Forests are often subject to multiple, compounded disturbances, representing both natural and human-induced processes. Predicting forest dynamics requires that we consider how these disturbances interact to affect species demography. Here we present results of an individual-based, spatially explicit forest simulator that we developed to analyze the compounded effects of hurricane disturbance and land use legacies on the dynamics of a subtropical forest. We used data from the 16-ha Luquillo Forest Dynamics Plot in Puerto Rico, together with a reconstruction of historical wind damage, to parameterize the simulator. We used the model to ask two questions. (1) What are the implications of variation in hurricane frequency and severity for the long-term dynamics of forest composition, diversity, and structure? Both storm severity and frequency had striking effects on forest dynamics, composition, and structure. The periodicity of disturbance also played an important role, with periods of high hurricane activity fostering the establishment of species that may become rare in the absence of severe storms and quiescent periods allowing these species to reach reproductive size. Species responses to hurricane disturbance could not be predicted from their life history attributes. However, species perceived to be primary forest species exhibited low temporal variation in abundance through the simulations. (2) How do hurricanes and legacies from human land use interact to determine community structure and composition? Our results suggest that, over time, regardless of the storm regime, land use legacies will become less apparent but will lead to a forest community that contains a mixture of secondary and primary forest species formerly dominant in areas of different land use. In the long term, hurricane disturbance generated two communities with slightly greater similarity than those not subject to storms. Thus, the inclusion of hurricane disturbance does not alter the prediction that land use legacies in this tropical forest will diminish over time. Our simulations also highlight the contingent effects of human legacies on subsequent community dynamics, including the response to hurricane disturbance, therefore supporting the notion that compounded disturbances can interact in ways that cannot be predicted by the study of single disturbances. The widespread importance of land use as a large-scale disturbance makes it imperative that it be addressed as a fundamental ecological process.

Abstract. Seedling recruitment and survival are critical bottlenecks in tree population dynamics and are likely to play central roles in shifts in species distributions under climate change. We use data from the Forest Inventory and Analysis program to quantify the relationships between two key climate variablesmean annual temperature and growing season water deficit-and rates of seedling recruitment and survival for the 50 most common tree species in the eastern United States. Our statistical models include the positive effects of conspecific adult abundance on recruitment and the potentially negative competitive effects of total canopy abundance on seedling survival. The tradeoff between these two effects creates a range from positive to negative conspecific density dependence, depending on the absolute and relative abundance of conspecific vs. heterospecific adults in a plot. Variation along the climate gradients mirrors patterns found previously in adult distributions. The clearest signal is in variation in the presence/absence of seedlings, while seedling density when present is only weakly related to local climate. The relatively narrow niche breadths for the presence of both seedlings and adults suggest that the frequency of occurrence of species within the landscape, rather than their relative abundance when present, will show the greatest response to climate change. Our analyses predict seedling survival as a function of mean annual temperature independent of the effects of competition and water deficit and thus provide an indication of the fundamental niche for seedling distribution along the temperature gradient. For more than half of the 50 species, their realized seedling niches are displaced to warmer climates. This reflects the prevalence of species in which survival declines with increasing temperature across a significant portion of at least their southern range. Our results show that when the effects of warmer climates are taken into account, seedling survival generally increases with increasing water deficit in the generally humid climates of the eastern United States. This result is consistent with recent global surveys of the relationship between net primary productivity and forest turnover rates, but contrasts with recent studies highlighting the potential impact of drought stress on tree mortality in more arid climates.

Citation: Canham, C. D., and L. Murphy. 2016. The demography of tree species response to climate: sapling and canopy tree growth. Ecosphere 7(10):e01474. 10. 1002/ecs2.1474 Abstract. Despite the clear need to predict the effects of climate change on the distribution and abundance of temperate tree species, there is still only a rudimentary understanding of how climate influences key demographic processes that determine the current distribution and abundance of tree species. We use data from the U.S. Forest Service Forest Inventory and Analysis (FIA) program to quantify the relationships between two key climate variables-mean annual temperature and effective growing season precipitation-and rates of sapling and canopy tree growth for the 50 most common tree species in the eastern United States. Our models include the effects of tree size, competition, and anthropogenic nitrogen (N) deposition, both to avoid confounding effects and to provide context for the importance of variation in climate relative to other factors known to influence tree growth. The 50 species show a broad range of relationships between size and growth, in contrast to predictions of metabolic theory. The 50 species differ widely in shade tolerance, and both saplings and canopy trees show a wide range of competitive responses to total stand basal area. The competitive responses of canopy trees were more sensitive than were saplings to the size of an individual relative to the median size of trees in the stand. As has been shown in other studies with FIA data, species responses to N deposition also varied widely and were related to the type of mycorrhizal association of the tree species. Relationships between the two climate variables and tree growth were surprisingly modest, and bore little obvious relationship to the distributions of the species along climate gradients. For over a quarter of the species, there was no statistical support for a relationship between 5-yr average growing season precipitation and 5-yr average growth, and for most of the remaining species, the relationship was effectively flat over a wide range of precipitation. Responses to regional variation in mean annual temperature were stronger, but again showed little obvious correlation with the distribution of abundance of most species along the temperature gradient.

Tropical forests play a critical role in carbon and water cycles at a global scale. Rapid climate change is anticipated in tropical regions over the coming decades and, under a warmer and drier climate, tropical forests are likely to be net sources of carbon rather than sinks. However, our understanding of tropical forest response and feedback to climate change is very limited. Efforts to model climate change impacts on carbon fluxes in tropical forests have not reached a consensus. Here, we use the Ecosystem Demography model (ED2) to predict carbon fluxes of a Puerto Rican tropical forest under realistic climate change scenarios. We parameterized ED2 with species-specific tree physiological data using the Predictive Ecosystem Analyzer workflow and projected the fate of this ecosystem under five future climate scenarios. The model successfully captured interannual variability in the dynamics of this tropical forest. Model predictions closely followed observed values across a wide range of metrics including aboveground biomass, tree diameter growth, tree size class distributions, and leaf area index. Under a future warming and drying climate scenario, the model predicted reductions in carbon storage and tree growth, together with large shifts in forest community composition and structure. Such rapid changes in climate led the forest to transition from a sink to a source of carbon. Growth respiration and root allocation parameters were responsible for the highest fraction of predictive uncertainty in modeled biomass, highlighting the need to target these processes in future data collection. Our study is the first effort to rely on Bayesian model calibration and synthesis to elucidate the key physiological parameters that drive uncertainty in tropical forests responses to climatic change. We propose a new path forward for model-data synthesis that can substantially reduce uncertainty in our ability to model tropical forest responses to future climate.