Recent reduction in the frequency of frost accounts for most of the increased growth of a high elevation spruce forest in northwestern China

Abstract

Key message Using process-based models in combination with dendrochronological measurements provides a way to explain recent increased tree growth in northwestern China. Abstract Dendrochronological studies of tree rings in a 250-year-old Qinghai spruce (Picea crassifolia) forest in the Qilian Mountains of northwestern China indicate a 60 % sustained increase in tree-ring growth between 1980 and 2009 compared with any time since 1785. Over the same period, the maximum, minimum, and average temperatures all increased by nearly 2°C during the growing season (May through September), the frequency of frost decreased 18 days, precipitation remained unchanged, while atmospheric concentrations of CO 2 increased by 48 ppm. To explain how the changes in climatic variables might cause the increase in tree growth, we parameterized a process-based growth model (3-PG, physiological processes predicting growth) with values from the literature and performed a series of sensitivity tests. The results of our analysis indicated that a reduction in frost frequency during the growing season, which allows stomata to remain open, enhanced gross photosynthesis by 42 %. Up to a 20 % increase in P G could be attributed to rising atmospheric CO 2 between 1980 and 2009, with half of this attributed to increased light interception from a simulated 0.4 increase in canopy leaf area index. The increase in average and maximum temperatures had little direct effect on gross photosynthesis with the optimum temperature set between 9 and 10°C. Indirectly, the increase in monthly average minimum temperature during the growing season, although small, crossed a threshold that reduced the impact of frost. Our analyses show the value of combining dendrochronological measurements with a process-based model to gain a more holistic understanding of how environmental factors interact to affect tree growth.

CO2 Fertilization Confounds Tree‐Ring Records of Regional Hydroclimate at Northeastern Qinghai‐Tibetan Plateau

“…The correlations of either raw or detrended TRW versus leaf internal CO 2 are all significant but not particularly strong ( R 2 averaged 0.31), consistent with the fact that while rising CO 2 does stimulate growth in 20% to 50% of forests worldwide (Gedalof & Berg, ; Peñuelas et al, ), including in arid central Asia (Waring & Gao, ), this stimulation can be partially or totally negated by unfavorable hydroclimate, particularly dry or warm conditions (Linares & Camarero, ; van der Sleen et al, ; Wang et al, ). Thus, one measure of forest vulnerability to climate warming and drought may be the degree to which CO 2 stimulation is observed in growth.…”

Investigating old‐growth ponderosa pine physiology using tree‐rings, δ13C, δ18O, and a process‐based model

“…This sub‐optimal predictive ability of the model is also likely due to the relatively long period (1895–2002, 107 yr) over which we applied the model compared to previous studies. Waring and Gao () compared the tree‐ring index of Picea crassifolia to normalized 3‐PG‐predicted diameter growth for 52 yr. Wei et al. () compared observed and predicted DBH of Abies grandis for 50 yr with sub‐optimal success.…”

“…, , Law et al. , Waring and Gao ). 3‐PG uses a simple light‐use efficiency relationship to estimate carbon assimilation (i.e., gross primary productivity, GPP) and the original model version assumes a constant fraction of GPP (0.47) is allocated to net primary productivity (NPP), which is then partitioned into below‐ and aboveground biomass.…”

“…A widely used stand-level process model is Physiological Principles in Predicting Growth (3-PG), developed by Landsberg and Waring (1997) and since modified by numerous other investigators (Xenakis et al 2008, Gonzalez-Benecke et al 2014, Wei et al 2014a, Almeida and Sands 2016, Forrester and Tang 2016, Meyer et al 2018. The 3-PG model utilizes environmental conditions, stand characteristics, and species-specific physiological and allometric measurements to accurately predict growth and productivity in changing environmental conditions and on diverse forested stands (Coops et al 1998, 2007, Waring and Gao 2016. 3-PG uses a simple light-use efficiency relationship to estimate carbon assimilation (i.e., gross primary productivity, GPP) and the original model version assumes a constant fraction of GPP (0.47) is allocated to net primary productivity (NPP), which is then partitioned into below-and aboveground biomass.…”

Divergent Growth Responses to Warming between Stand-Grown and Open-Grown Trees in a Dryland Montane Forest in Northwestern China

“…However, the warming effects on growth of stand-grown trees changed from positive to negative with decreasing elevation (Figures 3 and 6). This is inconsistent with the direct effect of increasing temperature on tree growth, because temperature increase before this at the optimum temperature (around 15-20 • C) for net photosynthesis should enhance tree growth [38,39]. However.…”