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Climate change and caribou: effects of summer weather on forage Elizabeth A. Lenart, R. Terry Bowyer, Jay Ver Hoef, and Roger W. Ruess

Abstract: In 1989, the Chisana caribou (Rangifer tarandus) herd in the northern Wrangell Mountains, Alaska, U.S.A., declined substantially in population size and productivity. Grasses, sedges, forbs, and willows (Salix spp.) are critical components of the diet of caribou in spring and summer, and the abundance and quality of forage are influenced by climate. To evaluate effects of climatic variation on caribou forage we conducted a field experiment in subarctic tundra where light, air temperature, and precipitation were manipulated. We used a plastic tarpaulin to increase air temperature and decrease precipitation. We also decreased light intensity with a shade cloth and increased precipitation by adding water to determine climatic effects on nutrient content and biomass of forage for caribou during the summers of 1994 and 1995. The most notable treatment effect on aboveground biomass was that shading resulted in higher nitrogen concentrations in all plant growth forms. In addition, shading consistently reduced biomass in forbs during mid and late season. Water treatment increased total plant biomass in the greenhouse plots during midseason in 1994 and in late spring in 1995. Water treatment also increased late-season biomass in control plots during 1994 but had no effect on biomass in shaded plots in either 1994 or 1995. A decline in nitrogen concentration in plants occurred throughout summer, a pattern that was not evident in in vitro dry matter digestibility. Climate variation and subsequent effects on forage plants have the potential to influence the population dynamics of caribou through effects on their food supply. Résumé : En 1989, le troupeau de caribous (Rangifer tarandus) du Chisana dans les monts Wrangell du nord, en Alaska, É.-U., a subi un déclin important de sa densité et de sa productivité. Les herbes, les laîches, les plantes herbacées non graminéennes et les saules (Salix spp.) sont des composantes essentielles de l’alimentation des caribous au printemps et en été et l’abondance et la qualité du brout sont influencées par le climat. Dans le but d’évaluer les effets des variations climatiques sur le brout des caribous, nous avons procédé à une expérience sur le terrain dans la toundra subarctique où la lumière, la température de l’air et les précipitations ont été manipulées. Nous avons utilisé une bâche de plastique pour augmenter la température de l’air et diminuer les précipitations; nous avons également diminué l’intensité lumineuse au moyen d’un rideau et augmenté les précipitations en ajoutant de l’eau dans le but de déterminer les effets du climat sur le contenu en nutriments et sur la biomasse du brout des caribous durant les étés de 1994 et 1995. L’effet le plus remarquable sur la biomasse hors terre a été que la diminution de l’intensité lumineuse a donné lieu à une augmentation des concentrations d’azote dans les plantes de toutes les formes de croissance. De plus, la diminution de la lumière s’accompagnait toujours d’une réduction de la biomasse des herbacées non graminéennes vers la mi-saison et en fin de saison. Le traitement à l’eau a augmenté la biomasse totale des plantes en serre à la mi-saison en 1994 et à la fin du printemps en 1995. Le traitement à l’eau a aussi fait augmenter la biomasse de fin de saison dans les parcelles témoins en 1994, mais n’a pas eu d’effet sur la biomasse dans les zones ombragées ni en 1994, ni en 1995. L’azote a diminué pendant tout l’été, un phénomène qui ne s’est pas reflété dans la digestibilité des matières sèches in vitro. La diminution de l’intensité lumineuse a fait augmenter les concentrations d’azote dans les plantes de toutes les formes de croissance et, en particulier, dans les graminoïdes. La variation climatique et ses effets subséquents sur les plantes du brout peuvent donc potentiellement influencer la dynamique des populations de caribous en affectant leurs ressources alimentaires. [Traduit par la Rédaction]

678 Lenart et al.

Received 14 February 2001. Accepted 11 February 2002. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 29 April 2002. E.A. Lenart.1,2 Institute of Arctic Biology and Alaska Department of Fish and Game, Division of Wildlife Conservation, 1300 College Road, Fairbanks, AK 99701, U.S.A., and Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks, Fairbanks, AK 99775, U.S.A. R.T. Bowyer and R.W. Ruess. Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska Fairbanks, Fairbanks, AK 99775, U.S.A. J. Ver Hoef. Alaska Department of Fish and Game, Division of Wildlife Conservation, 1300 College Road, Fairbanks, AK 99701, U.S.A. 1 2

Corresponding author (e-mail: [email protected]). Present address: Institute of Arctic Biology and Alaska Department of Fish and Game, Division of Wildlife Conservation, 1300 College Road, Fairbanks, AK 99701, U.S.A

