Long term trends of stand transpiration in a remnant forest during wet and dry years

Abstract

1Daily and annual rates of stand transpiration in a drought year and a non-drought year are 2 compared in order to understand the adaptive responses of a remnant woodland to drought. 3 Two methods were used to estimate stand transpiration. In the first, the ratio of sap velocity of 4 a few trees measured for several hundred days to the mean sap velocity of many trees measured 5 during brief sampling periods (generally 6-7 trees for 5 or 6 days), called the E sv method is 6 used to scale temporally from the few intensive study periods. The second method used the 7Penman-Monteith (P-M) equation (called the E PM method). Weather variables were used to 8 predict canopy conductance, which in turn was used to predict daily and annual stand 9 transpiration. Comparisons of daily transpiration estimated with the two methods showed 10 larger values for the E PM method during a drought year and smaller values for the E PM when 11 the rainfall was above average. Annual estimates of stand transpiration were similar using the 12 two methods. The E sv method produced an estimate of 318 mm (61 % of rainfall) in the 13 drought year and 443 mm (42 %) in the year having above average rainfall. The E PM method 14 estimated stand transpiration as 379 mm (73 %) and 398 mm (37 %) respectively for the two 15 years. Both estimates of annual stand transpiration demonstrated that the remnant forest 16 showed resilience to an extreme and long-term drought. More importantly, the annual 17 estimates showed that in dry years a larger proportion of rainfall was used as transpiration, and 18 groundwater recharge was absent but in years with above average rainfall recharge was 19 significantly increased. Changes in leaf area index were minimal between years and changes in 20 stomatal conductance were the dominant mechanism for adapting to the drought. The remnant 21 forest rapidly responded to increased water availability after the drought through a new flush of 22 leaves and increased stomatal conductance.

Environmental controls on seasonal ecosystem evapotranspiration/potential evapotranspiration ratio as determined by the global eddy flux measurements

“…During dry years or periods, a lack of precipitation may cause a reduction of the leaf area index, and K c will decrease. During rainy seasons, as leaf area index and stomatal conductance of trees and rain-fed crops increases, so does K c (Kar et al, 2006;Zeppel et al, 2008). Irrigation of cropland is a primary mechanism for increasing yield (Fereres and Soriano, 2007;Du et al, 2015), so the CRO may have a high monthly K c even at sites with a low precipitation.…”

Impacts of extreme precipitation and seasonal changes in precipitation on plants

“…Soil water content is driven in part by soil type, texture, soil depth and density. Microclimatic factors that influence evapotranspiration also determine whether changes in timing have a large impact Zeppel et al, 2008b). Further, current mean annual precipitation (MAP), which influences whether the site is xeric, mesic or hydric, will be a key factor in determining whether plant growth increases or declines Heisler-White et al, 2009).…”

Groundwater Dependent Ecosystems: Classification, Identification Techniques and Threats

“…Thus, an independent estimate of rates of water use are required in addition to analyses of the stable isotope composition of soil water, groundwater and xylem water. Methods to estimate rates of vegetation water use include eddy covariance ), measurement of rates of sapflow (Zeppel et al 2008) and remotely sensed estimates (Nagler et al 2009). When only a single isotope is analysed ( 2 H or 18 O) a linear mixing model can distinguish between only two potential sources of water (groundwater and soil water).…”