Photosynthetic phenological variation may promote coexistence among co-dominant tree species in a Madrean sky island mixed conifer forest

Stomatal, mesophyll and biochemical limitations to photosynthesis and their relationship with leaf structure over an elevation gradient in two conifers

Photosynthetic responses across complex elevational gradients provides insight into fundamental processes driving responses of plant growth and net primary production to environmental change. Gas exchange of needles and twig water potential were measured in two widespread coniferous tree species, Pinus contorta and Picea engelmannii, over an 800-m elevation gradient in southeastern Wyoming, USA. We hypothesized that limitations to photosynthesis imposed by mesophyll conductance (gm) would be greatest at the highest elevation sites due to higher leaf mass per area (LMA) and that estimations of maximum rate of carboxylation (Vcmax) without including gm would obscure elevational patterns of photosynthetic capacity. We found that gm decreased with elevation for P. contorta and remained constant for P. engelmannii, but in general, limitation to photosynthesis by gm was small. Indeed, estimations of Vcmax when including gm were equivalent to those estimated without including gm and no correlation was found between gm and LMA nor between gm and leaf N. Stomatal conductance (gs) and biochemical demand for CO2 were by far the most limiting processes to photosynthesis at all sites along the elevation gradient. Photosynthetic capacity (A) and gs were influenced strongly by differences in soil water availability across the elevation transect, while gm was less responsive to water availability. Based on our analysis, variation in gm plays only a minor role in driving patterns of photosynthesis in P. contorta and P. engelmannii across complex elevational gradients in dry, continental environments of the Rocky Mountains and accurate modeling of photosynthesis, growth and net primary production in these forests may not require detailed estimation of this trait value.

Bimodal cambial activity and false-ring formation in conifers under a monsoon climate

Tracking wood formation in semiarid regions during the seasonal march of precipitation extremes has two important applications. It can provide (i) insight into the adaptive capacities of trees to drought and (ii) a basis for a richer interpretation of tree-ring data, assisting in a deeper understanding of past and current climate. In the southwestern USA, the anatomical signature of seasonally bimodal precipitation is the 'false ring'-a band of latewood-like cells in the earlywood. These occur when a particularly deep drought during the early growing season ends abruptly with timely, mid-growing season monsoonal rains. Such conditions presented in southern Arizona in 2014, enabling us to explore false-ring formation in ponderosa pine (Pinus ponderosa Lawson and C. Lawson) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) in mixed-conifer forest at 2573 m above sea level. We ask: what were the cell-by-cell timings and durations in the phases of wood cell development in 2014? How do these seasonal patterns relate to strongly fluctuating environmental conditions during the growing season? We took weekly microcores from March through November from six ponderosa pine and seven Douglas-fir trees at a well-instrumented flux tower site. Thin sections were prepared, and we counted cells in cambial, expansion, cell wall thickening and mature phases. For ponderosa pine trees forming a false ring, the first impact of intensifying seasonal drought was seen in the enlarging phase and then, almost a month later, in cambial activity. In this species, recovery from drought was associated with recovery first in cambial activity, followed by cell enlargement. This timing raised the possibility that cell division may be affected by atmospheric moisture increases before soil recharge. In both species, the last false-ring cells matured during the summer rainy season. Bimodal cambial activity coincident with moisture availability was observed in both species, whether or not they formed a false ring. This deeper knowledge of the precise timing of both developmental and environmental events should help define mechanistic connections among these factors in creating bimodal growth patterns.

