TY - JOUR
AU - Barron-Gafford, Greg, A
AB - Abstract 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. Introduction Forests comprise 30% of land cover globally and are responsible for the majority of sustained carbon sequestration in the western USA (Bonan 2008, Schimel et al. 2002). Forest production is typically derived using remote sensing products, forest inventories and/or a variety of modeling approaches (Hayes et al. 2012, Baccini et al. 2017). However, forests represent a variety of tree-dominated landscapes with variations in density, carbon stocks and biodiversity that may not be captured at coarse scales (Pan et al. 2011). As a result, leaf- and landscape-level measurements of forest carbon assimilation using phenocams, eddy covariance towers and individual tree-scale measurements are necessary to promote mechanistic understanding of forest dynamics (Beer et al. 2010, Brown et al. 2016). Further, forests are variably sensitive to meteorological forcing as a result of their distribution across topographically complex landscapes with broad gradients in species composition, environmental conditions and resource availability (e.g., Xu et al. 2020). Given the broad diversity of forest structure and function, predicting the influence of current and projected climate on forest carbon cycling remains a grand ecological challenge (Myneni et al. 2001). In the western USA, water availability principally limits ecophysiological function (Huxman et al. 2004, Biederman et al. 2017). Additionally, strong elevational gradients of moisture availability and distinctive seasonal variation in the timing, form and amount of precipitation—characteristic of the western USA—shape the structure and function of forests (Pelletier et al. 2018). In particular, the accumulation of winter snowpack and the timing of snowmelt strongly regulate the water cycle of semiarid montane forests (Vivoni et al. 2008), which in turn feeds back to determine forest productivity (Monson et al. 2002, Hu et al. 2010, Knowles et al. 2018). As a result, semiarid forests are a widespread and important contributor to the regional annual carbon budget (Grünzweig et al. 2003, Rotenberg and Yakir 2010), but also the inter-annual variability that is critically important to the global terrestrial carbon flux (Ahlström et al. 2015). However, the degree to which these ecohydrological processes are subject to modification by topography remains an understudied question in complex terrain. This is due, in part, to challenges associated with eddy covariance measurements and mature canopy access in complex terrain. The influence of inter-seasonal precipitation variability on forest productivity can be mediated by topographic complexity, such as hillslope position, slope and aspect due to predictable differences in incoming solar radiation (Bennie et al. 2008, Pelletier et al. 2018). Incoming solar radiation strikes more directly on equator-facing aspects relative to pole-facing aspects such that evaporation is greater on south aspects (in the northern hemisphere), and these slopes experience earlier, more rapid snowmelt relative to adjacent north aspects in areas that receive snow (Hinckley et al. 2014, Yetemen et al. 2015). As a result, soils on south aspects also dry down earlier in the growing season relative to north aspects (Gutiérrez-Jurado et al. 2006, Geroy et al. 2011, Hinckley et al. 2014) (Figure 1). Whereas regional scale differences in incoming radiation contribute to widely noted differences in forest community composition and elevational distribution (Whittaker and Niering 1965, Nemani and Running 1989, Coblentz and Riitters 2004), the same differences can manifest in denser, more insulating canopy development on pole-facing aspects within a single catchment (Zou et al. 2007). Further, substrate thickness and architecture controls on subsurface hydrological storage and flow have been shown to vary by aspect in complex terrain (Grant and Dietrich 2017). Taken together, soil water availability on opposing north and south aspects depends on complex interactions between biotic and abiotic factors including both canopy and geologic structure. Figure 1. Open in new tabDownload slide The apparent migration of the sun creates predictable differences in the seasonal intensity and angle of incoming radiation (red arrows). Incoming blue arrows represent the magnitude of precipitation (snow or rain) for the (A) spring, (B) pre-monsoon, (C) monsoon, (D) fall and (E) winter periods, with no input represented during the pre-monsoon dry summer. Both incoming radiation and input of water contribute to differences