Journal of Hydrology 611 (2022) 128016 Available online 7 June 2022 0022-1694/Crown Copyright © 2022 Published by Elsevier B.V. All rights reserved. Research papers Seasonal fog enhances crop water productivity in a tropical rubber plantation Palingamoorthy Gnanamoorthy a , Qinghai Song a, b, * , Junbin Zhao c , Yiping Zhang a, b, * , Jing Zhang a, d , Youxing Lin a, b , Liguo Zhou a, b , Sadia Bibi a , Chenna Sun a, d , Hui Yu a, d , Wenjun Zhou a, b , Liqing Sha a, b , Shusen Wang e , S. Chakraborty f , Pramit Kumar Deb Burman f, g a CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun 666303, China b Center of Plant Ecology, Core Botanical Gardens, Chinese Academy of Sciences, Menglun 666303, China c Department of Biogeochemistry and Soil Quality, Division of Environment and Natural Resources, Norwegian Institute of Bioeconomy Research, 1433 Ås, Norway d University of Chinese Academy of Sciences, Beijing 100049, China e Canada Centre for Remote Sensing, Natural Resources Canada, Ottawa K1A0Y7, Canada f Indian Institute of Tropical Meteorology, Ministry of Earth Sciences, Pune 411008, India g Department of Atmospheric and Space Sciences, Savitribai Phule Pune University, Pune 411007, India A R T I C L E INFO This manuscript was handled by Marco Borga, Editor-in-Chief, with the assistance of Lixin Wang, Associate Editor Keywords: Canopy conductance Evapotranspiration Eddy covariance Net ecosystem CO 2 exchange Sap fow Transpiration ABSTRACT The rapid conversion of tropical rainforests into monoculture plantations of rubber (Hevea brasiliensis) in Southeast Asia (SEA) necessitates understanding of rubber tree physiology under local climatic conditions. Frequent fog immersion in the montane regions of SEA may affect the water and carbon budgets of the rubber trees and the plantation ecosystems. We studied the effect of fog on various plant physiological parameters in a mature rubber plantation in southwest China over 3 years. During the study period, an average of 141 fog events occurred every year, and the majority occurred during the dry season, when the temperature was relatively low. In addition to the low temperature, fog events were also associated with low vapor pressure defcit, atmospheric water potential, relative humidity and frequent wet-canopy conditions. We divided the dry season into cool dry (November-February) and hot dry (March-April) seasons and classifed days into foggy (FG) and non-foggy (non- FG) days. During the FG days of the cool dry season, the physiological activities of the rubber trees were sup- pressed where carbon assimilation and evapotranspiration showed reductions of 4% and 15%, respectively, compared to the cool dry non-FG days. Importantly, the unequal declines in carbon assimilation and evapo- transpiration led to enhanced crop water productivity (WP c ) on cool dry FG days but insignifcant WP c values were found between FG and non-FG days of the hot dry season. Our results suggest that, by regulating plant physiology, fog events during the cool dry season signifcantly reduce water demand and alleviate water stress for the trees through improved WP c . 1. Introduction The montane areas (>300 m elevation) of Southeast Asia (SEA) are rich in natural resources and considered to be one of the global biodi- versity hotspots and important regions of primary forest carbon stock (Fox et al., 2014; Blagodatsky et al., 2016). However, in recent decades, these areas have been experiencing severe biodiversity loss and envi- ronmental risks due to the conversion of natural tropical rainforests into monoculture plantations of rubber (Hevea brasiliensis) (Myers et al., 2000; Tan et al., 2011; Zakari et al., 2020). For example, in the southwest edge of China (Xishuangbanna), rubber trees were frst introduced in the late 1950s, and its land coverage reached 22% by 2010, forming a new ‘non-traditional’ environmental area (Zhu et al., 2004; Xu et al., 2014; Giambelluca et al., 2016). Such large-scale changes in land-use patterns in natural forests have led to serious environmental problems that have affected the regional and local forest carbon (Blagodatsky et al., 2016; Warren-Thomas et al., 2018) and water balances (Tan et al., 2011; Pfeifer et al., 2016). Predominantly, the rubber plantations reduce groundwater recharge due to the high rainwater runoff (Ma et al., 2019) and also introduce pollution due to * Corresponding authors. E-mail addresses: sqh@xtbg.ac.cn (Q. Song), yipingzh@xtbg.ac.cn (Y. Zhang). Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol https://doi.org/10.1016/j.jhydrol.2022.128016 Received 8 December 2021; Received in revised form 4 May 2022; Accepted 30 May 2022