The history of widespread decrease in oak dominance exemplified in a grassland-forest landscape

Climate-limited vegetation change in the conterminous United States of America

The effects of climate change on vegetation composition and distribution are evident in different ecosystems around the world. Although some climate-derived alterations on vegetation are expected to result in changes in lifeform fractional cover, disentangling the direct effects of climate change from different non-climate factors, such as land-use change, is challenging. By applying "Liebig's law of the minimum" in a geospatial context, we determined the climate-limited potential for tree, shrub, herbaceous, and non-vegetation fractional cover change for the conterminous United States and compared these potential rates to observed change rates for the period 1986 to 2018. We found that 10% of the land area of the conterminous United States appears to have climate limitations on the change in fractional cover, with a high proportion of these sites located in arid and semiarid ecosystems in the Southwest part of the country. The rates of change in lifeform fractional cover for the remaining area of the country are likely limited by non-climate factors such as the disturbance regime, land management, land-use history, soil conditions, and species interactions and adaptations.

Mesophication of Oak Landscapes: Evidence, Knowledge Gaps, and Future Research

Pyrophytic oak landscapes across the central and eastern United States are losing dominance as shade-tolerant, fire-sensitive, or opportunistic tree species encroach into these ecosystems in the absence of periodic, low-intensity surface fires. Mesophication, a hypothesized process initiated by intentional fire exclusion by which these encroaching species progressively create conditions favorable for their own persistence at the expense of pyrophytic species, is commonly cited as causing this structural and compositional transition. However, many questions remain regarding mesophication and its role in declining oak dominance. In the present article, we review support and key knowledge gaps for the mesophication hypothesis. We then pose avenues for future research that consider which tree species and tree traits create self-perpetuating conditions and under what conditions tree-level processes might affect forest flammability at broader scales. Our goal is to promote research that can better inform restoration and conservation of oak ecosystems experiencing structural and compositional shifts across the region.

Is climate an important driver of post-European vegetation change in the Eastern United States?

Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles. To help decipher their relative importance during Euro-American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance, and pyrogenicity classes and applied to comparative tree-census data. Our megadata analysis of 190 datasets determined the relative impacts of climate vs. altered disturbance regimes for various biomes across the eastern United States. As the Euro-American period (ca. 1500 to today) spans two major climatic periods, from Little Ice Age to the Anthropocene, vegetation changes consistent with warming were expected. In most cases, however, European disturbance overrode regional climate, but in a manner that varied across the Tension Zone Line. To the north, intensive and expansive early European disturbance resulted in the ubiquitous loss of conifers and large increases of Acer, Populus, and Quercus in northern hardwoods, whereas to the south, these disturbances perpetuated the dominance of Quercus in central hardwoods. Acer increases and associated mesophication in Quercus-Pinus systems were delayed until mid 20th century fire suppression. This led to significant warm to cool shifts in temperature class where cool-adapted Acer saccharum increased and temperature neutral changes where warm-adapted Acer rubrum increased. In both cases, these shifts were attributed to fire suppression rather than climate change. Because mesophication is ongoing, eastern US forests formed during the catastrophic disturbance era followed by fire suppression will remain in climate disequilibrium into the foreseeable future. Overall, the results of our study suggest that altered disturbance regimes rather than climate had the greatest influence on vegetation composition and dynamics in the eastern United States over multiple centuries. Land-use change often trumped or negated the impacts of warming climate, and needs greater recognition in climate change discussions, scenarios, and model interpretations.