in soil moisture on opposing aspects. Hypothesized water loss and carbon uptake is shown for all seasons, with environmentally driven responses differing on each aspect. Arrows are not drawn to scale. Figure 1. Open in new tabDownload slide The apparent migration of the sun creates predictable differences in the seasonal intensity and angle of incoming radiation (red arrows). Incoming blue arrows represent the magnitude of precipitation (snow or rain) for the (A) spring, (B) pre-monsoon, (C) monsoon, (D) fall and (E) winter periods, with no input represented during the pre-monsoon dry summer. Both incoming radiation and input of water contribute to differences in soil moisture on opposing aspects. Hypothesized water loss and carbon uptake is shown for all seasons, with environmentally driven responses differing on each aspect. Arrows are not drawn to scale. The Madrean ‘sky islands’ of northern Sonora (Mexico) and southern Arizona and New Mexico (USA) are a series of isolated mountain ranges between the Colorado Plateau to the north and the Sierra Madre Occidental to the south (Brusca et al. 2013, Potts et al. 2017). Individual sky islands within the archipelago rise sharply above desert lowlands to altitudes greater than 3000 m elevation, resulting in a vertical gradient of ecosystems more commonly found along broad latitudinal ranges. Among the Madrean sky islands, the Santa Catalina Mountains have long been the focus of plant ecology research examining the interplay between climate and topography (Shreve 1915, Whittaker and Niering 1965, Whittaker and Niering 1975). Diverse forest communities occupy the highest elevations of these mountains, which reflects the region’s distinctive hydroclimate. In addition to snowmelt, which dominates seasonal patterns of soil moisture availability in the Rocky Mountains, Madrean sky island forests are strongly influenced by summer convective storms associated with the North American Monsoon (Adams and Comrie 1997). The resulting bimodal seasonal moisture distribution affects carbon and water cycling by influencing patterns of photosynthetic phenology (Potts et al. 2017) and ecosystem carbon dioxide (CO2) exchange (Knowles et al. 2020). However, forecasted climate warming may also alter the amount and seasonal timing of precipitation in these montane forests (Melillo et al. 2014) with unknown implications for localized or species-specific ecohydrologic function. As a result, observed differences in physiological function on opposing aspects and among tree species can serve as useful proxies with which to simulate the effects of climate forcing on the abundance and productivity of the regional montane forest biome. The principal objective of this research was to constrain the effects of topographic complexity on species-specific physiological responses to seasonal variation in moisture availability (Chorover et al. 2007, Vose et al. 2011, Harpold et al. 2015). To address this, we characterized the seasonal variation in leaf water potential and leaf photosynthesis of two codominant conifer species, Pinus ponderosa and Pseudotsuga menziesii, growing on opposing north- and south-facing aspects of an instrumented headwater catchment in the Santa Catalina Mountains of southern Arizona, USA. We hypothesized that the magnitude of slope aspect-mediated differences in plant physiological performance would vary with seasonal shifts in sun angle, air temperature, soil moisture and tree species (Figure 1). In particular, we expected a strong influence of slope aspect on plant physiological performance during spring as a result of mild temperatures and potentially variable snow cover. Subsequently, we expected a diminished effect of slope aspect during the summer when sun angles are higher such that trees growing on opposing aspects would perform more similarly to one another (Figure 1). To assess leaf-level performance, we used a hand-held infrared gas analyzer to quantify the per-unit area rate of net CO2 uptake and rate of water loss from the needles. At the species level, we extended the conceptual framework of Potts et al. (2017) to predict the impact of slope aspect on species-specific patterns of seasonal photosynthesis according to thermal and moisture tolerances. We expected that, under warm and dry conditions, P. ponderosa would maintain higher rates of leaf photosynthesis on south-facing aspects and P. menziesii would maintain greater rates of leaf photosynthesis on north-facing aspects. Materials and methods Site description The study was conducted at the Mt Bigelow research site (32° 25′ 00′′ N, 110° 43′ 31′′ W, 2573 m elevation; AmeriFlux