Forest transpiration from sap flux density measurements in a Southeastern Coastal Plain riparian buffer system

Forested riparian buffers are prevalent throughout the Southeastern Coastal Plain Region of the United States (US). Vegetation in the riparian forest buffers in this region are primarily made up of slash pine (Pinus elliottii Engelm.), longleaf pine (Pinus palustris Mill.), and yellow poplar (Liriodendron tulipifera L.) trees. Because they make up a significant portion of the regional landscape, transpiration within these riparian buffers is believed to have an important impact on the hydrologic budget of regional watersheds. A riparian buffer along a first order stream in South-central Georgia US was selected for a sap flow study designed to provide measurements of tree transpiration. The forest provided a buffer zone that averaged 70 m in width from an upland field to the first order stream. Shallow water table conditions allowed direct interaction between the tree's root system and groundwater. Sap flux density, groundwater, and climatic data were collected to determine transpiration rates from different tree species and their relationship to potential ET rates and hydrologic and environmental conditions. Average sap flow rates ranged from 2 to 142 L day−1. Sap flow was related to tree diameter, solar flux density, and daily vapor pressure deficit. An exponential relationship was developed that related measured average daily sap flow rate (L day−1) to tree diameter at breast height (DBH) (mm) for the range of measured trees (56 < DBH < 390 mm) with a coefficient of 2.04 and an exponent of 0.01. On an area basis, the average transpiration for the studied 720 m2 study area was 1114 mm for the observation period from April to December. This represented 103% of the potential evapotranspiration for a reference grass (PET) for that same period. While landscape position, and subsequently access to groundwater, did not appear to strongly influence sap flow rates, reduced soil water in the vadose zone led to reductions in tree transpiration. The data indicate that transpiration within regional buffers uses a disproportionate amount of water on a per area basis compared to upland land covers, an important consideration when examining overall water consumption in regional watersheds.