Seedling Growth and Biomass Production under Different Light Availability Levels and Competition Types

Light availability is a crucial resource determining seedling survival, establishment, and growth. Competition for light is asymmetric, giving the taller individuals a competitive advantage for obtaining light resources. Species-specific traits, e.g., shade tolerance, rooting depth, and leaf morphology, determine their strategical growth response under limited resource availability and different competitive interactions. We established a controlled pot experiment using European beech, Norway spruce, and Douglas fir seedlings and applying three different light availability levels—10%, 20%, and 50%. The experiment’s main aim was to better understand the effects of light availability and competition type on the growth, growth allocation, and biomass production of recently planted seedlings. We planted four seedlings per pot in either monocultures or mixtures of two species. Relative height and diameter growth and aboveground woody biomass of seedlings increased with increasing light availability. All seedlings allocated more growth to height than diameter with decreasing light availability. Seedlings that reached on average greater height in the previous year allocated less growth to height in the following year. Additionally, there were general differences in growth allocation to the height between gymnosperms and angiosperms, but we did not find an effect of the competitor’s identity. Our mixture effect analysis trends suggested that mixtures of functionally dissimilar species are more likely to produce higher biomass than mixtures of more similar species such as the two studied conifers. This finding points towards increased productivity through complementarity.

Nitrogen Fertilization Increases Windstorm Damage in an Aggrading Forest

Storms are the most significant disturbance events in temperate forests. Forests impacted by nitrogen deposition may face more severe storm damage as changes in soil and wood chemistry impact tree growth allocation, wood strength, and species composition. To examine these potential effects of nitrogen deposition, we measured tree damage from a windstorm in an aggrading forest that is part of a nitrogen fertilization experiment. We discovered that within the nitrogen fertilization treatment area there was significantly more basal area and stems damaged when compared to the reference treatment, and the nitrogen fertilization treatment had more snapped and severely damaged trees. Additionally, the effect of treatment and amount of damage to trees was different depending on tree species. If our results are indicative of the large and globally-distributed regions of temperate forests impacted by nitrogen deposition, then the increased windstorm disturbance risk posed by climate change could be more significant due to the effects of nitrogen deposition.

Congeneric invasive versus native plants utilize similar inorganic nitrogen forms but have disparate use efficiencies

Aims Soil inorganic nitrogen (N) has long been recognized to play an important role in plant invasions. Whilst comparing the N use strategies of multiple invasive versus native plant congeners along an entire N gradient is key to understanding plant invasion success, there are few related studies. Methods We conducted a potted experiment with six invasive and native congeneric pairs, which were subjected to 11 nitrate/ammonium (NO3 -/NH4 +) ratios (i.e. 100% NO3 - at one end and 100% NH4 + at the other end), each with low and high N levels. Each species-N combination was replicated eight times, and thus there were 2112 pots in total. We measured the following traits: the total biomass, growth advantage, biomass allocation, leaf chlorophyll content, and low-N tolerance. Important Findings Invasive and native congeners grew well at any NO3 -/NH4 + ratios, and their responses of growth, allocation, and tolerance were approximately parallel along the 11 NO3 -/NH4 + ratios across two N levels. Plant invaders grew larger and had greater chlorophyll contents, higher root biomass allocation, and stronger low-N tolerance than their congeneric natives. These findings suggest that invasive and native plant congeners may utilize similar inorganic N forms (i.e. NO3 - and NH4 +) across an entire N composition gradient and that higher N use efficiencies could favor alien plants to invade new plant communities where congeneric natives are dominants.

Females and hermaphrodites of the gynodioecious Geranium maculatum respond similarly to soil nutrient availability

Sexual dimorphism in plant growth and/or reproductive responses to the surrounding environment has been documented in some plant species. In gynodioecious plants, it is especially important to understand whether females and hermaphrodites differ in their response to environmental stressors, as the fitness of females relative to hermaphrodites determines the extent to which these separate sexes are maintained in natural populations. Soil nutrient availability is of particular importance given the different nutrient requirements of male and female sexual functions in plants. Here, we evaluated and compared the growth of females and hermaphrodites of Geranium maculatum in response to varying levels of nutrients. Using a greenhouse experiment, we manipulated the overall nutrient, nitrogen, and phosphorus levels in the soil and measured growth, allocation, and leaf quality responses in both females and hermaphrodites. We found that sexes responded similarly in their growth and allocation responses to nutrient availability, albeit evidence that female leaf chlorophyll content may have increased more than that of hermaphrodites across soil nitrogen levels. Our findings demonstrate that hermaphrodites differ from females in terms of their physiological response to varying nutrient levels, however these physiological differences did not translate into meaningful growth or reproduction differences.

Impact of grazing intensities on reproduction patterns of elm trees (Ulmus pumila) in degraded sandy lands in China

The effect of grazing on patterns of reproduction in trees has been little reported. We explored the effects of grazing intensities on reproductive growth, allocation patterns, and duration in elm trees (Ulmus pumila L.) at the Horqin Sandy Land, a degraded area in northern China. Current-year shoots were selected from branches and harvested from individual elm trees subjected to one of four grazing intensities (heavy, moderate, light, and no grazing). Shoots, flower buds, flowers, seeds, leaf buds, and leaves were collected, dried, and weighed. Results showed that the biomass in heavy, moderate and light grazing treatments is significantly higher than in no grazing treatment (P < 0.05). The reproductive allocation of U. pumila in heavy grazing treatment was significantly higher from that in the no grazing treatment (P < 0.05). Additionally, we found that reproduction of U. pumila ended later in grazed plots, suggesting the duration of reproduction is extended with grazing disturbance. Our findings suggest that U. pumila may prolong it s duration of reproduction and alter its reproductive biomass in response to grazing. It is not clear whether these effects are related to damage to U. pumila trees by grazers or whether they are due to grazers affecting soil properties or plant competitors around U. pumila trees.

Growth and Tree Water Deficit of Mixed Norway Spruce and European Beech at Different Heights in a Tree and under Heavy Drought

Although several studies suggest that tree species in mixed stands resist drought events better than in pure stands, little is known about the impact on growth and the tree water deficit (TWD) in different tree heights at heavy drought. With dendrometer data at the upper and lower stem and coarse roots, we calculated the TWD and growth (ZGmax) (referring to the stem/root basal area) to show (1) the relationship of TWD in different tree heights (50% tree height (H50), breast height (BH), and roots) and the corresponding leaf water potential and (2) how mixture and drought influence the partitioning of growth and tree water. The analyses were made in a mature temperate forest of Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica (L.)). Half of the plots were placed under conditions of extreme drought through automatic closing roof systems within the stand. We found a tight relationship of leaf water potentials and TWD at all tree compartments. Through this proven correlation at all tree heights we were also able to study the differences of TWD in all tree compartments next to the growth allocation. Whereas at the beginning of the growing period, trees prioritized growth of the upper stem, during the course of the year the growth of lower stem became a greater priority. Growth allocation of mixed spruces showed a tendency of a higher growth of the roots compared to the BH. However, spruces in interspecific neighborhoods exhibited a lesser TWD in the roots as spruces in intraspecific neighborhood. Beeches in intraspecific neighborhoods showed a higher TWD in BH compared to H50 as beeches in interspecific neighborhoods. Mixture seems to enhance the water supply of spruce trees, which should increase the stability of this species in a time of climatic warming.