site ID US-MtB) within the Catalina-Jemez Critical Zone Observatory (Chorover et al. 2011, Knowles et al. 2020). This high-elevation mixed conifer forest is located within the Coronado National Forest and experiences a North American Monsoon moisture regime with precipitation maxima in both summer and winter. Atmospheric demand for moisture is high at this site, and semiarid conditions result from evapotranspiration (ET) that exceeds annual precipitation (Knowles et al. 2020). Between 2010 and 2018, the cumulative annual precipitation was 614 mm (10% snow) and the mean annual air temperature was 10.0 °C (Barron-Gafford 2009, J. Knowles unpublished data). The catchment is entirely below treeline and is codominated by P. menziesii and P. ponderosa, with an average canopy height of 10 m. Tree-ring analyses of P. menziesii suggest that overstory trees growing established in the late 1930s (Potts et al. 2017). Although individual trees are undisturbed, the US Forest Service selectively thinned the stand in 2012. The soil texture is sandy loam with 32% sand, 41% silt and 26% clay and a pH of 5.4 (Sánchez-Cañete et al. 2018). To address differences between individual productivity on north and south aspects, we measured photosynthetic rates of mature trees within a 1.5 ha headwater catchment (zero-order basin [ZOB]) located within the statistical measurement footprint of the Mt Bigelow eddy covariance tower (Knowles et al. 2020; Figure 2). The ZOB, defined as a catchment with no water inputs other than direct precipitation, drains east and has opposing north and south aspects with similar areas and slopes. The north aspect is offset 28° from geographic north and has a slope of ~14°; the south aspect is offset 158° from geographic north and has a slope of ~16°. We defined five seasons between 1 February 2016 and 31 January 2017 according to the following classification: spring (1 February 2016 to 30 April), pre-monsoon summer (1 May to 30 June), monsoon summer (1 July to 30 September), fall (1 October to 30 November) and winter (1 December to 31 January 2017). Figure 2. Open in new tabDownload slide Opposing aspects identified within the zero-order basin (ZOB—white outline). The mixed conifer forest at this site is located at 2573 m elevation, ~29 km northeast of Tucson, Arizona, USA. Map provided by the Santa Catalina Mountains & Jemez River basin critical zone observatory. Figure 2. Open in new tabDownload slide Opposing aspects identified within the zero-order basin (ZOB—white outline). The mixed conifer forest at this site is located at 2573 m elevation, ~29 km northeast of Tucson, Arizona, USA. Map provided by the Santa Catalina Mountains & Jemez River basin critical zone observatory. Environmental data Continuous above-ground meteorological and below-ground abiotic soil measurements at the Mt Bigelow eddy covariance site were established in 2009 (Knowles et al. 2020). These measurements include profiles of air temperature, relative humidity and vapor pressure deficit (VPD) at 1.5, 8.75, 16, 24 and 31 m above ground level (a.g.l.) (HMP-60, Vaisala, Helsinki, Finland), precipitation below (1 m a.g.l.) and above the canopy (30 m a.g.l.; TR-525 M, Texas Electronics, Dallas, TX, USA), and above-canopy net radiation (16 m a.g.l.; CNR-4, Kipp & Zonen, Delft, The Netherlands). In the ZOB below the main instrument tower, we continuously measured snow depth (Ultrasonic Depth Sensor, Judd Communications, Salt Lake City, UT, USA) and volumetric water content (VWC) and soil temperature (5TE, METER Group, Pullman, WA, USA) at 10, 30 and 60 cm below the soil surface on both north and south aspects. Below-canopy net all-wave radiation was measured at 1 m a.g.l. on both north and south aspects (CNR-4, Kipp & Zonen). We configured dataloggers (CR5000; Campbell Scientific, Logan, UT, USA) to sample all environmental data at 15 s and averaged those to 30-min means. Operations were powered by solar panels. Plant physiology data We sampled five mature individuals each from P. menziesii and P. ponderosa on both north and south aspects, for a total of 20 individuals. To characterize species-specific photosynthetic phenology, we conducted water potential and gas exchange measurements for all 20 individuals across five seasonal periods of varying ambient temperature and moisture conditions (Figures 3 and 4). Previous research at this site has shown that this number of replicates per species is adequate to capture intraspecific and interspecific variation at our site (Potts et al. 2017). The sampling dates for each seasonal period were 24 February 2016 (spring), 20 June 2016 (pre-monsoon