Ecological Indicators 117 (2020) 106615 Contents lists available at ScienceDirect Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind Species-specific drought resilience in juniper and fir forests in the central Himalayas T ⁎ Ouya Fanga,c, Hongyan Qiua, Qi-Bin Zhanga,b, a State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China b Tibet Agricultural and Animal Husbandry University, Linzhi, China c Swiss Federal Research Institute WSL, Birmensdorf, Switzerland ARTICLE INFO ABSTRACT Keywords: With increased frequency and intensity of drought occurrence in the changing climate, the drought resilience of Resistance forest trees is of widespread interest. Particularly, it is not clear as to how the resilience differs between tree Recovery species and whether or not such resilience changes over time. Understanding tree resilience to drought requires Juniperus tibetica observations not only from recent events but also from the historical past, information of which is usually hardly Abies spectabilis available. Here we defined historical drought based on isotope data and compared drought resilience in Juniperus Oxygen isotope tibetica and Abies spectabilis forest in the central Himalayas in five extreme droughts during the past two cen- Iso/anisohydry fi Tibetan plateau turies. We found that juniper trees had a stronger resistance than r trees in the three extreme droughts in the nineteenth century but this pattern reversed in the two drought events in the twentieth century. The length of response time to droughts and recovery time to pre-drought state were shorter in juniper trees than in fir trees. The proportion of declining trees showed a decreasing trend in fir trees but not in juniper trees. Our results indicate that the species-specific resilience might be related to the anisohydric (junipers) and isohydric (firs) strategies of stomatal regulation in response to droughts plants. The differences in species-specific drought re- silience should be taken into account when developing forest management policies against the influence of extreme droughts in future. 1. Introduction average elevation more than 3500 m above sea level. Juniper and fir are both long-lived and dominant tree species in high-mountain ecosys- Resilience is the capacity of ecosystems to withstand disturbance tems, which contribute to regional carbon stocks and forest pro- and maintain their structures and functions (Holling, 1973; Walker ductivity (Måren et al., 2015). Tree-ring oxygen isotopes collected from et al., 2004). Tree growth resilience to climate extremes has received fir trees in the central Himalayas showed that the region has been increasing attention in studies of forest response to changing climate subjected to drought extremes over the past two hundred years (Xu (Allen et al., 2016; Willis et al., 2018). It has been demonstrated that et al., 2018). The healthy growth of juniper and fir forests in this region trees could actively resist the influence of extreme events and recover to suggests that these two species developed strategies of resilience to the pre-disturbance growth state if the resilience threshold is not exceeded recurrent drought extremes. Yet it is not clear as to these strategies in (Hodgson et al., 2015; Gazol et al., 2018; Stevens-Rumann et al., 2018; the two species. Liu et al., 2019). Previous studies showed that tree resilience to climate In this study, we investigate the characteristics of drought resilience extremes varies among species due to differences in growth sensitivity, in Tibetan juniper and east Himalayan fir trees in two sites in the stomatal and physiological traits (Vitali et al., 2017; Gomes Marques central Himalayas. We use tree-ring data to quantify key components of et al., 2018; Serra-Maluquer et al., 2018; Duan et al., 2019; Rahman resilience (resistance, length of response time and length of recovery et al. 2019). Understanding the species-specific resilience is a pre- time) and compare them in the two species in the past two centuries. requisite for making management policy to mitigate climatic extremes The objectives of our study were to answer two questions: (a) What’s in forests. the differences in drought resilience between the two species? and (b) Tibetan juniper (Juniperus tibetica) and east Himalayan fir(Abies Has the resilience of the two species changed in the past two centuries? spectabilis) forests are distributed in the central Himalayas with the ⁎ Corresponding author at: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China. E-mail address: [email protected] (Q.