Photosynthetic sensitivity to historic meteorological variability for conifers in the eastern Sierra Nevada

Increased climatic variability can impact tree physiological processes beyond what is predicted from changes in mean conditions. We assessed the sensitivity of conifer saplings to spatial and temporal variability in meteorological conditions, taking advantage of the end of California's historic drought and the exceedingly wet winter of 2017. We sought to understand how very dry and very wet conditions constrain photosynthesis and growth in four regionally dominant conifers and whether sensitivity in these processes changes across a 500 m gradient in elevation. All species demonstrated phenotypic plasticity in response to temporal differences in precipitation on both inter-annual and seasonal timescales. Net photosynthesis in Pinus contorta decreased from an early season 2016 average of 12.4 to 6.89 μmol CO₂ m^-2 s^-1 later in the summer, but increased 14.1% between seasons in the wet year. By contrast, elevation had almost no effect on instantaneous photosynthetic gas exchange, CO₂ response curve parameters, or stem water potential in any of the years for any of the species. Effects of the heavy snow year (2017) on needle growth differed between elevations. Pinus contorta showed a 38.9% increase in average needle length at the lower two elevations but a 31.6% decrease at the highest site compared to the height of the drought. Despite these differences, biological variation was dampened compared to the physical variation between years, suggesting these trees can effectively withstand substantial meteorological variability. Our results show that these species demonstrated considerable ability to tolerate and recover from an extreme drought event.

Montane forest productivity across a semiarid climatic gradient

High-elevation montane forests are disproportionately important to carbon sequestration in semiarid climates where low elevations are dry and characterized by low carbon density ecosystems. However, these ecosystems are increasingly threatened by climate change with seasonal implications for photosynthesis and forest growth. As a result, we leveraged eddy covariance data from six evergreen conifer forest sites in the semiarid western United States to extrapolate the status of carbon sequestration within a framework of projected warming and drying. At colder locations, the seasonal evolution of gross primary productivity (GPP) was characterized by a single broad maximum during the summer that corresponded to snow melt-derived moisture and a transition from winter dormancy to spring activity. Conversely, winter dormancy was transient at warmer locations, and GPP was responsive to both winter and summer precipitation such that two distinct GPP maxima were separated by a period of foresummer drought. This resulted in a predictable sequence of primary limiting factors to GPP beginning with air temperature in winter and proceeding to moisture and leaf area during the summer. Due to counteracting winter (positive) and summer (negative) GPP responses to warming, leaf area index and moisture availability were the best predictors of annual GPP differences across sites. Overall, mean annual GPP was greatest at the warmest site due to persistent vegetation photosynthetic activity throughout the winter. These results indicate that the trajectory of this region's carbon sequestration will be sensitive to reduced or delayed summer precipitation, especially if coupled to snow drought and earlier soil moisture recession, but summer precipitation changes remain highly uncertain. Given the demonstrated potential for seasonally offsetting responses to warming, we project that decadal semiarid montane forest carbon sequestration will remain relatively stable in the absence of severe disturbance.

Topography influences species-specific patterns of seasonal primary productivity in a semiarid montane forest

Semiarid forests in the southwestern USA are generally restricted to mountain regions where complex terrain adds to the challenge of characterizing stand productivity. Among the heterogeneous features of these ecosystems, topography represents an important control on system-level processes including snow accumulation and melt. This basic relationship between geology and hydrology affects radiation and water balances within the forests, with implications for canopy structure and function across a range of spatial scales. In this study, we quantify the effect of topographic aspect on primary productivity by observing the response of two codominant native tree species to seasonal changes in the timing and magnitude of energy and water inputs throughout a montane headwater catchment in Arizona, USA. On average, soil moisture on north-facing aspects remained higher during the spring and early summer compared with south-facing aspects. Repeated measurements of net carbon assimilation (Anet) showed that Pinus ponderosa C. Lawson was sensitive to this difference, while Pseudotsuga menziesii (Mirb.) Franco was not. Irrespective of aspect, we observed seasonally divergent patterns at the species level where P. ponderosa maintained significantly greater Anet into the fall despite more efficient water use by P. menziesii individuals during that time. As a result, this study at the southern extent of the geographical P. menziesii distribution suggests that this species could increase water-use efficiency as a response to future warming and/or drying, but at lower rates of production relative to the more drought-adapted P. ponderosa. At the sub-landscape scale, opposing aspects served as a mesocosm of current versus anticipated climate conditions. In this way, these results also constrain the potential for changing carbon sequestration patterns from Pinus-dominated landscapes due to forecasted changes in seasonal moisture availability.