summer), 12 August 2016 (monsoon summer), 28 October 2016 (fall) and 24 January 2017 (winter). These sampling dates represented extreme endmembers of moisture and temperature on the landscape and were selected based on the results of previous physiological and ecohydrological work at this location (Potts et al. 2017, Knowles et al. 2020). Figure 3. Open in new tabDownload slide Micrometeorological conditions during the study period between January 2016 and January 2017. (A) Air temperature, (B) VPD calculated from air temperature and relative humidity and (C) cumulative daily precipitation. Solid vertical lines indicate ecophysiological measurement dates. Figure 3. Open in new tabDownload slide Micrometeorological conditions during the study period between January 2016 and January 2017. (A) Air temperature, (B) VPD calculated from air temperature and relative humidity and (C) cumulative daily precipitation. Solid vertical lines indicate ecophysiological measurement dates. Figure 4. Open in new tabDownload slide Aspect-specific environmental conditions between January 2016 and January 2017. (A) Daily mean incoming shortwave radiation (between 06:00 and 22:00 h local time) measured below the canopy, (B) soil temperature measured at 30 cm depth and (C) VWC measured at 30 cm depth for both aspects. The dashed lines in (C) on 9 February 2016 and 18 February 2016 represent the 9-day period when snow was absent on the south aspect, but remained on the north aspect. Solid vertical lines indicate ecophysiological measurement dates. Figure 4. Open in new tabDownload slide Aspect-specific environmental conditions between January 2016 and January 2017. (A) Daily mean incoming shortwave radiation (between 06:00 and 22:00 h local time) measured below the canopy, (B) soil temperature measured at 30 cm depth and (C) VWC measured at 30 cm depth for both aspects. The dashed lines in (C) on 9 February 2016 and 18 February 2016 represent the 9-day period when snow was absent on the south aspect, but remained on the north aspect. Solid vertical lines indicate ecophysiological measurement dates. Due to the physical constraints of measuring directly within the canopy, we cut samples from the canopies of individual trees to conduct measurements on the ground. Following on the protocol of Potts et al. (2017), we used an extension ladder and telescoping tree pruner to collect south-facing, sun-exposed mid-canopy branches from each individual. To maintain hydraulic conductivity, we placed cut branches in buckets of water and re-cut underwater (Huxman et al. 2003). We selected 1-year-old needles for all measurements, in order to mitigate leaf age-mediated changes to physiological capability. We acclimated samples to ambient conditions within a cuvette using a portable photosynthesis analyzer (LI-6400XT, LI-COR, Lincoln, NE, USA) configured with LED light sources (model 6400-18A). We set the sample cuvette light intensity to 1500 μmol photons m−2 s−1 (to represent ambient daytime quantum flux), VPD to between 1–2 kPa, and CO2 concentration to ambient environmental concentration (~400 p.p.m.); we set ambient atmospheric temperature to match each season’s conditions. We monitored leaf gas exchange parameters closely to ensure that samples within the cuvette reached stable and steady-state conditions before we made measurements of ambient net CO2 assimilation (Anet) and transpiration. Following the gas exchange measurements, we removed needles from inside the cuvette and trimmed them such that only the needle area within the cuvette remained. We then placed needles in re-sealable plastic bags and stored them on ice for transport to the laboratory. At that point, we measured leaf area with a leaf area meter (LI-3100-C, LI-COR, Lincoln, NE, USA) as described by Fites and Teskey (1988). We corrected all gas exchange measurements to account for each sample’s actual photosynthetic leaf area using LI-COR software. We coupled gas exchange measurements to the midday water potential for the same branch using a portable pressure chamber (Model 600, PMS Instrument Company, Albany, OR, USA). We also collected predawn water potential measurements (Ψpd) from separate branches on each individual tree during the morning before gas exchange measurements. Due to freezing temperatures during the winter, branches periodically froze before we could conduct water potential measurements, so no Ψpd data could be collected between 14 January 2017 and 27 January 2017. Data analysis We used non-parametric statistical tests due to the non-normal distribution and small sample size of each sub-grouping. In particular, we