This story highlights a current research project at the Hubbard Brook Experimental Forest. To read more about research projects at HBEF, visit Current Research page. Check back regularly to learn about new research projects.
The Influence of Moose and Soil Freezing on Forest Nutrient Cycling and Tree Species Success


  Contact Info:
  Contact Info:
Lynn Christenson
Institute of Ecosystem Studies
Box AB
Millbrook, NY 12545
phone: (845) 677-5343
fax: (845) 677-5976 phone: (315) 443-3434

  Adult cow browsing aspen  
  Adult cow (Willow) browsing
aspen at the Kenai Moose Research Centre, Alaska
  Experimental plot with snow removal in progress
  Moose fecal pellet grouping
  Experimental plot with snow removal in progress
  Experimental plot with snow removal in progress
  Experimental plot with snow removal in progress
  Hobble bush plot with snow removed
  Experimental plot with snow removal in progress
  Path followed to investigate snow depth and height of browse

THE WHITE MOUNTAINS of New Hampshire are home to an expanding moose population and the Hubbard Brook Experimental Forest is no exception. Moose within Hubbard Brook Valley appear to utilize upland, mixed forests during the winter season and then move to lower, wetland areas for the summer season outside the Experimental Forest. This seasonal pattern of forest utilization, coupled with studies on snow depth and soil freezing at Hubbard Brook, have led to many questions regarding moose impact to temperate forest ecosystems. Understanding the role that these large herbivores play in forest nutrient cycling dynamics is a major focus of our study.

Herbivores interact with their environment in a number of ways and influence nutrient cycling both directly and indirectly. Under some conditions herbivores can increase or accelerate nutrient cycling (Ritchie et al. 1998, Webb et al. 1995), while under other conditions they may decrease or slow the rate of nutrient cycling (Pastor et al. 1993, Kielland & Bryant 1998, Ritchie et al. 1998). One way or another, moose affect the availability of their food source either directly through browsing pressure or indirectly through fecal inputs and long-term tree species composition shifts. These large herbivores may also interact with other environmental factors, notably climate change. One potential outcome of global climate change is the delay or reduction of snow pack development in
the northeast US. These changes in snow pack may lead to increased soil freeze events experienced by NE temperate forests. Research at HBEF (Mitchell et al. 1996, Groffman et al. 2001) indicates that nutrient cycling dynamics and fine root mortality are influenced by soil freeze events associated with low snowpack conditions. This new study will explore the interactions of soil freezing on moose fecal pellet fate and plant response to soil freezing, browsing and fecal pellet additions. We are also evaluating how snow depth alters moose browsing behavior.

Some Methods Used
For tracing the fate of moose fecal nitrogen (N), 15N labeled moose droppings were produced. The Kenai Moose Research Centre in Alaska is home to 20 captive research animals, and 1 cow moose (‘Willow’) was fed a special diet of labeled browse and pelleted food supplement. Labeled moose droppings were collected and transported back to Hubbard Brook.

A total of 48, 2 m2 plots have been established on Kineo Mountain to assess the fate of moose fecal N under the canopy of three different plant species (balsam fir Abies balsamea, sugar maple Acer saccharum and hobble bush Viburnum sp), two browse treatments (no browse or mechanical browse) and two snow treatments (no snow removal or snow removal). The snow removal is being used during the 2003-2004 winter to enhance soil freezing due to the absence of the insulating snow pack. In this snow removal treatment, snow is being shoveled until >20 cm of soil frost is observed. All plots are equipped with tension lysimeters to collect soil water solutions and frost gauges for determining the depth of freezing.

To assess snow depth affects on moose browsing behavior, natural snow depth and height of browse are recorded along moose pathways. A snow manipulation is planned for winter 2004-2005 in areas where moose activity is high. Transects will have snow artificially removed, exposing twigs of understory trees. Control transects will be established next to manipulated transects, and vegetation browse will be evaluated during the spring season.

Groffman PM, Driscoll CT, Fahey TJ, Hardy JP, Fitzhugh RD and Tierney GL 2001 Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest. Biogeochem. 56:191-213

Kielland K and Bryant JP 1998 Moose herbivory in taiga: effects on biogeochemistry and vegetation dynamics in primary succession. Oikos 82:377-383

Mitchell MJ, Driscoll CT, Kahl JS, Likens GE, Murdoch PS and Pardo LH 1996 Climatic control of nitrate loss from forested watersheds in the northeastern United States. Environ. Sci. Tech. 30:2609-2612

Pastor J, Dewey B, Naiman RJ, McInnes PF and Cohen Y 1993 Moose browsing and soil fertility in the boreal forest of Isle Royale Park. Ecology 74(2) 467-480

Ritchie ME, Tilman D and Knops JMH 1998 Herbivore effects on plant and nitrogen dynamics in oak savanna. Ecology 79:165-177

Webb JR, Cosby BJ, Diviney FA Jr, Eshleman KN and Galloway JN 1995 Change in the acid-base status of an Appalcian Mountain catchment following forest defoliation by the gypsy moth. Water Air Soil Pollut 85:535-540

Date Prepared: January 2004