Forest responses to simulated elevated CO2 under alternate hypotheses of size- and age-dependent mortality

Tree Lifespans in a Warming World: Unravelling the Universal Trade-Off Between Growth and Lifespan in Temperate Forests

Tree growth and lifespan are key determinants of forest dynamics, and ultimately control carbon stocks. Warming and increasing CO2 have been observed to increase growth but such increases may not result in large net biomass gains due to trade-offs between growth and lifespan. A deeper understanding of the nature of the trade-off and its potential spatial variation is crucial to improve predictions of the future carbon sink. This study aims to identify key drivers of growth and lifespan, assess the universality of tree growth-lifespan trade-offs, explore the possible latitudinal patterns of trade-off strengths and their determinants, and project growth and lifespan under future climate scenarios. We analyzed 21,193 trees of 69 species (48 included in further analysis) at 445 sites (417 included in further analysis) in temperate forests in northeastern China to estimate early growth rate and tree lifespan. We find that temperature and human pressure enhance tree growth and reduce lifespan, while altitude increases lifespan. We further find evidence for growth-lifespan trade-offs at all studied levels, that is, among trees, among species and communities, and within species and communities. Trade-offs are stronger at colder, higher latitudes compared to warmer sites, because of larger variation in tree growth and climate, larger range sizes for individual species, and lower species' diversity for communities at high latitudes. We predict future increases in growth and reductions in tree lifespan in response to climate change for the 2050s. Taking growth lifespan trade-offs into account resulted in even larger predictions of decreases in tree lifespan of up to 8%. In conclusion, growth-lifespan trade-offs are universal, but the strengths may vary by environment and between different forests. Its effects are important to include in predictions of forest responses to global change and need to be considered more widely.

Demographic but not competitive time lags can transiently amplify climate-induced changes in vegetation carbon storage

How terrestrial ecosystems will accumulate carbon as the climate continues to change is a major source of uncertainty in projections of future climate. Under growth-stimulating environmental change, time lags inherent in population and community dynamic processes have been posed to dampen, or alternatively amplify, short-term carbon gain in terrestrial vegetation, but these outcomes can be difficult to predict. To theoretically frame this problem, we developed a simple model of vegetation dynamics that identifies the stage-structured demographic and competitive processes that could govern the timescales of carbon storage and loss. We show that demographic lags associated with growth-stimulating environmental change can allow a rapid increase in population-level carbon storage that is lost back to the atmosphere in later years. However, this transient carbon storage only emerges when environmental change increases the transition of adult individuals into a larger size class that suffers markedly higher mortality. Otherwise, demographic lags simply slow carbon accumulation. Counterintuitively, an analogous tradeoff between maximum adult size and survivorship in two-species models, coupled with environmental change-driven replacement, does not generate the transient carbon gain seen in the single-species models. Instead lags in competitive replacement slow the approach to the eventual carbon trajectory. Together, our results suggest that time lags inherent in demographic and compositional turnover tend to slow carbon accumulation in systems responding to growth-stimulating environmental change. Only under specific conditions will lagged demographic processes in such systems drive transient carbon accumulation, conditions that investigators can examine in nature to help project future carbon trajectories.

Time Effects of Global Change on Forest Productivity in China from 2001 to 2017

With global warming, the concentrations of fine particulate matter (PM_2.5) and greenhouse gases, such as CO₂, are increasing. However, it is still unknown whether these increases will affect vegetation productivity. Exploring the impacts of global warming on net primary productivity (NPP) will help us understand how ecosystem function responds to climate change in China. Using the Carnegie-Ames-Stanford Approach (CASA) ecosystem model based on remote sensing, we investigated the spatiotemporal changes in NPP across 1137 sites in China from 2001 to 2017. Our results revealed that: (1) Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) were significantly positively correlated with NPP (p < 0.01), while PM_2.5 concentration and CO₂ emissions were significantly negatively correlated with NPP (p < 0.01). (2) The positive correlation between temperature, rainfall and NPP gradually weakened over time, while the negative correlation between PM_2.5 concentration, CO₂ emissions and NPP gradually strengthened over time. (3) High levels of PM_2.5 concentration and CO₂ emissions had negative effects on NPP, while high levels of MAT and MAP had positive effects on NPP.

Population age structures, persistence and flowering cues in Cerberiopsis candelabra (Apocynaceae), a long-lived monocarpic rain-forest tree in New Caledonia

Cerberiopsis candelabra Vieill. is a long-lived, monocarpic (= semelparous) and mass-flowering rain-forest tree, endemic to New Caledonia. Population size structures suggest establishment has been episodic, followed by a recruitment gap that might signal population decline. Here, we use age structures based on tree rings to better assess population dynamics and persistence, and investigate influences of tree size, age and growth rate on flowering. Age structures of populations surveyed in 2007–2008 were unimodal, with establishment over c. 15–81 y, followed by a recruitment gap of c. 23–79 y. Seedling mortality was generally high. High densities of flowering trees or large-scale exogenous disturbances may be necessary for in-situ regeneration. There was no evidence of a simple flowering threshold: flowering in 2017 occurred across a wide range of tree size, age and growth rate. Instead, evidence suggested that size and age at flowering may vary among plants depending on their growth trajectory. Environmental triggers of flowering were not identified by dating tree establishment, but the last three mass-flowering events occurred in years of tropical cyclones. Regeneration and persistence might be facilitated if large-scale disturbances trigger flowering, improving reproductive efficiency by synchronising flowering and linking reproduction with environmental conditions that enhance seedling recruitment.

Forest carbon sink neutralized by pervasive growth-lifespan trade-offs

Land vegetation is currently taking up large amounts of atmospheric CO₂, possibly due to tree growth stimulation. Extant models predict that this growth stimulation will continue to cause a net carbon uptake this century. However, there are indications that increased growth rates may shorten trees' lifespan and thus recent increases in forest carbon stocks may be transient due to lagged increases in mortality. Here we show that growth-lifespan trade-offs are indeed near universal, occurring across almost all species and climates. This trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment. Thus, current tree growth stimulation will, inevitably, result in a lagged increase in canopy tree mortality, as is indeed widely observed, and eventually neutralise carbon gains due to growth stimulation. Results from a strongly data-based forest simulator confirm these expectations. Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.