used repeated-measures analysis of variance (ANOVA) for significance testing and the Wilcoxon Rank Sum test for post-hoc tests of equal means between data subsets. For all physiological parameters, we performed two-sided (difference of means not equal to 0) and one-sided (mean of north aspect greater than south) tests. We report significance results from both tests, but conclusions are based on the results of the one-sided tests as they more accurately address our hypotheses. We performed statistical analyses using R 4.0.0 (R Development Core Team 2020). Results Environmental conditions The study period was marked by typical weather: the winter months of December and January experienced episodic freeze events, maximum air temperatures occurred in June during the dry summer period, and the onset of monsoon precipitation occurred in early July (Figure 3). Air temperature increased during the spring (8.2 ± 3.6 °C), peaked at 29.8 °C (20.4 ± 4.3 °C) on 19 June during the pre-monsoon period, and gradually declined through the monsoon (17.0 ± 2.5 °C), fall (14.4 ± 2.1 °C) and winter (−4.3 ± 2.8 °C) (Figure 3A); the maximum VPD was 4.1 kPa on 19 June and followed a similar seasonal trend (Figure 3B). Precipitation was near average throughout the first half of 2016, but significantly (>1 SD) above average thereafter (Knowles et al. 2020) (Figure 3C). Cumulative precipitation from 1 February 2016 through 31 January 2017 was 883 mm. Mean annual below-canopy incoming shortwave radiation was 66% lower on the north aspect due to both the more oblique sun angle and greater shading from the canopy at the soil surface (Figure 4A). Reduced incoming shortwave radiation on the north aspect produced differences in the magnitude of snow accumulation and the timing of melt on the opposing slopes. This was supported by three data sources: Figure 5 shows a more rapid decline in snow depth on the south aspect (orange points) relative to the north aspect (blue line); there was evidence of diurnal melt cycles (high amplitude between midday and midnight soil moisture) between 18 January and 9 February on the south aspect, and between 9 February and 18 February on the north aspect (Figure 4C); and half-hour images taken by phenocams permanently installed at the site showed that snow became patchy and disappeared earlier in the season on the south aspect relative to the north (not shown). The maximum snow depth was 66 cm on the flat ridgetop immediately above the basin and 50 cm on the north aspect in the ZOB. Figure 5. Open in new tabDownload slide Snow depth measured on north and south aspects in 2016 (spring conditions during the study period) and 2017 (spring conditions following the winter measurement period). Despite inconsistent data from one sensor, a clear trend is evident in which snow melts more rapidly on the south aspect relative to the north aspect. Figure 5. Open in new tabDownload slide Snow depth measured on north and south aspects in 2016 (spring conditions during the study period) and 2017 (spring conditions following the winter measurement period). Despite inconsistent data from one sensor, a clear trend is evident in which snow melts more rapidly on the south aspect relative to the north aspect. During the spring measurement period, the average VWC on the north aspect (23.7 ± 3.8%) was higher than on the south aspect (17.6 ± 0.4%), and aspect-specific differences in soil moisture were generally maintained throughout late spring and early summer (Figure 4C). The onset of the monsoon occurred on 1 July with 37 mm precipitation, but soil moisture did not consistently rise above pre-monsoon levels until 2 August, after which point the VWC remained above 15% on both aspects for the remainder of the summer. This transition allowed for plant performance measurements under both typical dry summer and wet summer conditions. Overall, soil moisture throughout the wet summer period (22.4 ± 6.3% and 18.6 ± 0.9% on north and south aspects, respectively) was approximately double the dry summer period. Post-monsoon, the VWC declined at a similar rate on both north and south aspects throughout the fall and winter, thereby preserving greater soil moisture on north-facing slopes. Plant ecophysiology Seasonal predawn (Ψpd) and midday (Ψmd) water potentials for both P. menziesii and P. ponderosa were most negative (indicating greater potential plant stress) during the warm, dry early-summer season, and the least negative during the wet, late-summer monsoon season (Figure 6A). Despite high VPD in the early summer, water potentials did not reach established thresholds for moisture stress. In contrast, net