Geographic range predicts photosynthetic and growth response to warming in co-occurring

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1 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE2497 Geographic range predicts photosynthetic and growth response to warming in co-occurring tree species Site and species descriptions The experiment is located at two University of Minnesota field stations; the Cloquet Forestry Center, Cloquet MN ( N, W, 382 m a.s.l., MAT, 4.8 C mean annual temperature, 783 mm mean annual precipitation) and the Hubachek Wilderness Research Center, Ely, MN, ( N, W, 415 m a.s.l., MAT at nearest weather station (Tower, MN), 2.6 C mean annual temperature, 726 mm mean annual precipitation). Climate data for is from the Midwest Regional Climate Center. At both sites, warming experiments are located on coarse-textured upland soils in year old mixed aspen-birch-fir stands scattered with pine, spruce, and other species. Treatments are positioned in both closed ( 5-10% of full sunlight) and relatively open ( 40-60% of full sunlight) overstory conditions. Both closed and open plots are exposed to experimental treatments because regeneration in both habitat types is important in determining boreal forest canopy composition, given the spatial and temporal patterns of natural and anthropogenic disturbances 30,31. The eleven species include six native broadleaf (Acer rubrum, A. saccharum, Betula papyrifera, Populus tremuloides, Quercus macrocarpa and Q. rubrum), one naturalized broadleaf (Rhamnus cathartica), and four native needle leaved (Abies balsamea, Picea glauca, Pinus banksiana, and Pinus strobus) species, all of which are present in the ecotonal region. Local ecotypes of all species except Rhamnus were planted from plant material obtained from two Minnesota Department of Natural Resources nurseries in northern Minnesota. Exact locations of origin is NATURE CLIMATE CHANGE 1

2 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE2497 unknown, but seeds were collected between 46 0 and N latitude in northeastern Minnesota. Rhamnus seedlings were transplants dug up from forests in north central Minnesota. The center of the latitudinal range distribution (west of 86 degrees longitude and east of 98 degrees longitude) was defined as the mid-point between the northern and southern range limits and obtained for all native species from range maps in the U.S. Forest Service publications, the Silvics of North America, Volumes 1 and 2. The longitudes were chosen to span equally distant east and west of our two study sites. An alternative measure of the southern range limit of each species was defined as the latitude in central North America (also west of 86 degrees longitude and east of 98 degrees longitude) above which 95% of individuals in the U.S. are found; these values were calculated using U.S. Forest Service Forest Inventory Analysis (FIA) data. We use the center of the range in our paper because the southern range limit based on FIA data is incomplete due to lack of Canadian inventory information; it does not incorporate information from the entire range, and this is problematic especially for species with a large part of their range in Canada. Comprehensive range maps do not yet exist for Rhamnus cathartica. Its southern range limit was defined as the 95 th percentile of counties where it had been observed (west of 86 degrees longitude and east of 98 degrees longitude), using the USDA Invasive Plants Atlas. Its northern range limit was defined based on personal communication with invasive species experts in Minnesota and Ontario. Despite differences in sources for the two alternative continent-wide indices, the southern range limits for the two are linearly correlated (r=0.94, P<0.0001, and indistinguishable from a 1:1 line). Moreover, the relationships of the effect size of warming on net photosynthesis and growth to species ranges are similar whether the effect size is 2 NATURE CLIMATE CHANGE

3 DOI: /NCLIMATE2497 SUPPLEMENTARY INFORMATION related to the center of the range (Figs 1-2) or to the southern range limit developed from FIA data (data not shown). The local ecotonal distribution index represents the proportional relative abundance of each species in the north vs. southern zone of the boreal ecotone in northeastern Minnesota based on FIA data. As the border between the boreal and temperate zones in northeastern Minnesota trends northwest-southeast (e.g., Supplementary Fig. 1), we chose to use an index (% northern distribution at the ecotone) based on comparison of the fraction of proportional basal area of each species that occurs in the northeast of Minnesota (St. Louis, Lake, and Cook counties) with that in three adjacent counties to the southwest (Carleton, Aitken, Crow Wing). The index is defined as the fraction of total relative abundance (% of total basal area represented by each species) in the northeastern zone divided by that of the northeastern and southwestern zone combined. As an example, if a species had 15% of total basal area in the northeastern zone and 5% of total basal area in the southwestern zone, its % of ecotonal abundance in north value would be 75%. Local abundance data for Rhamnus cathartica was not available from the FIA data, and instead was obtained from a set of plots surveyed by the Minnesota Department of Natural Resources (Minnesota Department of Natural Resources Minnesota s releve and Native Plant Community worksheet databases. MNDNR St. Paul, MN, USA). Given much sparser sampling by the DNR than in the FIA database, and the fact that Rhamnus cathartica was very rare in the far north, we used the five most northeastern (rather than three) Minnesota counties as well as the same three counties to their south as for the other 10 species. Gas-exchange measurements NATURE CLIMATE CHANGE 3