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Introduction Caribou (Rangifer tarandus) herds in Alaska, U.S.A., and in Canada experience short- and long-term fluctuations in population size, some of which can be dramatic (White et al. 1981; Messier et al. 1988; Adams et al. 1996). Factors influencing fluctuations in caribou populations include adverse weather, poor forage quality and availability, intraspecific competition, insect harassment, and predation (Bergerud 1980; Leader-Williams 1980; White 1983; Skogland 1985; Russell et al. 1993; Dale et al. 1994; Boertje et al. 1996). Beginning in 1989, the Chisana caribou herd in the northern Wrangell Mountains of Alaska declined markedly in both population size and productivity. Other caribou herds in interior Alaska also experienced high rates of mortality of adults, low rates of recruitment of young, and low body mass of young during 1989–1992 (Valkenburg et al. 1996). Indeed, poor nutrition was hypothesized to have depressed numbers of caribou throughout interior Alaska (Valkenburg et al. 1996). Factors contributing to these demographic changes may have included a decline in the quality and availability of forage. In spring and summer, caribou forage selectively, choosing plants high in nutrients and low in secondary compounds (Klein 1970; Bryant et al. 1983; White 1983), including willow leaves (Salix spp.), lichens, forbs, and graminoids (Boertje 1984; Barten et al. 2001). Forage quality on caribou ranges may be influenced by annual variation in climate, including changes in irradiance, temperature, and precipitation (Chapin et al. 1995). These factors affect nutrient concentrations and anti-herbivore defenses of plants, and also can affect forage availability by influencing plant growth. For example, shortterm (≤3 years) field experiments involving simulated environmental changes (i.e., increased temperature, reduced irradiance) showed variable effects on growth and nutrient content in some species of plants that are important forages for caribou. Shading reduced growth of Carex bigellowii, Eriophorum vaginatum, Eriophorum angustifolium, and dwarf birch, Betula nana (Chapin and Shaver 1985; Shaver et al. 1986; Chapin et al. 1995). Elevated air temperatures increased growth of Salix pulchra but decreased growth of E. angustifolium (Chapin and Shaver 1985). Higher air temperatures decreased nutrient concentrations and increased phenolic content in aboveground shoots of E. vaginatum, Rumex acetosa, and Solidago virgaurea (Jonasson et al. 1986). Nevertheless, the indirect effects of higher temperatures on rates of nitrogen mineralization in soil and, thereby, availability of nutrients may be more important than the direct effects of temperature on plant growth (Chapin 1983). Low levels of soil moisture have limited net primary productivity and nitrogen content in some species of tundra plants (Webber 1978; Chapin et al. 1988). Yet Carex aquatilis exhibited reduced growth in an area with a higher water table (Peterson et al. 1984). Clearly there is potential for annual variation in climate to affect the quantity and quality of forage for caribou during summer. Forage quality and availability directly influence body condition of female caribou, which in turn affects production and survivorship of young (Leader-Williams 1980; Reimers 1983; Skogland 1985, 1986), a pattern common in cervids (Albon et al. 1986; Andersen and Linnell 1998; Keech et al. 2000). For example, Cameron et al. (1993), Cameron and Ver Hoef (1994), and Gerhart et al. (1996) noted a positive

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correlation between body mass of females in autumn and their subsequent rate of parturition. In wild female reindeer in Norway, pregnancy rates also varied directly with dressed body mass (Reimers 1983). Despite evidence for a positive relation between nutritional status and reproductive performance in caribou, there are few data on forage quality and availability, and specifically how those variables are affected by climate, especially in montane environments. Moreover, climatic fluctuations are thought to affect populations of northern ungulates (Eastland and White 1991; Post et al. 1997; Bowyer et al. 1998; Portier et al. 1998; Post and Stenseth 1998, 1999), including patterns of habitat selection (Eastland et al. 1989; Rachlow and Bowyer 1998). Clearly, data on relationships among climate and range quality are necessary components for understanding the population dynamics of caribou. Climatic factors are thought to reduce the quality (e.g., high air temperature, low precipitation, full sunlight) and quantity (e.g., low air temperature, low precipitation) of forage for caribou in summer (Webber 1978; Chapin and Shaver 1985; Shaver et al. 1986; Chapin et al. 1995). To determine effects of annual variation in climate on forage quality and abundance, we used a field experiment to test whether altered light, temperature, or water availability influenced nutrient content of forage for caribou. We tested whether the following conditions affected nitrogen content, aboveground biomass, in vitro dry matter digestibility (IVDMD), or tannin content: (i) changes in available sunlight; (ii) changes in amount of precipitation; (iii) changes in ambient temperature; (iv) changes in temperature and precipitation combined; and (v) changes in available sunlight and precipitation combined. We predicted that plants receiving reduced irradiance, and thereby lower temperature, would have lower aboveground biomass, higher nitrogen concentration and digestibility (Salisbury and Ross 1978), and lower tannin content (Bryant et al. 1983) than plants grown at higher irradiance and temperature. We also hypothesized that reduced irradiance would decrease rates of photosynthesis, and thereby total biomass, but increase percent nitrogen because of increases in the ratio of nitrogen content to biomass. Percent digestibility should increase under lower irradiance and temperature because less structural carbohydrate would result from reduced growth. We predicted that supplemental water would increase biomass and percent nitrogen of plants grown at high temperatures.

Materials and methods Study area We conducted this study in the eastern portion of Wrangell – St. Elias National Park and Preserve at Solo Mountain (61°50′N, 141°50′W) in interior Alaska, U.S.A., during the summers of 1994 and 1995. The study site was located at an elevation of 1524 m in an area inhabited by the Chisana caribou herd from post-calving through summer (Fig. 1). The Chisana herd ranges throughout the Nutzotin Mountains and north Wrangell Mountains from the Nabesna River east into Yukon Territory, Canada (Fig. 1), at elevations ranging from 800 to 2000 m (C. Gardner, personal communication). Parturition is restricted to higher elevations (1460–2000 m), with parturient females typically sequestering themselves away from other © 2002 NRC Canada

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Fig. 1. Range of the Chisana caribou herd and location of the study site in summer 1994 and summer 1995, Solo Mountain, Alaska, U.S.A.