carbon assimilation rates (Anet) were seasonally dependent on both aspect and species. Specifically, the P. ponderosa Anet was significantly higher on north aspects throughout the spring and the dry early summer period (Figure 6C). During the same time period, we observed no significant differences between Anet in P. menziesii individuals on north versus south aspects, and neither species demonstrated significant aspect-dependent Anet differences throughout the rest of the year. Seasonally, Anet was highest in spring and fall in both species, but diverged during the fall when the P. ponderosa Anet was twice as high as the P. menziesii Anet. The range of stomatal conductance to water (gsw) in P. ponderosa individuals was 2.4 times that of P. menziesii, reaching as high as 0.18 mol m−2 s−1 (Figure 7). Figure 6. Open in new tabDownload slide Seasonal ecophysiological measurements. Asterisks represent statistically significant differences between aspects, with one asterisk indicating a P-value <0.1 and two indicating a P-value <0.05. Black asterisks are for one-sided non-parametric tests, while gray are for two-sided. (A) Seasonal predawn water potential values (Ψpd) measured between 04:00 and 05:00 h (winter is not shown due to temperature constraints on the sampling methodology), (B) seasonal midday water potential values (Ψmd) measured between 11:00 and 12:00 h, (C) aspect-specific net carbon assimilation rates (Anet) measured fewer than 2 h after removal from tree (positive values represent net carbon uptake), (D) aspect-specific water loss from the needles, measured at the same time as Anet and (E) sample-specific instantaneous WUE, calculated as Anet per net transpiration. Figure 6. Open in new tabDownload slide Seasonal ecophysiological measurements. Asterisks represent statistically significant differences between aspects, with one asterisk indicating a P-value <0.1 and two indicating a P-value <0.05. Black asterisks are for one-sided non-parametric tests, while gray are for two-sided. (A) Seasonal predawn water potential values (Ψpd) measured between 04:00 and 05:00 h (winter is not shown due to temperature constraints on the sampling methodology), (B) seasonal midday water potential values (Ψmd) measured between 11:00 and 12:00 h, (C) aspect-specific net carbon assimilation rates (Anet) measured fewer than 2 h after removal from tree (positive values represent net carbon uptake), (D) aspect-specific water loss from the needles, measured at the same time as Anet and (E) sample-specific instantaneous WUE, calculated as Anet per net transpiration. Figure 7. Open in new tabDownload slide Carbon uptake (Anet) as a function of stomatal conductance to water (gsw). Pseudotsuga menziesii exhibited stomatal regulation regardless of season, whereas P. ponderosa demonstrated greater Anet as a result of greater gsw on the north aspect during spring and fall (light blue and orange circles). Pinus ponderosa Individuals on the south aspect and throughout the other seasons controlled water loss more closely to P. menziesii. Figure 7. Open in new tabDownload slide Carbon uptake (Anet) as a function of stomatal conductance to water (gsw). Pseudotsuga menziesii exhibited stomatal regulation regardless of season, whereas P. ponderosa demonstrated greater Anet as a result of greater gsw on the north aspect during spring and fall (light blue and orange circles). Pinus ponderosa Individuals on the south aspect and throughout the other seasons controlled water loss more closely to P. menziesii. Water-use efficiency (WUE), calculated as μmol CO2 assimilated per mmol H2O released, followed a seasonal pattern that closely mirrored Anet (Figure 6E). Relative to the spring and fall, there was a species-independent reduction in WUE during the dry summer period that resulted from the combination of both unchanged or increased transpiration and slightly decreased Anet. Water-use efficiency rebounded similarly for both species with the onset of the monsoon and remained similar throughout the fall. Notably, P. menziesii individuals were the most water-use efficient during the winter when P. ponderosa individuals were the least water-use efficient, but very low transpiration values (denominator of the WUE equation) may have confounded these data. There was no significant difference in WUE between individuals of the same species on north versus south aspects at any point during the study. Discussion This study characterized seasonal patterns of carbon assimilation mediated by topographical changes in soil moisture. Consistent with Hinckley et al. (2014) and Pelletier et al. (2018), we found