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

Abstract

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.

Effects of climate change on treeline trees in Sagarmatha (Mt. Everest, Central Himalaya)

“…The long‐term trends assessed for δ 13 C and its derived traits (Δ 13 C and iWUE) indicated that over the last 150 years a progressive increase in water‐use efficiency (WUE) has characterized the cost–benefit relationship of C assimilation by photosynthesis vs. water loss by transpiration in both species, consistent with a previous reports for other Himalayan timberline tree species (Huang et al. , 2017; Panthi, Fan, and van der Sleen, 2020). Mechanistically, plants can increase their WUE substantially in two different ways.…”

Growth-Climate Relationships and Long-Term Growth Trends of the Tropical Forest Tree Choerospondias axillaris (Anacardiaceae) in East-Central Thailand

“…The most significant evidence for climate change is rising temperatures and the increasing frequency of extreme events, such as extreme drought, violent storms, and severe flooding [1]. Climate change and related increases in climate extremes have affected tree growth and forest productivity globally [2,3], including in tropical regions [4,5], the Himalayas [6,7] and Europe [8,9]. Tree-ring analysis can provide important information on climate change [10].…”

Tree-Ring Isotopes Provide Clues for Sink Limitation on Treeline Formation on the Tibetan Plateau

“…Tree-ring δ 13 C is expected to be strongly affected by photosynthetic rates, which are governed both by temperature and by other factors at the upper treeline [35,47,48]. Following the theoretical principles of the dual-isotope model [28], we found that photosynthesis is greater at treeline than at low-elevation forests.…”