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Fig. 2. Design of the tundra-mat experiment at Solo Mountain in the summer range of the Chisana caribou herd, 14 June – 8 August 1994 and 13 June – 5 August 1995. Each treatment plot consists of eight 0.25-m2 subplots. “Clear tarpaulin” refers to the greenhouse treatment in the experiments.

caribou (Barten et al. 2001). During 1990–1995, post-parturient aggregations of 300–500 animals occurred at Solo Mountain. Predators within the range of this caribou herd include wolves (Canis lupus), grizzly bears (Ursus arctos), black bears (Ursus americanus), wolverines (Gulo gulo), golden eagles (Aquila chrysaetos), and coyotes (Canis latrans). Ungulates other than caribou in the study area include Dall’s sheep (Ovis dalli) and moose (Alces alces). Regional vegetation is a mosaic of white and black spruce (Picea glauca and Picea mariana) at lower elevations (700– 900 m), alpine tundra (predominantly a Carex–Dryas community) at intermediate elevations (1000–1550 m), and heath (Cassiope sp.), bare ground, and rugged talus slopes at high elevations (>1550–2000 m). Willows follow riparian drainages. Mountainsides are dominated by willow (principally S. pulchra), B. nana, and blueberry (Vaccinium vitas-idaea). Plant nomenclature follows Hultén (1968). The climate is typical of the subarctic region, with long cold winters and a short growing season. In summer (15 June – 15 August in 1981–1995), mean total precipitation was 139 mm, mean maximum temperature was 17.8°C, and mean temperature was 11.5°C (Nabesna Weather Station; Fig. 1). Snowfall in winter averaged 28.9 cm (1 October – 1 May in 1980–1995; Northway Weather Station). Experimental designs and field procedures We implemented our experiment in an area of representative habitat where the Chisana caribou herd was present

from post-parturition (19–26 June) through summer. The experiment, which involved tundra-mat habitat, was conducted in a tundra community of sedge and Dryas octopetala (slope <5%) consisting mainly of C. bigelowii, D. octopetala, Salix reticulata, Salix arctica, Lupinus arctica, and moss, with few lichens (Cladina sp.). In tundra-mat habitat, a 48 by 60 m grid consisting of 30 treatment plots (1.8 by 3.6 m) was established in June 1993 (Fig. 2). Percent cover of each species was estimated for all plots during 1993 prior to applying treatments to those plots in 1994. A principal-components analysis (PCA) was performed on estimates of percent cover, and these factor scores were used to compute a variogram that identified spatial autocorrelation in the grid. Based on the estimated autocorrelation in the variogram, treatments were assigned to plots by means of a simple genetic algorithm (Goldberg 1989) and simulated annealing (Geman and Geman 1984) to obtain an optimal spatial pattern that allowed maximum statistical power for detecting treatment effects (Ver Hoef and Cressie 1993). Six treatments (including one control) were applied to the 30 vegetation plots in tundra-mat habitat to simulate changes relative to extant summer conditions: a warm dry summer; a warm wet summer; a cloudy dry summer; and a cloudy wet summer. The six treatments were (1) greenhouse (clear plastic tarpaulin); (2) greenhouse with additional water; (3) shade (shade tarpaulin); (4) shade with additional water; (5) unaltered control (no tarpaulin); and (6) no tarpaulin but additional water. There were five replicates per treatment. Shade © 2002 NRC Canada

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tarpaulins allowed 40–70% of natural precipitation to pass through; greenhouse plots prevented natural precipitation from reaching the plot. Once each week, 30 L of water was added to the treatment plots to mimic above-average rainfall; additional water eliminated by the shade tarpaulin was not added back to that treatment. Treatments were applied to each plot from 14 June to 8 August 1994 and from 13 June to 5 August 1995. Each treatment plot included eight subplots, each 0.25 m2. Treatment plots were spaced 7.5 m apart horizontally and 9.8 m apart vertically (Fig. 2). Subplots were spaced 28.5 cm apart horizontally and 18 cm apart vertically, and were 28 cm from the edge of the tarpaulin. Clear and shade tarpaulins were 1.8 by 3.6 m and covered an entire plot. Tarpaulins were suspended 25–35 cm above the plant canopy in a tentlike fashion and opened at the sides to allow circulation of air. Clear tarpaulins were made from 0.5-mL polyethylene plastic. Shade tarpaulins were constructed of 50% Aluminet. We implemented a design in which plots were sampled in close proximity to one another to help reduce variance among plots. We selected that design over one in which plots were randomly located widely across the landscape, so that effects of treatments could be detected. We acknowledge that this reduces the area of inference for our experiment, but this methodology was essential to test effects of climatic variables on forages because of the variable nature of plant communities in areas inhabited by caribou (Barten et al. 2001). To assess aboveground biomass, four subplots were clipped in both 1994 and 1995. All vegetation in one subplot per treatment plot was clipped to ground level on the following dates: 10–11 June, 26–28 June, 19–22 July, and 8–12 August during 1994 and 5–8 June, 27–30 June, 20–22 July, and 6–9 August during 1995. We varied clipping dates between years so that plants would be clipped at the same phenological stage in both years. The first clipping was not done until plants were green (green-up) and there was sufficient aboveground biomass to obtain adequate samples (>2 g dry mass) for forage analyses. The second clipping was done when plants had been growing for approximately 2 weeks, the period being defined as late spring, based on plant phenology. The third clipping was done during peak biomass and the last clipping was done after plants senesced. Treatments were applied following the first clipping in each year. Plants were sorted immediately after clipping. Samples were subdivided into live and dead components, with live plants sorted further into three plant categories: forbs (e.g., L. arctica, D. octopetala, Astragulus sp., Thalictrum alpinum, Pedicularis capitata, Oxytropis nigrescens); graminoids (e.g., C. bigellowi); and prostrate willows (e.g., S. reticulata and S. arctica). Although percent cover of mosses and lichens was estimated, we removed mosses from analyses because they are not usually an important forage for caribou in summer (Boertje 1984) and their intake by caribou is probably incidental. Lichens also were removed from analyses because they were extremely low in abundance, consequently there was not adequate material for forage analyses. After sorting, plants were air-dried for 2–5 days and then stored in paper bags. In addition to aboveground biomass, percent cover of species was estimated visually on each subplot and ranked as follows: 1 (<1%); 2 (1–5%); 3 (6–10%); 4 (11– 25%); 5 (26–50%); 6 (51–75%); 7 (76–100%).

