Reliance on shallow soil water in a mixed-hardwood forest in central Pennsylvania.

Katie P Gaines,Jane W Stanley,Henry Lin,David R Woodruff,David M Eissenstat,Katherine A Mcculloh,Weile Chen,Frederick C Meinzer,Thomas S Adams,Nathan Phillips

doi:10.1093/treephys/tpv113

Abstract

We investigated depth of water uptake of trees on shale-derived soils in order to assess the importance of roots over a meter deep as a driver of water use in a central Pennsylvania catchment. This information is not only needed to improve basic understanding of water use in these forests but also to improve descriptions of root function at depth in hydrologic process models. The study took place at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. We asked two main questions: (i) Do trees in a mixed-hardwood, humid temperate forest in a central Pennsylvania catchment rely on deep roots for water during dry portions of the growing season? (ii) What is the role of tree genus, size, soil depth and hillslope position on the depth of water extraction by trees? Based on multiple lines of evidence, including stable isotope natural abundance, sap flux and soil moisture depletion patterns with depth, the majority of water uptake during the dry part of the growing season occurred, on average, at less than ∼60 cm soil depth throughout the catchment. While there were some trends in depth of water uptake related to genus, tree size and soil depth, water uptake was more uniformly shallow than we expected. Our results suggest that these types of forests may rely considerably on water sources that are quite shallow, even in the drier parts of the growing season.

Highlights

IntroductionDepth of water uptake, can affect plant productivity and the length of the growing season (Chapin et al 2002), in addition to playing an important role in determining drought stress (Milly and Dunne 1994, Sternberg et al 1996, Hubbert et al 2001), plant competition (Fernandez and Caldwell 1975, Sala et al 1989, Schwinning 2010), soil formation (Jenny 1980, Schenk 2005) and climate (Kleidon and Heimann 2000, Kleidon and L orenz 2001)
We addressed the following questions: (i) To what extent do trees in our study system rely on deep roots to maintain sap flux during dry portions of the growing season? (ii) What is the role of genus, tree size and slope position or soil depth on depth of water extraction?
Effective rooting depth has not been well studied in humid temperate forests

Summary

Introduction

Depth of water uptake, can affect plant productivity and the length of the growing season (Chapin et al 2002), in addition to playing an important role in determining drought stress (Milly and Dunne 1994, Sternberg et al 1996, Hubbert et al 2001), plant competition (Fernandez and Caldwell 1975, Sala et al 1989, Schwinning 2010), soil formation (Jenny 1980, Schenk 2005) and climate (Kleidon and Heimann 2000, Kleidon and L orenz 2001). The majority of roots are likely to be distributed in the top 30 cm of soil (Jackson et al 1996, Schenk and Jackson 2002a), deep roots have been observed in many water-limited ecosystems and in drought-adapted species (Sternberg et al 1996, Schenk and Jackson 2002b). Forests in humid temperate climates may have little need to rely on deep roots if precipitation is frequent. The occurrence of random droughts during the growing season even in humid forests (NOAA 2014) and the existence of dry microsites on rocky hillslopes and ridge tops could lead to a

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Results

Conclusion