Species Interactions and Abiotic Effects on Stand Structure of Bristlecone Pine (Pinus Longaeva) and Limber Pine (Pinus Flexilis)
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Species interactions and abiotic effects on stand structure of bristlecone pine (Pinus longaeva) and limber pine (Pinus flexilis) Selene Arellano1, Anna Douglas2, Neira Ibrahimovic3 , Janette Jin1 1University of California, Berkeley; 2University of California, Santa Cruz; 3University of California, Los Angeles ABSTRACT Plants experience a wide range of biotic and abiotic stresses based on their environmental conditions. Demographic stages—recruits, saplings, and adults—may react differently under stress. In this study, we examine how the factors of aspect, substrate, and competition affect the stand structure of Great Basin bristlecone pine (Pinus longaeva) and limber pine (Pinus flexilis), which are among the few trees that can withstand the stressful subalpine environment. We surveyed an even distribution of north and south- facing slopes with both dolomite and granite substrate in the White Mountains of Inyo National Forest, California. We recorded the density of different demographic stages of both tree species and tested how neighboring trees affected the cone production of bristlecones. We found that the effect of aspect and substrate depended on the demographic stage. We also found no evidence of competition between bristlecone and limber. Taken together, our results suggest that abiotic factors are more important than biotic factors in determining bristlecone and limber establishment. Overall, we suggest that abiotic factors are more influential in shaping subalpine plant communities. Keywords: bristlecone pine, limber pine, competition, abiotic stress, stand structure INTRODUCTION and have low levels of phosphorus, which is an essential element for plant development Plants may experience extreme stress (Wright and Mooney 1965, Malhotra et al. when they are growing in environments with 2018). Slope aspect is another stress- challenging abiotic or biotic conditions. inducing abiotic factor. South-facing slopes Abiotic stressors can include intense solar in the northern hemisphere have higher radiation, low soil moisture, nutrient solar radiation and drier soils, which limits a deficient substrate, or limited water plant’s water access. Therefore, more availability (Beasley and Klemmedson 1973, vegetative cover is generally associated with Harsch et al. 2009). Substrate is a prominent north-facing slopes regardless of season abiotic factor affecting a plant's growing (Toro Guerrero et al. 2016). Biotic conditions (Wright and Mooney 1965). For interactions, liKe interspecific competition, example, dolomite-derived soils are alKaline are another form of stress that can inhibit CEC Research | https://doi.org/10.21973/N37H45 Summer 2021 Vol. 5, Issue 5 1/13 plant growth (Godsoe et al. 2017). stressful environments because they are Interspecific competition can impact species better adapted to abiotic subalpine distributions when one species alters the conditions liKe dolomite substrate. As young growth rates and fitness of another species limbers are establishing themselves into (Abrams 1987, Godsoe et al. 2017). areas formerly occupied by bristlecone, Combinations of biotic and abiotic stress more interactions between bristlecone and are prevalent in the subalpine environment limber are occurring (Millar et al.). of the White Mountains, Inyo National Research on bristlecone and limber has Forest. The two co-dominant tree species in historically focused on dendrochronology, as the White Mountains are Great Basin bristlecone’s slow-growth strategies bristlecone pine (Pinus longaeva, hereafter preserve climate records (Furguson 1968). referred to as bristlecone) and limber pine Few studies have analyzed the stand (Pinus flexilis, hereafter referred to as structure and community dynamics of limber). While most plants cannot grow in forests dominated by bristlecone and limber extremely stressful environments, (Baker 1992). Moreover, there is conflicting bristlecone and limber are adapted to evidence about the relative importance of surviving harsh subalpine conditions abiotic and biotic factors on tree (Mooney et al. 1962). establishment. While some studies have Bristlecones are the oldest non-clonal found abiotic and biotic variables were living organisms on Earth (Furguson 1968). equally important in determining tree stand They are slow-growth trees mainly found in structure, other studies have found abiotic nutrient-poor dolomite substrate (Brown variables played a larger role (Meier et al. and Schoettle, 2008). Bristlecone forests are 2010, Godsoe et al. 2017). characterized by short warm seasons and In this study, we examined how aspect and limited moisture availability (Brunstein and substrate affected the stand