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Ambient temperature was recorded every 1.6 h from 14 June to 8 August 1994 and from 13 June to 5 August 1995 for one replicate per treatment with HOBO dataloggers. The HOBO-TEMP was placed approximately 25 cm above the ground in the center of the plot. Precipitation (±1 mm) was recorded with a rain gauge for one replicate per treatment. One soil core (4 cm diameter by 10 cm depth) was collected from the center of each treatment plot on 16 July 1994 and again on 14 July 1995. Cores were placed in plastic bags and stored in the field in a cooler with snow for 1 day, then air-freighted to Fairbanks, Alaska, and frozen for later analyses. Soil and plant analyses The litter layer was removed from each soil core prior to weighing. Cores were weighed frozen and weighed again after oven-drying for 48 h to estimate percent soil moisture as (frozen mass – dry mass)/frozen mass × 100. For our experiment, each vegetation sample was ovendried at <60°C for 48 h to constant mass and weighed to the nearest 0.01 g to estimate aboveground biomass. Leaves and flowers were included in analyses of forbs. Leaves and early buds were removed from prostrate willows, redried, and weighed. Woody material was not included because caribou seldom eat stems and twigs during summer (Boertje 1984). Samples of forbs (including leaves, stems, and flowers), graminoids, and prostrate willows (leaves and early buds) were ground in a Wiley Mill through 20-mesh (0.1 mm; for analysis of in vitro dry matter digestibility) and 40-mesh (0.05 mm; for nitrogen analysis) screens. These ground samples were stored in tightly sealed plastic bags. The nitrogen concentration in plant tissue was determined by combustion in a LECO CNS 2000 autoanalyzer at the Forest Soils Laboratory at the University of Alaska Fairbanks (Bremner and Mulvaney 1982). IVDMD was determined at the Institute of Arctic Biology at the University of Alaska Fairbanks using the method of Tilley and Terry (1963), with modifications recommended by Person et al. (1980). Rumen liquor was obtained from a fistulated reindeer at the Robert G. White Large Animal Research Station at the University of Alaska Fairbanks; the pasture diet of the reindeer was supplemented with a barley- and corn-based concentrate with crude protein ≥16%. An assay for proanthocyanidin (condensed tannin) was performed at the Institute of Arctic Biology, University of Alaska Fairbanks, following procedures outlined in Martin and Martin (1982). Tannin analyses were conducted on leaves from 120 samples of prostrate willows (S. reticulata, S. arctica, Salix hybrid) collected from tundra habitat on 20 July and 10 August 1994 and 21 July and 8 August 1995. The standard (reference sample) for all tannin assays was condensed tannin from S. pulchra. Statistical analyses Biomass, percent nitrogen, percent soil moisture, IVDMD, and tannin content from the spatially designed experiment (Fig. 2) were analyzed with the gls-variogram method (Ver Hoef and Cressie 1993) to detect differences among treatments. To analyze biomass from the tundra-mat habitat, we modified the gls-variogram for use with covariates, in a similar manner to analysis of covariance. We performed PCA on © 2002 NRC Canada

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Table 1. Mean, average maximum, and average minimum air temperatures (±1 SD), numbers of degree-days >5°C, and total precipitation in one shaded, one greenhouse, and one control plot, Solo Mountain, Alaska, U.S.A. Summer 1994 Environmental parameter

Summer 1995

Shade

Greenhouse

Control

Shade

Greenhouse

Control

9.6 ± 3.2 16.0 ± 4.6 2.8 ± 2.8 245 40.6

12.3 ± 3.8 24.0 ± 6.9 2.8 ± 2.9 385 0.0

10.8 ± 3.5 18.5 ± 5.3 2.7 ± 2.9 305 97.0

8.9 ± 2.9 15.6 ± 4.6 1.7 ± 2.7 209 118.9

10.9 ± 3.3 20.9 ± 5.4 1.7 ± 2.7 317 0.0

9.5 ± 3.2 16.7 ± 4.7 1.6 ± 2.8 245 163.3

a

Air temperature (°C) Mean Average maximum Average minimum No. of degree-days >5°C Total precipitationb (mm) a b

Collected from 14 June to 6 August (54 days). Collected from 14 June to 13 August (61 days).

