Hydrometeorology organizes intra-annual patterns of tree growth across time, space and species in a montane watershed

Diel and seasonal stem growth responses to climatic variation are consistent across species in a subtropical tree community

Understanding how intra-annual stem growth responds to atmospheric and soil conditions is essential for assessing the effects of climate extremes on forest productivity. In species-poor forests, such understanding can be obtained by studying stem growth of the dominant species. Yet, in species-rich (sub-)tropical forests, it is unclear whether these responses are consistent among species. We monitored intra-annual stem growth with high-resolution dendrometers for 27 trees belonging to 14 species over 5 yr in a montane subtropical forest. We quantified diel and seasonal stem growth patterns, verified to what extent observed growth patterns coincide across species and analysed their main climatic drivers. We found very consistent intra-annual growth patterns across species. Species varied in the rate but little in the timing of growth. Diel growth patterns revealed that - across species - trees mainly grew before dawn when vapour pressure deficit (VPD) was low. Within the year, trees mainly grew between May and August driven by temperature and VPD, but not by soil moisture. Our study reveals highly consistent stem growth patterns and climatic drivers at community level. Further studies are needed to verify whether these results hold across climates and forests, and whether they can be scaled up to estimate forest productivity.

Topographic Wetness Index calculation guidelines based on measured soil moisture and plant species composition

Soil moisture controls environmental processes and species distributions, but it is difficult to measure and interpolate across space. Topographic Wetness Index (TWI) derived from digital elevation model is therefore often used as a proxy for soil moisture. However, different algorithms can be used to calculate TWI and this potentially affects TWI relationship with soil moisture and species assemblages. To disentangle insufficiently-known effects of different algorithms on TWI relation with soil moisture and plant species composition, we measured the root-zone soil moisture throughout a growing season and recorded vascular plants and bryophytes in 45 temperate forest plots. For each plot, we calculated 26 TWI variants from a LiDAR-based digital terrain model and related these TWI variants to the measured soil moisture and moisture-controlled species assemblages of vascular plants and bryophytes. A flow accumulation algorithm determined the ability of the TWI to predict soil moisture, while the flow width and slope algorithms had only a small effects. The TWI calculated with the most often used single-flow D8 algorithm explained less than half of the variation in soil moisture and species composition explained by the TWI calculated with the multiple-flow FD8 algorithm. Flow dispersion used in the FD8 algorithm strongly affected the TWI performance, and a flow dispersion close to 1.0 resulted in the TWI best related to the soil moisture and species assemblages. Using downslope gradient instead of the local slope gradient can strongly decrease TWI performance. Our results clearly showed that the method used to calculate TWI affects study conclusion. However, TWI calculation is often not specified and thus impossible to reproduce and compare among studies. We therefore provide guidelines for TWI calculation and recommend the FD8 flow algorithm with a flow dispersion close to 1.0, flow width equal to the raster cell size and local slope gradient for TWI calculation.

Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A

In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies: Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesii to understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (ψ_x), xylem tissue vulnerability to cavitation (P₅₀, or ψ causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example, P. ponderosa grew in more xeric locations, while P. engelmannii were largely confined to more mesic locations, yet P. engelmannii had more negative P₅₀ values. P. menziesii had the lowest P₅₀ value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins; P. menziesii had the widest safety margin, P. engelmannii was intermediate, and P. ponderosa displayed the narrowest safety margin. All three species were also using water sources deeper than 30 cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species-specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.

Reduction in lumen area is associated with the δ18 O exchange between sugars and source water during cellulose synthesis

High temporal resolution measurements of wood anatomy and the isotopic composition in tree-rings have the potential to enhance our interpretation of climate variability, but the sources of variation within the growing season are still not well understood. Here we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸ O_cell ) to shifts in relative humidity (RH) in two treatments, one from high-low RH and the second one form low-high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both treatments. The measured δ¹⁸ O_cell along the ring was responsive to RH variations in both treatments. However, estimated δ¹⁸ O_cell did not agree with measured δ¹⁸ O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex ) was kept constant. We found that P_ex increased throughout the ring as LA decreased. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose is a better recorder of RH while late-season cellulose is a better recorder of the source water.

Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies

In temperate regions such as the American west, forest trees often exhibit growth sensitivity to climatic conditions of a particular season. For example, annual tree ring growth increments may correlate well with winter precipitation, but not with summer rainfall, suggesting that trees rely more on winter snow than summer rain. Because both the timing and character of seasonal western climate patterns are expected to change considerably over coming decades, variation in the importance of different seasonal moisture sources for trees can be expected to influence how different forest trees respond to climate change as a whole, with shifts in seasonality potentially benefitting some trees while challenging others. In this study, we inferred patterns of tree water use in Douglas fir trees from the Northern Rockies for 2 years using stable water isotopes, while simultaneously quantifying and tracking precipitation inputs to soil moisture across a vertical soil profile. We then coupled water source use with daily measurements of radial growth to demonstrate that soil moisture from winter precipitation accounted for 87.5% and 84% of tree growth at low and high elevations, respectively. We found that prevailing soil moisture conditions drive variation in the depth at which trees access soil water, which in turn determines which seasonal precipitation inputs are available to support tree growth and function. In general, trees at lower elevations relied more on winter precipitation sourced from deep soils while trees at higher elevations made better use of summer rains sourced from near-surface soil layers. As both the timing of seasons and phase of precipitation (rain vs. snow) are likely to change considerably across much of the west, such patterns in tree water use are likely to play a role in determining the evolution of forest composition and structure in a warming climate.

Summer dry-down modulates the isotopic composition of soil CO2 production in snow-dominated landscapes

In mountainous landscapes, soil moisture is highly dynamic due to the effects of topography and the temporal variability imposed by seasonal precipitation, including rainfall and snow. Soil moisture is known to affect ecosystem carbon exchange both aboveground and belowground, as well as the stable isotopic composition of exchanged CO₂. In this study we used an extensive suite of measurements to examine the effects of seasonal changes in soil moisture on the isotopic composition of soil CO₂ production at the landscape level. We show that the seasonal decline in soil moisture (i.e., summer dry-down) appeared to impose a trend in the δ¹³C of soil CO₂ production (δ_P) with more negative δ_P early in the growing season when soils were wet, and more positive δ_P as the growing season progressed and soils dried out. This seemingly generalizable pattern for a snow-dominated watershed is likely to represent the variability of recently assimilated C, tracked through the plant-soil system and imprinted in the respired CO₂. Thus, our observations suggest that, at least for mountainous environments, seasonal changes in δ_P are largely mediated by soil moisture and their spatial variability is partially organized by topography.

Moisture-driven xylogenesis in Pinus ponderosa from a Mojave Desert mountain reveals high phenological plasticity

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature-driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water-limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015-2016) showing opposite-sign anomalies between warm- and cold-season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July-August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.