Changes in tracheid and ray traits in fire scars of North American conifers and their ecophysiological implications

Annals of Botany 114: 223– 232, 2014 doi:10.1093/aob/mcu112, available online at www.aob.oxfordjournals.org Changes in tracheid and ray traits in fire scars of North American conifers and their ecophysiological implications Estelle Arbellay1,*, Markus Stoffel1,2, Elaine K. Sutherland3, Kevin T. Smith4 and Donald A. Falk5,6 1 Received: 28 February 2014 Returned for revision: 9 April 2014 Accepted: 29 April 2014 Published electronically: 18 June 2014 † Background and Aims Fire scars have been widely used as proxies for the reconstruction of fire history; however, little is known about the impact of fire injury on wood anatomy. This study investigates changes in tracheid and ray traits in fire scars of Douglas fir (Pseudotsuga menziesii), western larch (Larix occidentalis) and ponderosa pine (Pinus ponderosa), and discusses their ecophysiological implications for tree recovery from fire. † Methods Transverse and tangential microsections were prepared for light microscopy and image analysis. Measurements of tracheids and rays were made in the three spatial dimensions: axially (at different section heights), radially (in different rings) and tangentially (with increasing distance from the wound margin). † Key Results Changes were strongest in the first year after fire injury, with a decrease in tracheid size (by 25 –30 %) and an increase in tracheid density (by 21 –53 %) for the three species. In addition, an increase in ray size (by 5 –27 %) and an increase in ray density (by 19– 36 %) were found in P. menziesii and L. occidentalis. Changes were comparable along the fire-injured stem and were often most marked close to the fire scar. † Conclusions The differentiation after fire injury of narrower and more numerous tracheids expresses a trade-off between hydraulic safety and hydraulic efficiency, while that of larger and more numerous rays serves compartmentalization and wound closure, mechanical strength and defence responses. Pinus ponderosa does not generally produce more ray tissue after fire injury and thus appears to be more adapted to fire. Key words: Ecophysiology, conifer, fire scar, Larix occidentalis, western larch, Pinus ponderosa, ponderosa pine, Pseudotsuga menziesii, Douglas fir, ray, tracheid, wood anatomy. IN T RO DU C T IO N Fire scars form on trees when surface fires of low to moderate intensity heat the cambium to lethal temperatures around a portion of the bole (Gutsell and Johnson, 1996). Similarly to other types of mechanical injury to the cambium, tree response to fire injury includes (1) compartmentalization to resist the spread of decay and (2) production of wound xylem (or woundwood) to close the wound and restore continuity of the cambium (Smith and Sutherland, 1999, 2001). Wound closure occurs from the growth of new, healthy vascular tissues that converge inward from both sides of the wound to shield the exposed xylem (Larson, 1994; Fink, 1999). Fire scars provide macroscopic – at the sample level – evidence of past forest fires that has been used to determine the occurrence, extent and frequency of events (Heyerdahl et al., 2001; Swetnam et al., 2009; Farris et al., 2010; Falk et al., 2011). Nevertheless, limitations of fire scars (Van Horne and Fulé, 2006) have motivated the exploration of alternative proxies for the reconstruction of fire history. In this context, both scarred and unscarred trees have been studied for microscopic – at the tissue level – evidence of past forest fires, including anomalies in wood anatomy (Bigio et al., 2010; Bravo, 2010; Kames et al., 2011) and wood chemistry (Bukata et al., 2008; Beghin et al., 2011; Pearson et al., 2011). At the same time, a growing body of research on mechanically injured trees has presented results on the wood anatomical response of conifers and broad-leaved trees to flooding (Ballesteros et al., 2010a, b; Arbellay et al., 2012b) and mass-movement processes (Bollschweiler et al., 2008; Stoffel and Hitz, 2008; Schneuwly et al., 2009a, b; Arbellay et al., 2010, 2012a, 2013). However, the impact of fire injury on wood anatomy has been assessed more rarely and conifers have yet to be investigated, as existing studies on the topic have been conducted exclusively with broadleaved trees (Bigio et al., 2010; Bravo, 2010). This study elucidates the wood anatomical response of conifers to fire injury. Tracheids and rays were analysed for changes in size and density in fire scars of three widely distributed North American conifers: Douglas fir (Pseudotsuga menziesii), western larch (Larix occidentalis) and ponderosa pine (Pinus ponderosa). Changes were measured axially (at different section heights), radially (in different rings) and tangentially (with increasing distance from the wound margin). This work determines the spatio-temporal extent of these changes, discusses their ecophysiological implications for tree recovery # The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: Dendrolab.ch, Institute of Geological Sciences, University of Berne, Berne, Switzerland, 2Climatic Change and Climate Impacts, Institute for Environmental Sciences, University of Geneva, Geneva, Switzerland, 3USDA Forest Service, Rocky Mountain Research Station, Missoula, MT, USA, 4USDA Forest Service, Northern Research Station, Durham, NH, USA, 5School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, USA and 6Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ, USA * For correspondence. Present address: Tree-Ring Lab at UBC, University of British Columbia, Vancouver, Canada. E-mail 224 Arbellay et al. — Ecophysiology of tracheids and rays in fire scars of conifers from fire, and evaluates their potential as new proxy evidence for tree exposure to fire. MAT E RI ALS A ND METH O DS Field campaign and sample preparation TA B L E 1. Characteristics of the six trees analysed SH ¼ 25 cm SH ¼ 50 cm SH ¼ 75 cm SH ¼ 100 cm Age (years) P. menziesii PSM1 PSM2 LAO1 LAO2 PIP1 PIP2 L. occidentalis P. ponderosa TC (cm) WS (%) TC (cm) WS (%) TC (cm) WS (%) TC (cm) WS (%) 71 61 49 71 68 71 58 54 53 46 37 31 68 57 44 68 66 68 53 47 64 50 42 46 64 52 39 64 61 67 53 27 62 53 34 37 60 51 42 62 59 62 52 20 52 16 34 31 97 47 108 124 97 156 SH, section height above the ground surface; TC, tree circumference at section height; WS, wound size at section height, i.e. percentage of cambium killed. B C A NX D 1 0 2 4 100 75 WX 8 Bark 50 4 cm 25 0 cm F I G . 1. Study design for wood anatomical analysis. (A) Tracheids and rays were analysed in cross-sections taken at four different section heights (25, 50, 75 and 100 cm above the ground surface) along the fire-injured stem. (B) Cross-sections were sectioned with a chisel to obtain a wood block where tracheids and rays could be compared between normal xylem (NX) and wound xylem (WX) within 4 cm from the wound margin, (...truncated)