medians of cover classes for each species and used factor 1 as a covariate for the biomass analyses to correct for changes in species composition across the sampling grid. The glsvariogram uses the underlying spatial variation (autocorrelation) to estimate treatment contrasts with greater precision than classical ANOVA (Ver Hoef and Cressie 1993). Ver Hoef and Cressie (1993) suggested comparing the test statistic with a standard normal distribution. Because of our small sample sizes, however, we simulated the null distribution for the gls-variogram for a variety of autocorrelation values and obtained a significant level: Z = 2.4 versus the traditional 1.96 from the standard normal distribution. We used the model Yijkm = Rij + Eijkm, where Rij is treatment effect; i = 1 (added water) or 2 (no water); j = 1 (shade), 2 (greenhouse), or 3 (control); Eijkm is a spatially explicit error term; k is row 1, 2, 3, 4, 5, or 6; and m is column 1, 2, 3, 4, or 5. We tested the following contrasts: (i) main effects of water (R11 + R12+ R13 – R21 – R22 – R23); (ii) main effects of control versus shade (R13 + R23 – R11 – R21); (iii) main effects of control versus greenhouse (R13 + R23 – R12 – R22); (iv) main effects of shade versus greenhouse (R11 + R21 – R12 – R22); (v) control versus shade for watered plots (R13 – R11); (vi) control versus greenhouse for watered plots (R13 – R12); (vii) shade versus greenhouse for watered plots (R11 – R12); (viii) control versus shade for unwatered plots (R23 – R21); (ix) control versus greenhouse for unwatered plots (R23 – R22); (x) shade versus greenhouse for unwatered plots (R21 – R22); (xi) shade–greenhouse interaction for watered plots (R11 – R12 – R21 + R22); (xii) shade with water versus shade with unwatered plots (R11 – R21); (xiii) greenhouse with watered plots versus greenhouse with unwatered plots (R12 – R22); and (xiv) control with watered plots versus control with unwatered plots (R13 – R23). Because there is a 1/20 chance of a test being significant (P = 0.05) by chance alone, and there were 168 tests, we used the following rules when interpreting data: if only one contrast per plant category per analysis (percent nitrogen, biomass, IVDMD) per clipping period was significant, or if only 1 of 14 contrasts was significant for only one time period, we attributed those outcomes to chance.

Results Total precipitation at the study site was higher in 1995 than in 1994, and average maximum temperature was higher in 1994 than in 1995 (Table 1). Changes in mean temperature between 1994 and 1995 were reflected in differences in degree-days (Table 1). Green-up occurred approximately 5–7 days earlier in 1995 than in 1994. Relative to controls, air

temperatures increased in greenhouse plots (11.9 ± 3.8°C (mean ± 1 SD)) and decreased in shade plots (9.6 ± 3.2°C; Table 1) over the 2 years. Indeed, our experimental manipulation produced the substantial variation that might be expected under a markedly changing climate. Our results are reported as contrasts from main effects (i.e., include both watered and unwatered plots per tarpaulin treatment) unless otherwise noted. No significant differences in aboveground biomass occurred among treatments for graminoids in 1994, but there was a main effect of water during the second sampling (Table 2). During sampling in late spring (28 June) in 1995, however, aboveground biomass in greenhouse plots was significantly higher (Tables 2 and 3; P < 0.05) than in both shaded and control plots, and was higher in greenhouse treatments with water than in those without water. Forb biomass in shaded plots was significantly lower than in greenhouse and control plots during both peak biomass and senescence in 1994 and during senescence in 1995 (Tables 2 and 3). In 1995, control plots were significantly higher in biomass than shaded plots during peak biomass. In 1994, the greenhouse treatment with water was significantly higher in biomass than the greenhouse treatment without water during the 20 July sampling, and the control with water was higher than the control without water during the 9 August sampling (P < 0.05). The greenhouse treatment with water was also higher in biomass than the greenhouse without water during the 28 June 1995 sampling. Although summer 1995 was wetter, most precipitation occurred in July and August. For prostrate willows, a main effect of water and some differences in biomass were detected during the first sampling in 1994, which was done prior to treatment; differences also occurred during the second sampling (including an effect of water), which was unexpected because treatments had been applied for a short time (~2 weeks; Table 2). In the third sampling (20 July), control plots were higher in biomass than both the shaded and greenhouse plots, but no differences occurred during the last sampling. In 1995, no significant differences occurred for prostrate willows (Tables 2 and 3). Soil moisture was significantly higher in shaded plots than in greenhouse plots during the 16 July 1994 sampling (36.6 ± 4.1% (mean ± 1 SD) for shade and 32.7 ± 4.9% for greenhouse). No significant differences in moisture occurred among treatments in 1995, and no differences occurred between years for control plots. Shading tended to increase the nitrogen content relative to the greenhouse treatment in both experiments. Main effects © 2002 NRC Canada

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30.19 ± 10.87a 16.39 ± 7.43b 28.08 ± 14.04a 30.39 ± 14.60a 19.83 ± 6.95b 29.93 ± 7.44a 12.02 ± 7.57 13.30 ± 6.67 19.45 ± 8.17 7.32 ± 3.38 6.80 ± 3.06 6.96 ± 2.89

1.00 ± 0.90 1.32 ± 0.65 0.64 ± 0.37

Note: Values followed by a different letter are significantly different (P < 0.05) within each sampling period; “H” indicates a main effect of water. Statistical results were obtained from glsvariogram and PCA that corrected for species composition changing across the grid.

