Using whole-tree sapflow measurements to assess the effects of land use/cover change on stand transpiration in a seasonal montane cloud forest zone in central Veracruz, Mexico

Background/Question/Methods Information on whole tree and stand-level transpiration is essential for accurate assessments of water balance and hydrological processes, yet few data on transpiration exist for tropical montane cloud forests, where high species diversity combined with complex climatic and topographical conditions make data collection and up-scaling particularly challenging. To address this gap, whole-tree seasonal sapflow was determined for nine tree species growing in three different land use/cover types in a seasonal montane cloud forest (MCF) zone in Central Veracruz: a species diverse old-growth MCF, and 19-year old naturally regenerating MCF dominated by Alnus jorullensis, and a 7-year old Pinus patula reforestation. Granier-type thermal dissipation probes, in conjunction with heat pulse sensors (for data validation), were used to measure daily sapflow velocity (Fd) rate in 2-5 individuals of each species. Fd values were scaled to the whole-tree level (Ft) using tree sapwood area of the sampled individuals, and to the stand level (Et) using estimated total sapwood area per unit ground area per species summed over all species (functional grouping were used to derive estimates for species for which Fd values were not obtained directly with sapflow sensors). Results/Conclusions Results showed a clear ranking among individual species in their Fd rates, with the early successional Pinus patula and Alnus jorullensis having the highest Fd rates (15.7 l dm-2 day-1 and 13.0 l dm-2 day-1, respectively), a group of six mature cloud forest species (Alchornea latifolia, Clethra macrophylla, Quercus lancifolia, Q. ocoteifolia, Hedyosmum mexicanum, and Ternstroemia sylvatica) having intermediate Fd rates (6.8 to 11.7 l dm-2 day-1), and Litsea sp., also from the mature MFC, having the lowest Fd rate (3.5 l dm-2 day-1). Comparing annual Et across land use/cover types, the pine reforestation had the highest annual Et (1009 mm year-1), the mature MCF had intermediate annual Et (858 mm year-1), and the regenerating MCF had the lowest annul Et (832 mm year-1). These are amongst the first annual Et data derived from whole-tree sapflow data for MCF regions on a global basis, and suggest that conversion of mature MCF vegetation to young, rapidly growing tree plantations can increase total water use by the vegetation, at least within shorter time periods of 7 years. On the other hand, Et rates of naturally regenerating vegetation relatively quickly (within approximately 20 years) resemble rates observed for mature old growth MCF.