Daniel J. Chmura

Phenology Differs among Norway Spruce Populations in Relation to Local Variation in Altitude of Maternal Stands in the Beskidy Mountains

An investigation of phenological and growth traits was conducted in a lowland plantation, comprised of 23 Norway spruce (Picea abies (L.) Karst.) seed stands from the Istebna region in the Beskidy Mountains of Poland. Significant differences were found among population in growth initiation, growth termination and the length of shoot elongation period. The two latter traits were highly correlated and were attributed to the altitude of the maternal stand. The tested populations were grouped based on phenology. Progeny of lower-alttitude stands ceased growth later, had a longer duration of shoot growth and greater current leader length compared to progeny of higher-altitude stands. Individual populations, however, did not differ in current-year growth and total tree height, implying higher within-population variation in growth traits than in phenology. The similar growth capacity of all tested populations suggests considerable gene flow between maternal stands, although differences in phenology imply the adaptation of progenies to the altitudinal environment of seed origin. These results suggest that within tested populations, selection is possible based on phenology alone, without considerable reduction of early height growth; final decisions, however, should be based on the environmental conditions of the planting site.

Seedling growth and biomass allocation in relation to leaf habit and shade tolerance among 10 temperate tree species

Initial growth of germinated seeds is an important life history stage, critical for establishment and succession in forests. Important questions remain regarding the differences among species in early growth potential arising from shade tolerance. In addition, the role of leaf habit in shaping relationships underlying shade tolerance-related differences in seedling growth remains unresolved. In this study we examined variation in morphological and physiological traits among seedlings of 10 forest tree species of the European temperate zone varying in shade tolerance and leaf habit (broadleaved winter-deciduous species vs needle-leaved conifers) during a 10-week period. Seeds were germinated and grown in a controlled environment simulating an intermediate forest understory light environment to resolve species differences in initial growth and biomass allocation. In the high-resource experimental conditions during the study, seedlings increased biomass allocation to roots at the cost of leaf biomass independent of shade tolerance and leaf habit. Strong correlations between relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), specific leaf area (SLA) and leaf mass fraction (LMF) indicate that physiology and biomass allocation were equally important determinants of RGR as plant structure and leaf morphology among these species. Our findings highlight the importance of seed mass- and seed size-related root morphology (specific root length-SRL) for shade tolerance during early ontogeny. Leaf and plant morphology (SLA, LAR) were more successful in explaining variation among species due to leaf habit than shade tolerance. In both broadleaves and conifers, shade-tolerant species had lower SRL and greater allocation of biomass to stems (stem mass fraction). Light-seeded shade-intolerant species with greater SRL had greater RGR in both leaf habit groups. However, the greatest plant mass was accumulated in the group of heavy-seeded shade-tolerant broadleaves. The results of our study suggest that the combinations of plant attributes enhancing growth under high light vary with shade tolerance, but differ between leaf habit groups.

Woody plants optimise stomatal behaviour relative to hydraulic risk

Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbon-maximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle.

Limited variation found among Norway spruce half-sib families in physiological response to drought and resistance to embolism

Projections of future climates suggest that droughts (Ds) may become more frequent and severe in many regions. Genetic variation, especially within populations in traits related to D resistance, is poorly investigated in forest trees, but this knowledge is necessary to better understand how forests will respond to water shortages. In this study, we investigated variability among seven open-pollinated half-sib families of a single population and two population-level progenies of Norway spruce (Picea abies (L.) H. Karst.) in their gas exchange response to imposed D and xylem vulnerability to embolism. During their third growing season, saplings were subjected to three treatments-control (C), D (for 19 weeks) and broken drought (BD, 54 days without watering starting in mid-July, then well-watered). In response to D, all families reduced their stomatal conductance (gs) and light-saturated rates of photosynthesis (Amax) in a similar way. After rewatering, the xylem water potential (Ψ) recovered in the BD treatment, but gs and Amax remained lower than in C. Needle starch concentration was altered in both D treatments compared with C. Xylem of D-exposed trees was more vulnerable to embolism than in C. The minimum attained safety margin remained positive for all families, indicating that no catastrophic hydraulic failure occurred in stem xylem during D. Significant family variation was found for Ψ early in the D (midday Ψ between -1.2 and -1.8 MPa), and for needle damage, but not for sapling mortality. Family variation found at the initial stages of D, and not afterward, suggests that all families responded similarly to greater D intensity, exhibiting the species-specific response. Limited variation at the family level indicates that the response to D and the traits we examined were conservative within the species. This may limit breeding opportunities for increased D resistance in Norway spruce in light of expected climatic changes.

Plant water potential improves prediction of empirical stomatal models.

Climate change is expected to lead to increases in drought frequency and severity, with deleterious effects on many ecosystems. Stomatal responses to changing environmental conditions form the backbone of all ecosystem models, but are based on empirical relationships and are not well-tested during drought conditions. Here, we use a dataset of 34 woody plant species spanning global forest biomes to examine the effect of leaf water potential on stomatal conductance and test the predictive accuracy of three major stomatal models and a recently proposed model. We find that current leaf-level empirical models have consistent biases of over-prediction of stomatal conductance during dry conditions, particularly at low soil water potentials. Furthermore, the recently proposed stomatal conductance model yields increases in predictive capability compared to current models, and with particular improvement during drought conditions. Our results reveal that including stomatal sensitivity to declining water potential and consequent impairment of plant water transport will improve predictions during drought conditions and show that many biomes contain a diversity of plant stomatal strategies that range from risky to conservative stomatal regulation during water stress. Such improvements in stomatal simulation are greatly needed to help unravel and predict the response of ecosystems to future climate extremes.

Environmental and genetic effects on crown shape in young loblolly pine plantations

Tree crown shape is an important trait affecting the light environment in forest canopies. We examined genetic and environmental effects on outer crown shape of young single-family stands of loblolly pine (Pinus taeda L.). Crown di- ameter profiles were measured after canopy closure at four experimental sites in the southeastern US. The two examined families of contrasting aboveground productivity differed in crown length but not in their outer crown shapes or crown shape ratios, defined as the ratio of crown diameter to crown length. Within each site, intensive silvicultural treatment, consisting of fertilization and control of competing vegetation, had little effect upon crown shape. A strongly significant effect of site on crown shape parameters was found in the family grown at all four experimental sites; however, density differences among the experimental series likely accounted for a part of the across-sites variation in crown shape. In con- trast to other studies on crown shape in trees, and to findings at age 2 years in the same stands, we conclude that family effects on the outer crown shape were small compared with the environmental effects in these 5-year-old pine plantations, following canopy closure.