This document describes a new model called Spe-CN that simulates carbon and nitrogen cycling in northeastern U.S. forests. The model is parameterized for individual tree species and allows the user to input scenarios about species composition changes and harvest regimes over time. Model runs show that stands dominated by different tree species vary significantly in predicted nitrogen leaching and retention. Changes in tree species and harvest strategies also impact nitrogen retention levels, even with constant nitrogen deposition levels. The model can help identify forest management strategies that maintain production, maximize nutrient retention, and minimize losses under changing environmental conditions.

1. Impacts of Tree Species and Harvest Regimes on Nitrogen Retention
in Northeastern U.S. Forests
Gary Lovett and Katherine Crowley
Background
Forests regulate nutrient movement from the atmosphere to surface- and
groundwater, and differences in nutrient retention among stands of
different tree species can produce wide variation in nitrogen (N) leaching.
Forest N retention is a dynamic variable controlled by interactions among
silvicultural practices, changing atmospheric inputs of N, and tree species
change caused by such factors as climate shifts and invasive pests. Our
goal is to elucidate how silvicultural practices and environmental changes
interact to influence forest production, N retention, and C storage.
The Model
We developed a new model of forest carbon (C) and N cycling , called
Spe-CN, that is parameterized for individual tree species in the
northeastern US and allows the user to input scenarios of species change.
The model simulates pools of C and N in plants, detritus, and soil; it
includes species but not individual trees. The user sets scenarios such as:
• Single- or multi-species stand
• Rate and type of change in species over time
• Changes in N deposition over time
• Disturbance or harvest regime
Structure of the Spe-CN Model
(1) Stands dominated by different tree
species vary considerably in predicted
nitrate leaching, and therefore in the
amount of N retained in plants and soils.
Model runs: Simulated leaching is for 2010
Initial stands: Dominated by single species
Harvest: 80% harvest in 1910 (details below)
N deposition: Catskill Mountains of New York (details below),
stabilizing at a range of hypothetical 2010 levels
Results
(2) Changes in tree species composition and different harvest strategies may cause forest stands to
retain very different amounts of N, even when N deposition remains constant. The predicted
magnitude and direction of the changes in N retention vary with species identity and harvest regime.
c. Initial harvest (1910)
followed by a clearcut
(2060) with regeneration of
the maple-oak stand:
Spe-CN predicts that N
retention will remain
around 65% in the
regenerating maple-oak
stand, then decrease as the
stand ages.
a. Harvest (1910) and
recovery with no further
disturbance: Because maple
continues to dominate with
no further management,
Spe-CN predicts N leaching
will increase and N retention
fall to <40%.
Conclusion
Species composition, N deposition, and
forest management interact to
influence N retention in ecosystems.
Ecosystem models can help identify
management strategies that maintain
forest production, maximize nutrient
retention, and minimize nutrient losses
in the context of multiple interacting
environmental changes.
Cary Institute of Ecosystem Studies
Millbrook, NY 12545
LovettG@caryinstitute.org
Thanks to our funders:
b. Initial harvest (1910)
followed by selective harvest
(2060) for maple with
replacement by red oak:
Spe-CN predicts that with a
transition to red oak, leaching
will decrease and N retention
increase to >85%.
RO – red oak, WP – white pine, BF – balsam fir, EH – eastern hemlock, WA – white ash, RS – red spruce,
YB – yellow birch, BB – black birch, AB – American beech, PB – paper birch, SM – sugar maple, RM – red maple
Active
SOM C & NPassive
SOM C & N
NH4
+ NO3
-
NO3
- leaching
Impose scenarios of harvest,
disturbance, species change
HUMUS C & N
Sp 1 Sp 2 Sp n
PLANT C & N
(Fol., roots, wood)
CWD
C & N
Fol., roots,
fine wood
Coarse
wood
C
N
C&N
Sp 1 Sp 2 Sp n
LITTER C & N
LA NLA
Myco
N
CO2
Moisture
Temp. N
deposition
Impose scenarios
of climate change
Impose scenarios
of N emissions
NPP
0.0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6 7 8 9 10 11 12
NO3
-leaching(gm-2y-1ofN)
Species
0.25 0.45 0.65 0.85 1.05 1.25
RO WP BF EH WA RS YB BB AB PB SM RM
N dep 2010:
0
20
40
60
80
100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1800 2000 2200 2400
Nretention(%)
Nlostorgained(gNm-2y-1)
Year
Leaching Deposition Retained % Retention
0
20
40
60
80
100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1800 2000 2200 2400
Nretention(%)
Nlostorgained(gNm-2y-1) Year
Leaching Deposition Retained % Retention
0
20
40
60
80
100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1800 2000 2200 2400
Nretention(%)
Nlostorgained(gNm-2y-1)
Year
Leaching Deposition Retained % Retention
Harvest &
recovery
Peak N
deposition Ongoing sugar
maple stand
(no further
harvest)
Harvest &
recovery
Selective maple harvest
with replacement by
red oak
Harvest &
recovery Clearcut with regeneration
of maple-oak stand
Starting conditions:
Initial stand: 80% sugar maple – 20% red oak
Harvest: 80% harvest in 1910 (90% removal of
biomass, 10% forest floor disturbance)
N deposition: Catskill Mountains (Peak 1990 N
deposition 1.1 g N m-2 yr-1; 0.67 g N m-2 yr-1 by
2010; constant thereafter
NO3
- leaching from single-species stands at
a range of N deposition levels
For description of model, see Crowley, K. F., G. M. Lovett, M. A. Arthur, and K. C. Weathers. 2016. Long-term effects of pest-induced tree
species change on carbon and nitrogen cycling in northeastern US forests: A modeling analysis. Forest Ecol. Management 372:269-290.