Although stand age has been shown to strongly affect forest ecosystem processes, little is known about the role that forest stand age plays in tree radial growth processes. The knowledge on geographical patterns of the stand age effect on radial growth along climatic gradients is also scarce. Based on dendrological methods and tree-ring cores from 2276 trees at 14 sampling plots, we confirmed that differences in stand age could result in radial growth dissimilarities of Qinghai spruce (Picea crassifolia Kom.), a moisture-sensitive forest tree species endemic to the Tibetan Plateau. However, the effect that stand age has on the radial growth of Qinghai spruce is not consistent. There is evident dissimilarity in the radial growth dynamics of Qinghai spruce from different-aged forest stands in semi-humid regions, but this dissimilarity is minimal in regions under higher drought limitation. Additionally, we observed significant negative correlations in temporal changes of growth concordance of Qinghai spruce from different-aged stands and regional moisture conditions at each study site. It can therefore be concluded that Qinghai spruce will exhibit greater stand age related growth dissimilarities under lower drought limitation. Findings from this study can improve our understanding of biogeographical patterns of moisture-sensitive tree growth that will be necessary to improve future projections of forest dynamics and to guide forest management under a changing climate.

Understanding the spatial-temporal tree growth variability and its associative climatic response is fundamental in the assessment of forest vulnerability and the appraisal of forest risk under climate change. Based on 4219 tree-ring cores from 23 sample plots, this study demonstrates divergent radial growth dynamics of Qinghai spruce (Picea crassifolia Kom.) across a moisture gradient. On the one hand, contrary to the warming-induced growth enhancement of trees in cold regions reported in previous studies, our study find that Qinghai spruce growth has persistently declined under recent warming trends in multiple cold and moist sites that are representative of the Tibetan Plateau, China. On the other hand, Qinghai spruce growth exhibited W-shaped dynamic trajectories that were similar to changes in regional precipitation at all warmer and drier sites investigated in this study. Along with differences in regional evaporative change, differences in the growth-climate response can explain divergences in Qinghai spruce growth observed throughout various hydrological niches. Drought is believed to be the primary limiting factor of Qinghai spruce growth in all regions, resulting in a lower radial growth rate in warmer and drier regions. However, Qinghai spruce growth exhibited a higher dependency on the negatively-affected temperature in cold and moist regions, while the growth dependency respective to the positively-affected precipitation was higher than the temperature in warmer and drier regions. Results from this study are intended to add to the growing knowledgebase of forest response under persistent climate change.

Drought-related forest growth declines are observed globally in main forest types, especially with repeatedly hot droughts. Therefore, quantifying forest resilience and identifying the factors driving resilience in response to extreme drought with the consideration of atmospheric CO2 fertilization is crucial for the accurate assessment of forest dynamics under current climate change, particularly for the widespread and climate-sensitive spruce forests in the arid Tianshan Mountains, China. Here, we explored the growth response of Schrenk spruce (Picea schrenkiana) to six extreme drought events since 1900, and investigated how tree resilience in pure stands is related to local drought intensity, cambial age (CA), and intrinsic water-use efficiency (iWUE). Specifically, we found that spruce trees had a mean resistance (Rt) value of less than 1, with iWUE contributing less to Rt variation. The results are in agreement with the drought-induced limitations on tree growth in response to increasing CO2, in spite of rising iWUE trends. However, increased iWUE has significant and positive impacts on the recovery (17%) and resilience (15%) of trees, suggesting that increased iWUE enhances the restoration of Schrenk spruce growth after extreme drought events. The growth resilience indices of Schrenk spruce showed that juvenile and adult trees exhibit different strategies to mitigate the drought influences. This study indicated that tree age, climate conditions, and variation in iWUE should be considered simultaneously in drought resilience evaluations to assess forest dynamics objectively in relation to climate change (i.e., drought) and propose appropriate forest management strategies.

Larch-dominant communities are the most extensive high-latitude forests in Eurasia and are experiencing the strongest impacts from warming temperatures. We analyzed larch (Larix dahurica Turcz) growth index (GI) response to climate change. The studied larch-dominant communities are located within the permafrost zone of Northern Siberia at the northern tree limit (ca. N 67A degrees 38', E 99A degrees 07'). Methods included dendrochronology, analysis of climate variables, root zone moisture content, and satellite-derived gross (GPP) and net (NPP) primary productivity. It was found that larch response to warming included a period of increased annual growth increment (GI) (from the 1970s to ca. 1995) with a follow on GI decline. Increase in GI correlated with summer air temperature, whereas an observed decrease in GI was caused by water stress (vapor pressure deficit and drought increase). Water stress impact on larch growth in permafrost was not observed before the onset of warming (ca. 1970). Water limitation was also indicated by GI dependence on soil moisture stored during the previous year. Water stress was especially pronounced for stands growing on rocky soils with low water-holding capacity. GPP of larch communities showed an increasing trend, whereas NPP stagnated. A similar pattern of GI response to climate warming has also been observed for Larix sibirica Ledeb, Pinus sibirica Du Tour, and Abies sibirica Ledeb in the forests of southern Siberia. Thus, warming in northern Siberia permafrost zone resulted in an initial increase in larch growth from the 1970s to the mid-1990s. After that time, larch growth increment has decreased. Since ca. 1990, water stress at the beginning of the vegetative period became, along with air temperature, a main factor affecting larch growth within the permafrost zone.

The boreal forest accounts for approximately 22% of the Northern Hemisphere landmass with nearly 40% of this huge biome growing on continuously frozen soils. Projected climate change leading to degradation of permafrost and increasing drought situation at high latitudes in Eurasia will seriously affect productivity of forests on permafrost. Here we present the results of an on-going research of tree radial growth in the midst of the permafrost zone in Siberia, Russia (Tura region, 64 degrees N, 100 degrees E, 140-610 m a.s.1.). Tree-ring width and density chronologies of Gmelin larch and Siberian spruce from a great variety of sites characterized by different thermo-hydrological regime of soils are analyzed. The obtained results reveal that current tree radial growth and tree-ring structure in permafrost region in Siberia are largely dependent on local site conditions and may be constrained by low air and soil temperatures as well as soil water availability. Varying climatic responses and seasonal radial growth of trees at different habitats indicate a range of possible scenarios of further development of northern larch stands. Forest fire is another important factor strongly affecting tree stand dynamics and forest ecosystem functioning in the continuous permafrost zone. Analysis of tree-ring parameters indicate that post-fire dynamics of tree-ring structure is in accordance with the changes in habitat conditions caused by removal by fire and then gradual recovery of ground vegetation resulting in an alteration in soil active layer depth. In general, the results of this multi-proxy analysis for trees growing under various conditions in the continuous permafrost zone in Siberia allow assumptions about changes in tree productivity, stand dynamics and therefore carbon uptake under projected climate change and permafrost degradation.