that soil moisture conditions were heavily influenced by sun exposure (incoming shortwave radiation) in complex terrain, such that equatorial-facing slopes were depleted of key moisture resources more quickly than poleward-facing slopes (Figure 4C). This was evidenced by lower rates of snowmelt and ET that extended the impact of precipitation pulses on north-facing slopes. The topographic moisture effect manifested in significant ecophysiological differences during the spring and fall when P. ponderosa carbon assimilation on the north-facing aspect was, on average, double that of individuals on the south-facing aspect. At the leaf scale, aspect-driven differences in stomatal conductance could have also contributed to reduced Anet on south-facing slopes where within-canopy VPDs are likely to be higher (Fekedulegn et al. 2003). The combination of ecophysiology- and terrain-driven impacts on water and nutrient cycling highlights the interconnected relationships between abiotic state and biotic function at the ecosystem scale. This is especially important for semiarid forests in the southwestern USA, where increasing air temperature and changing monsoon precipitation dynamics are expected to alter ecosystem structure and function (Szejner et al. 2016, Pascale et al. 2019). P. ponderosa maintained greater Anet than P. menziesii throughout the year, and P. ponderosa photosynthetic activity was strongly aspect-dependent, whereas P. menziesii was not (Figure 6C). Differences in the rate of carbon uptake were strongly dependent on gsw, especially on the north aspect where P. ponderosa individuals maintained elevated water and CO2 exchange rates during both spring and fall (Figure 7). In contrast, P. menziesii limited gsw throughout all seasons, which also inhibited Anet. However, despite lower overall Anet throughout the year, P. menziesii persistently used water more efficiently than P. ponderosa (Figure 6E). Species-level differences in ‘connectivity’ between soil moisture variability and plant response could have resulted from different rooting strategies: P. menziesii has been shown to develop shallow, fibrous roots in montane ecosystems, whereas P. ponderosa generally develops deeper roots in the same environment (Berndt and Gibbons 1958). In this way, root structure may factor into the physiological differences shown in Figure 6 by way of increased photosynthetic capacity for P. ponderosa relative to P. menziesii. Mechanistically, this could result from delayed snow melt and reduced solar radiation inputs on the north aspect in spring and subsequent deeper melt water infiltration into the soil column (e.g., Fan et al. 2017). Irrespective of process, it appears that efficient water use allows P. menziesii to successfully codominate this landscape. Inter-annual meteorological variability represents another potentially important control on the patterns of plant physiological activity shown by this work. Unlike previous studies at this site (Brown-Mitic et al. 2007, Potts et al. 2017), our data indicate that carbon assimilation was maintained during the dry summer period. However, the timing and magnitude of seasonal precipitation is highly variable in this ecosystem, and 2016 was the wettest year since continuous precipitation measurements began in 2009. Further, carry-over effects from antecedent conditions can be expressed over multiple seasons or years and could be affecting interpretation of these results (Peltier et al. 2016). Similarly, relatively lesser winter Anet during our study period contrasts greater winter Anet from previous work (Grünzweig et al. 2003, Potts et al. 2017). Together, these current and previous results underscore the impact of inter-annual variability on plant and ecosystem function and the need for longer-term monitoring at instrumented sites. We attribute reduced winter Anet to uncommonly low winter air temperatures during our study period that may have limited or intermittently terminated photosynthetic activity (Bowling et al. 2018). Measurements during periods of extreme environmental conditions such as these contribute to understanding the plasticity of evergreen conifer species across potential climate scenarios. Individual leaf-level measurements provide a more comprehensive perspective on carbon and water exchange from sky island montane forests and add context to the seasonal dynamics previously found at this site (Knowles et al. 2020). Observed Anet peaks during the spring and the monsoon/fall support the bimodal intra-annual distribution of gross primary productivity demonstrated by Knowles et al. (2020) and suggest that our sample