchange (e.g., vapor pressure deficit-VPD, solar radiation, air temperature, cloud cover). The relative importance of each driver on plant water balance will vary across many factors including the two examined in this study, species and landscape position. Tropical montane cloud forests (TMCFs) have highly variable solar radiation and atmospheric water demand creating unique environments for comparing drivers of tree water use across these environmental periods. Within a TMCF, this study examined the influence of soil moisture at two depths, air vapor pressure deficit, solar radiation, air temperature, and leaf wetness on tree level sap velocity by three dominant TMCF tree species growing at three slope positions (upslope, midslope, lowslope). Specifically, sap flow (heat ratio method) and environmental parameters were measured in 4-6 trees across these three sites during the dry to wet transition period of May and June in the TMCF of Veracruz, Mexico. Data were analyzed using multiple regression models to assess the influence of selected environmental parameters on tree level sap flow.
Sap velocities were greater in trees in upslope positions. To explain variation in sap velocity, we developed multiple regression models that included potential water sources (2 depths of soil moisture and leaf wetness) and environmental drivers (air VPD, solar radiation, and temperature). Overall model fit was strong and sap velocities were predominantly driven by air temperature, VPD, and solar radiation. During clear daytime periods sap velocities were predominantly explained by variation in air temperature. However during fog periods, the variation in sap velocity was explained by VPD (as much as 91%) with a greater influence at the upslope and midslope sites which experience more frequent fog events. There was also significant non-zero sap velocities during night time periods that was predominantly explained by VPD during clear nights and VPD and leaf wetness during night fog events. Overall VPD explained greater variation at the exposed upslope site while solar radiation and air temperature were a greater influence at the lowslope position. Across all models, soil moisture explained very little variation. This experiment demonstrates that the drivers of sap velocity in a tropical cloud forest vary depending on the position in the landscape, time period, and presence of fog, and species.