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

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 (δ13 C) to quantify long-term trends in Ci /Ca ratio and iWUE (δ13 C-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 δ13 C-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.

Contrasting stomatal sensitivity to temperature and soil drought in mature alpine conifers.

Conifers growing at high elevations need to optimize their stomatal conductance (gs ) for maximizing photosynthetic yield while minimizing water loss under less favourable thermal conditions. Yet the ability of high-elevation conifers to adjust their gs sensitivity to environmental drivers remains largely unexplored. We used 4 years of sap flow measurements to elucidate intraspecific and interspecific variability of gs in Larix decidua Mill. and Picea abies (L.) Karst along an elevational gradient and contrasting soil moisture conditions. Site- and species-specific gs response to main environmental drivers were examined, including vapour pressure deficit, air temperature, solar irradiance, and soil water potential. Our results indicate that maximum gs of L. decidua is >2 times higher, shows a more plastic response to temperature, and down-regulates gs stronger during atmospheric drought compared to P. abies. These differences allow L. decidua to exert more efficient water use, adjust to site-specific thermal conditions, and reduce water loss during drought episodes. The stronger plasticity of gs sensitivity to temperature and higher conductance of L. decidua compared to P. abies provide new insights into species-specific water use strategies, which affect species' performance and should be considered when predicting terrestrial water dynamics under future climatic change.

Nighttime warming alleviates the incidence of juniper forest growth decline on the Tibetan Plateau.

Recent warming over the Tibetan Plateau (TP) is approximately twice the global-mean surface temperature increase and poses a threat to the healthy growth of forests. Although many studies have focused on whether recent climate warming has caused forest growth decline on the TP, it remains unclear how asymmetric warming, that is faster increasing nighttime temperature than daytime, impacts forest growth decline. We explored this question by using a ring-width index series from 1489 juniper trees (Juniperus prezwalskii and J. tibetica) at 50 sites on the TP. We calculated the percentage of trees with growth decline (PTD) to reconstruct historical forest growth decline and employed a piecewise structural equation meta-model (pSEM) and linear mixed model (LMM) to explore influencing factors. We found that the PTD has decreased since the late 19th century, with an abrupt decreasing trend since the 1980s. Results of the pSEM show that winter minimum temperature has a stronger indirect negative effect on the variation in PTD (ß = -0.24, p < 0.05) compared to that of the weak indirect positive effect of summer maximum temperature (ß = 0.16, p < 0.05). The results of LMM show that the variation in PTD is directly negatively (p < 0.001) affected by both winter minimum temperature and summer total precipitation, but the former has a greater independent contribution than the latter (with 17.7% vs 2.5% of variances independently explained, respectively). These results suggest that increased winter minimum temperature substantially mitigates the growth decline in juniper forests on the TP. As the minimum temperature generally occurs at night, we conclude that the asymmetric increase in nighttime temperature has decreased the incidence of juniper forest growth decline on the TP under climate warming. This alleviating effect of nighttime warming is likely due to reduced low-temperature constraints and reduced damage to tree growth.