40.73 ± 1348a 20.17 ± 10.03b 31.93 ± 8.76b 32.70 ± 16.85ab 26.20 ± 9.30a 37.21 ± 10.33b 26.12 ± 10.46 21.77 ± 6.75 22.37 ± 8.62

3.30 ± 2.41 3.87 ± 1.96 4.16 ± 2.52 ± 1.49 ± 2.42 ± 1.49

4.64 ± 3.48 4.99 ± 1.10 3.85 ± 2.79

12.50 ± 7.48 8.61 ± 3.26 11.25 ± 5.61

4.50 ± 2.12 5.98 ± 3.10 4.26 ± 2.62 4.46 ± 2.12a 3.70 ± 1.65b 2.78 ± 1.47b

8.41 ± 3.75 8.46 ± 3.20 8.15 ± 3.62 4.74 ± 4.82 3.71 ± 1.03 2.83 ± 2.12 8.51 ± 6.33 7.07 ± 3.0 7.68 ± 2.11

H 4.28 7.31 6.77 H 2.05 3.32 2.31 H 1.73 ± 0.85a 1.93 ± 1.41b 2.63 ± 2.0ab

Prostrate willow Greenhouse Shade Control Graminoids Greenhouse Shade Control Forbs Greenhouse Shade Control

26 June 10 June Plant and treatment

± 2.44b ± 3.03ab ± 2.68a

5.19 ± 2.81b 6.69 ± 2.80b 9.09 ± 3.39a

1.86 ± 1.30 1.79 ± 0.68 1.34 ± 0.89

10.35 ± 5.90 9.27 ± 6.47 7.55 ± 2.57

7 August 22 July 28 June 6 June 20 July

9 August

1995 1994

Clipping period

Table 2. Biomass (g/0.25 m2; mean ± 1 SD) measured in the tundra-mat experiment, Solo Mountain.

3.87 ± 2.71 4.81 ± 2.45 3.60 ± 2.34

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7.16 ± 2.80 6.94 ± 3.66 7.26 ± 2.96

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(watered and unwatered plots combined) of shaded plots were significantly higher in nitrogen (P < 0.05) than those of greenhouse plots for all plant growth forms on all but one sampling date in 1995 for prostrate willows, and for forbs and prostrate willows during senescence in 1994 (Table 3, Figs. 3 and 4). In addition, shaded plots were higher in nitrogen than control plots in several clippings, and control plots occasionally were significantly higher than greenhouse plots (Table 3, Figs. 3 and 4). For prostrate willows, the control without water was significantly higher in nitrogen than the control with water on 7 August 1995 (P < 0.05). Analyses of IVDMD in graminoids indicated that digestibility was significantly higher (P < 0.05) in greenhouse plots and control plots than in shaded plots during the onset of senescence in 1994, and significantly higher in greenhouse plots than in shaded or control plots during senescence in 1995 (Table 3, Figs. 5 and 6). For forbs, there was a main effect of water, and IVDMD was higher in the control with water than in the control without water during senescence in 1994 (P < 0.05) (Fig. 5). No significant differences in IVDMD were detected in 1995; however, IVDMD was higher in greenhouse plots than in shaded plots during senescence (Fig. 6). In prostrate willows, IVDMD was significantly higher in greenhouse plots than in shaded plots during senescence in 1995 (Fig. 6). In addition, for prostrate willows, IVDMD was significantly higher in the control with water than in the control without water, and was also higher in the greenhouse with water than in the greenhouse without water during senescence in 1994, which was similar to forbs. There was also a main effect of water during senescence in 1994 (Fig. 5). IVDMD was significantly higher in prostrate willows in control plots than in greenhouse plots, and there was a shade versus greenhouse interaction for water during late spring 1994. Also, IVDMD was significantly higher in control plots than in shaded plots during senescence in 1994. No significant differences in tannin content were detected in our tundra-mat experiment.

Discussion Although we detected some differences between watered and unwatered plots, in general we observed no consistent main effects of water in tundra-mat habitat. That outcome may have been influenced by differences in summer weather between 1994 and 1995. Summer 1994 was substantially drier and warmer than summer 1995. In 1995, precipitation may have been adequate to saturate the soil adjacent to our plots, thereby providing subsurface water (Table 1). In addition, green-up occurred approximately 5 days later in 1994 than in 1995. Aboveground biomass of live forbs was lower in shaded plots; thus, cloudy summers may decrease growth of forbs, especially during July and August (Table 2). In addition, there was some indication that water would limit growth of forbs during a dry summer. Biomass of graminoids (e.g., C. bigelowii) was also lower in the shaded plots during late spring (28 June) in 1995 (Table 2). Similarly, Chapin and Shaver (1985) reported that shade reduced growth of C. bigelowii in a wet tussock tundra, and Bø and Hjeljord (1991) concluded that growth of graminoids in southeastern Norway was delayed in a cloudy, wet June. Rachlow and Bowyer (1998) © 2002 NRC Canada

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671 Table 3. Significance of gls-variogram (P < 0.05, n = 10 per mean) in biomass, percent nitrogen, and percent digestibility analyses of the three plant categories in response to shade (S), greenhouse (G), and control (C) in 1995 tundra-mat experiment, Solo Mountain. Plant category and clipping period Prostrate willow Green-up Late spring Peak biomass Senescence Graminoids Green-up Late spring Peak biomass Senescence Forbs Green-up Late spring Peak biomass Senescence

Aboveground biomass

Percent nitrogen

In vitro dry matter digestibility

ns ns ns ns

ns S > G; S > C S > G; S > C S > G; S > C

ns ns ns G>S

ns G > S; G > C ns ns

S S S S

> > > >

G G; C > G G; S > C G; S > C

ns ns ns G > S; G > C

ns ns C>S G > S; G > C

S S S S

> > > >

G; S > C G; G; S > C G; S > C

ns ns ns ns

Note: Plants were collected at green-up (6 June 1995), in late spring (28 June 1995), at peak biomass (22 July 1995), and at senescence (7 August 1995); ns, no significant difference.

