Biogeography of Alaska paper birch (Betula neoalaskana): latitudinal patterns in chemical defense and plant architecture

Abstract

25The latitudinal herbivory-defense hypothesis (LHDH) predicts that plants near the equator will 26 be more heavily defended against herbivores than are plants at higher latitudes. Although this 27 idea is widely found in the literature, recent studies have called this biogeographic pattern into 28 question. We sought to evaluate the LHDH in a high-latitude terrestrial ecosystem where fire and 29 mammalian herbivores may contribute to selection for higher levels of defensive chemistry. To 30 address this objective, we collected seeds of Alaska paper birch (Betula neoalaskana) from nine 31 locations along two north-south transects between 55° N and 62° N latitudes in western, interior 32Canada. The birch seeds were planted in pots in a common garden in Madison, Wisconsin, 33 USA. From the resulting seedlings, we determined levels of chemical defense by assessing the 34 density of resin glands, which have been shown to be negatively correlated with browsing. To 35 assess plant architectural traits such as height, mean individual leaf area, and root-to-shoot ratio, 36we harvested a subset of the birch seedlings. Further, we used these traits to examine growth-37 defense tradeoffs. Contrary to the LHDH, we found a positive correlation between chemical 38 defense and latitude. Investigating relationships with fire, we found a strong positive correlation 39 between resin gland density and percent of area annually burned (PAAB) around each collection 40 location and also between PAAB and latitude. Additionally, birch seedlings originating from 41 higher latitudes were shorter, smaller-leaved, and rootier than their lower-latitude counterparts. 42Growth-defense tradeoffs were observed in negative correlations between resin gland density 43 and height and leaf size. Seedlings with higher resin gland densities also allocated less biomass 44 to shoots and more to roots. These results further call into question the LHDH and provide 45 specific information about latitudinal trends in plant defense at high, northern latitudes where fire 46 3 is a major ecosystem driver and mammals are the main herbivores of deciduous trees such as 47 birches. We propose that these interconnected relationships are the key drivers of the positive 48 correlation between defense and latitude in B. neoalaskana.

Population Variation, Environmental Gradients, and the Evolutionary Ecology of Plant Defense against Herbivory

“…For example, resource limitation may enforce stronger trade-offs for highly defended species from stressful habitats compared with species originating from less stressful habitats (Hahn and Maron 2016;Robinson and Strauss 2018). Chemically defended species like milkweeds (Woods et al 2012) and mints (Rokaya et al 2016) or slow-growing species like boreal trees (Stevens et al 2016) and sagebrush (Pratt and Mooney 2013) may be more likely to exhibit growth-defense trade-offs across populations than less defended fast-growing species. In our study, although we found that most pairwise correlations were neutral or positive, we did find some evidence for trade-offs between growth and a chemical defense (i.e., cardenolides) in the common garden.…”

“…Similarly, among milkweed species, tropical species tend to be more chemically defended, whereas temperate species tend to be less defended but more clonal (Pellissier et al 2016). Information on the habitat in which a species evolved, therefore, seems critical for describing intraspecific growthdefense strategies (Rokaya et al 2016;Stevens et al 2016;Robinson and Strauss 2018) and, importantly, correlations among growth and defense may differ depending on the traits being compared.…”

“…On the basis of its intuitive appeal and widespread empirical support (Endara and Coley 2011;Massad et al 2011), the RAH has also been invoked to explain intraspecific patterns in plant defense allocation. Although some studies support the RAH (e.g., Woods et al 2012;Burghardt 2016;Stevens et al 2016), there is generally poor support for it within species (Hahn and Maron 2016). The RAH has also been extended to describe intraspecific variation in growth and defense at both the physiological level and the evolutionary level (Herms and Mattson 1992).…”

“…First, measuring traits in situ among populations distributed across a climate gradient can inform how climate influences natural phenotypic variation in plant defense traits, including both environmentally induced (i.e., plasticity) and genetically based contributions (Castillo et al 2013;Abdala-Roberts et al 2016a, 2016b. Second, because geographically separated populations have experienced repeated bouts of selection imposed by abiotic and biotic factors under their unique environmental conditions (Agrawal 2011), common-garden experiments can reveal evolved differences in defense traits among populations as set by the environment (i.e., genetic differentiation; Anstett et al 2015;Stevens et al 2016;Kooyers et al 2017;Tomiolo et al 2017). As such, among-population studies that combine field observations with common gardens have the potential to evaluate how variation in resource availability and herbivore pressure might underpin patterns of defense, both plastically and genetically, within species (Pennings et al 2009;Woods et al 2012;Rasmann et al 2014a;Baskett and Schemske 2018).…”

Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

“…For example, plant species growing in nutrientpoor soils invest more in chemical defences relative to species growing in nutrient-rich soils (Coley et al 1985, Coley andBarone 1996), and such effects may operate concurrently with herbivory along latitudinal gradients (Johnson andRasmann 2011, Abdala-Roberts et al 2016a). Further, the effect of abiotic factors may not vary latitudinally or may show latitudinal trends opposite to those of herbivory which may lead to unexpected latitudinal trends in plant defence investment (Abdala-Roberts et al 2016a, Stevens et al 2016. Therefore, addressing the simultaneous effects of multiple abiotic drivers is necessary for understanding how abiotic forcing influences plant traits, herbivores, or both along latitudinal gradients.…”