Forest understorey plant responses to long-term experimental warming, light and nitrogen addition

Impact of coppicing on microclimate and understorey vegetation diversity in an ancient Mediterranean oak forest

Coppicing is one of the oldest silvicultural practices and is still widely applied to produce renewable energy from broadleaf forests. However, the consequences on microclimate and understorey vegetation are still poorly understood, especially in Mediterranean oak forests. With the ongoing changes in the climate system and global biodiversity loss, a better understanding of how the forest temperature buffering capacity and below-canopy plant community are impacted by coppicing is crucial. Here we quantify microclimate and understorey vegetation changes in adjacent ancient coppice-with-standards and high forest stands dominated by oaks in Italy, where these systems have been applied for a long time. Air and soil temperatures were recorded for 2.5 years, and nested vegetation plots were used to analyse coppicing effects on species composition, taxonomic, phylogenetic, and functional diversity. Coppicing significantly reduced the forest temperature buffering capacity. The mean of the daily maximum temperatures over the entire period was 1.45 °C higher in the coppiced sites, whereas the mean of the daily minimum temperatures was 0.62 °C lower than in the high forest. Coppicing increased understorey species richness by favouring generalist taxa, but significantly decreased the proportion of forest specialists. The understorey community in coppiced forests consisted of more warm-adapted species. Moreover, coppicing also led to a loss of phylogenetic evenness and to shifts in diversity and community weighted mean Leaf Dry Matter content, pointing to habitat filtering and acclimation processes. In sum, we show that coppicing affects microclimate and understory vegetation in a direction that can exacerbate the effects of climate change, negatively affecting the oak forest specialist flora and its phylogenetic evenness.

Multiple drivers of functional diversity in temperate forest understories: Climate, soil, and forest structure effects

In macroecology, shifting from coarse- to local-scale explanatory factors is crucial for understanding how global change impacts functional diversity (FD). Plants possess diverse traits allowing them to differentially respond across a spectrum of environmental conditions. We aim to assess how macro- to microclimate, stand-scale measured soil properties, forest structure, and management type, influence forest understorey FD at the macroecological scale. Our study covers Italian forests, using thirteen predictors categorized into climate, soil, forest structure, and management. We analyzed five traits (i.e., specific leaf area, plant size, seed mass, belowground bud bank size, and clonal lateral spread) capturing independent functional dimensions to calculate the standardized effect size of functional diversity (SES-FD) for all traits (multi-trait) and for single traits. Multiple regression models were applied to assess the effect of predictors on SES-FD. We revealed that climate, soil, and forest structure significantly drive SES-FD of specific leaf area, plant size, seed mass, and bud bank. Forest management had a limited effect. However, differences emerged between herbaceous and woody growth forms of the understorey layer, with herbaceous species mainly responding to climate and soil features, while woody species were mainly affected by forest structure. Future warmer and more seasonal climate could reduce the diversity of resource economics, plant size, and persistence strategies of the forest understorey. Soil eutrophication and acidification may impact the diversity of regeneration strategies; canopy closure affects the diversity of above- and belowground traits, with a larger effect on woody species. Multifunctional approaches are vital to disentangle the effect of global changes on functional diversity since independent functional specialization axes are modulated by different drivers.

Forest understorey flowering phenology responses to experimental warming and illumination

Species are altering their phenology to track warming temperatures. In forests, understorey plants experience tree canopy shading resulting in light and temperature conditions, which strongly deviate from open habitats. Yet, little is known about understorey phenology responses to forest microclimates. We recorded flowering onset, peak, end and duration of 10 temperate forest understorey plant species in two mesocosm experiments to understand how phenology is affected by sub-canopy warming and how this response is modulated by illumination, which is related to canopy change. Furthermore, we investigated whether phenological sensitivities can be explained by species' characteristics, such as thermal niche. We found a mean advance of flowering onset of 7.1 d per 1°C warming, more than previously reported in studies not accounting for microclimatic buffering. Warm-adapted species exhibited greater advances. Temperature sensitivity did not differ between early- and later-flowering species. Experimental illumination did not significantly affect species' phenological temperature sensitivities, but slightly delayed flowering phenology independent from warming. Our study suggests that integrating sub-canopy temperature and light availability will help us better understand future understorey phenology responses. Climate warming together with intensifying canopy disturbances will continue to drive phenological shifts and potentially disrupt understorey communities, thereby affecting forest biodiversity and functioning.

Temperate forest understory vegetation shifts after 40 years of conservation

Understanding how vegetation composition and diversity respond to global changes is crucial for effective ecosystem management and conservation. This study evaluated shifts in understory vegetation after 40 years of conservation within Drawa National Park (NW Poland), to check which plant communities changed the most, and whether vegetation shifts reflect global change symptoms (climate change and pollution) or natural forest dynamics. Using ordination and generalized mixed-effects linear models, we assessed changes in alpha diversity metrics, accounting for taxonomic, functional, and phylogenetic aspects within 170 quasi-permanent plots, surveyed in 1973-85 and resurveyed in 2015-19. We found an overall homogenization of forest vegetation and specific shift patterns in certain forest associations. In coniferous and nutrient-poor broadleaved forests, the overall number of species increased due to the replacement of functionally distinct or specialized species with more ubiquitous species that could exploit increased resource availability. In riparian forests and alder carrs we found either shifts from riparian forest to alder carrs or to mesic broadleaved forests. The most stable communities were fertile broadleaved forests. Our study quantified shifts in taxonomic, functional, and phylogenetic diversity after 40 years of conservation and provides important insights into the shifts in vegetation composition in temperate forest communities. In coniferous and nutrient-poor broadleaved forests we found an increase in species richness and replacement of functionally distinct or specialized species by ubiquitous species, indicating increased resource availability. Shifts between wet broadleaved forests and transition into mesic forests suggest water limitation, which can be related to climate change. The most stable were fertile broadleaved forests fluctuating due to natural stand dynamics. The findings highlight the need for ongoing monitoring and management of ecological systems to preserve their diversity and functionality in the face of global changes.

Negative effects of winter and spring warming on the regeneration of forest spring geophytes

The climate is changing rapidly, provoking species to shift their ranges poleward and upslope. We currently lack a mechanistic understanding of the effect of warmer temperatures on plants, especially for seasonally distinct patterns. Spring geophytes are emblematic forest plants that have a short aboveground lifecycle in the first half of the year and are thus particularly sensitive to winter and spring warming. We set up a warming experiment with separate and combined winter and spring warming on seedlings of three European spring geophytes: Anemone nemorosa, Hyacinthoides non-scripta and Ornithogalum pyrenaicum. Seedling emergence and plant height were recorded at the end of winter and spring treatment, when also biomass of the root, shoot and storage organ was determined. We found negative effects of combined winter and spring warming on seedling emergence. The weight of the storage organ proved to be the best indicator of seedling performance and was negatively affected by separate winter warming in Anemone and by spring warming in Hyacinthoides. Successful seedling emergence was jeopardized by the absence of a cold period, while seedling performance seemed to be negatively influenced directly by higher temperatures through a phenological shift. Our findings confirm that warmer winter and spring temperatures could hamper regeneration of spring geophytes.