also reported reduced growth of grasses and forbs in alpine habitat in a cool, overcast summer. We unexpectedly detected differences in biomass among treatments in prostrate willows during the first sampling in 1994, but these did not indicate a trend (Table 2). Shade decreased aboveground biomass but increased nitrogen concentration (Fig. 4). Other studies have reported higher protein content in S. pulchra and B. nana grown in shade relative to full sun (Bø and Hjeljord 1991; Molvar et al. 1993), and higher nitrogen concentration in Eriophorum sp. growing at lower temperatures than in those growing at higher ones (Jonasson et al. 1986). In contrast, Chapin et al. (1995) reported higher nitrogen concentration in their greenhouse plots, which they attributed to indirect effects of higher temperatures increasing rates of mineralization in soil, perhaps because their treatments were warmer than ours. Although our shade treatment may have been extreme (i.e., 50% shade for the entire summer), we hypothesize that as long as temperatures remain high enough for plant growth (≥5°C), higher nitrogen concentrations in aboveground live biomass of caribou forage should be produced during a cloudy summer than during a clear summer in this type of vegetation. In addition, the soilmoisture level may be higher in a cloudy summer, because the shaded plots were significantly higher in soil moisture than were the greenhouse plots during the 1994 sampling. There is good evidence that a cloudy summer favors increasing nitrogen concentration in caribou forage, but there is some indication from our experiments that it would have a negative effect on percent digestibility (Figs. 5 and 6). In tundra-mat habitat, percent digestibility was lower in shade plots near the end of the season for graminoids, forbs, and prostrate willows in 1994 and 1995 (Figs. 5 and 6). We hypothesize that in the greenhouse plots, sugars (i.e., soluble carbohydrates) accumulated in the plant by the end of the season because of higher photosynthetic rates resulting from higher temperatures (Table 1). Percent digestibility, however, was substantially lower in all plant categories during senes-

cence in 1994 (the dry summer) than in 1995 (Fig. 6). Thus, a very warm summer may have negative effects on IVDMD by accelerating senescence. Bowyer et al. (1998) reported similar effects when a dry spring reduced the quality of willows for moose in interior Alaska. Leaves wilted early and nitrogen content and IVDMD peaked early in the season and declined rapidly thereafter (Bowyer et al. 1998). There is evidence that higher percent digestibility is associated with shade. Molvar et al. (1993) reported higher IVDMD and nitrogen concentration in S. pulchra in natural shady versus sunny sites. Other factors (e.g., sex and age of plant, secondary compounds; Bryant et al. 1983; Reichardt et al. 1990; Klein and Bay 1994) also may influence digestibility and interact with or mask effects of climate. Secondary compounds, particularly tannins, may reduce intake and digestion of some foods by herbivores (Bryant et al. 1983; Robbins et al. 1987) by binding to proteins and inhibiting absorption (Zucker 1983). Jonasson et al. (1986) and Bø and Hjeljord (1991) reported lower concentrations of secondary metabolites in a cool than in a warm summer. Differences in tannin concentration were not detected in tundra-mat habitat, which included S. arctica and S. reticulata. Those willow species may not have high concentrations of tannins, and the cloudy summer in 1995 may have naturally reduced tannin concentrations and masked effects of treatments. Nutritional effects Forage quality and availability during summer can affect physical condition and reproduction in reindeer and caribou (Reimers 1983; Cameron et al. 1993; Gerhart et al. 1996) and thereby influence population size (Leader-Williams 1980; Crête and Huot 1993). Caribou are nitrogen-deficient at the end of winter (McEwan and Whitehead 1970), like other arctic ungulates (Rachlow and Bowyer 1991, 1994; Barboza and Bowyer 2000). Female caribou need to replenish their reserves throughout summer to conceive that autumn (Reimers 1983; White 1983; Skogland 1985; Cameron et al. 1993). © 2002 NRC Canada

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Fig. 3. Percent nitrogen (mean + 1 SE; n = 10, including both watered and unwatered plots) by plant category (prostrate willow (a), graminoids (b), and forbs (c)) in tundra-mat habitat in response to shade, greenhouse, and control treatments, Solo Mountain, in 1994. Plants were collected at green-up (10 June), in late spring (26 June), at peak biomass (20 July), and at senescence (9 August). Treatments were established from 14 June to 8 August. A different letter above the bar indicates a significant difference (P < 0.05) within periods.

This short period for replenishing fat and protein reserves is influenced by climate. As was observed in our experiment, temperature and irradiance can influence the nitrogen content

of caribou forage, with a higher nitrogen concentration in aboveground biomass with lower irradiance and temperature. Therefore, during a cloudy summer, caribou may be able to © 2002 NRC Canada

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Fig. 4. Percent nitrogen (mean + 1 SE; n = 10, including both watered and unwatered plots) by plant category (prostrate willow (a), graminoids (b), and forbs (c)) in tundra-mat habitat in response to shade, greenhouse, and control treatments, Solo Mountain, in 1995. Plants were collected at green-up (6 June), in late spring (28 June), at peak biomass (22 July), and at senescence (7 August). Treatments were established from 13 June to 5 August. A different letter above the bar indicates a significant difference (P < 0.05) within periods.

acquire more nitrogen in fewer bites; but they are also probably acquiring more dead matter (particularly graminoids) from the previous year, and possibly expending more energy foraging because less green biomass is available (Boertje

1990). Thus, there could be a trade-off between availability and biomass (and perhaps digestibility) as nutritional requirements change throughout summer. Indirect effects of weather via insect harassment also may influence foraging © 2002 NRC Canada

