Long‐term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

• Published in: Global change biology Vol. 26; no. 3; pp. 1778 - 1794
• Main Authors: Panthi, Shankar, Fan, Ze‐Xin, Sleen, Peter, Zuidema, Pieter A.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.03.2020
• Subjects: Abies; Abies spectabilis; Carbon Dioxide; Carbon isotopes; central Himalaya; Climate Change; Climate effects; Climate models; Dendrochronology; Drying; elevation gradients; Environmental changes; Environmental effects; Evergreen trees; Forests; Growth; high‐elevation forests; Himalayan fir (Abies spectabilis); intrinsic water‐use efficiency (iWUE); Isotopes; long‐term growth trends; Moisture; Physiological responses; Physiology; Regions; Sensitivity analysis; Temperature effects; tree rings; Trees; Trends; Vapor pressure; Vapour pressure; central Himalaya; long-term growth trends; elevation gradients; intrinsic water-use efficiency (iWUE); Himalayan fir (Abies spectabilis); high-elevation forests; tree rings; climate change
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.14910

High‐elevation forests are experiencing high rates of warming, in combination with CO2 rise and (sometimes) drying trends. In these montane systems, the effects of environmental changes on tree growth are also modified by elevation itself, thus complicating our ability to predict effects of future climate change. Tree‐ring analysis along an elevation gradient allows quantifying effects of gradual and annual environmental changes. Here, we study long‐term physiological (ratio of internal to ambient CO2, i.e., Ci/Ca and intrinsic water‐use efficiency, iWUE) and growth responses (tree‐ring width) of Himalayan fir (Abies spectabilis) trees in response to warming, drying, and CO2 rise. Our study was conducted along elevational gradients in a dry and a wet region in the central Himalaya. We combined dendrochronology and stable carbon isotopes (δ13C) to quantify long‐term trends in Ci/Ca ratio and iWUE (δ13C‐derived), growth (mixed‐effects models), and evaluate climate sensitivity (correlations). We found that iWUE increased over time at all elevations, with stronger increase in the dry region. Climate–growth relations showed growth‐limiting effects of spring moisture (dry region) and summer temperature (wet region), and negative effects of temperature (dry region). We found negative growth trends at lower elevations (dry and wet regions), suggesting that continental‐scale warming and regional drying reduced tree growth. This interpretation is supported by δ13C‐derived long‐term physiological responses, which are consistent with responses to reduced moisture and increased vapor pressure deficit. At high elevations (wet region), we found positive growth trends, suggesting that warming has favored tree growth in regions where temperature most strongly limits growth. At lower elevations (dry and wet regions), the positive effects of CO2 rise did not mitigate the negative effects of warming and drying on tree growth. Our results raise concerns on the productivity of Himalayan fir forests at low and middle (<3,300 m) elevations as climate change progresses. Himalayan forests are experiencing exceptionally rapid warming during the last decades, yet knowledge on their physiological and growth responses to climate change is limited. Using dendrochronology and tree‐ring stable carbon‐isotope data of Himalayan fir, we found intrinsic water‐use efficiency has increased consistently at all elevations in both dry and wet regions during the past century. Warming has favored tree growth at higher elevations in wet regions, but growth reductions at low/middle elevations in both regions suggest that negative effects of warming and drying overruled positive CO2‐fertilization effects, thus raising concerns on the productivity of Himalayan fir forests under future climate change.