4 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE2497 In situ measures of net photosynthesis were made using six Li-Cor 6400 portable photosynthesis systems (Li-Cor, Lincoln, NE). Measurements were made throughout the growing seasons (June to September) of 2009 through Not all species were measured each year due to the timeconsuming nature of the measurements. Fully expanded, healthy upper canopy leaves were chosen from individuals planted in a combination of ambient, +3.4 C, open, and closed plots at both sites. Light was maintained in the leaf chamber at saturating levels using the LED light source. Airflow was set at 500 µmol s -1 and CO 2 reference concentrations were set at 400 µmol mol -1. Growth measurements We measured diameter at a marked point on the stem (5 cm above soil surface) in both east-west and north-south orientations, and total height of the leader, on each sapling in September or October of each year. We harvested the aboveground biomass of a subset (N=790 in 2011) of saplings in 2011 across species, sites, canopies, and warming treatments, and used these to build a regression relationship (R 2 = 0.95, P<0.0001) to estimate 2011 aboveground biomass values for all saplings from their diameter and leader heights. These biomass values were used in subsequent analyses. Diameter and height data (and thus biomass estimates) from more than 70% of the trees originally planted in 2008 were recorded in 2011 and used in analyses (n=4090); the remainder either had died, been removed after sampling for physiology or plant-insect studies, been removed after being damaged during the course of the experiment (e.g. by falling branch, by a researcher in the plot, etc.), or been deemed unusable due to a data entry error or missing value. Mortality in the plots was slight to modest and did not influence the results. The single species with greatest (but still moderate) mortality across all treatments, Pinus banksiana, was 4 NATURE CLIMATE CHANGE

5 DOI: /NCLIMATE2497 SUPPLEMENTARY INFORMATION intermediate in its response to warming and had little influence on comparisons such as in Figures 1 and 2. Moreover, minor differences in survival responses to warming treatments, would have, if anything further separated responses of the most-positive versus most- negative responders. This is because the boreal species with the most negative responses to warming had slightly greater warming-related mortality; thus growth assessments could be interpreted as slightly too positive for these species in the warmed treatments as the poorest growing individuals were likely those that died. This makes our results conservative in the sense that any bias likely suggests the two most boreal species may have performed even worse than our results indicate. Given that the patterns in results are already quite clear we don t think it necessary to take up more space in either the main text or supplemental materials with this point. Statistical analysis Multi-factor analyses of variance (ANOVAs) were used to compare light saturated photosynthetic rates (A sat ) to treatment combinations. Models included the following independent variables: site, species, overstory condition, warming treatment, and all interactions among variables. Plot was added to each model as a random effect. Models were initially run separately by year since different subsets of species were measured each year; additionally, models were run across years and seasons (shoulder seasons, midsummer) for the subset of species measured across years. These analyses found no interactions between treatments (or treatments x species) and site, canopy conditions, or seasons or years- in other words, responses to warming were consistent across canopies, sites, and years. Given that, we also analyzed all species responses to warming, site, and canopy conditions, pooled across years. This approach is also supported by examination of the conditions during which species were measured (Supplementary Table 2). NATURE CLIMATE CHANGE 5