-B. Zhang). https://doi.org/10.1016/j.ecolind.2020.106615 Received 20 January 2020; Received in revised form 18 May 2020; Accepted 7 June 2020 1470-160X/ © 2020 Elsevier Ltd. All rights reserved. O. Fang, et al. Ecological Indicators 117 (2020) 106615 Extreme drought event T1 T2 V1 V3 Resistance Recovery V2 Fig. 2. Definition of tree resilience. V1-3: tree-ring index before, during and after the extreme drought event; T1-2: Time to resist and recover. coefficients between the tree-ring chronologies and scPDSI from Oc- tober of prior growth year to September of the growth year were cal- Fig. 1. Location of sampling sites and meteorological station. CM: site wth culated for the period 1967 to 2005 to identify the climate-growth re- Juniperus tibetica; QCM: site with Abies spectabilis; Ganesh: site with tree-ring lationships. oxygen isotope record published by Xu (2018); The crosses describe the grid of Tree-ring cellulose δ18O sequence from fir trees in Ganesh, ap- the CRU scPDSI data (84.5–86°E, 28–29°N) used in this study. proximately 30 km from site QCM and 40 km from site CM, was proved to have significant correlation with scPDSI from June to September (Xu 2. Materials and methods et al., 2018). The data of tree-ring cellulose δ18O, spanning from 1801 to 2000 were downloaded at the National Oceanic and Atmospheric 2.1. Study sites and tree-ring data Administration (https://www.ncdc.noaa.gov/paleo/study/22551). Linear trend in δ18O data was eliminated and the years that the de- Two sites in natural high-mountain forests in the central Himalayas trended δ18O values dropped below two standard deviation of the series were chosen in this study (Fig. 1). One site is Cangmu (CM) where the were defined as extreme droughts. dominant tree species is Tibetan juniper and the geographical location is 85°12′57″ E, 28°31′59″ N. The other site is Qiucangmu (QCM) where the dominant tree species is east Himalayan fir and the geographical 2.3. Quantification of tree resilience location is 85°13′42″ E, 28°26′20″ N. The growing season of the two species is from May to late September or early October according to the We defined three components of tree resilience (Fig. 2). Tree re- relevant studies (Liang et al., 2010; Li et al., 2019; Xu et al., 2019). sistance (Rt) is the reduction in tree growth during disturbance relative Regional annual mean temperature is 3.8 ± 0.5 ℃ (mean ± SD) and to pre-disturbance, that is, Rt21=−(V V )/V1. Length of response time annual total precipitation is 638.6 ± 139.8 mm (mean ± SD) based (T1) is the number of years that tree-ring width reached the minimum on climate data from years 1967–2018 in the Nielamu national me- value since the extreme drought, reflecting lag effect of the drought. teorological station (Fig. 1). Maximum temperature and precipitation Length of recovery time (T2) is the number of years that trees return both occur in July. from the lowest growth to the pre-disturbance state. Increment cores were collected from relatively old juniper trees in Where V1 is the 10-year average of ring indices before the event CM site and fir trees in QCM site, which are approximately 10 km apart year; V2 is the lowest value in ring indices from the event year to three in distance and located within the same valley with similar soils and years after the event. V3 is the 5-year running average of tree-ring in- climate conditions. The sampling was conducted in elevation between dices that is equal or greater than V1.T1 is the time from V1 to V2, and ’ 3400 and 3500 m on the south slope in CM, and between 3500 and T2 is the time from V2 to V3. If a tree sT2 is longer than four years, this 3600 m on the north slope in QCM. We intentionally selected sites that tree is considered as a growth declining tree (Anderegg et al., 2015; Wu had minimal evidence of past human activities, including trees with an et al., 2017). All these indices were calculated at the individual tree intact large diameter class. Increment cores was extracted at breast level for each extreme event in this study. height of trees and in the direction parallel to the contour. In order to ensure enough individual trees to be sampled in the remote area, we 3. Results collected one core per tree from a total number of 20 and 30 trees in the fi two sites, respectively. The core samples were dried, xed and sanded 3.1. Tree growth in extreme droughts to bring the rings clearly visible. Ring widths were measured using a Lintab 6 system (Frank Rinntech, Heidelberg, Germany) with an accu- The longest tree-ring series at the site CM and QCM are from 1781 racy of 0.001 mm. Tree rings were crossdated and quality-checked by to 2005 and 1783 to 2005, respectively. Tree-ring chronologies at the standard dendrochronological techniques (Schweingruber 1988; two sites show positive correlation with scPDSI in all months of the year Holmes 1983). Growth trend in tree-ring series was removed using a from 1967 to 2005.