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Fig. 5. In vitro dry matter digestibility (IVDMD; mean + 1 SE; n = 10, including both watered and unwatered plots) by plant category (prostrate willow (a), graminoids (b), and forbs (c)) in tundra-mat habitat in response to shade, greenhouse, and control treatments, Solo Mountain, in 1994. Plants were collected at green-up (10 June), in late spring (26 June), at peak biomass (20 July), and at senescence (9 August). Treatments were established from 14 June to 8 August. A different letter above the bar indicates a significant difference (P < 0.05) within periods; “H” indicates a main effect of water.

conditions; consequently, a cloudy summer could decrease insect harassment (Thomas and Kilaan 1990; Russell et al. 1993; Helle and Kojola 1994; Mörschel and Klein 1997), which would allow more time for foraging. We hypothesize

that in a caribou population below K, a cloudy summer would be more favorable than a clear, hot summer. We experimentally demonstrated that short-term variation in climate can affect nutrient quality, particularly nitrogen © 2002 NRC Canada

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Fig. 6. IVDMD (mean + 1 SE; n = 10, including both watered and unwatered plots) by plant category (prostrate willow (a), graminoids (b), and forbs (c)) in tundra-mat habitat in response to shade, greenhouse, and control treatments, Solo Mountain, in 1995. Plants were collected at green-up (6 June), in late spring (28 June), at peak biomass (22 July), and at senescence (7 August). Treatments were established from 13 June to 5 August. A different letter above the bar indicates a significant difference (P < 0.05) within periods.

content, in aboveground biomass of caribou forage. That outcome is only meaningful, however, when considered in relation to the availability of forage for individual caribou. Climate variability and subsequent effects on population dynamics of ungulates cannot be understood readily without considering the relation of the population to K (Kie et al.

2002). Predation has been reported as regulating ungulate populations in arctic ecosystems (Gasaway et al. 1983, 1992; Van Ballenberghe and Ballard 1994; Bowyer et al. 1998), and may affect the population dynamics of caribou (Bergerud 1980; Whitten et al. 1992; Dale et al. 1994; Crête and Desrosiers 1995; Adams et al. 1996; Barten et al. 2001). In addition, © 2002 NRC Canada

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Boertje et al. (1996) suggested that direct and indirect effects of adverse weather increased vulnerability to predation and influenced population size in other caribou herds in interior Alaska during the early 1990s. Thus, future studies of effects of climate on caribou populations also should consider the role of predation. Caribou, forage, and global warming Global warming is predicted to have more pronounced effects at northern latitudes (Lashof and Ahuja 1990). Effects of annual variation in weather, however, likely will have different outcomes from long-term changes in climate. For example, Chapin et al. (1995) determined that short-term responses (3 years) were poor predictors of longer term changes, at least in composition of plant communities. That outcome most likely occurred because of warming of soils and concomitant increases in nutrient cycling (Nadelhoffer et al. 1992; Chapin et al. 1995). Increased nutrient cycling could cause species composition within tundra ecosystems to change rapidly, with fast-growing species with high nutrient requirements becoming more common (Berendse and Jonasson 1992). Thus, a mosaic of taiga forest and shrublands would eventually displace arctic tundra (Bryant and Reichardt 1992). Such a change likely would affect caribou populations adversely by creating competition with browsing ungulates and eliminating food sources, especially in winter. Nonetheless, effects of global warming on forage quality and abundance, insect harassment in summer, snow conditions in winter and early spring, and their influence on caribou populations would likely be manifested long before the composition of plant communities changed. If these weather variables are tracked, even short-term results could provide transitional information on possible responses of caribou populations to effects of global warming. Because productivity in caribou populations is strongly related to their forage, global warming holds great potential to alter the population dynamics of these large mammals. Indeed, Bowyer et al. (1998) argued that climate change would likely affect populations of arctic ungulates long before it brought about changes in the composition of plant communities. Our data support that contention and suggest that more research on the relationship between climate change and population dynamics of large mammals is needed.

Acknowledgements We thank field and laboratory technicians D. Bennet, J. Bennet, R. DeLong, D. Grangaard, B. Hunter, C. Ihl, S. Kennedy, D. Lambert, L. Rossow, B. Scotton, K. Taylor, and S. Warner and pilots J. Coady, T. Overly, P. Valkenburg, and P. Zackowski. We appreciated the time given to us by Alaska Department of Fish and Game (ADFG) biologists R. Boertje, C. Gardner, and P. Valkenburg. Other staff at ADFG, D. Reed, T. Boudreau, R. Cameron, M. Johnson, and L. McCarthy, provided advice and assistance with editing and budgets. Biologists K. Jenkins and W. Route at Wrangell – St. Elias National Park and Preserve provided logistical and proposal advice. Individuals at the University of Alaska Fairbanks (UAF) provided insights throughout the project, particularly J. Bryant, K. Kielland, R. Kedrowski; we thank the Forest Soils Laboratory at UAF for use of their laboratory.

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We especially thank C. Gardner for his understanding of the Chisana caribou herd and T. Overly and individuals at Pioneer Outfitters in Chisana for providing logistical support, anecdotal observations, and an interesting experience. We especially thank S. Murley for field assistance and her constant dedication and enthusiasm to the project, and R. Delong for his support.

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© 2002 NRC Canada

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Climate change and caribou: effects of summer weather ...

mance in caribou, there are few data on forage quality and availability, and .... methodology was essential to test effects of climatic vari- ables on ...... Agronomy and Soil Science Society of America, Madison, Wis. pp. ... and machine learning.

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