The Effects of Topography on Forest Cover in Subalpine Forest Jason Farrell Mountain Research Station | University of Colorado Boulder Winter Ecology Spring 2016 Jason Farrell Spring 2016 “The Effects of Topography on Forest Cover in Subalpine Forest” pptx: 1.7 MB Vegetation, topography, tree density, slope, aspect **Note: all images by me, unless noted otherwise

Background Soil moisture and aspect = forest cover Climate change = unknown changes Predict growth given climate change? What areas have the most growth now? Does the topography of the land have an influence on forest cover (the amount of trees)? After elevation, the most significant abiotic factor on forest cover in the subalpine is wetness1, followed by aspect (Bader and Ruijten 2008). While competition is a significant factor on forest growth patterns at lower elevations (with less extreme abiotic conditions), topography is still important in determining forest cover (Callaway 1998). Differences in slope and aspect result in variable soil properties, notably soil moisture, and because resulting microclimates affect subalpine forest cover, namely tree density (Bader and Ruijten 2008). Previous studies correlate increased soil moisture with higher tree density (Tyagi et al. 2013), and some mountain systems with warm, dry summers have shown increased tree density where snowpack is highest, especially on topographic convexities (Rochefort and Peterson). Having a deeper understanding of where and why trees thrive (and thus act as carbon stocks) will allow for better understanding of which areas will see increased or decreased growth in light of current and future changes in climate and soil moisture (Adams et al. 2014). This is significant due to the ecosystem services that subalpine forests provide, such as habitat, watershed protection, and carbon sequestration, and the need of governmental agencies to manage land with a subalpine zone.

Experiment H0 = tree density/basal area will have no significant relationship with aspect, angle, drainage character, or snow depth H1 = tree density/basal area will have a significant relationship with one or more of the following: aspect, angle, drainage character, or snow depth This project will examine the affects that slope aspect, slope angle, drainage character (a proxy for soil moisture), and snow depth have on tree density and basal area. Given that aspect, angle, and snow depth can also be proxies (or indirectly affect) soil moisture in some manner, the project’s main goal is to determine if tree density is affected by soil moisture, which is best accomplished by performing a variety of tests. The hypotheses are:   H0 = tree density will have no significant relationship with aspect, angle, drainage character, or snow depth H1 = tree density will have a significant relationship with one or more of the following: aspect, angle, drainage character, or snow depth Pictures (left to right) http://avalanche.state.co.us/forecasts/backcountry-avalanche/front-range/ http://www.justgetout.net/subaru/pages/email/?bid=03f30013-37bf-43bc-bc6d-5e0b6b505c23 http://www.library.arizona.edu/exhibits/swetc/azso/body.1_div.1.html http://pubs.usgs.gov/of/2004/1069/

Setup Definitions: Tree density: number living trees of trees per m2 (pic 1) Basal area: DBH height, biomass estimate (pic 2) Snow depth: surrogate indicating soil moisture Drainage character: relative to surrounding landscape, either flat, concave, or convex (pic 3) Procedure Site Selection Walk north of MRS to access south, east, and north facing slopes (pic 4) 15 sites in total Identified by drainage character; 5 flat samples, 5 concave, 5 convex Use transect tape to measure 10m2 plot Measure aspect, angle, elevation from center of plot Measure snow depth at all 4 corners and center, then find average depth for each plot Count all living trees (having needles/buds) Due to time and logistical constraints, random selection is not present in this project. This creates bias that may mean that the data does not accurately reflect the actual tree density of the subalpine, even after statistical tests are performed. Pictures (left to right): http://landcover.usgs.gov/luhna/chap7.php http://oak.snr.missouri.edu/nr3110/topics/basalarea.php

Results (Slope, Snow Depth) Linear regression Density vs Slope: no relation, p = 0.75588 Density vs Snow depth: no relation, p = 0.72664 Basal area vs Slope: no relation, p = 0.90518 Basal area vs Snow depth: no relation, p = 0.63383

Results (Aspect, Drainage) 2 Factor Analysis of Variance (ANOVA) Examined aspect and drainage effects on tree density and basal area 3 categories each Aspect = N (316-45 deg), E (46-135 deg), S (136-225) Drainage = mid, bottom, top slope Aspect nor drainage had significant effect on tree density Aspect no effect on basal area Drainage did have effect on basal area P = 0.01354 Overall trend shows that while aspect is variable, a clear trend is seen with drainage character; means/SE are far enough apart, not due to chance Bottom slopes had the most biomass, followed by mid slope and top slopes

Results (Aspect, Drainage) Slope is tied to drainage character Top/bottom slopes tend to be flatter than mid slopes Shows trend that basal area increases w/ drainage character

Results (Aspect, Drainage) Aspect, no matter the slope, is (surprisingly) uncorrelated with basal area

Discussion Drainage Character: bottom vs. top slope Top slope Possible that bottom slopes supported more biomass because water collects there, whereas top slopes tend to shed water Previous studies correlate increased soil moisture with higher tree density (Tyagi et al. 2013) However it is also possible that sampling bias interferes with reality, and there may be no correlation on a larger scale Anecdotally (based on my observations), bottom slopes tended to have a higher number of smaller/thinner trees Mid slopes had larger, more spaced trees Top slopes were variable, but often more sparse with exposed ground Top slope Mid slope Bottom slope

Discussion Snow Depth: potential significance? While there was no significant relationship between basal area and snow depth, when the top left data point was excluded from analysis the p value dropped to 0.0865 While still not significant, given that bottom slopes accumulate more snow, it is possible with larger sampling that snow depth would show a relationship with basal area Note the general trend Aspect did not have a significant influence on any measure of forest cover It is likely that aspect is more significant on a meso-scale, not a micro-scale Slope also had no effect on forest cover, although this could be due to sampling homogeneity; not enough variation in slope angles

Conclusion Basal area is better measure of forest cover H0 true for slope, aspect, snow depth H1 true for drainage character Basal area is likely a more accurate proxy for forest cover/tree density, as it measures biomass Forest cover appears to be related directly to the water accumulating potential of the land Logically would accumulate more snow/rain water (my data does not quite show this to be true) No significant effect of slope, aspect, or snow depth Significant effect on forest cover due to drainage character

References Adams, H. , H. Barnard, and A. Loomis. 2014. Topography alters tree growth–climate relationships in a semi-arid forested catchment. Ecosphere 5(11): 148. Bader, M. and J. Ruijten. 2008. A topography-based model of forest cover at the alpine tree line in the tropical Andes. Journal of Biogeography 35: 711–723. R. Callaway. 1998. Competition and facilitation on elevation gradients in subalpine forests of the northern Rocky Mountains, USA. Oikos 82(3): 561-573. R. Rochefort and D. Peterson. 1996. The temporal and spatial distribution of trees in subalpine meadows of Mount Rainier National Park, Washington, U.S.A. Arctic and Alpine Research 28(1): 52-59. Tyagi, J. V., N Qazi, S. Rai, and M. Singh. 2013. Analysis of soil moisture variation by forest cover structure in lower western Himalayas, India. Journal of Forestry Research 24(2): 317-324.