Comparison of morphological and physiological response to drought stress among temperate forest understory forbs and graminoids

Abstract

Drought stress can profoundly affect plant growth and physiological vitality, yet there is a notable scarcity of controlled drought experiments focused on herbaceous species of the forest understory. In this study, we collected seeds from five forb and four graminoid species growing in the temperate forest understory of the Czech Republic. These seeds were germinated under controlled glasshouse conditions and subjected to moderate drought stress for five weeks. We assessed biomass partitioning, stomatal and leaf morphology, leaf gas exchange, minimum leaf conductance (gmin), and chlorophyll fluorescence parameters. The comparison of two ecological guilds revealed that graminoids exhibited a higher root-to-shoot ratio, improved water-use efficiency, greater carboxylation efficiency, and enhanced non-photochemical quenching under drought conditions compared to forbs. In contrast, forbs had significantly lower gmin, along with higher total biomass and total leaf area. Despite these differences in morpho-physiological functional traits, both groups experienced a similar relative reduction in biomass during drought stress. Key predictors of biomass accumulation under drought included photochemical quenching, stomatal traits, total leaf area and gmin. A negative correlation between biomass and gmin suggests that plants with lower residual water losses after stomatal closure can accumulate more biomass under drought stress. Additionally, gmin was positively correlated with guard cell length, suggesting that larger stomata contribute to higher residual water loss. Graminoids exhibited morpho-physiological modifications that enhanced drought resistance, indicating a greater emphasis on stress tolerance as a survival strategy. In contrast, forbs maintained higher biomass and total leaf area, reflecting a competitive strategy for maximizing resource acquisition.