structure of Yamaguchi 2018, LaMarche and StocKton bristlecone and limber in the White 1974). Bristlecones have a more restricted Mountains. We predicted there would be elevational range (2200–3700 m) than more trees from all demographic stages on limbers (2290–3350 m) and are usually north-facing slopes due to better growing found in drier parts of the White Mountains conditions (Wright and Mooney 1965). We (Brown and Schoettle 2008). While also predicted there would be overall more bristlecones typically encounter minimal bristlecone on dolomite and more limber on interspecific competition from other plants granite due to previous research reporting due to the harsh abiotic conditions they the same trends (Wright and Mooney 1965). endure in the subalpine (Beasley and Finally, we predicted there would be Klemmedson 1973), they may experience interspecific competition between competition from limbers (Smithers 2017). It bristlecone and limber because limber are is possible bristlecones inhabit harsher known to be a more competitively dominant environments because the presence of species (Windmuller-Campione and Long limber prevents them from living on more 2016). Our study provides implications for nutrient-rich granite soils (Billings and stand structure and community dynamics of Thompson 1957 , Wright and Mooney 1965). old-growth trees in stressful environments. Alternatively, bristlecones may inhabit more CEC Research | https://doi.org/10.21973/N37H45 Summer 2021 Vol. 5, Issue 5 2/13 METHODS cold, moist conditions on north-facing slopes (Ferguson 1968). The hillsides are typically 2.1 Study System dominated by granite, dolomite, and quartzite substrates (Wright and Mooney We conducted our study in the White 1965). Average temperatures in the White Mountains of Inyo National Forest, California Mountains range from -14.4°C to 18.3°C, and (37°N, 118°W; 3048–3353 m) from August average precipitation is 1235 mm. 1–6, 2021 (Fig. 1). The rain shadow effect in Bristlecone and limber are two dominant the White Mountains creates a harsh tree species able to withstand the strenuous subalpine habitat characterized by dry, subalpine environment. warm conditions on south-facing slopes, and Figure 1: Map of White Mountain survey sites. We surveyed a total of 52 900 m2 plots in the White Mountains of California. The red/pink colors represent forests dominated by granite substrate. The indigo/violet colors represent forests dominated by dolomite substrate. The darker shades of red and indigo represent north-facing slopes and the corresponding lighter shades of pink and violet represent south-facing slopes. 2.2 Site Level Surveys substrate, and the remainder were dominated by dolomite substrate. For each To examine how abiotic factors affected set of 26 substrate-specific plots, half of the the stand structure of bristlecone and sites were north-facing slopes and half of the limbers, we surveyed 52 plots. Of these sites were south-facing slopes. Every plot plots, half were dominated by granite CEC Research | https://doi.org/10.21973/N37H45 Summer 2021 Vol. 5, Issue 5 3/13 had an area of 900 m2 that we demarcated number of steps, and then selecting the tree with two 30 m transects laid perpendicularly closest to where we landed. At times when to each other. We laid the transects out there were fewer than two trees of a specific randomly along the mountainside and species in our plot, we only surveyed the surveyed 3–5 plots on the north-facing slope trees present. For every tree we selected, we and 3–5 plots on the south-facing slope counted the number of cones on the tree as before continuing the process at a new site. a measure of reproductive fitness. For large Upon surveying each plot, we counted and trees, cone number was visually estimated recorded the total number of live recruits, by counting the number of cones in one saplings and adults for both bristlecone and section of the tree, then scaling by the limber. All cone-bearing trees were classified number of equivalent sections on the tree. as adults. The non-cone bearing trees were We also measured the circumference of the distinguished by size, with trees less than tree, the distance to the nearest neighboring half a meter being classified as recruits and adult tree, and the circumference of the trees over half a meter being classified as neighboring tree. We noted whether the saplings (Fig. 2). neighboring tree was a bristlecone or limber. Overall, we surveyed 90 bristlecones and 46 limbers, for a total of 136 trees. 2.4 Statistical Analysis All statistical analyses were conducted using JMP v.16 Pro software. We ran a two- way ANOVA to test whether the interaction between demographic stage and species had an effect on tree density. We also ran two- way ANOVAs to test whether the substrate, Figure 2: Recruit, sapling, and adult demographic stage of bristlecone