6 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE2497 The two species (Abies, Picea) with the most negative photosynthetic responses to warming were measured in only one year, but they were measured under slightly cooler ambient conditions, slightly lower warming treatment temperature elevations, and similar cross-warming treatment difference in soil moisture conditions, than the other species measured in more years (Supplementary Table 2); thus on average they were measured under conditions where warming would tend to have more positive effects. Hence our estimate of the difference in the response of Abies and Picea to warming (versus other species) is slightly conservative- i.e. if measured under identical conditions one would expect the differences to be more slightly pronounced as Abies and Picea should have even more negative responses to greater warming under warmer ambient conditions. Results using only 2011 data were similar to the results using all available data. For analysis of growth, we used the initial stem diameter and height (in 2008) as covariates. All statistical analyses were conducted in JMP statistical analysis software (JMP 10.0, SAS Institute, Cary, NC). To test whether the differential response of species near their warm vs. cold range margins were similar in the two different canopy conditions, we used analysis of covariance (also known as separate slopes analysis). As this requires tests for differences in slope of contrasting linear relations, we compared the linearized slopes of response to warming vs. geographic range metrics (from Figures 1 and 2) for open versus understory plots. The slopes were linearized by taking the square of both the effect size and independent variables. In none of the four cases (two response measurements, two indices of species distribution) did the slopes differ by canopy type (P values testing for slope differences ranged from 0.72 to 0.97). Tests for whether the 6 NATURE CLIMATE CHANGE

7 DOI: /NCLIMATE2497 SUPPLEMENTARY INFORMATION intercepts of the relations in Figures 1 and 2 differed by canopy were also made by removing the non-significant interaction term (canopy x range index) and testing for differences in the overall elevations of the lines. These also were not significant, with P values ranging from 0.08 to 0.16; although the P values were in some cases near to marginal significance and all cases, the trend was for the effect size to be slightly more positive on average in closed than opened conditions. Supplementary figure legends Supplementary Figure 1. Ranges of the 10 native species in North America. Ranges from E.L. Little, Jr.; USGS web site. Supplementary Figure 2. Location of all 11 species on both measures of geographic range; the center of the species range and the % of regional population occurring north of the local ecotone. Correlation: r= 0.90, P< Supplementary Figure 3. Mean photosynthetic rates and sapling stem biomass for all species at ambient and elevated temperatures, averaged across sites and canopy conditions. NATURE CLIMATE CHANGE 7

8 Supplementary Fig. 1 Abies balsamea Betula papyrifera Picea glauca Pinus banksiana Populus tremuloides Acer rubrum Acer saccharum Pinus strobus Quercus macrocarpa Quercus rubra

9 DOI: /NCLIMATE2497 SUPPLEMENTARY INFORMATION % Abundance north of ecotone Acer rubrum Quercus rubra Acer saccharum Quercus macrocarpa Pinus strobus Rhamnus cathartica Pinus banksiana Populous tremuloides Picea glauca Abies balsamea Betula papyrifera Center of range ( o N) Supplementary Fig. 2 NATURE CLIMATE CHANGE 9

10 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE ambient o C a Stem biomass (g) b 12 Acer rubrum Quercus rubra Acer saccharum Rhamnus cathartica Quercus macrocarpa Pinus strobus Pinus banksiana Betula papyrifera Populus tremuloides Asat (µmol m -2 s -1 ) Abies balsamea Picea glauca NATURE CLIMATE CHANGE

11 DOI: /NCLIMATE2497 SUPPLEMENTARY INFORMATION Supplementary Table 1. Annual climate means for the two sites prior to and during the experiment. Tower (Ely) weather station is 43 km from the research site. Cloquet weather station is 3 km from the research site. Standard deviation (SD) is among years. Weather Station Prior to experiment ( ) During experiment ( ) Mean precipitation Mean temperature Mean precipitation Mean temperature (mm) (SD) ( o C) (SD) (mm) (SD) ( o C) (SD) Cloquet (138.5) 4.8 (1.0) (117.8) 5.1 (0.8) Tower (Ely) (135.5) 2.6 (1.0) (123.6) 3.6 (0.5) NATURE CLIMATE CHANGE 11

12 SUPPLEMENTARY INFORMATION DOI: /NCLIMATE2497 Supplementary Table 2. Average conditions ( ) under which lightsaturated photosynthesis was measured for each species. The ambient leaf temperature (temp) is for the ambient plants that were measured; the Delta is the difference in leaf temperature between those ambient plants and the warmed plants for which photosynthesis was measured; and volumetric water content (VWC) shows the average soil moisture for the ambient and warmed treatments during those measurements. Species Ambient temp o C Delta Ambient VWC Warmed VWC Closed Open Closed Open Closed Open Closed Open Abies balsamea Acer rubrum Acer saccharinum Betula papyrifera Picea glauca Pinus banksiana Pinus strobus Populous tremuloides Quercus macrocarpa Quercus rubra Rhamnus cathartica NATURE CLIMATE CHANGE