Evolutionary trade-offs in the chemical defense of floral and fruit tissues across genus Cornus

PREMISE

Defense investment in plant reproductive structures is relatively understudied compared to the defense of vegetative organs. Here the evolution of chemical defenses in reproductive structures is examined in light of the optimal defense, apparency, and resource availability hypotheses within the genus Cornus using a phylogenetic comparative approach in relation to phenology and native habitat environmental data.

METHODS

Individuals representing 25 Cornus species were tracked for reproductive phenology over a full growing season at the Arnold Arboretum of Harvard University. Floral, fruit, and leaf tissue was sampled to quantify defensive chemistry as well as fruit nutritional traits relevant to bird dispersal. Native habitat environmental characteristics were estimated using locality data from digitized herbarium records coupled with global soil and climate data sets.

RESULTS

The evolution of later flowering was correlated with increased floral tannins, and the evolution of later fruiting was correlated with increased total phenolics. Leaves were found to contain the highest tannin activity, while inflorescences contained the highest total flavonoids. Multiple aspects of fruit defensive chemistry were correlated with fruit nutritional traits. Floral and fruit defensive chemistry were evolutionarily correlated with aspects of native habitat temperature, precipitation, and soil characteristics.

CONCLUSIONS

Results provide tentative support for the apparency hypothesis with respect to both flower and fruit phenology, while relative concentrations of secondary metabolites across organs provide mixed support for the optimal defense hypothesis. The evolution of reproductive defense with native habitat provides, at best, mixed support for the resource availability hypothesis.

Size-dependent variation in leaf functional traits and nitrogen allocation trade-offs in Robinia pseudoacacia and Cornus controversa

Tree species vary in how they invest resources to different functions throughout their life histories, and investigating the detailed patterns of ontogenetic changes in key functional traits will aid in predicting forest dynamics and ecosystem processes. In this context, we investigated size-dependent changes in key leaf functional traits and nitrogen (N) allocation trade-offs in black locust (Robinia pseudoacacia L., an N-fixing pioneer species) and giant dogwood (Cornus controversa Hemsl., a mid-successional species), which have different life-history strategies, especially in their light use. We found that the leaf mass per area and leaf carbon concentrations increased linearly with tree size (diameter at breast height, DBH), whereas leaf N concentrations decreased nonlinearly, with U- and hump-shaped patterns in black locust and giant dogwood, respectively. We also discovered large differences in N allocation between the two species. The fraction of leaf N invested in cell walls was much higher in black locust than in giant dogwood, while the opposite was true for the light harvesting N fraction. Furthermore, these fractions were related to DBH to varying degrees: the cell wall N fraction increased with DBH for both species, whereas the light harvesting N fraction of giant dogwood decreased nonlinearly and that of black locust remained constant. Instead, black locust reduced the fraction of leaf N invested in other N pools, resulting in a smaller fraction compared to that of giant dogwood. On the other hand, both species had similar fraction of leaf N invested in ribulose-1,5-bisphosphate carboxylase/oxygenase across tree size. This study indicated that both species increased leaf mechanical toughness through characteristic changes in N allocation trade-offs over the lifetimes of the trees.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

Soil CO2 efflux (Fsoil) is commonly considered equal to soil CO2 production (Rsoil), and both terms are used interchangeably. However, a non-negligible fraction of Rsoil can be consumed in the subsurface due to a host of disparate, yet simultaneous processes. The ratio between CO2 efflux/O2 influx, known as the apparent respiratory quotient (ARQ), enables new insights into CO2 losses from Rsoil not previously captured by Fsoil. We present the first study using continuous ARQ estimates to evaluate annual CO2 losses of carbon produced from Rsoil. We found that up to 1/3 of Rsoil was emitted directly to the atmosphere, whereas 2/3 of Rsoil was removed by subsurface processes. These subsurface losses are attributable to dissolution in water, biological activities and chemical reactions. Having better estimates of Rsoil is key to understanding the true influence of ecosystem production on Rsoil, as well as the role of soil CO2 production in other connected processes within the critical zone.