1 INTRODUCTION
About 50-75% of annual precipitation that falls in southern forests in
the U.S. returns the atmosphere through the process of
evapotranspiration (ET), the sum of tree transpiration, canopy
interception, and soil evaporation (Sun et al., 2001; Sun et al., 2016).
Natural (e.g. climate change and variability, drought) and anthropogenic
stressors (e.g. land use change, urbanization), and forest management
(e.g., thinning, prescribed burning) affect water quantity and water
quality, and ecosystem productivity (Sun et al., 2011) by directly
altering forest transpiration process, a key component of ET in forests
(Domec et al., 2012). Water use by trees or stand is naturally variable
due to the large differences in tree species, age, stand structure (leaf
area index), and climate (Sun et al., 2011). Given the issues with
landcover change, urbanization, and an increase in extreme events from
climate change facing the Piedmont region in the U.S. (Wear and Greis,
2013), an improved understanding of tree transpiration at the species
level within a forested watershed is needed. Such information is useful
for developing foundational steps toward improving watershed-level
estimates of transpiration in mixed forests and reliable ecohydrological
model to predict the effects of environmental change on water and carbon
resources (Liu et al., 2020; Li et al., 2020a, 2020b).
Land use change and fire regimes have produced highly variable species
composition in the U.S. southeastern Piedmont. The region supports about
12.5 million hectares of forest land or 62% of the total land area,
with 52% of forest lands covered by upland hardwoods and 34% by pines
(Rummer and Hafer, 2014). The upland hardwood forest types consist of a
mixture of white oak (Quercus alba ), red oak (Quercus
rubra ), hickories (Carya spp. ), sweetgum (Liquidambar
styraciflua ), and tulip poplar (Liriodendron tulipifera ). The
oak-pine forest type is dominated by loblolly (Pinus taeda ),
Virginia (Pinus virginiana ) and shortleaf (Pinus Echinata )
pines, and southern red oak (Quercus falcata ). Although many
state and federal regulatory agencies require forests in the source
water supply zone of a watershed be protected to slow runoff and
maintain minimal discharge levels, 17% of total forest area could be
lost by 2060 due to high urbanization and low timber prices with most of
the losses occurring in the upland hardwood (Rummer and Hafer, 2014).
Given the variability in species transpiration, the type of trees
removed from the landscape will likely impact soil moisture, streamflow
in headwater catchments, and downstream water supply (Moore et al.,
2004; Swank and Vose, 1994).
Quantifying spatial variations in species-specific sap flux density
(i.e., g H2O cm-2day-1) and daily tree-level transpiration (i.e., kg
day-1 or liters day-1) is essential
to improve stand-scaled transpiration estimates and total ET at the
watershed level. Sap flux density and transpiration rates can vary
widely by species (Boggs et al., 2015; Yi et al., 2017). For example,
tree transpiration in a temperate pine-hardwood riparian buffer forest
can range from 2 to 142 liters day-1 and can increase
nonlinearly with increasing tree diameter (Bosch et al., 2014).
Transpiration can vary across plant species due to tree xylem structure
(Ford et al., 2011), responses to vapor pressure deficit (VPD) (Moore et
al., 2017), and age (Brantley et al., 2019). Tulip poplar, a species
with diffuse porous xylem and large amount of sapwood, can use up to
three-fold more water than oaks (Quercus spp. ), a species with a
narrow sapwood characterized by a ring porous structure (Ford et al.,
2011).
Species level transpiration can also differ across topography or zones
due to landscape level variations in microclimate and soil moisture
(Emanuel et al., 2010; 2011). Hawthorne and Miniat (2016) found that
transpiration per unit leaf area in hickory species was sensitive to the
topographic position during a wet year, producing 56% less water at a
cove site when compared to an upland site. However, chestnut oak
(Quercus prinus ) decreased 41% from a wet year to a dry year but
was not influenced by topographic position. Bosch et al. (2014) also
found that species topographical position does not consistently
influence tree transpiration rates across species.
At the stand level, linking species transpiration and soil moisture
helps further refine watershed-level estimates of transpiration (Oishi
et al., 2010) and improve our understanding of how forests respond to
water stresses, including drought (Vose et al., 2016, Yi et al., 2017).
Sap flux density and tree-level transpiration measurements have advanced
dramatically during the last decade (Poyatos et al., 2020). However,
questions are still being raised about how to improve watershed-level
transpiration, given the variations in species composition, topography,
and soil moisture. In addition, variability in tree transpiration across
heterogeneous watersheds is typically not considered (or well captured)
by models and may be one contributing factor to over or under
predictions of water loss from the forest at local and regional scales.
Therefore, the objectives of this study are to 1) quantify tree sap flux
density and transpiration across three zones (riparian buffer,
mid-hillslope, and upland-hillslope) in a wet and dry year, 2) analyze
the relationships between sap flux density and vapor pressure deficit,
and 3) compare watershed-level transpiration derived from three zones.