collection scheme accurately represented seasonal changes throughout the year (Figure 6). However, the spring and pre-monsoon P. ponderosa transpiration maximum, especially on north aspects, contrasts the singular monsoon ET peak characterized by Knowles et al. (2020), and thereby constrains the seasonal transpiration fraction of landscape-scale ET. Further, Knowles et al. (2020) showed peak WUE during the dry pre-monsoon season, whereas the current study suggests that trees used water less efficiently during this time relative to the other non-winter seasons. Given that Knowles et al. (2020) invoked the potential for moisture limitation during the dry summer period, our data may indicate the presence of a low moisture threshold for efficient stomatal regulation, especially on south aspects (Figure 6E). In this case, total precipitation was only 99.9 mm prior to our 20 June pre-monsoon sample collection, and previous work has highlighted the potential for springtime drought to amplify foresummer moisture limitation to growth (Knowles et al. 2018, Lian et al. 2020). Despite seasonal differences in Anet and transpiration, similar water-use patterns between species throughout the year suggest that both P. ponderosa and P. menziesii have adapted to fill their respective niches. A primary focus of ecosystem science is to understand and effectively describe the spatial and temporal patterns of ecosystem processes. New ecosystem monitoring tools and expanded sensor networks have dramatically improved our ability to bound estimates of ecosystem responses to projected climate conditions, but these data can be limited in resolution and/or coverage across space and time, and therefore insensitive to variation resulting from community structure and topography (Baatz et al. 2018, Brown et al. 2016). Further, efforts to model the forest response to environmental change include landscape scale processes, but often exclude finer-scale dynamics that may be equally important to forested ecosystem function. For example, the current study contributes to a better understanding of the relationship between slope aspect and microclimate with implications for the montane forest response to disturbance in the southwestern USA (Williams et al. 2010, Williams et al. 2013). Additionally, results of this study illustrate that species dominance within a mixed community can change the overall ecosystem carbon balance, which may not be evident from larger scale flux tower or remotely sensed data. These results add new insight into when and where fluxes of carbon and water on the landscape are variably dependent on multiscale interrelationships between the biotic, abiotic and time domains. Montane vegetation communities are subject to landscape-scale differences in soil structure, soil water retention and solar energy input that do not apply to vegetation in flat terrain (Zapata-Rios et al. 2015). The importance of snowpack to montane forest productivity is well established (Trujillo et al. 2012, Knowles et al. 2017), but the current study further identifies aspect-driven differences in photosynthetic assimilation, evidenced by increased Anet of P. ponderosa individuals growing on north relative to south facing aspects. This result constrains the potential for reduced growing season productivity as a result of lower projected snowpack in the western USA, insofar as the opposing aspects function as a mesocosm of current versus anticipated environmental conditions. Snow on south aspects ablated more rapidly than snow on north aspects, and Pinus individuals growing on north-facing slopes were more sensitive to snowmelt moisture as a result. Hence, this study joins a growing body of literature to suggest that earlier snowmelt could yield lower warm season carbon sequestration from Pinus-dominated landscapes in the future due to moisture-induced reductions in productivity during drought (Hu et al. 2010, Knowles et al. 2018). This work contributes to a more dynamic understanding of the relationship between plants and their environment, and the anticipated ecophysiological response to changing climatic conditions throughout the Pinus domain. Acknowledgments The authors wish to thank J. 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TI - Topography influences species-specific patterns of seasonal primary productivity in a semiarid montane forest
JO - Tree Physiology
DO - 10.1093/treephys/tpaa083
DA - 2020-10-07
UR - https://www.deepdyve.com/lp/oxford-university-press/topography-influences-species-specific-patterns-of-seasonal-primary-wR06f8Wjd8
SP - 1343
EP - 1354
VL - 40
IS - 10
DP - DeepDyve
ER -